Transcript star
Chapter 26 The Universe Stars A star is a large, glowing ball of gas in space, which generates energy through nuclear fusion in its core. The closest star to Earth is the sun, which is considered to be a fairly average star. Stars Parallax Stars are so far away that astronomers cannot measure their distances directly. Astronomers are able to observe stars from two different positions–opposite sides of Earth’s orbit. Nearby stars appear to move against the moredistant background stars. Stars • The apparent change in position of an object with respect to a distant background is called parallax. • Astronomers measure the parallax of nearby stars to determine their distance from Earth. Stars With the invention of the telescope, astronomers could measure the positions of stars with much greater accuracy. • The closer a star is to Earth, the greater is its parallax. • Astronomers have measured the parallax of many nearby stars and determined their distances from Earth. Stars The Light-Year Because stars are so far apart, it’s not practical to measure their distances in units that might be used on Earth, such as kilometers. • A light-year is the distance that light travels in a vacuum in a year, which is about 9.5 trillion kilometers. • Proxima Centauri, the closest star to the sun, is about 4.3 light-years away. Stars Astronomers classify stars by their color, size, and brightness. Other important properties of stars include their chemical composition and mass. Stars Most stars have a chemical makeup that is similar to the sun, with hydrogen and helium together making up 96 to 99.9 percent of the star’s mass. Stars Color and Temperature A star’s color indicates the temperature of its surface. • The hottest stars, with surface temperatures above 30,000 K, appear blue. • The surfaces (photospheres) of relatively cool red stars are still a toasty 3000 K or so. • Stars with surface temperatures between 5000 and 6000 K appear yellow, like the sun. Stars Brightness Astronomers have discovered that the brightness of stars can vary by a factor of more than a billion. Stars that look bright may actually be farther away than stars that appear dim. Stars The sun appears very bright to us because it is much closer than other stars. The brightness of a star as it appears from Earth is called its apparent brightness. The apparent brightness of a star decreases as its distance from you increases. Stars Absolute brightness is how bright a star really is. A star’s absolute brightness is a characteristic of the star and does not depend on how far it is from Earth. You can calculate a star’s absolute brightness if you know its distance from Earth and its apparent brightness. Stars Size and Mass Once astronomers know a star’s temperature and absolute brightness, they can estimate its diameter and then calculate its volume. The masses of many stars can be determined by observing the gravitational interaction of stars that occur in pairs. For most stars, there is a relationship between mass and absolute brightness. Stars Composition A spectrograph is an instrument that spreads light from a hot glowing object into a spectrum. Astronomers can use spectrographs to identify the various elements in a star’s atmosphere. Stars This is the spectrum of a star. The dark absorption lines indicate the presence of various elements in the star. Stars H-R diagrams are used to estimate the sizes of stars and their distances, and to infer how stars change over time. Stars Stars can be classified by locating them on a graph showing two easily determined characteristics. Such a graph is called a Hertzsprung-Russell diagram, or H-R diagram. An H-R diagram is a graph of the surface temperature, or color, and absolute brightness of a sample of stars. Stars The horizontal axis shows the surface temperatures of stars. A star’s color is directly related to its surface temperature. The hottest blue stars are on the left and the coolest red stars are on the right. Surface temperatures of stars range from less than 3000 K to more than 30,000 K. Stars The vertical axis of the H-R diagram shows absolute brightness, with the brightest stars at the top and the faintest at the bottom. The absolute brightnesses of stars vary even more than temperature, ranging from about one ten-thousandth to a million times that of the sun. Stars • A star’s placement on an H-R diagram indicates its absolute brightness and surface temperatur e (or color). Stars Main-Sequence Stars Stars occur only in certain places on the H-R diagram. Most stars are found along a diagonal band running from the bright hot stars on the upper left to the dim cool stars on the lower right. Astronomers call this diagonal band on the H-R diagram the main sequence. About 90% of all stars are found on the main sequence. The sun lies near the middle of this band. Stars Giants and Dwarfs In general, two factors determine a star’s absolute brightness: its size and its surface temperature. An H-R diagram shows a star’s absolute brightness and surface temperature. • If you compare two stars at the same temperature, the brighter one must be larger. • Hotter stars are brighter than cooler stars of the same size. Stars The very bright stars at the upper right of the HR diagram are called supergiants. Supergiants are much brighter than mainsequence stars of the same temperature, so they must be very large compared with mainsequence stars. Stars Supergiants range in size from 100 to 1000 times the diameter of the sun. Just below the supergiants on the H-R diagram are the giants—large, bright stars that are smaller and fainter than supergiants Stars Below the main sequence in the lower part of the H-R diagram are white dwarfs. • A white dwarf is the small, dense remains of