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Stars and the Universe

C H A P T E R

28WORDS TO KNOW

big bang galaxy nuclear fusioncosmic background radiation light-year redshiftDoppler effect luminosity starfrequency Milky Way galaxy

This chapter will help you answer the following questions:1 How do astronomers investigate stars?2 Are stars different from other celestial objects?3 How do stars generate energy?4 What kinds of stars are there?5 How do stars change?6 What are the structure, history, and future of the universe?

HOW DO ASTRONOMERS INVESTIGATE STARS?

Stars are extremely hot and have no solid surface. Scientists cannotsend instruments to land on stars. Any devices would melt andprobably vaporize long before reaching the visible surface of a star.Furthermore, other than the sun, stars are too far away to reach byspacecraft. With our present technology, it would take tens or evenhundreds of thousands of years for a spacecraft to reach even the

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next nearest star. Therefore, most of the information astronomershave about stars comes from light and other electromagnetic en-ergy they radiate into space.

The Electromagnetic Spectrum

To most of us, visible light is the most important kind of electro-magnetic radiation. Light allows us to see. Visible light can be bro-ken up into its component colors by a prism. This is shown inFigure 28-1.

Our eyes are sensitive to only a narrow band of wavelengths thatwe know as visible light. However, there are other electromagneticradiations, which make up the electromagnetic spectrum. Lookback at Figure 19-12 on page 452 from the Earth Science ReferenceTables. It shows the full range of electromagnetic radiation, alsoknown as the electromagnetic spectrum.

STUDENT ACTIVITY 28-1 —MAKING A SPECTRUM

You can separate sunlight into its spectrum of colors with a glassprism. This works best in a darkened room where windows face

the sun. Close the shades so that a narrow slit of direct sunlight en-ters the room. Place the prism near the narrow opening that admitssunlight. The prism will bend the light beam and separate it into itscolors. You may need to rotate the glass prism to project a visiblespectrum. The spectrum can be projected onto a sheet of whitepaper. The stronger the light and the closer the paper is held to theprism, the brighter the spectrum will be. To increase the size of thespectrum, move the paper screen away from the prism. What twochanges in the spectrum do you observe as the paper is moved awayfrom the prism?

Elements can absorb and emit light. Each element has its owncharacteristic absorption lines, or absorption spectrum. Stars arecomposed primarily of hydrogen and helium. White light thatpasses through these gases shows dark lines in the orange, yellow,green, and blue areas of the spectrum that characterize hydrogen

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and helium. These spectral lines correspond to the energy thatelectrons absorb when they move to higher energy levels withinthe atoms. The atoms give off the same colors when the electronsfall to lower or inner energy levels. Each element has a unique setof energy levels. Therefore, these “spectral fingerprints” allow as-tronomers to identify the composition of distant stars. Visit thefollowing Web site to decode cosmic spectra: http://www.pbs.org/wgbh/nova/origins/spectra.html

Optical Telescopes

Astronomers use optical telescopes to study the stars. (See Figure1-5 on page 10.) These telescopes concentrate the light of stars.Optical telescopes allow them to see objects that are too dim to beseen with unaided eyes. Some people think that the most impor-tant feature of a telescope is how much it magnifies. However, thestars are so distant that even the most powerful telescopes shownearly all stars as points of light. When an image is magnified toomuch, it becomes dim, unclear, or fuzzy.

Other factors are more important than magnification in tele-scope construction and use. First is the size, or diameter, of thefront lens (or light-gathering mirror).This determines the dimmestobject that can be observed. The larger the diameter of the frontlens, the dimmer the objects you can see. The farther astronomerslook into space, the dimmer the objects become.

FIGURE 28-1. A beamof white light enters thisglass prism from the top.Some of the light isreflected. However, thelight that passes throughthe glass is split into arainbow of colors. Thisshows that white light isactually a mixture ofcolors.


The second factor is the quality of optics of the telescope. If thelenses or mirrors are not made with great precision, magnified im-ages will not be sharp. Earth’s atmosphere is also a limiting factor.This is why major observatories are built on high mountains,where the atmosphere is thin and has less effect on the light. Fig-ure 28-2l shows several buildings containing large telescopes ona mountaintop in Arizona. The Hubble Space Telescope (Figure28-2r) orbits Earth above the distorting effects of Earth’s atmos-phere. This telescope was a major step forward in observational as-tronomy. Visit the following Web site to see the universe throughthe Hubble Space Telescope: http://news.xinhuanet.com/english/2006-12/18/content_5501941.htm

STUDENT ACTIVITY 28-2 —MAKING A TELESCOPE

You can build a simple telescope using two convex lenses. A tubeto hold the lenses at the proper distance and alignment is help-

ful but not necessary. By using lenses with more or less curvature,you can change magnification. Moving the lens that is closest toyour eye adjusts the focus.

