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L_0313 | star classification | T_1714 | Think about how the color of a piece of metal changes with temperature. A coil of an electric stove will start out black, but with added heat will start to glow a dull red. With more heat, the coil turns a brighter red, then orange. At extremely high temperatures the coil will turn yellow-white, or even blue-white (its hard to imagine a stove coil getting that hot). A stars color is also determined by the temperature of the stars surface. Relatively cool stars are red, warmer stars are orange or yellow, and extremely hot stars are blue or blue-white (Figure 1.1). | text | null |
L_0313 | star classification | T_1715 | Color is the most common way to classify stars. Table 1.1 shows the classification system. The class of a star is given by a letter. Each letter corresponds to a color, and also to a range of temperatures. Note that these letters dont match the color names; they are left over from an older system that is no longer used. Class O B A F G K M Color Blue Blue-white White Yellowish-white Yellow Orange Red Temperature Range 30,000 K or more 10,000-30,000 K 7,500-10,000 K 6,000-7,500 K 5,500-6,000 K 3,500-5,000 K 2,000-3,500 K Sample Star Zeta Ophiuchi Rigel Altair Procyon A Sun Epsilon Indi Betelgeuse, Proxima Cen- tauri For most stars, surface temperature is also related to size. Bigger stars produce more energy, so their surfaces are hotter. Click image to the left or use the URL below. URL: Click image to the left or use the URL below. URL: | text | null |
L_0314 | star constellations | T_1716 | When you look at the sky on a clear night, you can see dozens, perhaps even hundreds, of tiny points of light. Almost every one of these points of light is a star, a giant ball of glowing gas at a very, very high temperature. Stars differ in size, temperature, and age, but they all appear to be made up of the same elements and to behave according to the same principles. | text | null |
L_0314 | star constellations | T_1717 | People of many different cultures, including the Greeks, identified patterns of stars in the sky. We call these patterns constellations. Figure 1.1 shows one of the most easily recognized constellations. Why do the patterns in constellations and in groups or clusters of stars, called asterisms, stay the same night after night? Although the stars move across the sky, they stay in the same patterns. This is because the apparent nightly motion of the stars is actually caused by the rotation of Earth on its axis. The patterns also shift in the sky with the seasons as Earth revolves around the Sun. As a result, people in a particular location can see different constellations in the winter than in the summer. For example, in the Northern Hemisphere Orion is a prominent constellation in the winter sky, but not in the summer sky. This is the annual traverse of the constellations. | text | null |
L_0314 | star constellations | T_1718 | Although the stars in a constellation appear close together as we see them in our night sky, they are not at all close together out in space. In the constellation Orion, the stars visible to the naked eye are at distances ranging from just 26 light-years (which is relatively close to Earth) to several thousand light-years away. Click image to the left or use the URL below. URL: | text | null |
L_0314 | star constellations | T_1719 | There is no reason to think that the alignment of the stars has anything to do with events that happen on Earth. The constellations were defined by people who noticed that patterns could be made from stars, but the patterns do not reflect any characteristics of the stars themselves. When scientific tests are done to provide evidence in support of astrological ideas, the tests fail. When a scientific idea fails, it is abandoned or modified. Astrologers do not change or abandon their ideas. Click image to the left or use the URL below. URL: | text | null |
L_0315 | star power | T_1720 | The Sun is Earths major source of energy, yet the planet only receives a small portion of its energy. The Sun is just an ordinary star. Many stars produce much more energy than the Sun. The energy source for all stars is nuclear fusion. | text | null |
L_0315 | star power | T_1721 | Stars are made mostly of hydrogen and helium, which are packed so densely in a star that in the stars center the pressure is great enough to initiate nuclear fusion reactions. In a nuclear fusion reaction, the nuclei of two atoms combine to create a new atom. Most commonly, in the core of a star, two hydrogen atoms fuse to become a helium atom. Although nuclear fusion reactions require a lot of energy to get started, once they are going they produce enormous amounts of energy (Figure 1.1). In a star, the energy from fusion reactions in the core pushes outward to balance the inward pull of gravity. This energy moves outward through the layers of the star until it finally reaches the stars outer surface. The outer layer of the star glows brightly, sending the energy out into space as electromagnetic radiation, including visible light, heat, ultraviolet light, and radio waves (Figure 1.2). | text | null |
L_0315 | star power | T_1722 | In particle accelerators, subatomic particles are propelled until they have attained almost the same amount of energy as found in the core of a star (Figure 1.3). When these particles collide head-on, new particles are created. This process simulates the nuclear fusion that takes place in the cores of stars. The process also simulates the conditions A diagram of a star like the Sun. that allowed for the first helium atom to be produced from the collision of two hydrogen atoms in the first few minutes of the universe. The SLAC National Accelerator Lab in California can propel particles a straight 2 mi (3.2 km). The CERN Particle Accelerator presented in this video is the worlds largest and most powerful particle accelerator. The accelerator can boost subatomic particles to energy levels that simulate conditions in the stars and in the early history of the universe before stars formed. Click image to the left or use the URL below. URL: | text | null |
L_0315 | star power | T_1722 | In particle accelerators, subatomic particles are propelled until they have attained almost the same amount of energy as found in the core of a star (Figure 1.3). When these particles collide head-on, new particles are created. This process simulates the nuclear fusion that takes place in the cores of stars. The process also simulates the conditions A diagram of a star like the Sun. that allowed for the first helium atom to be produced from the collision of two hydrogen atoms in the first few minutes of the universe. The SLAC National Accelerator Lab in California can propel particles a straight 2 mi (3.2 km). The CERN Particle Accelerator presented in this video is the worlds largest and most powerful particle accelerator. The accelerator can boost subatomic particles to energy levels that simulate conditions in the stars and in the early history of the universe before stars formed. Click image to the left or use the URL below. URL: | text | null |
L_0316 | states of water | T_1723 | Water is simply two atoms of hydrogen and one atom of oxygen bonded together (Figure 1.1). The hydrogen ions are on one side of the oxygen ion, making water a polar molecule. This means that one side, the side with the hydrogen ions, has a slightly positive electrical charge. The other side, the side without the hydrogen ions, has a slightly negative charge. Despite its simplicity, water has remarkable properties. Water expands when it freezes, has high surface tension (because of the polar nature of the molecules, they tend to stick together), and others. Without water, life might not be able to exist on Earth and it certainly would not have the tremendous complexity and diversity that we see. | text | null |
L_0316 | states of water | T_1724 | Water is the only substance on Earth that is present in all three states of matter - as a solid, liquid or gas. (And Earth is the only planet where water is abundantly present in all three states.) Because of the ranges in temperature in specific locations around the planet, all three phases may be present in a single location or in a region. The three phases are solid (ice or snow), liquid (water), and gas (water vapor). See ice, water, and clouds (Figure 1.2). (a) Ice floating in the sea. Can you find all three phases of water in this image? (b) Liquid water. (c) Water vapor is invisible, but clouds that form when water vapor condenses are not. Click image to the left or use the URL below. URL: | text | null |
L_0316 | states of water | T_1724 | Water is the only substance on Earth that is present in all three states of matter - as a solid, liquid or gas. (And Earth is the only planet where water is abundantly present in all three states.) Because of the ranges in temperature in specific locations around the planet, all three phases may be present in a single location or in a region. The three phases are solid (ice or snow), liquid (water), and gas (water vapor). See ice, water, and clouds (Figure 1.2). (a) Ice floating in the sea. Can you find all three phases of water in this image? (b) Liquid water. (c) Water vapor is invisible, but clouds that form when water vapor condenses are not. Click image to the left or use the URL below. URL: | text | null |
L_0318 | stratosphere | T_1729 | There is little mixing between the stratosphere, the layer above the troposphere, and the troposphere below it. The two layers are quite separate. Sometimes ash and gas from a large volcanic eruption may burst into the stratosphere. Once in the stratosphere, it remains suspended there for many years because there is so little mixing between the two layers. | text | null |
L_0318 | stratosphere | T_1730 | In the stratosphere, temperature increases with altitude. What is the heat source for the stratosphere? The direct heat source for the stratosphere is the Sun. The ozone layer in the stratosphere absorbs high energy ultraviolet radiation, which breaks the ozone molecule (3-oxygens) apart into an oxygen molecule (2-oxygens) and an oxygen atom (1- oxygen). In the mid-stratosphere there is less UV light and so the oxygen atom and molecule recombine to from ozone. The creation of the ozone molecule releases heat. Because warmer, less dense air sits over cooler, denser air, air in the stratosphere is stable. As a result, there is little mixing of air within the layer. There is also little interaction between the troposphere and stratosphere for this reason. | text | null |
L_0318 | stratosphere | T_1731 | The ozone layer is found within the stratosphere between 15 to 30 km (9 to 19 miles) altitude. The ozone layer has a low concentration of ozone; its just higher than the concentration elsewhere. The thickness of the ozone layer varies by the season and also by latitude. Ozone is created in the stratosphere by solar energy. Ultraviolet radiation splits an oxygen molecule into two oxygen atoms. One oxygen atom combines with another oxygen molecule to create an ozone molecule, O3 . The ozone is unstable and is later split into an oxygen molecule and an oxygen atom. This is a natural cycle that leaves some ozone in the stratosphere. The ozone layer is extremely important because ozone gas in the stratosphere absorbs most of the Suns harmful ultraviolet (UV) radiation. Because of this, the ozone layer protects life on Earth. High-energy UV light penetrates cells and damages DNA, leading to cell death (which we know as a bad sunburn). Organisms on Earth are not adapted to heavy UV exposure, which kills or damages them. Without the ozone layer to absorb UVC and UVB radiation, most complex life on Earth would not survive long. Click image to the left or use the URL below. URL: Click image to the left or use the URL below. URL: | text | null |
