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CHAPTER 1 Astronomy: Horizons 10th edition Michael Seeds The Formation and Structure of Stars CHAPTER 1 The Formation and Structure of Stars • In this chapter, you will learn how gravity creates stars from the thin gas of space and how nuclear reactions inside stars generate energy. • You will learn how the flow of that energy outward toward the surface of the star balances gravity and makes the stars stable. CHAPTER 1 The Formation and The Interstellar Medium Structure of Stars • It is a common misconception to imagine that space is empty— a vacuum. – In fact, if you glance at Orion’s sword on a winter evening, you can see the Great Nebula in Orion—a glowing cloud of gas and dust. CHAPTER 1 The Formation and The Interstellar Medium Structure of Stars • Such evidence shows that the space between the stars is filled with lowdensity gas and dust called the interstellar medium. – About 75 percent of the mass of the gas is hydrogen. – Around 25 percent is helium. – There are also traces of carbon, nitrogen, oxygen, calcium, sodium, and heavier atoms. CHAPTER 1 The Formation and The Interstellar Medium Structure of Stars – Roughly 1 percent of the mass is made up of microscopic dust called interstellar dust. – The dust seems to be made mostly of carbon and silicates (rocklike minerals) mixed with or coated with frozen water. – The average distance between dust grains is about 150 m. CHAPTER 1 The Formation and The Interstellar Medium Structure of Stars • How do astronomers know there is an interstellar medium, and how do they know its properties? – In some cases, the interstellar medium is easily visible as clouds of gas and dust—as in the case of the Great Nebula in Orion. – Astronomers call such a cloud a nebula—from the Latin word for cloud. – Such nebulae are visible evidence of an interstellar medium. CHAPTER 1 The Formation and The Interstellar Medium Structure of Stars • There are three kinds of nebulae. • There are three points to note about nebulae. • One, very hot stars can excite clouds of gas and dust to emit light. – This reveals that the clouds contain mostly hydrogen gas at very low densities. CHAPTER 1 The Formation and The Interstellar Medium Structure of Stars • Two, where dusty clouds reflect the light of slightly cooler stars, you see evidence that the dust in the clouds is made up of very small particles. CHAPTER 1 The Formation and The Interstellar Medium Structure of Stars • Three, some dense clouds of gas and dust are detectable only where they are silhouetted against background regions filled with stars or bright nebulae. CHAPTER 1 The Formation and The Interstellar Medium Structure of Stars • If a cloud is less dense, the starlight may be able to penetrate it, and stars can be seen through the cloud. – However, the stars look dimmer because the dust in the cloud scatters some of the light. CHAPTER 1 The Formation and The Interstellar Medium Structure of Stars • Shorter wavelengths are scattered more easily than longer wavelengths, so redder photons are more likely to make it through the cloud. • Thus, the stars look slightly redder than they should—an effect called interstellar reddening. – This is the same process that makes the setting sun look redder. CHAPTER 1 The Formation and The Interstellar Medium Structure of Stars • Distant stars are dimmed and reddened by intervening gas and dust—clear evidence of an interstellar medium. – At near-infrared wavelengths, stars are more easily seen through the dusty interstellar medium because those longer wavelengths are scattered less often. – The interstellar medium is very cold—10 to 50 K. CHAPTER 1 The Formation and The Interstellar Medium Structure of Stars • X-ray observations can detect regions of very hot gas apparently produced by exploding stars. CHAPTER 1 The Formation and The Interstellar Medium Structure of Stars • Infrared observations can detect dust in the interstellar medium. – Although the dust grains are very small and very cold, there are huge numbers of grains in a cloud, and each grain emits infrared radiation.. – Some molecules in the cold gas emit in the infrared—so infrared observations can detect very cold clouds of gas. CHAPTER 1 The Formation and The Interstellar Medium Structure of Stars • Ultraviolet observations have been able to map the distribution of hydrogen in the interstellar medium. • Furthermore, radio astronomers can study the radio emissions of specific molecules in the interstellar medium. CHAPTER 1 The Formation of Stars from The Formation and the Interstellar Medium Structure of Stars • The thermal energy in clouds of gas causes them to resist collapse. – Temperature is a measure of the motion of the atoms or molecules in a material—in a hot gas, the atoms move more rapidly than do those in a cool gas. – Although the interstellar clouds are very cold, even at a temperature of only 10 K, the average hydrogen atom moves about 0.5 km/s (1,100 mph). – This thermal motion would make the cloud drift apart if gravity were too weak to hold it together. CHAPTER 1 The Formation of Stars from The Formation and the Interstellar Medium Structure of Stars • Other factors