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FIGURE 28-2. (l.) The largest optical telescopes are placed in observatories.The observatories are built on remote mountains to lift them above as much of atmospheric pollution as possible. (r.) The Hubble Space Telescope orbitsEarth far above the distorting effects of Earth’s atmosphere. The Hubbletelescope has taken spectacular images of distant stars and galaxies,including the Eagle Nebula pictured on page 604.

Radio Telescopes

Radio telescopes gather long-wavelength radio energy rather thanlight. Radio telescopes, like those in Figure 28-3, are not affectedby our atmosphere or by clouds of dust and gas in space that blocklight. They also can detect objects that do not produce light. Radiotelescopes do not form sharp images. This makes it difficult to tellthe exact position of a radio source. However, using radio tele-scopes astronomers make observations that would not be possi-ble with optical telescopes.

Other Telescopes

Other kinds of telescopes use electromagnetic wavelengths shorterthan visible light, such as x-rays and gamma rays. These telescopesmust be located in orbit above Earth’s atmosphere, which filtersout these forms of radiation.

Technology has changed the ways astronomers use telescopes.The first telescopes were used for direct observations. If astrono-mers wanted a permanent record of what they saw, they had to drawit by hand. Film cameras allowed astronomers to take picturesthrough their telescopes. Today, the more advanced telescopes useelectronic sensors like those in digital cameras along with comput-ers to create better quality images than ever before possible.

FIGURE 28-3.This group of radiotelescopes inwestern NewMexico uses 27giant receivingsurfaces to amplifyweak radio signalsfrom nearly anydirection in space.

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Spectroscope

The spectroscope is one of the most important tools that astrono-mers use. A spectroscope separates light into its component col-ors (wavelengths). A spectroscope works something like a prism.When starlight passes through a spectroscope, dark lines appearin certain parts of the spectrum. These dark lines are producedwhen certain wavelengths of light are absorbed by gaseous ele-ments within the outer parts of the star.

THE SEARCH FOR EXTRATERRESTRIAL LIFE

In 1996, scientists announced that a meteorite recovered from theice of Antarctica might contain evidence of life outside Earth. Sci-entists studied the meteorite with a microscope. They saw whatmight be fossilized bacteria. They also identified organic com-pounds thought to be the result of biological processes near thisfossil.

Most scientists agree that the meteorite came from Mars. How-ever, no evidence of life on Mars has been found by remote obser-vations or by spacecraft. If life exists there, it is probably primitive,microscopic organisms. These organisms most likely live underthe planet’s surface. Some scientists suggest that the fossil-likeshape and the compounds thought to be of biological origin couldbe the result of inorganic processes. Scientists are still discussingwhether the features in this meteorite are our first direct evidenceof life outside Earth.

ET, Phone Earth

Another attempt to discover extraterrestrial life is project SETI(Search for Extraterrestrial Intelligence). SETI seeks to find radiotransmissions coming from outside our solar system. Consideringthe billions of stars in the universe, it seems possible that planetswith conditions similar to those on Earth could be orbiting somestars. It also seems possible that on some of these planets theremay be technological civilizations like our own.

The huge distances in space may keep people from travelingamong the stars. However, radio waves can travel long distances at

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the speed of light. Radio waves are long-wavelength electromag-netic radiation that we cannot see.Astronomers use large radio tele-scopes to explore the universe using radio waves. Radio waves canpenetrate dust and clouds of gas that prevent visual observations.

How would scientists know if they received signals from an-other civilization? The signals probably would not be in a famil-iar language. Astronomers look for patterns in radio transmissionsthat have no known natural source. These patterns could be sig-nals from another civilization. For several decades, some of thelargest radio telescopes have been listening for intelligent trans-missions. Figure 28-3 on page 685 is a group of radio telescopesdesigned to receive long-wave electromagnetic radiation.

At first, analyzing these signals was a problem. How could scien-tists separate intelligent communications from the large amount ofradio noise generated by stars? This is where computers come in.Computers can quickly find patterns in radio signals. Faster andmore powerful computers have let astronomers scan far more ob-servations than humans could ever analyze. However, since SETIbegan in about 1985, no signals identified as likely forms of intelli-gent communication have been found.

What Could This Mean for Humans?

If scientists did detect intelligent communications, how could itaffect Earth? Perhaps the information would provide us new in-sights into mathematics, science, or technology. Perhaps peoplecould learn more about the promise and the dangers of a devel-oping civilization. On the other hand, there is always the dangerof conquest by an unknown military power. The future is alwaysunknown. Fortunately, we have learned to apply new technologiesand discoveries to improve our lives. Experience has taught that,in the long run, the developments of science, for the most part,benefit humans.