L_0319 | streams and rivers | T_1732 | Streams are bodies of water that have a current; they are in constant motion. Geologists recognize many categories of streams depending on their size, depth, speed, and location. Creeks, brooks, tributaries, bayous, and rivers are all streams. In streams, water always flows downhill, but the form that downhill movement takes varies with rock type, topography, and many other factors. Stream erosion and deposition are extremely important creators and destroyers of landforms. Rivers are the largest streams. People have used rivers since the beginning of civilization as a source of water, food, transportation, defense, power, recreation, and waste disposal. With its high mountains, valleys and Pacific coastline, the western United States exhibits nearly all of the features common to rivers and streams. The photos below are from the western states of Montana, California and Colorado. | text | null |
L_0319 | streams and rivers | T_1733 | A stream originates at its source. A source is likely to be in the high mountains where snows collect in winter and melt in summer, or a source might be a spring. A stream may have more than one source. Two streams come together at a confluence. The smaller of the two streams is a tributary of the larger stream (Figure 1.1). The confluence between the Yellowstone River and one of its tributaries, the Gar- diner River, in Montana. The point at which a stream comes into a large body of water, like an ocean or a lake, is called the mouth. Where the stream meets the ocean or lake is an estuary (Figure 1.2). The mouth of the Klamath River creates an estuary where it flows into the Pacific Ocean in California. The mix of fresh and salt water where a river runs into the ocean creates a diversity of environments where many different types of organisms create unique ecosystems. | text | null |
L_0319 | streams and rivers | T_1733 | A stream originates at its source. A source is likely to be in the high mountains where snows collect in winter and melt in summer, or a source might be a spring. A stream may have more than one source. Two streams come together at a confluence. The smaller of the two streams is a tributary of the larger stream (Figure 1.1). The confluence between the Yellowstone River and one of its tributaries, the Gar- diner River, in Montana. The point at which a stream comes into a large body of water, like an ocean or a lake, is called the mouth. Where the stream meets the ocean or lake is an estuary (Figure 1.2). The mouth of the Klamath River creates an estuary where it flows into the Pacific Ocean in California. The mix of fresh and salt water where a river runs into the ocean creates a diversity of environments where many different types of organisms create unique ecosystems. | text | null |
L_0319 | streams and rivers | T_1734 | As a stream flows from higher elevations, like in the mountains, towards lower elevations, like the ocean, the work of the stream changes. At a streams headwaters, often high in the mountains, gradients are steep (Figure 1.3). The stream moves fast and does lots of work eroding the stream bed. Headwaters of the Roaring Fork River in Colorado. As a stream moves into lower areas, the gradient is not as steep. Now the stream does more work eroding the edges of its banks. Many streams develop curves in their channels called meanders (Figure 1.4). As the river moves onto flatter ground, the stream erodes the outer edges of its banks to carve a floodplain, which is a flat, level area surrounding the stream channel (Figure 1.5). Base level is where a stream meets a large body of standing water, usually the ocean, but sometimes a lake or pond. Streams work to down cut in their stream beds until they reach base level. The higher the elevation, the farther the stream is from where it will reach base level and the more cutting it has to do. The ultimate base level is sea level. | text | null |
L_0319 | streams and rivers | T_1734 | As a stream flows from higher elevations, like in the mountains, towards lower elevations, like the ocean, the work of the stream changes. At a streams headwaters, often high in the mountains, gradients are steep (Figure 1.3). The stream moves fast and does lots of work eroding the stream bed. Headwaters of the Roaring Fork River in Colorado. As a stream moves into lower areas, the gradient is not as steep. Now the stream does more work eroding the edges of its banks. Many streams develop curves in their channels called meanders (Figure 1.4). As the river moves onto flatter ground, the stream erodes the outer edges of its banks to carve a floodplain, which is a flat, level area surrounding the stream channel (Figure 1.5). Base level is where a stream meets a large body of standing water, usually the ocean, but sometimes a lake or pond. Streams work to down cut in their stream beds until they reach base level. The higher the elevation, the farther the stream is from where it will reach base level and the more cutting it has to do. The ultimate base level is sea level. | text | null |
L_0319 | streams and rivers | T_1735 | A divide is a topographically high area that separates a landscape into different water basins (Figure 1.6). Rain that falls on the north side of a ridge flows into the northern drainage basin and rain that falls on the south side flows into the southern drainage basin. On a much grander scale, entire continents have divides, known as continental divides. A green floodplain surrounds the Red Rock River as it flows through Montana. (a) The divides of North America. In the Rocky Mountains in Colorado, where does a raindrop falling on the western slope end up? How about on the eastern slope? (b) At Triple Divide Peak in Montana water may flow to the Pacific, the Atlantic, or Hudson Bay depending on where it falls. Can you locate where in the map of North America this peak sits? | text | null |
L_0319 | streams and rivers | T_1735 | A divide is a topographically high area that separates a landscape into different water basins (Figure 1.6). Rain that falls on the north side of a ridge flows into the northern drainage basin and rain that falls on the south side flows into the southern drainage basin. On a much grander scale, entire continents have divides, known as continental divides. A green floodplain surrounds the Red Rock River as it flows through Montana. (a) The divides of North America. In the Rocky Mountains in Colorado, where does a raindrop falling on the western slope end up? How about on the eastern slope? (b) At Triple Divide Peak in Montana water may flow to the Pacific, the Atlantic, or Hudson Bay depending on where it falls. Can you locate where in the map of North America this peak sits? | text | null |
L_0320 | supervolcanoes | T_1736 | Supervolcano eruptions are extremely rare in Earths history. Its a good thing because they are unimaginably large. A supervolcano must erupt more than 1,000 cubic km (240 cubic miles) of material, compared with 1.2 km3 for Mount St. Helens or 25 km3 for Mount Pinatubo, a large eruption in the Philippines in 1991. Not surprisingly, supervolcanoes are the most dangerous type of volcano. | text | null |
L_0320 | supervolcanoes | T_1737 | The exact cause of supervolcano eruptions is still debated. However, scientists think that a very large magma chamber erupts entirely in one catastrophic explosion. This creates a huge hole or caldera into which the surface collapses (Figure 1.1). The caldera at Santorini in Greece is so large that it can only be seen by satellite. | text | null |
L_0320 | supervolcanoes | T_1738 | The largest supervolcano in North America is beneath Yellowstone National Park in Wyoming. Yellowstone sits above a hotspot that has erupted catastrophically three times: 2.1 million, 1.3 million, and 640,000 years ago. Yellowstone has produced many smaller (but still enormous) eruptions more recently (Figure 1.2). Fortunately, current activity at Yellowstone is limited to the regions famous geysers. Click image to the left or use the URL below. URL: The Yellowstone hotspot has produced enormous felsic eruptions. The Yellowstone caldera collapsed in the most recent super eruption. | text | null |
L_0320 | supervolcanoes | T_1739 | A supervolcano could change life on Earth as we know it. Ash could block sunlight so much that photosynthesis would be reduced and global temperatures would plummet. Volcanic eruptions could have contributed to some of the mass extinctions in our planets history. No one knows when the next super eruption will be. Interesting volcano videos are seen on National Geographic Videos, Environment Video, Natural Disasters, Earth- quakes: One interesting one is Mammoth Mountain, which explores Hot Creek and the volcanic area it is a part of in California. Click image to the left or use the URL below. URL: | text | null |
L_0321 | surface features of the sun | T_1740 | The Suns surface features are quite visible, but only with special equipment. For example, sunspots are only visible with special light-filtering lenses. | text | null |
L_0321 | surface features of the sun | T_1741 | The most noticeable surface features of the Sun are cooler, darker areas known as sunspots (Figure 1.1). Sunspots are located where loops of the Suns magnetic field break through the surface and disrupt the smooth transfer of heat from lower layers of the Sun, making them cooler, darker, and marked by intense magnetic activity. Sunspots usually occur in pairs. When a loop of the Suns magnetic field breaks through the surface, a sunspot is created where the loop comes out and where it goes back in again. Sunspots usually occur in 11-year cycles, increasing from a minimum number to a maximum number and then gradually decreasing to a minimum number again. | text | null |
L_0321 | surface features of the sun | T_1742 | There are other types of interruptions of the Suns magnetic energy. If a loop of the Suns magnetic field snaps and breaks, it creates solar flares, which are violent explosions that release huge amounts of energy (Figure 1.2). A strong solar flare can turn into a coronal mass ejection. A solar flare or coronal mass ejection releases streams of highly energetic particles that make up the solar wind. The solar wind can be dangerous to spacecraft and astronauts because it sends out large amounts of radiation that can harm the human body. Solar flares have knocked out entire power grids and disturbed radio, satellite, and cell phone communications. (a) Sunspots. (b) A close-up of a sunspot taken in ultraviolet light. | text | null |
L_0321 | surface features of the sun | T_1742 | There are other types of interruptions of the Suns magnetic energy. If a loop of the Suns magnetic field snaps and breaks, it creates solar flares, which are violent explosions that release huge amounts of energy (Figure 1.2). A strong solar flare can turn into a coronal mass ejection. A solar flare or coronal mass ejection releases streams of highly energetic particles that make up the solar wind. The solar wind can be dangerous to spacecraft and astronauts because it sends out large amounts of radiation that can harm the human body. Solar flares have knocked out entire power grids and disturbed radio, satellite, and cell phone communications. (a) Sunspots. (b) A close-up of a sunspot taken in ultraviolet light. | text | null |
L_0321 | surface features of the sun | T_1743 | Another highly visible feature on the Sun are solar prominences. If plasma flows along a loop of the Suns magnetic field from sunspot to sunspot, it forms a glowing arch that reaches thousands of kilometers into the Suns atmosphere. Prominences can last lengths of time ranging from a day to several months. Prominences are also visible during a total solar eclipse. Most of the imagery comes from SDOs AIA instrument; different colors represent different temperatures, a common technique for observing solar features. SDO sees the entire disk of the Sun in extremely high spatial and temporal resolution, allowing scientists to zoom in on notable events such as flares, waves, and sunspots. | text | null |