can help a cloud resist its own gravity. • Observations show that clouds are turbulent with currents of gas pushing through and colliding with each other. • Also, magnetic fields in clouds may resist being squeezed. – The thermal motion of the atoms, turbulence in a cloud, and magnetic fields resist gravity, and only the densest clouds are likely to contract. CHAPTER 1 The Formation of Stars from The Formation and the Interstellar Medium Structure of Stars • The densest interstellar clouds contain from 103 to 105 atoms/cm3, include from a few hundred thousand to a few million solar masses, and have temperatures as low as 10 K. – In such clouds, hydrogen can exist as molecules (H2) rather than as atoms. – These clouds are called molecular clouds, and the largest are called giant molecular clouds. CHAPTER 1 The Formation of Stars from The Formation and the Interstellar Medium Structure of Stars • Stars form in these clouds when the densest parts of the clouds become unstable and contract under the influence of their own gravity. – Most clouds, though, do not appear to be gravitationally unstable and will not contract to form stars on their own. CHAPTER 1 The Formation of Stars from The Formation and the Interstellar Medium Structure of Stars • Nevertheless, a stable cloud colliding with a shock wave —the astronomical equivalent of a sonic boom—can be compressed and disrupted into fragments. CHAPTER 1 The Formation of Stars from The Formation and the Interstellar Medium Structure of Stars • Theoretical calculations show that some of these fragments can become dense enough to collapse under the influence of their own gravity and form stars. CHAPTER 1 The Formation of Stars from The Formation and the Interstellar Medium Structure of Stars • Thus, most interstellar clouds may not collapse and form stars—until they are triggered by the compression of a shock wave. – Happily for this hypothesis, space is filled with shock waves. CHAPTER 1 The Formation of Stars from The Formation and the Interstellar Medium Structure of Stars • Supernova explosions produce shock waves that compress the interstellar medium. – Recent observations show young stars forming at the edges of such shock waves. CHAPTER 1 The Formation of Stars from The Formation and the Interstellar Medium Structure of Stars • Another source of shock waves may be the birth of very hot stars. – A massive star is so luminous and hot that it emits vast amounts of ultraviolet photons. – When such a star is born, the sudden blast of light, especially ultraviolet radiation, can ionize and drive away nearby gas—forming a shock wave that could compress nearby clouds and trigger further star formation. – Even the collision of two interstellar clouds can produce a shock wave and trigger star formation. CHAPTER 1 The Formation of Stars from The Formation and the Interstellar Medium Structure of Stars • Once begun, star formation can spread like a grass fire. • Astronomers have found a number of giant molecular clouds in which stars are forming in a repeating cycle. – Both high-mass and low-mass stars form in such a cloud. – However, when the massive stars form, their intense radiation or eventual supernovae explosions push back the surrounding gas and compress it. CHAPTER 1 The Formation of Stars from The Formation and the Interstellar Medium Structure of Stars • This compression can trigger the formation of more stars, some of which will also be massive. – Thus, a few massive stars can drive a continuing cycle of star formation in a giant molecular cloud. CHAPTER 1 The Formation of Stars from The Formation and the Interstellar Medium Structure of Stars • Although low-mass stars do form in such clouds along with massive stars, they also form in smaller clouds of gas and dust. – However, as they have lower luminosities and do not develop quickly into supernovae, low-mass stars alone cannot drive a continuing cycle of star formation. CHAPTER 1 The Formation of Stars from The Formation and the Interstellar Medium Structure of Stars • Such a collapsing cloud of gas does not form a single object. – Due to instabilities, it fragments—producing 10 to 1,000 stars. • Stars held in a stable group by their combined gravity are called a star cluster. CHAPTER 1 The Formation of Stars from The Formation and the Interstellar Medium Structure of Stars • An association is a group of stars that are not gravitationally bound to one another. – Thus, an association drifts apart in a few million years. – The youngest associations are rich in young stars, including O and B stars ie. the hottest stars. CHAPTER 1 The Formation and The Formation of Protostars Structure of Stars • To continue the story of star formation, you need to focus on a single fragment of a collapsing cloud as it forms a star. – You might be wondering how the unimaginably cold gas of an interstellar cloud can heat up to form a star. – The answer is gravity. CHAPTER 1 The Formation and The Formation of Protostars Structure of Stars • Once part of a cloud is triggered to collapse, gravity draws each atom toward the center. • At first, the atoms fall unopposed—they hardly ever collide with each other. • Collisions convert the inward