WHAT IS A STAR?

A star is a massive object in space that creates energy and radiatesit as electromagnetic radiation. The sun is a star. If you comparethe sun with the thousands of stars known to astronomers, the

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sun appears to be a typical star. Actually, most of the stars we ob-serve in the night sky are larger and brighter than the sun. At thesame distances as the visible stars, there are far more stars smallerthan the sun that are too dim to see from Earth. Observations ofthe sun give astronomers insights into most other stars and theirobservations of other stars help them understand the sun. Neverlook directly at the sun.

STUDENT ACTIVITY 28-3 —LIGHT INTENSITY AND DISTANCE

You can use a light meter to measure the change in light inten-sity with distance. Place a lightbulb in a dark room. Measure the

intensity of the light at different distances from the lightbulb. Makea data table of the light intensity at various distances from the light-bulb. Graph your data. What other factor shows a similar change ofintensity with distance?

Starlight

For centuries, astronomers have wondered how the sun could pro-duce the large amounts of energy it radiates into space. They knewthat the brightest light sources on Earth produced light by chemi-cal changes such as burning. Fuels such as wood and coal rapidlycombine with oxygen in the atmosphere to produce heat and light.If the sun burned coal or wood to produce energy, it would run outof fuel very quickly. As scientists became aware of the age the solarsystem, it became clear that a very different process was takingplace in the sun.

Advances in science in the early twentieth century showed thatmatter could be changed into energy. You may have heard of Al-bert Einstein’s famous equation E ! mc2, in which E is energy, mis mass, and c is the speed of light. The speed of light is a very largenumber. Therefore, the square of that number is enormous. Thepoint of this formula is that great amounts of energy are createdby the conversion of a small amount of mass. In the nearly 5 bil-lion years since the solar system originated, scientists estimate thatthe sun has only lost about one-third of 1 percent of its total mass.

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Nuclear Fusion in Stars

Most of the mass of the sun or a similar star is hydrogen, the light-est element. When four hydrogen nuclei join to make a heliumnucleus, they lose about 1 percent of their mass. The processby which light elements join to make heavier elements is callednuclear fusion. (See Figure 28-4.) While 1 percent may seem likea small loss of mass, it is enough to create a great amount of en-ergy. However, nuclear fusion can occur only under extreme con-ditions of heat and pressure. Although the gas giant Jupiter is thelargest planet in our solar system, it is not quite large enough tosupport fusion and become a star.

Energy Escapes from Stars

Energy created deep in the sun moves to the sun’s visible surfaceby radiation and convection. From the solar surface, the energyescapes as electromagnetic radiation. The surface temperature ofthe star determines the kind of electromagnetic energy it radiates


Proton

Neutron

PositronEnergy

Nutrino

Gamma Ray

1 Helium Atom

4 Hydrogen Atoms

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! !

"

" "

FIGURE 28-4. Nuclear fusion within the sun converts four protons (hydrogennuclei) into one atom of helium. This process occurs in several steps. Thesun’s mass is decreasing by about 4 million tons/s. This loss of mass isexpected to continue for another 5 billion years with plenty of mass left over.


into space. The sun is a yellow star because its roughly 6000°Csurface radiates most intensely as yellow light in the visible part ofthe spectrum.

Based on observations of other stars, astronomers predict thatthe sun will continue to radiate energy as it now does for about an-other 5 billion years.

HOW ARE STARS CLASSIFIED?

In the early twentieth century, astronomers in Denmark and theUnited States discovered a way to classify stars. They based theirclassification on the amount of electromagnetic energy the starsgenerate and their temperature. The total energy output of a staris its luminosity, or absolute brightness. Apparent brightness, orstellar magnitude, is how bright the star looks as seen from Earth.The closer a star is, the brighter it appears to us. The brightnessof a star depends on its absolute magnitude, or luminosity, and itsdistance from the observer.

You may have noticed that when you turn off an incandescentlightbulb the color of the hot wire briefly changes to red before itgoes dark. Red is the coolest color of light visible to our eyes. If amaterial is heated beyond red-hot, it becomes white and thenblue. Continued heating would push the radiation into the ultra-violet part of the spectrum and beyond.

Hertzsprung-Russell Diagram

Astronomers use the Hertzsprung-Russell, or H-R, diagram toclassify stars. This graph was named in honor of the two men whodeveloped it. Figure 28-5 is from the Earth Science Reference Tables.Unlike other graphs, this graph is usually plotted with the temper-atures decreasing to the right along the bottom axis. (Usually, val-ues increase to the right as well as upward on the vertical axis.)Visit the following Web site to compare different magnitude lev-els with lightbulbs: http://www.ioncmaste.ca/homepage/resources/web_resources/CSA_Astro9/files/multimedia/unit2/magnitudes/magnitudes.html

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Main Sequence Stars

When plotted on this graph, most stars fall into distinct groups.The greatest number of stars fall into an elongated group thatruns across the luminosity and temperature diagram from theupper left to the lower right. This region of the graph is the mainsequence. The position of a star along the main sequence is pri-marily a function of the mass of the star.