L_0321 | surface features of the sun | T_1744 | The video above was taken from the SDO, the most advanced spacecraft ever designed to study the Sun. During its five-year mission, SDO will examine the Suns magnetic field and also provide a better understanding of the role the Sun plays in Earths atmospheric chemistry and climate. Since just after its launch on February 11, 2010, SDO is providing images with clarity 10 times better than high-definition television and will return more comprehensive science data faster than any other solar-observing spacecraft. The Solar Dynamics Observatory is a NASA spacecraft launched in early 2010 is obtaining IMAX-like images of the Sun every second of the day, generating more data than any NASA mission in history. The data will allow researchers to learn about solar storms and other phenomena that can cause blackouts and harm astronauts. Click image to the left or use the URL below. URL: | text | null |
L_0323 | sustainable development | T_1750 | Can society change and get on a sustain- able path? A topic generating a great deal of discussion these days is sustainable development. The goals of sustainable development are to: help people out of poverty. protect the environment. use resources no faster than the rate at which they are regenerated. Science can be an important part of sustainable development. When scientists understand how Earths natural systems work, they can recognize how people are impacting them. Scientists can work to develop technologies that can be used to solve problems wisely. An example of a practice that can aid sustainable development is fish farming, as long as it is done in environmentally sound ways. Engineers can develop cleaner energy sources to reduce pollution and greenhouse gas emissions. Citizens can change their behavior to reduce the impact they have on the planet by demanding products that are produced sustainably. When forests are logged, new trees should be planted. Mining should be done so that the landscape is not destroyed. People can consume less and think more about the impacts of what they do consume. And what of the waste products of society? Will producing all that we need to keep the population growing result in a planet so polluted that the quality of life will be greatly diminished? Will warming temperatures cause problems for human populations? The only answer to all of these questions is, time will tell. Click image to the left or use the URL below. URL: | text | null |
L_0326 | testing hypotheses | T_1757 | How do you test a hypothesis? In this example, we will look into the scientific literature to find data in studies that were done using scientific method. To test Hypothesis 1 from the concept "Development of Hypotheses," we need to see if the amount of CO2 gas released by volcanoes over the past several decades has increased. There are two ways volcanoes could account for the increase in CO2 : There has been an increase in volcanic eruptions in that time. The CO2 content of volcanic gases has increased over time globally. To test the first hypothesis, we look at the scientific literature. We see that the number of volcanic eruptions is about constant. We also learn from the scientific literature that volcanic gas compositions have not changed over time. Different types of volcanoes have different gas compositions, but overall the gases are the same. Another journal article states that major volcanic eruptions for the past 30 years have caused short-term cooling, not warming! Hypothesis 1 is wrong! Volcanic activity is not able to account for the rise in atmospheric CO2 . Remember that science is falsifiable. We can discard Hypothesis 1. | text | null |
L_0326 | testing hypotheses | T_1758 | Hypothesis 2 states that the increase in atmospheric CO2 is due to the increase in the amount of fossil fuels that are being burned. We look into the scientific literature and find this graph in the Figure 1.1. Global carbon dioxide emissions from fos- sil fuel consumption and cement produc- tion. The black line represents all emis- sion types combined, and colored lines show emissions from individual fossil fu- els. Fossil fuels have added an increasing amount of carbon dioxide to the atmosphere since the beginning of the Industrial Revolution in the mid 19th century. Hypothesis 2 is true! Click image to the left or use the URL below. URL: | text | null |
L_0327 | the hertzsprung russell diagra | T_1759 | The Hertzsprung-Russell diagram (often referred to as the H-R diagram) is a scatter graph that shows various classes of stars in the context of properties such as their luminosity, absolute magnitude, color, and effective temperature. Created around 1910 by Ejnar Hertzsprung and Henry Norris Russell, the diagram provided a great help in understanding stellar evolution. There are several forms of the Hertzsprung-Russell diagram, and the nomenclature is not very well defined. The original diagram displayed the spectral type of stars on the horizontal axis and the absolute magnitude on the vertical axis. The form below shows Kelvin temperature along the horizontal axis going from high temperature on the left to low temperature on the right and luminosity on the vertical axis. We can think of the luminosity as brightness in multiples of the Sun. A luminosity of 100 on the axis would mean 100 times as bright as the Sun. Most of the stars occupy a region in the diagram along a line called the Main Sequence. During that stage, stars are fusing hydrogen into helium in their cores. The position of the Sun in the main sequence is shown in the diagram. You should note that the axial scales for this diagram are not linear. The vertical scale is logarithmic, each line is 100 times greater than the previous line. On the horizontal axis, as we move to the right, the temperature reduces by between 1,000 and 10,000 degrees K between each line. If all other factors were the same, the highest temperature stars would also be the most luminous (the brightest). In the main sequence of stars, we see that as the temperature increases to the left, the luminosity also increases, demonstrating that the hottest stars in this grouping are also the brightest. There are stars, however, that are less bright than their temperature would predict. This group of stars is called white dwarfs. These stars are less bright than expected because of their very small size. These dwarf stars are only one one-thousandth the size of stars in the main sequence. There are also stars that are much brighter than their temperature would predict. This group of stars are called red giants. They are brighter than their temperature would predict because they are much larger than stars in the main sequence. These stars have expanded to several thousand times the size of stars in the main sequence. Stars that are reddish in color are cooler than other stars while stars that are bluish in color are hotter than other stars. A white dwarf is a stellar remnant that is very dense. A white dwarfs mass is comparable to the Sun and its volume is comparable to that of Earth. The very low brightness of a white dwarf comes from the emission of stored heat energy. White dwarfs are thought to be the final evolutionary state of any star whose mass is not great enough to become a neutron star. Approximately 97% of the stars in our galaxy will become neutron stars. After the hydrogen-fusing lifetime of a main-sequence star of low or medium mass ends, it will expand to a red giant which fuses helium to carbon and oxygen in its core. If a red giant has insufficient mass to generate the core temperatures required to fuse carbon, around 1 billion K, an inert mass of carbon and oxygen will build up at its center. After blowing off its outer layers to form a planetary nebula, the core will be left behind to form the remnant white dwarf. White dwarfs are composed of carbon and oxygen. A white dwarf is very hot when it is formed, but since it has no source of energy (no further fusion reactions), it will gradually radiate away its energy and cool down. Over a very long time, a white dwarf will cool to temperatures at which it will no longer emit significant light, and it will become a cold black dwarf. A red giant star is a star with a mass like the Sun that is in the last phase of its life, when Hydrogen fusion reactions in the core decrease due to the lack of fuel. With the gravitational collapse of the core, the fusion reactions now occur in a shell surrounding the core. The outer layer of the star expands enormously up to 1000 times the size of the Sun. When the Sun becomes a red giant, its volume will include the orbit of Mercury and Venus and maybe even Earth. The increased size increases the luminosity even though the outer layer cools to only 3000 K or so. The cooler outer layer causes it to be a red star. After a few more million years, the star evolves into a white dwarf-planetary nebula system. | text | null |
L_0328 | the history of astronom | T_1760 | null | text | null |
L_0328 | the history of astronom | T_1761 | The Astronomy of the ancient Greeks was linked to mathematics, and Greek astronomers sought to create geomet- rical models that could imitate the appearance of celestial motions. This tradition originated around the 6th century BCE, with the followers of the mathematician Pythagoras (~580 - 500 BCE). Pythagoras believed that everything was related to mathematics and that through mathematics everything could be predicted and measured in rhythmic patterns or cycles. He placed astronomy as one of the four mathematical arts, the others being arithmetic, geometry and music. While best known for the Pythagorean Theorem, Pythagoras did have some input into astronomy. By the time of Pythagoras, the five planets visible to the naked eye - Mercury, Venus, Mars, Jupiter and Saturn - had long been identified. The names of these planets were initially derived from Greek mythology before being given the equivalent Roman mythological names, which are the ones we still use today. The word planet is a Greek term meaning wanderer, as these bodies move across the sky at different speeds from the stars, which appear fixed in the same positions relative to each other. For part of the year Venus appears in the eastern sky as an early morning object before disappearing and reappearing a few weeks later in the evening western sky. Early Greek astronomers thought this was two different bodies and assigned the names Phosphorus and Hesperus to the morning and evening apparitions respectively. Pythagoras is given credit for being the first to realize that these two bodies were in fact the same planet, a notion he arrived at through observation and geometrical calculations. Pythagoras was also one of the first to think that the Earth was round, a theory that was finally proved around 330 BCE by Aristotle. (Although, as you are probably aware, many people in 1642 CE still believed the earth to be flat.) Aristotle (384 BCE - 322 BCE) demonstrates in his writings that he knew we see the moon by the light of the sun, how the phases of the moon occur, and understood how eclipses work. He also knew that the earth was a sphere. Philosophically, he argued that each part of the earth is trying to be pulled to the center of the earth, and so the earth would naturally take on a spherical shape. He then pointed out observations that support the idea of a spherical earth. First, the shadow of the earth on the moon during a lunar eclipse is always circular. The only shape that always casts a circular shadow is a sphere. Second, as one traveles more north or south, the positions of the stars in the sky change. There are constellations visible in the north that one cannot see in the south and vice versa. He related this to the curvature of the earth. Aristotle talked about the work of earlier Greeks, who had developed an earth centered model of the planets. In these models, the center of the earth is the center of all the other motions. While it is not sure if the earlier Greeks actually thought the planets moved in circles, it is clear that Aristotle did. Aristotle rejected a moving earth for two reasons. Most importantly he didnt understand inertia. To Aristotle, the natural state for an object was to be at rest. He believed that it takes a force in order for an object to move. Using Aristotles ideas, if the earth were moving through space, if you tripped, you