velocities of the atoms into random motions. CHAPTER 1 The Formation and The Formation of Protostars Structure of Stars • The initial collapse of the gas forms a dense core of gas. • As more gas falls in, a warm protostar develops buried deep in the dusty gas. • When a star or protostar changes, it is said to evolve. • You can follow that evolution in the H-R diagram. CHAPTER 1 The Formation and The Formation of Protostars Structure of Stars • The dusty cloud also hides the protostar from sight during its contraction. – If you could see it, it would be a luminous red object a few thousand times larger than the sun. – You would plot it in the red-giant region of the H-R diagram. CHAPTER 1 The Formation and The Formation of Protostars Structure of Stars • The hot gas inside the protostar resists gravity. • Throughout its contraction, the protostar converts its gravitational energy into thermal energy. – Half of this thermal energy radiates into space. – The remaining half, though, raises the internal temperature. CHAPTER 1 The Formation and The Formation of Protostars Structure of Stars • As the internal temperature climbs, the collapsing gas becomes ionized— becoming a mixture of positively charged atomic nuclei and free electrons. – When the center gets hot enough, nuclear reactions begin generating energy, the protostar halts its contraction, and—having absorbed part of its cocoon of gas and dust and blown away the rest—it becomes a stable, main-sequence star. CHAPTER 1 The Formation and The Formation of Protostars Structure of Stars • The time a protostar takes to contract from a cool interstellar gas cloud to a main-sequence star depends on its mass. – The more massive the star, the stronger its gravity and the faster it contracts. CHAPTER 1 The Formation and The Formation of Protostars Structure of Stars – The sun took about 30 million years to reach the main sequence. – In contrast, a 15-solarmass star can contract in only 160,000 years. – Conversely, a star of 0.2 solar mass takes 1 billion years to reach the main sequence. CHAPTER 1 The Formation and Observations of Star Formation Structure of Stars • Unfortunately, a protostar is not easy to observe. – The protostar stage is less than 0.1 percent of a star’s total lifetime. – Protostars form deep inside clouds of dusty gas that absorb any light the protostar might emit. CHAPTER 1 The Formation and Observations of Star Formation Structure of Stars • Only when the protostar is hot enough to drive away its enveloping cloud of gas and dust do you see it. – The birth line in the H-R diagram shows where contracting protostars first become visible. CHAPTER 1 The Formation and Observations of Star Formation Structure of Stars • Protostars cross the birth line shortly before they reach the main sequence. – Thus, the early evolution of a protostar is hidden from sight. CHAPTER 1 The Formation and Observations of Star Formation Structure of Stars • Although astronomers cannot see protostars at visible wavelengths before they cross the birth line, you can detect them in the infrared. – The dust absorbs light from the protostar and grows warm. – Warm dust radiates great amounts of infrared. – Infrared observations made by orbiting telescopes reveal many bright sources of infrared radiation that are protostars buried in dust clouds. CHAPTER 1 The Formation and Observations of Star Formation Structure of Stars – Observations of small globules of gas and dust within larger nebulae show how star formation begins. CHAPTER 1 The Formation and Fusion in Stars Structure of Stars • All main-sequence stars fuse hydrogen into helium to generate energy. – The sun fuses hydrogen using a chain of reactions— the proton-proton chain. – Upper main-sequence stars are more massive than the sun and can fuse hydrogen more efficiently. – Also, aging stars fuse fuels other than hydrogen. CHAPTER 1 The Formation and The CNO Cycle Structure of Stars • Main-sequence stars more massive than the sun fuse hydrogen into helium using the CNO (carbon–nitrogen–oxygen) cycle. – This is a hydrogen-fusion process that uses carbon, nitrogen, and oxygen as stepping-stones. CHAPTER 1 The Formation and The CNO Cycle Structure of Stars • The cycle begins with a carbon nucleus, transforms it first into a nitrogen nucleus, then into an oxygen nucleus, and then back to a carbon nucleus. – The carbon is unchanged in the end. – However, along the way, four hydrogen nuclei are fused to make a helium nucleus plus energy, just as in the proton–proton chain. CHAPTER 1 The Formation and The CNO Cycle Structure of Stars • The cycle requires a higher temperature because it begins with a carbon nucleus combining with a hydrogen nucleus. – A carbon nucleus has a charge six times higher than hydrogen—so the Coulomb barrier is high. – Thus, temperatures higher than 16,000,000 K are required to make the cycle work. CHAPTER 1 The Formation and The CNO Cycle Structure of Stars • The center of the sun is not quite hot enough. • In stars more massive than about 1.1 solar masses, the cores are hotter. – Those stars use the CNO cycle and not the less efficient proton–proton chain. CHAPTER 1 The Formation and Heavy-Element Fusion Structure of Stars • At later stages in the life of a star, when it has exhausted its hydrogen fuel, it may fuse other nuclear fuels. – The ignition of these fuels also requires high temperatures—because the nuclei have large positive charges. – The particles must travel at high velocities—to overcome the electrostatic repulsion and force the particles close enough together to react. – Helium fusion requires a temperature of at least 100 million K. CHAPTER 1 The Formation and Heavy-Element Fusion Structure of Stars • You can summarize the helium-fusion process in two steps. 