RED DWARF STARS The smallest stars, such as Barnard’s Star,are red dwarf stars, which are barely large enough to support nu-clear fusion. They are red because they are relatively cool. Thesestars are so dim that even the red dwarfs closest to us are difficultto see without a telescope. In fact, about 80 percent of the nightstars closest to Earth are too dim to be visible to the unaided eye.

FIGURE 28-5.When stars areplotted on a graph according to their energyoutput (luminosity)and surfacetemperature (whichdetermines thestar’s color), moststars fall into thegroups labeledhere.

This leads astronomers to infer that red dwarf stars are more nu-merous than all other groups of stars.

Small stars last longer than larger stars. The lower temperatureand pressure in these stars allow them to conserve hydrogen fueland continue nuclear fusion much longer than larger stars. Thecombination of small size and slow production of energy makesthem very dim.

BLUE SUPERGIANT STARS At the other end of the main se-quence are the blue supergiants. These massive stars last a smallfraction of the lives of the smaller stars. The extreme conditions oftemperature and pressure at the center of these stars cause rapiddecrease in their large quantities of hydrogen. Some of them are amillion times brighter than the sun. They are also much hotterthan the sun, giving them a blue color. These largest stars are notnearly as common as the smaller stars, in part because they burnout quickly. The most massive stars last less than one-thousandthof the life of the sun. Rigel, a bright star in the winter constella-tion Orion, is 10,000 times as luminous as the sun. The blue colorof Rigel is apparent if you compare it with Betelgeuse, anotherbright star in Orion. Betelgeuse is a red giant star on the oppositeside of the same constellation.

White dwarfs, red giants, and the supergiants are the most com-mon star groups outside the main sequence. Most other stars fallinto one of theses three groups on the temperature-luminositygraph.

HOW DO STARS EVOLVE?

Different sizes of stars have different life cycles. However, the evo-lution of stars can be illustrated by considering a star about thesize of the sun.

Birth of a Star

Star formation begins when a cloud of gas (mostly hydrogen)and dust begins to come together (condense) under the influenceof gravity. There are two sources for the material in the cloud.Some of it is hydrogen and helium left over from the formation

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of the universe about 14 billion years ago. The rest is the debrisfrom the explosions of massive stars that formed earlier in thehistory of the universe. This initial phase takes about 50 millionyears. (The process is faster for larger stars and slower for smallerstars.)

As the material comes together, heat is generated by the col-lapse of the matter and by friction. This heat causes the tempera-ture to rise until it is hot enough and the pressure is high enoughto support nuclear fusion. When fusion starts, the star becomesvisible. The star produces and radiates large amounts of energy.With binoculars or a small telescope, you can see several youngstars below the belt of Orion shining through a giant cloud of gasthat surrounds them. Visit the following Web site to watch the lifecycle of a star: http://aspire.cosmic-ray.org/labs/star_life/support/HR_animated_real.html

Middle Age

The star becomes less luminous after it fully condenses. An aver-age star spends most of its life on the main sequence region of theluminosity and temperature graph. Gravitational pressure bal-anced by heat from nuclear fusion keeps the star from shrinkingfurther. Middle age is the longest and most stable phase of stellarevolution.

Death of an Average Star

After about 10 billion years, a star the size of the sun runs low onhydrogen. Fusion slows. The core of helium collapses, whichcauses the outer part of the star to expand quickly. The star be-comes a red giant. Fusion of helium and other heavier elementsreplaces hydrogen fusion. The outer shell of gases expands andcools in the red giant stage. Eventually, all that is left is a dense,hot core, which is a white dwarf star.

Death of a Massive Star

Stars with more than about 10 times the mass of the sun end theirperiod in the main sequence more violently. These stars create avariety of heavier elements before they collapse. The collapse



process of larger stars generates so much energy that these starsend their life in an explosion called a supernova. They briefly gen-erate more energy than the billions of stars that make up thewhole galaxy. Most of the mass of the star is blown into space. Thecore of the star may form an extremely dense object called a neu-tron star. Some stars are so massive that they form an object withgravity so strong that not even light can escape: a black hole. Blackholes cannot radiate energy, but they can be detected because en-ergy is given off by matter that falls into the black hole. They canalso be located by their gravitational effects on other objects. As-tronomers have recently detected a black hole at the center of theMilky Way and many other galaxies.