a low- or medium-mass star. • White dwarfs are hot but dimmer than main-sequence stars of the same temperature. Stars • The diameter of a red giant is typically 10–100 times that of the sun and more than 1000 times that of a white dwarf. Life Cycle of Stars A star is formed when a contracting cloud of gas and dust becomes so dense and hot that nuclear fusion begins. Life Cycle of Stars A nebula is a large cloud of gas and dust spread out over a large volume of space. • Some nebulas are glowing clouds lit from within by bright stars. • Other nebulas are cold, dark clouds that block the light from more-distant stars beyond the nebulas. Stars form in the densest regions of nebulae. Gravity pulls a nebula’s dust and gas into a denser cloud. As the nebula contracts, it heats up. Life Cycle of Stars A contracting cloud of gas and dust with enough mass to form a star is called a protostar. As a protostar contracts, its internal pressure and temperature continue to rise. Pressure from fusion supports the star against the tremendous inward pull of gravity. Life Cycle of Stars A star’s mass determines the star’s place on the main sequence and how long it will stay there. Life Cycle of Stars Stars spend about 90 percent of their lives on the main sequence. In all main-sequence stars, nuclear fusion converts hydrogen into helium at a stable rate. There is an equilibrium between the outward thermal pressure from fusion and gravity’s inward pull. The amount of gas and dust available when a star forms determines the mass of each young star. Life Cycle of Stars The most massive stars have large cores and therefore produce the most energy. High-mass stars become the bluest and brightest mainsequence stars. These blue stars are about 300,000 times brighter than the sun. Because blue stars burn so brightly, they use up their fuel relatively quickly and last only a few million years. Life Cycle of Stars Stars similar to the sun occupy the middle of the main sequence. A yellow star like the sun has a surface temperature of about 6000 K and will remain stable on the main sequence for about 10 billion years. Life Cycle of Stars Small nebulas produce small, cool stars that are long-lived. A star can have a mass as low as a tenth of the sun’s mass. The gravitational force in such low-mass stars is just strong enough to create a small core where nuclear fusion takes place. This lower energy production results in red stars, the coolest of all visible stars. A red star, with a surface temperature of about 3500 K, may stay on the main sequence for more than 100 billion years. Life Cycle of Stars The dwindling supply of fuel in a star’s core ultimately leads to the star’s death as a white dwarf, neutron star, or black hole. Life Cycle of Stars When a star’s core begins to run out of hydrogen, gravity gains the upper hand over pressure, and the core starts to shrink. • The core temperature rises enough to cause the hydrogen in a shell outside the core to begin fusion. • The energy flowing outward increases, causing the outer regions of the star to expand. The expanding atmosphere moves farther from the hot core and cools to red. • The star becomes a red giant. Life Cycle of Stars • The collapsing core grows hot enough for helium fusion to occur, producing carbon, oxygen, and heavier elements. • In helium fusion, the star stabilizes and its outer layers shrink and warm up. • The final stages of a star’s life depend on its mass. Life Cycle of Stars Low- and Medium-Mass Stars Low-mass and medium-mass stars, which can be as much as eight times as massive as the sun, eventually turn into white dwarfs. • Stars remain in the giant stage until their hydrogen and helium supplies dwindle and there are no other elements to fuse. • The energy coming from the star’s interior decreases. • With less outward pressure, the star collapses. Life Cycle of Stars • The dying star is surrounded by a glowing cloud of gas, called a planetary nebula. • As the dying star blows off much of its mass, only its hot core remains. • This dense core is a white dwarf. A white dwarf is about the same size as Earth but has about the same mass as the sun. • White dwarfs don’t undergo fusion, but glow faintly from leftover thermal energy. Life Cycle of Stars High-Mass Stars The life cycle of high-mass stars is very different from the life cycle of lower-mass stars. • As high-mass stars evolve from hydrogen fusion to the fusion of other elements, they grow into brilliant supergiants, which create new elements, the heaviest being iron. • A high-mass star dies quickly because it consumes fuel very rapidly. Life Cycle of Stars • As fusion slows in a high-mass star, pressure decreases. • Gravity eventually overcomes the lower pressure, leading to a dramatic collapse of the star’s outer layers. • This collapse produces a supernova, an explosion so violent that the dying star becomes more brilliant than an entire galaxy. Life Cycle of Stars Supernovas produce enough energy to create elements heavier than iron. • These elements, and lighter ones such as carbon and oxygen, are ejected into space by the explosion. • As a supernova spews material into space, its core continues to collapse. Life Cycle of Stars If the remaining core of a supernova has a mass less than about three times the sun’s mass, it will become a neutron star, the dense remnant of a high-mass star that has exploded as a supernova. • In a neutron star, electrons and protons are crushed together by the star’s enormous gravity to form neutrons. • Neutron stars are much smaller and denser than white dwarfs. Life Cycle of Stars A neutron star spins more and more rapidly as it contracts. Some neutron stars spin hundreds of turns per second! • Neutron stars emit steady beams of radiation in narrow