would not be in contact with the earth, and so would get left behind in space. Since this obviously does not happen, the earth must not move. This misunderstanding of inertia confused scientists until the time of Galileo. A second, but not as important, reason Aristotle rejected a moving earth is that he recognized that if the earth moved and rotated around the sun, there would be an observable parallax of the stars. One cannot see stellar parallax with the naked-eye, so Aristotle concluded that the earth must be at rest. (The stars are so far away, that one needs a good telescope to measure stellar parallax, which was first measured in 1838.) Aristotle believed that the objects in the heavens are perfect and unchanging. Since he believed that the only eternal motion is circular with a constant speed, the motions of the planets must be circular. This came to be called The Principal of Uniform Circular Motion. Aristotle and his ideas became very important because they became incorporated into the Catholic Churchs theology in the twelfth century by Thomas Aquinas. In the early 16th century, the Church banned new interpretations of scripture and this included a ban on ideas of a moving earth. Claudius Ptolemy (90 - 168 CE) was a citizen of Egypt which was under Roman rule during Ptolemys lifetime. During his lifetime he was a mathematician, astronomer, and geographer. His theories dominated the worlds understanding of astronomy for over a thousand years. While it is known that many astronomers published works during this time, only Ptolemys work The Almagest survived. In it, he outlined his geometrical reasoning for a geocentric view of the Universe. As outlined in the Almagest, the Universe according to Ptolemy was based on five main points: 1) the celestial realm is spherical, 2) the celestial realm moves in a circle, 3) the earth is a sphere, 4) the celestial realm orbit is a circle centered on the earth, and 5) earth does not move. Ptolemy also identified eight circular orbits surrounding earth where the other planets existed. In order, they were the moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn, and the sphere of fixed stars. A serious problem with the earth-centered system was the fact that at certain times in their orbits, some of the planets appeared to move in the opposite direction of their normal movement. This reverse direction movement is referred to as retrograde motion. If the earth was to remain motionless at the center of the system, some very intricate designs were necessary to explain the movement of the retrograde planets. In the Ptolemaic system, each retrograde planet moved by two spheres. The Ptolemaic system had circles within circles that produced epicycles. In the sketch above on the left, the red ball moved clockwise in its little circle while the entire orbit also orbited clockwise around the big circle. This process produced a path like that shown in the sketch above on the right. As the red ball moved around its path, at some times it would be moving clockwise and then for a short period, it would move counterclockwise. This motion was able to explain the retrograde motion noted for some planets. | text | null |
L_0328 | the history of astronom | T_1762 | It was not until 1543, when Copernicus (1473 - 1543) introduced a sun-centered design (heliocentric), that Ptolemys astronomy was seriously questioned and eventually overthrown. Copernicus studied at the University of Bologna, where he lived in the same house as the principal astronomer there. Copernicus assisted the astronomer in some of his observations and in the production of the annual astrological forecasts for the city. It is at Bologna that he probably first encountered a translation of Ptolemys Almagest that would later make it possible for Copernicus to successfully refute the ancient astronomer. Later, at the University of Padua, Copernicus studied medicine, which was closely associated with astrology at that time due to the belief that the stars influenced the dispositions of the body. Returning to Poland, Copernicus secured a teaching post at Wroclaw, where he primarily worked as a medical doctor and manager of Church affairs. In his spare time, he studied the stars and the planets (decades before the telescope was invented), and applied his mathematical understanding to the mysteries of the night sky. In so doing, he developed his theory of a system in which the Earth, like all the planets, revolved around the sun, and which simply and elegantly explained the curious retrograde movements of the planets. Copernicus wrote his theory in De Revolutionibus Orbium Coelestium (On the Revolutions of the Celestial Orbs). The book was completed in 1530 or so, but it wasnt published until the year he died, 1543. It has been suggested that Copernicus knew the publication would incur the wrath of the Catholic church and he didnt want to deal with problems so he didnt publish his theory until he was on his death bed. Legend has it that a copy of the printers proof was placed in his hands as he lay in a coma, and he woke long enough to recognize what he was holding before he died. Tycho Brahe (1546 - 1601) was born in a part of southern Sweden that was part of Denmark at the time. While attending the university to study law and philosophy, he became interested in astronomy and spent most evenings observing the stars. One of Tycho Brahes first contributions to astronomy was the detection and correction of several serious errors in the standard astronomical tables. Then, in 1572, he discovered a supernova located in the constellation of Cassiopeia. Tycho built his own instruments and made the most complete and accurate observations available without the use of a telescope. Eventually, his fame led to an offer from King Frederick II of Denmark & Norway to fund the construction of an astronomical observatory. The island of Van was chosen and in 1576, construction began. Tycho Brahe spent twenty years there, making observations on celestial bodies. During his life, Tycho Brahe did not accept Copernicus model of the universe. He attempted to combine it with the Ptolemaic model. As a theoretician, Tycho was a failure but his observations and the data he collected was far superior to any others made prior to the invention of the telescope. After Tycho Brahes death, his assistant, Johannes Kepler used Tycho Brahes observations to calculate his own three laws of planetary motion. In 1600, Johannes Kepler (1571 - 1630) began working as Tychos assistant. They recognized that neither the Ptolemaic (geocentric) or Copernican (heliocentric) models could predict positions of Mars as accurately as they could measure them. Tycho died in 1601 and after that Kepler had full access to Tychos data. He analyzed the data for 8 years and tried to calculate an orbit that would fit the data, but was unable to do so. Kepler later determined that the orbits were not circular but elliptical. | text | null |
L_0328 | the history of astronom | T_1763 | 1. The orbits of the planets are elliptical. 2. An imaginary line connecting a planet and the sun sweeps out equal areas during equal time intervals. (Therefore, the earths orbital speed varies at different times of the year. The earth moves fastest in its orbit when closest to the sun and slowest when farthest away.) Keplers Second Law of Planetary Motion was calculated for Earth, then the hypothesis was tested using data for Mars, and it worked! 3. Keplers Third Law of Planetary Motion showed the relationship between the size of a planets orbit radius, R ( 12 the major axis), and its orbital period, T . R2 = T 3 This law is true for all planets if you use astronomical units (that is, distance in multiples of earths orbital radium and time in multiples of earth years). Keplers three laws replaced the cumbersome epicycles to explain planetary motion with three mathematical laws that allowed the positions of the planets to be predicted with accuracies ten times better than Ptolemaic or Copernican models. | text | null |
L_0328 | the history of astronom | T_1764 | Galileo Galilei (1564-1642) was a very important person in the development of modern astronomy, both because of his contributions directly to astronomy, and because of his work in physics. He provided the crucial observations that proved the Copernican hypothesis, and also laid the foundations for a correct understanding of how objects moved on the surface of the earth and of gravity. One could, with considerable justification, view Galileo as the father both of modern astronomy and of modern physics. Galileo did not invent the telescope, but he was the first to turn his telescope toward the sky to study the heavens systematically. His telescope was poorer than even a cheap modern amateur telescope, but what he observed in the heavens showed errors in Aristotles opinion of the universe and the worldview that it supported. Observations through Galileos telescope made it clear that the earth-centered and earth doesnt move solar system of Aristotle was incorrect. Since church officials had made some of Aristotles opinions a part of the religious views of the church, proving Aristotles views to be incorrect also pointed out flaws in the church. Galileo observed four points of light that changed their positions around the planet Jupiter and he concluded that these were moons in orbit around Jupiter. These observations showed that there were new things in the heavens that Aristotle and Ptolemy had known nothing about. Furthermore, they demonstrated that a planet could have moons circling it that would not be left behind as the planet moved around its orbit. One of the arguments against the Copernican system had been that if the moon were in orbit around the Earth and the Earth in orbit around the Sun, the Earth would leave the Moon behind as it moved around its orbit. Galileo used his telescope to show that Venus, like the moon, went through a complete set of phases. This observation was extremely important because it was the first observation that was consistent with the Copernican system but not the Ptolemaic system. In the Ptolemaic system, Venus should always be in crescent phase as viewed from the Earth because the sun is beyond Venus, but in the Copernican system Venus should exhibit a complete set of phases over time as viewed from the Earth because it is illuminated from the center of its orbit. It is important to note that this was the first empirical evidence (coming almost a century after Copernicus) that allowed a definitive test of the two models. Until that point, both the Ptolemaic and Copernican models described the available data. The primary attraction of the Copernican system was that it described the data in a simpler fashion, but here finally was conclusive evidence that not only was the Ptolemaic universe more complicated, it also was incorrect. As each new observation was brought to light, increasing doubt was cast on the old views of the heavens. It also raised the credibility issue: could the authority of Aristotle and Ptolemy be trusted concerning the nature of the Universe if there were so many things in the Universe about which they had been unaware and/or incorrect? Galileos challenge of the Churchs authority through his refutation of the Aristotelian concept of the Universe eventually got him into deep trouble. Late in his life he was forced, under threat of torture, to publicly recant his Copernican views and spent his last years under house arrest. Galileos life is a sad example of the conflict between the scientific method and unquestioned authority. Sir Isaac Newton (1642-1727), who was born the same year that Galileo died, would build on Galileos ideas to demonstrate that the laws of motion in the heavens and the laws of motion on the earth were the same. Thus Galileo began, and Newton completed, a synthesis of astronomy and physics in which astronomy was recognized as but a part of physics, and that the opinions of Aristotle were almost completely eliminated from both. Many scientists consider Newton to be a peer of Einstein in scientific thinking. Newtons accomplishments had even greater scope than those of Einstein. The poet Alexander Pope wrote of Newton: Nature and Natures laws lay hid in night; God said, Let Newton be! and all was light. In terms of astronomy, Newton gave reasons for and corrections to Keplers Laws. Kepler had proposed three Laws of Planetary motion based on Tycho Brahes data. These Laws were supposed to apply only to the motions of the planets. Further, they were purely empirical, that is, they worked, but no one knew why they worked. Newton changed all of that. First, he demonstrated that the motion of objects on the Earth could be described by three new Laws of motion, and then he went on to show that Keplers three Laws of Planetary Motion were but special cases of Newtons three Laws when his gravitational force was postulated to exist between all masses in the Universe. In fact, Newton showed that Keplers Laws of planetary motion were only approximately correct, and supplied the quantitative corrections that with careful observations proved to be valid. | text | null |