WHAT IS THE STRUCTURE OF THE UNIVERSE?

Early astronomers noticed fuzzy objects in the night sky. Theycalled these objects nebulae (singular, nebula). The word nebulacomes from the Latin word for cloud. Unlike the stars, these ob-jects looked like dim fuzzy patches of light.

Nebulae and Galaxies

Telescopes revealed that some nebulae are regions of gas and dustwhere stars are forming. In addition, some nebulae were fartheraway than any known stars. Astronomers eventually realized thatsome nebulae are huge groups of stars held together by gravity.These objects are called galaxies. The whole Andromeda galaxy isvisible as a small, faint patch of light high in the autumn sky. Pow-erful telescopes revealed that the Andromeda galaxy, like billionsof other galaxies, is a gigantic group of billions of stars. Galaxiesare separated by vast distances that contain relatively few stars.Figure 28-6 is a typical spiral galaxy. Visit the following Web siteto learn what a billion (or a trillion) pennies would look like:http://www.kokogiak.com/megapenny/default.asp

Astronomers have discovered that collisions of galaxies are rel-atively common. However, the matter within the galaxies is sospread out that the two galaxies generally pass right through eachother without individual stars colliding.

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The Milky Way

Astronomers realized that all the stars visible in the night sky area part of the group of stars called the Milky Way galaxy. The sunand solar system are part of the Milky Way galaxy. This name camefrom the faint, white band of light (Figure 28-7 on page 696) thatcan be seen stretching across the sky on very dark, moonlessnights. (You cannot see the Milky Way in cities. In these areas lightpollution prevents the night sky from being dark enough to let itbe seen.) This broad band is actually made of thousands of stars.

Radio telescopes helped astronomers map the Milky Way gal-axy. Astronomers estimate that it is composed of roughly 100 bil-lion stars. Clouds of dust and gas obscure most of them. Theseclouds are also a part of our galaxy. The center of the galaxy islocated in the direction of the summer constellation Sagittarius.The shape of the Milky Way galaxy, like NGC 4414 and the An-dromeda galaxy, is a flattened spiral. The sun and solar system arelocated about two-thirds of the way from the center to the outeredge, as shown in Figure 28-6. As stars orbit the center of thegalaxy, inertia keeps gravity from drawing them together.

Orbiting the center of the galaxy is another one of Earth’s cyclicmotion in space. Our planet rotates on its axis in a 24-h cycle. It

FIGURE 28-6. Galaxy NGC 4414 is a typical spiral galaxy composed ofbillions of stars. The Milky Way galaxy and its relatively nearby twin, theAndromeda galaxy, are similar spiral galaxies. The location of Earth and thesolar system in the Milky Way galaxy is in one of the spiral arms far from thegalactic center. The white arrow in this image shows such a position.

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also revolves around the sun each year. The solar system revolvesaround the center of the Milky Way galaxy in about 220 millionyears. Although this is a long time, the Milky Way galaxy is so largethat this motion is actually about 10 times faster than Earth’s rev-olution in its orbit around the sun.

Clusters and Superclusters

Galaxies are not the largest structures in the universe. The MilkyWay and Andromeda galaxies are part of a cluster of about 30galaxies. This cluster is called the Local Group. Astronomers arenow mapping superclusters of galaxies and even larger structuresof matter.Why the matter of the universe is so unevenly distributedis one of the most important questions that astronomers are tryingto answer. Visit the following Web site to model galaxy collisionson your computer: http://burro.cwru.edu/JavaLab/GalCrashWeb/main.html

WHAT IS THE HISTORY OF THE UNIVERSE?

When you look at very distant objects in the universe, you arelooking back in time. This is because light has a limited speed. Youlearned in an earlier chapter that you could estimate the distanceto a lightning strike by counting the seconds between seeing the

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FIGURE 28-7. On dark, moonlessnights, ancient people saw aband of faint light crossing thesky. They called it the Milky Waybecause it reminded them ofspilled milk. They did not realizethat they were actually seeingmillions of distant stars, many ofthem much brighter than the sun.


flash and hearing the thunder. Due to light’s speed, you see theflash at essentially the same time it occurred. Light travels so fastthat it could circle Earth about 7 times in 1 s. Visit the followingWeb site to watch the PowerPoint Presentation—Can We Rewindthe Clock to Catch a Glimpse Near the Beginning of Time? http://www.bnl.gov/physics/colloquium/PHENIX_Final.ppt

Using Light as a Yardstick

Distances in space are so great that light cannot reach Earth in-stantly. Light and other electromagnetic energy take about 3 s totravel from the moon to Earth. It takes about 8 minutes for this en-ergy to travel from the sun to Earth. Light takes more than 4 yearsto arrive from the nearest star, Proxima Centauri. The most distantobject visible to the unaided eye is the Andromeda Galaxy. Lightfrom the Andromeda galaxy takes about 2 million years to reach us.