cones. • A spinning neutron star that appears to give off strong pulses of radio waves is called a pulsar. Life Cycle of Stars Pulsars emit steady beams of radiation that appear to pulse when the spinning beam sweeps across Earth. Life Cycle of Stars If a star’s core after a supernova explosion is more than about three times the sun’s mass, its gravitational pull is very strong. The core collapses beyond the neutron-star stage to become a black hole. A black hole is an object whose surface gravity is so great that even electromagnetic waves, traveling at the speed of light, cannot escape from it. Groups of Stars A group of stars that appear to form a pattern as seen from Earth is called a constellation. The stars in a constellation are generally not close to one another. They just happen to lie in the same general direction of the sky as seen from Earth. Groups of Stars Astronomers have determined that more than half of all stars are members of star systems. Most stars occur in groups of two or more. • A star system is a group of two or more stars that are held together by gravity. • A star system with two stars is called a binary star. The two stars orbit each other. Groups of Stars Sometimes the smaller star in a binary star is too dim to be seen easily from Earth but can still be detected from the motion of the other star. If one star passes in front of the other, blocking some of the light from reaching Earth, the star system is called an eclipsing binary. The brightness of an eclipsing binary varies over time in a regular pattern. Groups of Stars There are three basic kinds of star clusters: open clusters, associations, and globular clusters. Studying star clusters is useful because all the stars formed together in the same nebula, so they are about the same age and the same distance from Earth. Astronomers plot the stars of a cluster on an H-R diagram to estimate the cluster’s age. Groups of Stars An open cluster has a disorganized or loose appearance and contains no more than a few thousand stars that are well spread out. Open clusters often contain bright supergiants and gas and dust clouds. Associations are temporary groupings of bright, young stars. In time, gravity from nearby stars breaks these groups apart. Associations are typically larger than open clusters. Groups of Stars A globular cluster is a large group of older stars. Globular clusters usually lack sufficient amounts of gas and dust to form new stars. They are spherical and have a dense concentration of stars in the center. Globular clusters can contain more than a million stars. Globular clusters usually do not have short-lived blue stars because these stars have already died out. Astronomers estimate that the oldest globular clusters are about 12 billion years old. Thus, the universe must be at least that old. Groups of Stars Astronomers classify galaxies into four main types: spiral, barred-spiral, elliptical, and irregular. A galaxy is a huge group of individual stars, star systems, star clusters, dust, and gas bound together by gravity. • There are billions of galaxies in the universe. • The largest galaxies consist of more than a trillion stars. Galaxies vary widely in size and shape. Groups of Stars Spiral and Barred-Spiral Galaxies Spiral galaxies have a bulge of stars at the center, with arms extending outward like a pinwheel. • These spiral arms contain gas, dust, and many bright young stars. • The Milky Way is a spiral galaxy. Groups of Stars Some spiral galaxies have a bar through the center with the arms extending outward from the bar on either side. These are called barredspiral galaxies. Groups of Stars Elliptical Galaxies Elliptical galaxies are spherical or oval, with no trace of spiral arms. • Elliptical galaxies come in a wide range of sizes. • Elliptical galaxies have very little gas or dust between stars. They contain only old stars. Groups of Stars Irregular Galaxies A small fraction of all galaxies are known as irregular galaxies. Irregular galaxies have a disorganized appearance. They have many young stars and large amounts of gas and dust. Irregular galaxies come in many shapes, are typically smaller than other types of galaxies, and are often located near larger galaxies. Groups of Stars The Milky Way Galaxy The Milky Way galaxy has an estimated 200 to 400 billion stars and a diameter of more than 100,000 light years. Every individual star that you can see with the unaided eye is in our galaxy. The solar system lies in the Milky Way’s disk within a spiral arm, about two thirds of the way from the center. Groups of Stars In a side view, the Milky Way appears as a flat disk with a central bulge. An overhead view of the Milky Way shows its spiral shape. Location of solar system Central bulge Nucleus Overhead View of Our Galaxy Disk of spiral arms containing mainly young stars Halo containing oldest stars Central bulge containing mainly older stars Nucleus Side View of Our Galaxy Groups of Stars The Milky Way’s flattened disk shape is caused by its rotation. The sun takes about 220 million years to complete one orbit around the galaxy’s center. Recent evidence suggests that there is a massive black hole at our galaxy’s center. Stars are forming in the galaxy's spiral arms. Groups of Stars Quasars By studying their spectra, astronomers have determined that quasars are the enormously bright centers of distant, young galaxies. Quasars produce more light than hundreds of galaxies the size of the Milky Way. What makes a quasar so bright? The most likely explanation involves matter spiraling into a supermassive black hole with the mass of a billion suns. The Expanding Universe Absorption lines of a galaxy shift toward the blue end of the spectrum when it moves toward Earth. The lines