L_0328 | the history of astronom | T_1765 | The Big Bang Theory is the dominant and highly supported theory of the origin of the universe. It states that the universe began from an initial point which has expanded over billions of years to form the universe as we now know it. In 1922, Alexander Friedman found that the solutions to Einsteins general relativity equations resulted in an expanding universe. Einstein, at that time, believed in a static, eternal universe so he added a constant to his equations to eliminate the expansion. Einstein would later call this the biggest blunder of his life. In 1924, Edwin Hubble was able to measure the distance to observed celestial objects that were thought to be nebula and discovered that they were so far away they were not actually part of the Milky Way (the galaxy containing our sun). He discovered that the Milky Way was only one of many galaxies. In 1927, Georges Lemaitre, a physicist, suggested that the universe must be expanding. Lemaitres theory was supported by Hubble in 1929 when he found that the galaxies most distant from us also had the greatest red shift (were moving away from us with the greatest speed). The idea that the most distance galaxies were moving away from us at the greatest speed was exactly what was predicted by Lemaitre. In 1931, Lemaitre went further with his predictions and by extrapolating backwards, found that the matter of the universe would reach an infinite density and temperature at a finite time in the past (around 15 billion years). This meant that the universe must have begun as a small, extremely dense point of matter. At the time, the only other theory that competed with Lemaitres theory was the Steady State Theory of Fred Hoyle. The steady state theory predicted that new matter was created which made it appear that the universe was expanding but that the universe was constant. It was Hoyle who coined the term Big Bang Theory which he used as a derisive name for Lemaitres theory. George Gamow (1904 - 1968) was the major advocate of the Big Bang theory. He predicted that cosmic microwave background radiation should exist throughout the universe as a remnant of the Big Bang. As atoms formed from sub-atomic particles shortly after the Big Bang, electromagnetic radiation would be emitted and this radiation would still be observable today. Gamow predicted that the expansion of the universe would cool the original radiation so that now the radiation would be in the microwave range. The debate continued until 1965 when two Bell Telephone scientists stumbled upon the microwave radiation with their radio telescope. | text | null |
L_0329 | thermosphere and beyond | T_1766 | The density of molecules is so low in the thermosphere that one gas molecule can go about 1 km before it collides with another molecule. Since so little energy is transferred, the air feels very cold (See opening image). | text | null |
L_0329 | thermosphere and beyond | T_1767 | Within the thermosphere is the ionosphere. The ionosphere gets its name from the solar radiation that ionizes gas molecules to create a positively charged ion and one or more negatively charged electrons. The freed electrons travel within the ionosphere as electric currents. Because of the free ions, the ionosphere has many interesting characteristics. At night, radio waves bounce off the ionosphere and back to Earth. This is why you can often pick up an AM radio station far from its source at night. | text | null |
L_0329 | thermosphere and beyond | T_1768 | The Van Allen radiation belts are two doughnut-shaped zones of highly charged particles that are located very high the atmosphere in the magnetosphere. The particles originate in solar flares and fly to Earth on the solar wind. Once trapped by Earths magnetic field, they follow along the fields magnetic lines of force. These lines extend from above the Equator to the North Pole and also to the South Pole, then return to the Equator. | text | null |
L_0329 | thermosphere and beyond | T_1769 | When massive solar storms cause the Van Allen belts to become overloaded with particles, the result is the most spectacular feature of the ionosphere the nighttime aurora (Figure 1.1). The particles spiral along magnetic field lines toward the poles. The charged particles energize oxygen and nitrogen gas molecules, causing them to light up. Each gas emits a particular color of light. (a) Spectacular light displays are visible as the aurora borealis or northern lights in the Northern Hemisphere. (b) The aurora australis or southern lights encircles Antarctica. What would Earths magnetic field look like if it were painted in colors? It would look like the aurora! This QUEST video looks at the aurora, which provides clues about the solar wind, Earths magnetic field and Earths atmosphere. Click image to the left or use the URL below. URL: | text | null |
L_0329 | thermosphere and beyond | T_1770 | There is no real outer limit to the exosphere, the outermost layer of the atmosphere; the gas molecules finally become so scarce that at some point there are no more. Beyond the atmosphere is the solar wind. The solar wind is made of high-speed particles, mostly protons and electrons, traveling rapidly outward from the Sun. | text | null |
L_0331 | tides | T_1779 | Tides are the daily rise and fall of sea level at any given place. The pull of the Moons gravity on Earth is the primary cause of tides and the pull of the Suns gravity on Earth is the secondary cause (Figure 1.1). The Moon has a greater effect because, although it is much smaller than the Sun, it is much closer. The Moons pull is about twice that of the Suns. To understand the tides it is easiest to start with the effect of the Moon on Earth. As the Moon revolves around our planet, its gravity pulls Earth toward it. The lithosphere is unable to move much, but the water is pulled by the gravity and a bulge is created. This bulge is the high tide beneath the Moon. On the other side of the Earth, a high tide is produced where the Moons pull is weakest. These two water bulges on opposite sides of the Earth aligned with the Moon are the high tides. The places directly in between the high tides are low tides. As the Earth rotates beneath the Moon, a single spot will experience two high tides and two low tides approximately every day. High tides occur about every 12 hours and 25 minutes. The reason is that the Moon takes 24 hours and 50 minutes to rotate once around the Earth, so the Moon is over the same location every 24 hours and 50 minutes. Since high tides occur twice a day, one arrives each 12 hours and 25 minutes. What is the time between a high tide and the next low tide? The gravity of the Sun also pulls Earths water towards it and causes its own tides. Because the Sun is so far away, its pull is smaller than the Moons. Some coastal areas do not follow this pattern at all. These coastal areas may have one high and one low tide per day or a different amount of time between two high tides. These differences are often because of local conditions, such as the shape of the coastline that the tide is entering. The gravitational attraction of the Moon to ocean water creates the high and low tides. | text | null |
L_0331 | tides | T_1780 | The tidal range is the difference between the ocean level at high tide and the ocean level at low tide (Figure 1.2). The tidal range in a location depends on a number of factors, including the slope of the seafloor. Water appears to move a greater distance on a gentle slope than on a steep slope. | text | null |
L_0331 | tides | T_1781 | If you look at the diagram of high and low tides on a circular Earth above, youll see that tides are waves. So when the Sun and Moon are aligned, what do you expect the tides to look like? Waves are additive, so when the gravitational pull of both bodies is in the same direction, the high tides are higher and the low tides lower than at other times through the month (Figure 1.3). These more extreme tides, with a greater tidal range, are called spring tides. Spring tides dont just occur in the spring; they occur whenever the Moon is in a new-moon or full-moon phase, about every 14 days. Neap tides are tides that have the smallest tidal range, and they occur when the Earth, the Moon, and the Sun form a 90o angle (Figure 1.4). They occur exactly halfway between the spring tides, when the Moon is at first or last quarter. How do the tides add up to create neap tides? The Moons high tide occurs in the same place as the Suns low tide and the Moons low tide in the same place as the Suns high tide. At neap tides, the tidal range is relatively small. The tidal range is the difference between the ocean level at high tide and low tide. Studying ocean tides rhythmic movements helps scientists understand the ocean and the Sun/Moon/Earth system. This QUEST video explains how tides work, and visits the oldest continually operating tidal gauge in the Western Hemisphere. Click image to the left or use the URL below. URL: | text | null |
L_0331 | tides | T_1781 | If you look at the diagram of high and low tides on a circular Earth above, youll see that tides are waves. So when the Sun and Moon are aligned, what do you expect the tides to look like? Waves are additive, so when the gravitational pull of both bodies is in the same direction, the high tides are higher and the low tides lower than at other times through the month (Figure 1.3). These more extreme tides, with a greater tidal range, are called spring tides. Spring tides dont just occur in the spring; they occur whenever the Moon is in a new-moon or full-moon phase, about every 14 days. Neap tides are tides that have the smallest tidal range, and they occur when the Earth, the Moon, and the Sun form a 90o angle (Figure 1.4). They occur exactly halfway between the spring tides, when the Moon is at first or last quarter. How do the tides add up to create neap tides? The Moons high tide occurs in the same place as the Suns low tide and the Moons low tide in the same place as the Suns high tide. At neap tides, the tidal range is relatively small. The tidal range is the difference between the ocean level at high tide and low tide. Studying ocean tides rhythmic movements helps scientists understand the ocean and the Sun/Moon/Earth system. This QUEST video explains how tides work, and visits the oldest continually operating tidal gauge in the Western Hemisphere. Click image to the left or use the URL below. URL: | text | null |