Astronomers use light to measure the universe. A light-year isthe distance that any form of electromagnetic energy can travel in1 year. That distance is about 10 trillion km (6 trillion mi). Al-though the light-year may sound like a measure of time, it is a mea-sure of distance.

When astronomers look at distant objects in space, they seethem as they were when the light started its journey toward Earth.The deeper astronomers look into space, the farther back in timethey see. At present, astronomers estimate that the most distantobjects they can see are about 13 billion light-years away. As-tronomers can see what the universe looked like about a billionyears after its origin. They estimate the origin occurred about 14billion years ago.

Redshift

Astronomer Edwin Hubble examined the spectra of distant galax-ies in the early 1900s. He compared the dark absorption lines, orabsorption spectra, of light from these faraway galaxies to theabsorption spectra of nearby stars. Nearby stars had absorptionspectra similar to those produced in the laboratory. However, thedark lines in the spectra of distant galaxies were shifted toward thered end of the spectrum. Hubble reasoned that the motion of dis-tant galaxies away from Earth causes the redshift of spectral lines.Figure 28-8, on page 698, illustrates the redshift of spectral lines.


If the galaxies were moving toward Earth, the spectral lineswould shift toward the blue end of the spectrum. The shift towardthe red end of the spectrum indicates that the galaxies are movingaway from Earth. Visit the following Web site to see the absorp-tion lines of elements in the Periodic Table: http://jersey.uoregon.edu/vlab/elements/Elements.html

You can experience a similar change with sound. If you standnext to an automobile racetrack, the high-pitched sound of the ap-proaching car changes to a lower pitch as the car speeds past you.Pitch is related to frequency, which is a measure of how manywaves pass a given point in a given period of time. This apparentchange in frequency and wavelength of energy that occurs whenthe source of a wave is moving relative to an observer is called theDoppler effect. It was named for Christian Johann Doppler, thescientist who explained it in 1842. The change in the frequencyand wavelength of sound waves is similar to the changes thatHubble observed with light. The motion of a star away from us ata significant fraction of the speed of light extends the electromag-netic waves. The extended (stretched) waves make the star lookredder than if it were not receding. Figure 28-9 shows how the mo-tion of a source toward or away from an observer affects the wave-length. The greater the redshift, the faster the object is movingaway. Astronomers have found that the most distant galaxiesare moving away the fastest. Visit the following Web site to watchanimated simulations of the Doppler effect on sound: http://www.colorado.edu/physics/2000/applets_New.html

Two other factors supported Hubble’s hypothesis of an ex-panding universe. The redshift of light of distant galaxies is ob-served in all directions. In addition, the dimmer galaxies, which are

FIGURE 28-8. When welook at the signatureelements of stars, the darklines that mark hydrogenand other elements areclearly visible. The moredistant stars and galaxiesshow the dark linesshifted toward the red endof the spectrum.


thought to be dim because they are farther from Earth, showedgreater redshift. He also reasoned that this kind of motion is acharacteristic of an explosion. You might think that the motion ofdistant galaxies away from Earth in all directions means that weare at the center of the expansion. However, from every positionwithin the expanding matter of an explosion, matter is movingaway in all directions.

STUDENT ACTIVITY 28-4 —A MODEL OF THE BIG BANG

Inflate a round balloon to the size of a tennis ball. Draw severalsmall dots on the balloon’s surface. Notice that as the balloon is

inflated more, the dots always move apart. If observers were locatedanywhere on the surface of the balloon or even inside the balloon,as the balloon is inflated, they would see the dots moving away inall directions. No matter what location is chosen, it would appearthat the observer is at the center of the expansion. Therefore, thereis no way that astronomers can find the center of the universe.

In the 1960s, Arno Penzias and Robert Wilson were working onlong-distance radio communications for the Bell Telephone Com-pany. They built a special outdoor receiver to detect weak radio

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FIGURE 28-9. The Doppler effect is a difference in wave frequency dependingupon whether the source is moving toward or away from the observer. Thisoccurs with both sound and light waves. Distant galaxies appear more red tous because the light waves are spread out by the motion of these galaxiesaway from us.


signals. However, the device picked up annoying radio signals thatthey could not eliminate.They investigated the source of these radiosignals and found that the energy they were picking up was billionsof years old. They were listening to the origin of the universe. Theseradio signals, the cosmic background radiation, are weak electro-magnetic radiation left over from the formation of the universe.