shift to the red end of the spectrum when a galaxy moves away from Earth. The Expanding Universe When you observe a star that is ten light-years away, you are seeing the star as it was ten years ago, because light took ten years to travel from the star to Earth. Images of galaxies that are billions of light-years away show how those galaxies looked billions of years ago. The Expanding Universe The observed red shift in the spectra of galaxies shows that the universe is expanding. The Doppler effect can be used to determine how fast stars or galaxies are approaching or moving away from Earth. • When a star or galaxy is approaching Earth, the lines in its spectrum are shifted toward the shorter (bluer) wavelengths. • When the star or galaxy is moving away, the lines in its spectrum shift toward the longer (redder) wavelengths. The larger the observed shift, the greater is the speed. The Expanding Universe In the mid-1920s, Edwin Hubble discovered that the light from most galaxies undergoes a red shift—that is, their light is shifted toward the red wavelengths. This red shift showed that nearly all galaxies are getting farther away from Earth. The Expanding Universe Hubble also found that more-distant galaxies have greater red shifts. This relationship, called Hubble’s Law, says that the speed at which a galaxy is moving away is proportional to its distance from us. The most distant observed galaxies are moving away at more than 90 percent of the speed of light! The Expanding Universe The space between the galaxies is expanding in all directions. The universe as a whole is becoming larger. Hubble’s law expresses the relationship between the velocity that a galaxy is moving away from Earth and its distance from us. The ratio of these variables is a constant called Hubble’s constant. Hubble’s constant can be estimated by finding the slope of a graph of velocity versus distance for a set of galaxies. The Expanding Universe Hubble’s constant is one of the most important and debated numbers in astronomy. It expresses how fast the universe is expanding, and can be used to estimate the age of the universe. The Expanding Universe Astronomers theorize that the universe came into being at a single moment, in an event called the big bang. The existence of cosmic microwave background radiation and the red shift in the spectra of distant galaxies strongly support the big bang theory. The Expanding Universe The motion of galaxies indicates that the universe is expanding uniformly. The big bang theory states that the universe began in an instant, billions of years ago, in an enormous explosion. The Expanding Universe After the Big Bang The universe expanded and cooled down after the big bang. • After a few hundred thousand years of expansion, the universe was cool enough for atoms to form. • Gravity pulled atoms together into gas clouds that eventually evolved into stars in young galaxies. • The sun and solar system formed about 4.6 billion years ago, when the universe was about two thirds of its present size. The Expanding Universe The universe began with the big bang 13.7 billion years ago. The first stars and galaxies formed 200 million years later. The solar system, and Earth, formed about 9 billion years after the big bang. Big bang occurred 13.7 billion years ago. First stars and galaxies formed 200 million years after big bang. Solar system formed 4.6 billion years ago. Earth today The Expanding Universe Evidence for the Theory In 1965, Arno Penzias and Robert Wilson, using a radio telescope, noticed a faint distant glow in every direction. • Today this glow is called the cosmic microwave background radiation. • This glow is energy produced during the big bang, still traveling throughout the universe. The Expanding Universe The big bang theory describes how the expansion and cooling of the universe over time could have led to the present universe of stars and galaxies. It offers the best current scientific explanation of the expansion of the observable universe. Variations of the theory continue to be proposed and are being tested with new observations. The Expanding Universe Age of the Universe Since astronomers know how fast the universe is expanding now, they can infer how long it has been expanding. If you traveled backward in time, all of the matter in the universe would be at its starting point 13 to 14 billion years ago. Recent measurements of the microwave background radiation have led to a more precise age. Astronomers now estimate that the universe is 13.7 billion years old. The Expanding Universe Dark matter cannot be seen directly, but its presence can be detected by observing its gravitational effects on visible matter. To have a gravitational force strong enough to reverse the expansion, there must be sufficient mass in the universe. If there is less than this amount of mass, the universe will continue to expand. The Expanding Universe Much of the matter in the universe can’t be seen by astronomers. Dark matter is matter that does not give off radiation. Galaxies like ours may contain as much as ten times more dark matter than visible matter. The Expanding Universe There are many unanswered questions about dark matter. Astronomers don’t know what it is made of or how it is distributed through the universe. Much of the mass of the universe may be composed of dark matter. The Expanding Universe In the past few years, astronomers have discovered that the rate of expansion of the universe may be increasing. Galaxies appear to be moving apart faster now than expected. The reason for this is uncertain. A mysterious force called dark energy is theorized to be causing the rate of expansion to increase. If the expansion is accelerating, it’s likely that the universe will expand forever.