L_0331 | tides | T_1781 | If you look at the diagram of high and low tides on a circular Earth above, youll see that tides are waves. So when the Sun and Moon are aligned, what do you expect the tides to look like? Waves are additive, so when the gravitational pull of both bodies is in the same direction, the high tides are higher and the low tides lower than at other times through the month (Figure 1.3). These more extreme tides, with a greater tidal range, are called spring tides. Spring tides dont just occur in the spring; they occur whenever the Moon is in a new-moon or full-moon phase, about every 14 days. Neap tides are tides that have the smallest tidal range, and they occur when the Earth, the Moon, and the Sun form a 90o angle (Figure 1.4). They occur exactly halfway between the spring tides, when the Moon is at first or last quarter. How do the tides add up to create neap tides? The Moons high tide occurs in the same place as the Suns low tide and the Moons low tide in the same place as the Suns high tide. At neap tides, the tidal range is relatively small. The tidal range is the difference between the ocean level at high tide and low tide. Studying ocean tides rhythmic movements helps scientists understand the ocean and the Sun/Moon/Earth system. This QUEST video explains how tides work, and visits the oldest continually operating tidal gauge in the Western Hemisphere. Click image to the left or use the URL below. URL: | text | null |
L_0334 | tree rings ice cores and varves | T_1789 | In locations where summers are warm and winters are cool, trees have a distinctive growth pattern. Tree trunks display alternating bands of light-colored, low density summer growth and dark, high density winter growth. Each light-dark band represents one year. By counting tree rings it is possible to find the number of years the tree lived (Figure 1.1). The width of these growth rings varies with the conditions present that year. A summer drought may make the tree grow more slowly than normal and so its light band will be relatively small. These tree-ring variations appear in all trees in a region. The same distinctive pattern can be found in all the trees in an area for the same time period. Scientists have created continuous records of tree rings going back over the past 2,000 years. Wood fragments from old buildings and ancient ruins can be age dated by matching up the pattern of tree rings in the wood fragment in Cross-section showing growth rings. question and the scale created by scientists. The outermost ring indicates when the tree stopped growing; that is, when it died. The tree-ring record is extremely useful for finding the age of ancient structures. | text | null |
L_0334 | tree rings ice cores and varves | T_1790 | Besides tree rings, other processes create distinct yearly layers that can be used for dating. On a glacier, snow falls in winter but in summer dust accumulates. This leads to a snow-dust annual pattern that goes down into the ice (Figure gather allows them to determine how the environment has changed as the glacier has stayed in its position. Analyses of the ice tell how concentrations of atmospheric gases changed, which can yield clues about climate. The longest cores allow scientists to create a record of polar climate stretching back hundreds of thousands of years. | text | null |
L_0334 | tree rings ice cores and varves | T_1791 | Lake sediments, especially in lakes that are located at the end of glaciers, also have an annual pattern. In the summer, the glacier melts rapidly, producing a thick deposit of sediment. These alternate with thin, clay-rich layers deposited in the winter. The resulting layers, called varves, give scientists clues about past climate conditions (Figure 1.3). A warm summer might result in a very thick sediment layer while a cooler summer might yield a thinner layer. | text | null |
L_0335 | troposphere | T_1792 | The temperature of the troposphere is highest near the surface of the Earth and decreases with altitude. On average, the temperature gradient of the troposphere is 6.5o C per 1,000 m (3.6o F per 1,000 ft) of altitude. Earths surface is the source of heat for the troposphere. Rock, soil, and water on Earth absorb the Suns light and radiate it back into the atmosphere as heat, so there is more heat near the surface. The temperature is also higher near the surface because gravity pulls in more gases. The greater density of gases causes the temperature to rise. Notice that in the troposphere warmer air is beneath cooler air. This condition is unstable since warm air is less dense than cool air. The warm air near the surface rises and cool air higher in the troposphere sinks, so air in the troposphere does a lot of mixing. This mixing causes the temperature gradient to vary with time and place. The rising and sinking of air in the troposphere means that all of the planets weather takes place in the troposphere. | text | null |
L_0335 | troposphere | T_1793 | Sometimes there is a temperature inversion, in which air temperature in the troposphere increases with altitude and warm air sits over cold air. Inversions are very stable and may last for several days or even weeks. Inversions form: Over land at night or in winter when the ground is cold. The cold ground cools the air that sits above it, making this low layer of air denser than the air above it. Near the coast, where cold seawater cools the air above it. When that denser air moves inland, it slides beneath the warmer air over the land. Since temperature inversions are stable, they often trap pollutants and produce unhealthy air conditions in cities (Figure 1.1). Smoke makes a temperature inversion visible. The smoke is trapped in cold dense air that lies beneath a cap of warmer air. At the top of the troposphere is a thin layer in which the temperature does not change with height. This means that the cooler, denser air of the troposphere is trapped beneath the warmer, less dense air of the stratosphere. Air from the troposphere and stratosphere rarely mix. Click image to the left or use the URL below. URL: | text | null |
L_0336 | tsunami | T_1794 | Tsunami are deadly ocean waves from the sharp jolt of an undersea earthquake. Less frequently, these waves can be generated by other shocks to the sea, like a meteorite impact. Fortunately, few undersea earthquakes, and even fewer meteorite impacts, generate tsunami. | text | null |
L_0336 | tsunami | T_1795 | Tsunami waves have small wave heights relative to their long wavelengths, so they are usually unnoticed at sea. When traveling up a slope onto a shoreline, the wave is pushed upward. As with wind waves, the speed of the bottom of the wave is slowed by friction. This causes the wavelength to decrease and the wave to become unstable. These factors can create an enormous and deadly wave. Landslides, meteorite impacts, or any other jolt to ocean water may form a tsunami. Tsunami can travel at speeds of 800 kilometers per hour (500 miles per hour). | text | null |
L_0336 | tsunami | T_1796 | Since tsunami are long-wavelength waves, a long time can pass between crests or troughs. Any part of the wave can make landfall first. In 1755 in Lisbon, Portugal, a tsunami trough hit land first. A large offshore earthquake did a great deal of damage on land. People rushed out to the open space of the shore. Once there, they discovered that the water was flowing seaward fast and some of them went out to observe. What do you think happened next? The people on the open beach drowned when the crest of the wave came up the beach. Large tsunami in the Indian Ocean and more recently Japan have killed hundreds of thousands of people in recent years. The west coast is vulnerable to tsunami since it sits on the Pacific Ring of Fire. Scientists are trying to learn everything they can about predicting tsunamis before a massive one strikes a little closer to home. Although most places around the Indian Ocean did not have warning systems in 2005, there is a tsunami warning system in that region now. Tsunami warning systems have been placed in most locations where tsunami are possible. Click image to the left or use the URL below. URL: | text | null |
L_0337 | types of air pollution | T_1797 | The two types of air pollutants are primary pollutants, which enter the atmosphere directly, and secondary pollutants, which form from a chemical reaction. | text | null |
L_0337 | types of air pollution | T_1798 | Some primary pollutants are natural, such as volcanic ash. Dust is natural but exacerbated by human activities; for example, when the ground is torn up for agriculture or development. Most primary pollutants are the result of human activities, the direct emissions from vehicles and smokestacks. Primary pollutants include: Carbon oxides include carbon monoxide (CO) and carbon dioxide (CO2 ) (Figure 1.1). Both are colorless, odorless gases. CO is toxic to both plants and animals. CO and CO2 are both greenhouse gases. Nitrogen oxides are produced when nitrogen and oxygen from the atmosphere come together at high temper- atures. This occurs in hot exhaust gas from vehicles, power plants, or factories. Nitrogen oxide (NO) and nitrogen dioxide (NO2 ) are greenhouse gases. Nitrogen oxides contribute to acid rain. Sulfur oxides include sulfur dioxide (SO2 ) and sulfur trioxide (SO3 ). These form when sulfur from burning coal reaches the air. Sulfur oxides are components of acid rain. Particulates are solid particles, such as ash, dust, and fecal matter (Figure 1.2). They are commonly formed from combustion of fossil fuels, and can produce smog. Particulates can contribute to asthma, heart disease, and some types of cancers. Lead was once widely used in automobile fuels, paint, and pipes. This heavy metal can cause brain damage or blood poisoning. High CO2 levels are found in major metropolitan areas and along the major interstate highways. Particulates from a brush fire give the sky a strange glow in Arizona. | text | null |
L_0337 | types of air pollution | T_1798 | Some primary pollutants are natural, such as volcanic ash. Dust is natural but exacerbated by human activities; for example, when the ground is torn up for agriculture or development. Most primary pollutants are the result of human activities, the direct emissions from vehicles and smokestacks. Primary pollutants include: Carbon oxides include carbon monoxide (CO) and carbon dioxide (CO2 ) (Figure 1.1). Both are colorless, odorless gases. CO is toxic to both plants and animals. CO and CO2 are both greenhouse gases. Nitrogen oxides are produced when nitrogen and oxygen from the atmosphere come together at high temper- atures. This occurs in hot exhaust gas from vehicles, power plants, or factories. Nitrogen oxide (NO) and nitrogen dioxide (NO2 ) are greenhouse gases. Nitrogen oxides contribute to acid rain. Sulfur oxides include sulfur dioxide (SO2 ) and sulfur trioxide (SO3 ). These form when sulfur from burning coal reaches the air. Sulfur oxides are components of acid rain. Particulates are solid particles, such as ash, dust, and fecal matter (Figure 1.2). They are commonly formed from combustion of fossil fuels, and can produce smog. Particulates can contribute to asthma, heart disease, and some types of cancers. Lead was once widely used in automobile fuels, paint, and pipes. This heavy metal can cause brain damage or blood poisoning. High CO2 levels are found in major metropolitan areas and along the major interstate highways. Particulates from a brush fire give the sky a strange glow in Arizona. | text | null |
L_0337 | types of air pollution | T_1799 | Any city can have photochemical smog, but it is most common in sunny, dry locations. A rise in the number of vehicles in cities worldwide has increased photochemical smog. Nitrogen oxides, ozone, and several other compounds are some of the components of this type of air pollution. Photochemical smog forms when car exhaust is exposed to sunlight. Nitrogen oxide is created by gas combustion in cars and then into the air (Figure 1.3). In the presence of sunshine, the NO2 splits and releases an oxygen ion (O). The O then combines with an oxygen molecule (O2 ) to form ozone (O3 ). This reaction can also go in reverse: Nitric oxide (NO) removes an oxygen atom from ozone to make it O2 . The direction the reaction goes depends on how much NO2 and NO there is. If NO2 is three times more abundant than NO, ozone will be produced. If nitric oxide levels are high, ozone will not be created. The brown color of the air behind the Golden Gate Bridge is typical of California cities, because of nitrogen oxides. Ozone is one of the major secondary pollutants. It is created by a chemical reaction that takes place in exhaust and in the presence of sunlight. The gas is acrid-smelling and whitish. Warm, dry cities surrounded by mountains, such as Los Angeles, Phoenix, and Denver, are especially prone to photochemical smog. Photochemical smog peaks at midday on the hottest days of summer. Ozone is also a greenhouse gas. | text | null |