The Big Bang

The outward motions of distant galaxies and the cosmic back-ground radiation are evidence that the universe began as an eventnow called the big bang. The name was first proposed as a joketo make fun of the theory, but the name stuck. This theory pro-poses that at its origin, the universe was a concentration of mat-ter so dense that the laws of nature as we know them today didnot apply. This matter expanded explosively, forming the universe.Even the most extreme conditions that exist within the largeststars do not compare with the beginning of the universe.

Experiments and the theories of physics confirm that the speedof light is the greatest velocity possible for matter or energy. Thespeed of light is about 300 million m/s. Like the temperature ofabsolute zero (0 K), the speed of light is one of the absolute lim-its known to science. Astronomers reason that the universe is ex-panding at this rate.

Using the speed of light, astronomers work back to the time ofthe big bang. They estimate that the universe began with the bigbang 14 billion years ago. Expanding outward in all directions, theuniverse today could be as much as 28 billion light-years across.This is so gigantic that it is nearly impossible for humans to com-prehend how large this is. Earth, the solar system, and even thehuge Milky Way Galaxy are incredibly small compared with thesize of the universe. Visit the following Web site and take A Cos-mic Journey: A History of Scientific Cosmology and click on TheExpanding Universe: http://www.aip.org/history/cosmology/

WHAT IS THE FUTURE OF THE UNIVERSE?

From our point of view, Earth, as a planet, is very stable. Earth’sorbit around the sun is unchanging. Scientists expect the sun tocontinue its energy production on the main sequence for billionsof years. Collisions with large objects from space, such as those

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thought to mark the ends of past geologic eras, are possible.Fortunately, these events are becoming less likely as the solar sys-tem ages. However, the very-long-term future of the universe isnot clear.

The Future of the Universe

Scientists once thought the universe had three possible futures.Some scientists proposed that the expansion of the universe mightbe slowing due to gravity. However, it is possible that there is notenough gravity to stop the expansion. In this case, the universewould continue to expand without limit. Other scientists pro-posed that there may be enough gravity to just stop the expansion,leading to a steady state. If the universe has enough gravity to re-verse its expanding phase, it could fall back together in a very dis-tant event that some astronomers call the “big crunch.”

A good way to understand this is to consider a baseball thrownstraight up. Gravity brings the ball back to the ground. However, ifyou could propel the ball fast enough, it would continue upwardinto space and never return to Earth. Figure 28-10 illustrates thebig bang followed by the big crunch. Visit the following Web site to

FIGURE 28-10. Scientists know that the universe has expanded over the past 14billion years. The expansion began with the “big bang.” Will expansion stop andgravity pull the universe back into a black hole? Recent evidence seems to favor amodel with continued growth and even accelerating expansion.

watch the video of Neil deGrasse Tyson discussing Death by BlackHole: http://fora.tv/2008/02/19/Neil_DeGrasse_Tyson_Death_by_Black_Hole

In recent years, astronomers have found evidence that the uni-verse is not only expanding, but that it is expanding at an increas-ing rate. What force could work against the force of gravity tocause this? It is as if you threw a ball up into the air and it did notfall back to Earth. In fact, it is as if the ball flew upward faster andfaster with time. This would be surprising, indeed. Astronomersfind these observations just as surprising.

Astronomers have named the mysterious cause of this acceler-ating expansion “dark energy.” However, they cannot explain it.Nor can they explain the source of gravitational force that holdsthe rapidly spinning galaxies from breaking apart. This force is at-tributed to the gravitational attraction of “dark matter,” which as-tronomers think makes up about 90 percent of the matter in theuniverse. Dark matter and dark energy—the mysteries of sciencejust keep coming.

The ultimate future of the universe depends upon the balancebetween the expansion of the big bang, gravity, and dark energy. Todate, astronomers have not been able to determine which one willdominate. This remains one of many questions that guide scien-tific investigation.

CHAPTER REVIEW QUESTIONS

Part A

1. When astronomers investigate the most distant galaxies they see evidence that

(1) the most distant galaxies are moving toward Earth(2) the most distant galaxies are moving away from Earth(3) the most distant galaxies appear the brightest(4) only the smallest galaxies are visible at these distances

2. Compared with our sun, the star Betelgeuse is

(1) smaller, hotter and less luminous(2) smaller, cooler and more luminous(3) larger, hotter and less luminous(4) larger, cooler and more luminous


3. The symbols below are used to represent different parts or objects in space.

Which diagram below best shows the relationship between our solar system,the universe, and our galaxy?