L_0338 | types of fossilization | T_1800 | Most fossils are preserved by one of five processes outlined below (Figure 1.1): | text | null |
L_0338 | types of fossilization | T_1801 | Most uncommon is the preservation of soft-tissue original material. Insects have been preserved perfectly in amber, which is ancient tree sap. Mammoths and a Neanderthal hunter were frozen in glaciers, allowing scientists the rare opportunity to examine their skin, hair, and organs. Scientists collect DNA from these remains and compare the DNA sequences to those of modern counterparts. | text | null |
L_0338 | types of fossilization | T_1802 | The most common method of fossilization is permineralization. After a bone, wood fragment, or shell is buried in sediment, mineral-rich water moves through the sediment. This water deposits minerals into empty spaces and Five types of fossils: (a) insect preserved in amber, (b) petrified wood (permineralization), (c) cast and mold of a clam shell, (d) pyritized ammonite, and (e) compression fossil of a fern. produces a fossil. Fossil dinosaur bones, petrified wood, and many marine fossils were formed by permineralization. | text | null |
L_0338 | types of fossilization | T_1802 | The most common method of fossilization is permineralization. After a bone, wood fragment, or shell is buried in sediment, mineral-rich water moves through the sediment. This water deposits minerals into empty spaces and Five types of fossils: (a) insect preserved in amber, (b) petrified wood (permineralization), (c) cast and mold of a clam shell, (d) pyritized ammonite, and (e) compression fossil of a fern. produces a fossil. Fossil dinosaur bones, petrified wood, and many marine fossils were formed by permineralization. | text | null |
L_0338 | types of fossilization | T_1803 | When the original bone or shell dissolves and leaves behind an empty space in the shape of the material, the depression is called a mold. The space is later filled with other sediments to form a matching cast within the mold that is the shape of the original organism or part. Many mollusks (clams, snails, octopi, and squid) are found as molds and casts because their shells dissolve easily. | text | null |
L_0338 | types of fossilization | T_1804 | The original shell or bone dissolves and is replaced by a different mineral. For example, calcite shells may be replaced by dolomite, quartz, or pyrite. If a fossil that has been replace by quartz is surrounded by a calcite matrix, mildly acidic water may dissolve the calcite and leave behind an exquisitely preserved quartz fossil. | text | null |
L_0338 | types of fossilization | T_1805 | Some fossils form when their remains are compressed by high pressure, leaving behind a dark imprint. Compression is most common for fossils of leaves and ferns, but can occur with other organisms. Click image to the left or use the URL below. URL: Click image to the left or use the URL below. URL: | text | null |
L_0342 | universe | T_1825 | The study of the universe is called cosmology. Cosmologists study the structure and changes in the present universe. The universe contains all of the star systems, galaxies, gas, and dust, plus all the matter and energy that exists now, that existed in the past, and that will exist in the future. The universe includes all of space and time. | text | null |
L_0342 | universe | T_1826 | What did the ancient Greeks recognize as the universe? In their model, the universe contained Earth at the center, the Sun, the Moon, five planets, and a sphere to which all the stars were attached. This idea held for many centuries until Galileos telescope helped people recognize that Earth is not the center of the universe. They also found out that there are many more stars than were visible to the naked eye. All of those stars were in the Milky Way Galaxy. In the early 20th century, an astronomer named Edwin Hubble (Figure 1.1) discovered that what scientists called the Andromeda Nebula was actually over 2 million light years away many times farther than the farthest distances that had ever been measured. Hubble realized that many of the objects that astronomers called nebulas were not actually clouds of gas, but were collections of millions or billions of stars what we now call galaxies. Hubble showed that the universe was much larger than our own galaxy. Today, we know that the universe contains about a hundred billion galaxies about the same number of galaxies as there are stars in the Milky Way Galaxy. (a) Edwin Hubble used the 100-inch reflecting telescope at the Mount Wilson Observatory in California to show that some distant specks of light were galaxies. (b) Hubbles namesake space telescope spotted this six galaxy group. Edwin Hubble demonstrated the existence of galaxies. Click image to the left or use the URL below. URL: Click image to the left or use the URL below. URL: | text | null |
L_0343 | uranus | T_1827 | Uranus (YOOR-uh-nuhs) is named for the Greek god of the sky. From Earth, Uranus is so faint that it was unnoticed by ancient observers. William Herschel first discovered the planet in 1781. Although Uranus is very large, it is extremely far away, about 2.8 billion km (1.8 billion mi) from the Sun. Light from the Sun takes about 2 hours and 40 minutes to reach Uranus. Uranus orbits the Sun once about every 84 Earth years. Uranus has a mass about 14 times the mass of Earth, but it is much less dense than Earth. Gravity at the surface of Uranus is weaker than on Earths surface, so if you were at the top of the clouds on Uranus, you would weigh about 10% less than what you weigh on Earth. | text | null |
L_0343 | uranus | T_1828 | Like Jupiter and Saturn, Uranus is composed mainly of hydrogen and helium, with an outer gas layer that gives way to liquid on the inside. Uranus has a higher percentage of icy materials, such as water, ammonia (NH3 ), and methane (CH4 ), than Jupiter and Saturn. When sunlight reflects off Uranus, clouds of methane filter out red light, giving the planet a blue-green color. There are bands of clouds in the atmosphere of Uranus, but they are hard to see in normal light, so the planet looks like a plain blue ball. | text | null |
L_0343 | uranus | T_1829 | Most of the planets in the solar system rotate on their axes in the same direction that they move around the Sun. Uranus, though, is tilted on its side, so its axis is almost parallel to its orbit. In other words, it rotates like a top that was turned so that it was spinning parallel to the floor. Scientists think that Uranus was probably knocked over by a collision with another planet-sized object billions of years ago. | text | null |
L_0343 | uranus | T_1830 | Uranus has a faint system of rings (Figure 1.1). The rings circle the planets equator, but because Uranus is tilted on its side, the rings are almost perpendicular to the planets orbit. This image from the Hubble Space Tele- scope shows the faint rings of Uranus. The planet is tilted on its side, so the rings are nearly vertical. Uranus has 27 known moons and all but a few of them are named for characters from the plays of William Shakespeare. The five biggest moons of Uranus Miranda, Ariel, Umbriel, Titania, and Oberon are shown in Figure 1.2. These Voyager 2 photos have been resized to show the relative sizes of the five main moons of Uranus. Click image to the left or use the URL below. URL: | text | null |
L_0343 | uranus | T_1830 | Uranus has a faint system of rings (Figure 1.1). The rings circle the planets equator, but because Uranus is tilted on its side, the rings are almost perpendicular to the planets orbit. This image from the Hubble Space Tele- scope shows the faint rings of Uranus. The planet is tilted on its side, so the rings are nearly vertical. Uranus has 27 known moons and all but a few of them are named for characters from the plays of William Shakespeare. The five biggest moons of Uranus Miranda, Ariel, Umbriel, Titania, and Oberon are shown in Figure 1.2. These Voyager 2 photos have been resized to show the relative sizes of the five main moons of Uranus. Click image to the left or use the URL below. URL: | text | null |
L_0344 | uses of water | T_1831 | Humans use six times as much water today as they did 100 years ago. People living in developed countries use a far greater proportion of the worlds water than people in less developed countries. What do people use all of that water for? | text | null |
L_0344 | uses of water | T_1832 | Besides drinking and washing, people need water for agriculture, industry, household uses, and recreation (Figure Water use can be consumptive or non-consumptive, depending on whether the water is lost to the ecosystem. Non-consumptive water use includes water that can be recycled and reused. For example, the water that goes down the drain and enters the sewer system is purified and then redistributed for reuse. By recycling water, the overall water consumption is reduced. Consumptive water use takes the water out of the ecosystem. Can you name some examples of consumptive water use? | text | null |
L_0344 | uses of water | T_1833 | Some of the worlds farmers still farm without irrigation by choosing crops that match the amount of rain that falls in their area. But some years are wet and others are dry. For farmers to avoid years in which they produce little or no food, many of the worlds crops are produced using irrigation. Water used for home, industrial, and agricultural purposes in different regions. Globally more than two-thirds of water is for agriculture. | text | null |
L_0344 | uses of water | T_1834 | Three popular irrigation methods are: Overhead sprinklers. Trench irrigation: canals carry water from a water source to the fields. Flood irrigation: fields are flooded with water. All of these methods waste water. Between 15% and 36% percent of the water never reaches the crops because it evaporates or leaves the fields as runoff. Water that runs off a field often takes valuable soil with it. | text | null |
L_0344 | uses of water | T_1835 | A much more efficient way to water crops is drip irrigation (Figure 1.2). With drip irrigation, pipes and tubes deliver small amounts of water directly to the soil at the roots of each plant or tree. The water is not sprayed into the air or over the ground, so nearly all of it goes directly into the soil and plant roots. | text | null |
L_0344 | uses of water | T_1836 | Why do farmers use wasteful irrigation methods when water-efficient methods are available? Many farmers and farming corporations have not switched to more efficient irrigation methods for two reasons: 1. Drip irrigation and other more efficient irrigation methods are more expensive than sprinklers, trenches, and flooding. 2. In the United States and some other countries, the government pays for much of the cost of the water that is used for agriculture. Because farmers do not pay the full cost of their water use, they do not have any financial incentive to use less water. What ideas can you come up with to encourage farmers to use more efficient irrigation systems? | text | null |
L_0344 | uses of water | T_1837 | Aquaculture is a different type of agriculture. Aquaculture is farming to raise fish, shellfish, algae, or aquatic plants (Figure 1.3). As the supplies of fish from lakes, rivers, and the oceans dwindle, people are getting more fish from aquaculture. Raising fish increases our food resources and is especially valuable where protein sources are limited. Farmed fish are becoming increasingly common in grocery stores all over the world. Workers at a fish farm harvest fish they will sell to stores. Growing fish in a large scale requires that the fish stocks are healthy and protected from predators. The species raised must be hearty, inexpensive to feed, and able to reproduce in captivity. Wastes must be flushed out to keep animals healthy. Raising shellfish at farms can also be successful. | text | null |