4. Astronomers have observed that light from a faint galaxy designated TN J0924-2201 has its spectral lines shifted toward the red end of the spectrum. This shiftis evidence that the

(1) galaxy contains only very hot stars(2) galaxy contains only relatively cool stars(3) galaxy is moving toward Earth(4) galaxy is moving away from Earth

5. Which list shows stars in order of increasing surface temperature?

(1) Barnard’s Star, Polaris, Sirius, Rigel(2) Aldebaran, sun, Rigel, Procyon B(3) Rigel, Polaris, Aldebaran, Barnard’s Star(4) Procyon B, Alpha Centauri, Polaris, Betelgeuse

6. Most scientists believe that the Milky Way galaxy is

(1) spherical in shape(2) 4.6 million years old(3) composed of stars the revolve around Earth(4) one of billions of galaxies in the universe


7. Which star gives off the least light?

(1) Deneb(2) Proxima Centauri(3) the sun(4) Sirius

8. Approximately how much brighter than the star Aldebaran is the star Betel-geuse?

(1) Betelgeuse is approximately twice as bright as Aldebaran.(2) Betelgeuse is approximately 10 times as bright as Aldebaran.(3) Betelgeuse is approximately 100 times as bright as Aldebaran.(4) Betelgeuse is approximately 10,000 times as bright as Aldebaran.

Part B

Base your answers to questions 9 through 12 on the table below, which containsinferred information about the evolution of the universe.

Data Table

Average Temperature Time From the Beginning Stage Description of the Universe of the Universe (°C) of the Universe

1 The size of an atom ? 0 s

2 The size of a grapefruit ? 10"43 s

3 “Hot soup” of electrons 1027 10"32 s

4 Cooling allows protons and neutrons to form 1013 10"6s

5 Still too hot to allow atoms to form 108 3 minutes

6 Electrons combine with protons and neutrons, 10,000 300,000 yearsforming hydrogen and helium atoms; light emission begins

7 Hydrogen and helium form giant clouds (nebulae) "200 1 billion yearsthat will become galaxies; first stars form

8 Galaxy clusters form and first stars die; heavy "270 13.7 billion yearselements are thrown into space, forming new stars and planets.


9. How soon did protons and neutrons form after the beginning of the universe?

(1) 0.000000006 s(2) 0.000006 s(3) 0.6 s(4) "6 s

10. What is the most appropriate title for this data table?

(1) Evidence of the Law of Superposition(2) Biological Evolution of Life-forms(3) History According to the Big Bang Theory(4) Progressive Shrinking of the Universe

11. According to the table, which statement best describes the changes in the tem-perature of the universe?

(1) The temperature of the universe is known to have decreased continuously.(2) It seems likely that the temperature of the universe has decreased since an

age of 10"32 s.(3) The temperature of the universe is known to have increased continuously.(4) It seems likely that the temperature of the universe has increased since an

age of 10"32 s.

12. Approximately how old was the universe when planets formed?

(1) 10"32 s(2) 13,700,000 s(3) 13,700,000 years(4) 13,700,000,000 years


13. The diagram below represents the spectral lines of hydrogen observed in a lab-oratory experiment and hydrogen lines observed from three stars.

What is the best inference that can be made concerning the movement ofgalaxies A, B, and C?

(1) Galaxy A is moving away from Earth, but galaxies B and C are movingtoward Earth.

(2) Galaxy B is moving away from Earth, but galaxies A and C are movingtoward Earth.

(3) Galaxies A, B, and C are all moving toward Earth.(4) Galaxies A, B, and C are all moving away from Earth.

Part C

Base your answers to questions 14 and 15 on the passage below and on yourknowledge of Earth science.

The Future of the Sun

Hydrogen gas is the main source of fuel that powers the nuclear reactions thatoccur within the sun. But like any fuel, the sun’s hydrogen is in limited supply.As the hydrogen gas is used up, scientists predict that the helium created as anend product of earlier nuclear reactions will begin to fuel different nuclearreactions. When this happens, the sun is expected to become a red giant starwith a radius that would extend out past the orbit of Venus, and possibly as faras Earth. Conditions on Earth would certainly become impossible for humansand other life-forms. No need to worry at this time. This event is not expectedto happen for at least a few billion years.

14. What specific nuclear reaction converts hydrogen into helium within the sun?

Galaxy ASpectral Lines

Galaxy BSpectral Lines

Galaxy CSpectral Lines

Blue

Blue

Blue

Red

Red

Red

LaboratoryHydrogen

Spectral Lines

Blue Red


15. According to this paragraph, what is the range of estimates for the solar radiuswhen it becomes a red giant? (Use kilometers as your units of measure.)

16. Explain how a red star such as Aldebaran can give off more light than the sun,which is much hotter than Aldebaran.

Base your answer to question 17 on the incomplete table below.

Star Classification

Star Color Classification

Sun Yellow Main sequence

Procyon B

17. What is the color of the star Procyon B, and to what group of stars does it belong?

18. What do x-rays, infrared radiation, and radio waves have in common?

19. What force could slow the expansion of the universe?

20. What is the surface temperature of the North Star?


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