L_0344 | uses of water | T_1838 | For some species, aquaculture is very successful and environmental harm is minimal. But for other species, aqua- culture can cause problems. Natural landscapes, such as mangroves, which are rich ecosystems and also protect coastlines from storm damage, may be lost to fish farms (Figure 1.4). For fish farmers, keeping costs down may be a problem since coastal land may be expensive and labor costs may be high. Large predatory fish at the 4th or 5th trophic level must eat a lot, so feeding large numbers of these fish is expensive and environmentally costly. Farmed fish are genetically different from wild stocks, and if they escape into the wild they may cause problems for native fish. Because the organisms live so close together, parasites are common and may also escape into the wild. Shrimp farms on the coast of Ecuador are shown as blue rectangles. Mangrove forests, salt flats, and salt marshes have been converted to shrimp farms. | text | null |
L_0344 | uses of water | T_1839 | Industrial water use accounts for an estimated 15% of worldwide water use, with a much greater percentage in developed nations. Industrial uses of water include power plants that use water to cool their equipment and oil refineries that use water for chemical processes. Manufacturing is also water intensive. | text | null |
L_0344 | uses of water | T_1840 | Think about all the ways you use water in a day. You need to count the water you drink, cook with, bathe in, garden with, let run down the drain, or flush down the toilet. In developed countries, people use a lot of water, while in less developed countries people use much less. Globally, household or personal water use is estimated to account for 15% of world-wide water use. Some household water uses are non-consumptive, because water is recaptured in sewer systems, treated, and returned to surface water supplies for reuse. Many things can be done to lower water consumption at home. Convert lawns and gardens to drip-irrigation systems. Install low-flow shower heads and low-flow toilets. In what other ways can you use less water at home? | text | null |
L_0344 | uses of water | T_1841 | People love water for swimming, fishing, boating, river rafting, and other activates. Even activities such as golf, where there may not be any standing water, require plenty of water to make the grass on the course green. Despite its value, the amount of water that most recreational activities use is low: less than 1% of all the water we use. Many recreational water uses are non-consumptive including swimming, fishing, and boating. Golf courses are the biggest recreational water consumer since they require large amounts for irrigation, especially because many courses are located in warm, sunny, desert regions where water is scarce and evaporation is high. | text | null |
L_0344 | uses of water | T_1842 | Environmental use of water includes creating wildlife habitat. Lakes are built to create places for fish and water birds (Figure 1.5). Most environmental uses are non-consumptive and account for an even smaller percentage of water use than recreational uses. A shortage of this water is a leading cause of global biodiversity loss. Click image to the left or use the URL below. URL: | text | null |
L_0345 | venus | T_1843 | Venus thick clouds reflect sunlight well, so Venus is very bright. When it is visible, Venus is the brightest object in the sky besides the Sun and the Moon. Because the orbit of Venus is inside Earths orbit, Venus always appears close to the Sun. When Venus rises just before the Sun rises, the bright object is called the morning star. When it sets just after the Sun sets, it is the evening star. Of the planets, Venus is most similar to Earth in size and density. Venus is also our nearest neighbor. The planets interior structure is similar to Earths, with a large iron core and a silicate mantle (Figure 1.1). But the resemblance between the two inner planets ends there. | text | null |
L_0345 | venus | T_1844 | Venus rotates in a direction opposite the other planets and opposite to the direction it orbits the Sun. This rotation is extremely slow, only one turn every 243 Earth days. This is longer than a year on Venus it takes Venus only 224 days to orbit the Sun. Diagram of Venuss interior, which is simi- lar to Earths. | text | null |
L_0345 | venus | T_1845 | Venus is covered by a thick layer of clouds, as shown in pictures of Venus taken at ultraviolet wavelengths (Figure This ultraviolet image from the Pioneer Venus Orbiter shows thick layers of clouds in the atmosphere of Venus. Venus clouds are not made of water vapor like Earths clouds. Clouds on Venus are made mostly of carbon dioxide Click image to the left or use the URL below. URL: The atmosphere of Venus is so thick that the atmospheric pressure on the planets surface is 90 times greater than the atmospheric pressure on Earths surface. The dense atmosphere totally obscures the surface of Venus, even from spacecraft orbiting the planet. | text | null |
L_0345 | venus | T_1846 | Since spacecraft cannot see through the thick atmosphere, radar is used to map Venus surface. Many features found on the surface are similar to Earth and yet are very different. Figure 1.3 shows a topographical map of Venus produced by the Magellan probe using radar. This false color image of Venus was made from radar data collected by the Magellan probe between 1990 and 1994. What features can you identify? Most of the volcanoes are no longer active, but scientists have found evidence that there is some active volcanism (Figure 1.4). Think about what you know about the geology of Earth and what produces volcanoes. What does the presence of volcanoes suggest about the geology of Venus? What evidence would you look for to find the causes of volcanism on Venus? This image of the Maat Mons volcano with lava beds in the foreground was gen- erated by a computer from radar data. The reddish-orange color is close to what scientists think the color of sunlight would look like on the surface of Venus. Venus also has very few impact craters compared with Mercury and the Moon. What is the significance of this? Earth has fewer impact craters than Mercury and the Moon, too. Is this for the same reason that Venus has fewer impact craters? Its difficult for scientists to figure out the geological history of Venus. The environment is too harsh for a rover to go there. It is even more difficult for students to figure out the geological history of a distant planet based on the information given here. Still, we can piece together a few things. On Earth, volcanism is generated because the planets interior is hot. Much of the volcanic activity is caused by plate tectonic activity. But on Venus, there is no evidence of plate boundaries and volcanic features do not line up the way they do at plate boundaries. Because the density of impact craters can be used to determine how old a planets surface is, the small number of impact craters means that Venus surface is young. Scientists think that there is frequent, planet-wide resurfacing of Venus with volcanism taking place in many locations. The cause is heat that builds up below the surface, which has no escape until finally it destroys the crust and results in volcanoes. Click image to the left or use the URL below. URL: | text | null |
L_0345 | venus | T_1846 | Since spacecraft cannot see through the thick atmosphere, radar is used to map Venus surface. Many features found on the surface are similar to Earth and yet are very different. Figure 1.3 shows a topographical map of Venus produced by the Magellan probe using radar. This false color image of Venus was made from radar data collected by the Magellan probe between 1990 and 1994. What features can you identify? Most of the volcanoes are no longer active, but scientists have found evidence that there is some active volcanism (Figure 1.4). Think about what you know about the geology of Earth and what produces volcanoes. What does the presence of volcanoes suggest about the geology of Venus? What evidence would you look for to find the causes of volcanism on Venus? This image of the Maat Mons volcano with lava beds in the foreground was gen- erated by a computer from radar data. The reddish-orange color is close to what scientists think the color of sunlight would look like on the surface of Venus. Venus also has very few impact craters compared with Mercury and the Moon. What is the significance of this? Earth has fewer impact craters than Mercury and the Moon, too. Is this for the same reason that Venus has fewer impact craters? Its difficult for scientists to figure out the geological history of Venus. The environment is too harsh for a rover to go there. It is even more difficult for students to figure out the geological history of a distant planet based on the information given here. Still, we can piece together a few things. On Earth, volcanism is generated because the planets interior is hot. Much of the volcanic activity is caused by plate tectonic activity. But on Venus, there is no evidence of plate boundaries and volcanic features do not line up the way they do at plate boundaries. Because the density of impact craters can be used to determine how old a planets surface is, the small number of impact craters means that Venus surface is young. Scientists think that there is frequent, planet-wide resurfacing of Venus with volcanism taking place in many locations. The cause is heat that builds up below the surface, which has no escape until finally it destroys the crust and results in volcanoes. Click image to the left or use the URL below. URL: | text | null |
L_0350 | water distribution | T_1867 | Water is unevenly distributed around the world. Large portions of the world, such as much of northern Africa, receive very little water relative to their population (Figure 1.1). The map shows the number of months in which there is little rainfall in each region. In developed nations, water is stored, but in underdeveloped nations, water storage may be minimal. Over time, as population grows, rainfall totals will change, resulting in less water per person in some regions. In 2025, many nations, even developed nations, are projected to have less water per person than now | text | null |
L_0350 | water distribution | T_1868 | Water scarcity is a problem now and will become an even larger problem in the future as water sources are reduced or polluted and population grows. In 1995, about 40% of the worlds population faced water scarcity. Scientists estimate that by the year 2025, nearly half of the worlds people wont have enough water to meet their daily needs. Nearly one-quarter of the worlds people will have less than 500 m3 of water to use in an entire year. That amount is less water in a year than some people in the United States use in one day. Some regions have very little rainfall per month. | text | null |
L_0350 | water distribution | T_1869 | Droughts occur when a region experiences unusually low precipitation for months or years (Figure 1.2). Periods of drought may create or worsen water shortages. Human activities can contribute to the frequency and duration of droughts. For example, deforestation keeps trees from returning water to the atmosphere by transpiration; part of the water cycle becomes broken. Because it is difficult to predict when droughts will happen, it is difficult for countries to predict how serious water shortages will be each year. Extended periods with lower than normal rainfall cause droughts. | text | null |
L_0350 | water distribution | T_1870 | Global warming will change patterns of rainfall and water distribution. As the Earth warms, regions that currently receive an adequate supply of rain may shift. Regions that rely on snowmelt may find that there is less snow and the melt comes earlier and faster in the spring, causing the water to run off and not be available through the dry summers. A change in temperature and precipitation would completely change the types of plants and animals that can live successfully in that region. | text | null |
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