4He + 4He → 8Be + γ 8Be + 4He → 12C + γ – As a helium nucleus is called an alpha particle, these reactions are commonly known as the triple-alpha process. CHAPTER 1 The Formation and Heavy-Element Fusion Structure of Stars • At temperatures above 600,000,000 K, carbon fuses rapidly in a complex network of reactions, as depicted in the figure. – Each arrow represents a different nuclear reaction. CHAPTER 1 The Formation and Heavy-Element Fusion Structure of Stars • Reactions at still higher temperatures can convert magnesium, aluminum, and silicon into yet heavier atoms. – These reactions involving heavy elements will be important in the study of the deaths of massive stars. CHAPTER 1 The Pressure–Temperature Thermostat The Formation and Structure of Stars • Nuclear reactions in stars manufacture energy and heavy atoms under the supervision of a built-in thermostat that keeps the reactions from erupting out of control. – That thermostat is the relation between gas pressure and temperature. CHAPTER 1 The Pressure–Temperature Thermostat The Formation and Structure of Stars • In a star, nuclear reactions generate just enough energy to balance the inward pull of gravity. • Consider what would happen if the reactions produced too much energy. – As the star balances gravity by generating energy, the extra energy would force it to expand. – The expansion would lower the central temperature and density and slow the nuclear reactions until the star regained stability. CHAPTER 1 The Pressure–Temperature Thermostat The Formation and Structure of Stars • Thus, the star has a built-in regulator that keeps the nuclear reactions from occurring too rapidly. • The same thermostat keeps the reactions from dying down. – Suppose the nuclear reactions began making too little energy. – Then, the star would contract slightly—increasing the central temperature and density and increasing the nuclear energy generation. CHAPTER 1 The Pressure–Temperature Thermostat The Formation and Structure of Stars • The stability of a star depends on the relation between gas pressure and temperature. • Nuclear fusion at the centers of stars heats their interiors, creates high gas pressures, and thus balances the inward forces of gravity. – If an increase or decrease in temperature produces a corresponding change in pressure, then the thermostat is functioning correctly— and the star is stable. CHAPTER 1 The Formation and Hydrostatic Equilibrium Structure of Stars • When you think about a star, it is helpful to think of it as if it were made up of layers. – The weight of each layer must be supported by the layer below. – The deeper layers must support the weight of all the layers above. – As the inside of a star is made up of gas, the weight pressing down on a layer must be balanced by the gas pressure in the layer. CHAPTER 1 The Formation and Hydrostatic Equilibrium Structure of Stars • If the pressure is too low, the weight from above will compress the layer. • If the pressure is too high, the layer will expand and lift the layers above. – This balance between weight and pressure is called hydrostatic equilibrium. CHAPTER 1 The Formation and Hydrostatic Equilibrium Structure of Stars • The figure shows this hydrostatic balance in the imaginary layers of a star. – The weight pressing down on each layer is shown by lighter red arrows—which grow larger with increasing depth because the weight grows larger. CHAPTER 1 The Formation and Hydrostatic Equilibrium Structure of Stars – The pressure in each layer is shown by darker red arrows—which must grow larger with increasing depth to support the weight. CHAPTER 1 The Formation and Hydrostatic Equilibrium Structure of Stars • The pressure in a gas depends on the temperature and density of the gas. – Near the surface, there is little weight pressing down. – So, the pressure need not be high. – Deeper in the star, the pressure must be higher. – That means that the temperature and density of the gas must also be higher. CHAPTER 1 The Formation and Hydrostatic Equilibrium Structure of Stars • Hydrostatic equilibrium informs you that stars have to be hot inside to maintain the pressure needed to support their own weight. – The layers are hot because energy flows outward from the core of the star, where it is made by nuclear fusion. • Hydrostatic equilibrium is closely related to the pressure–temperature thermostat. CHAPTER 1 The Formation and Energy Transport Structure of Stars • The law of energy transport states that energy must flow from hot regions to cooler regions by conduction, convection, or radiation. CHAPTER 1 The Formation and Energy Transport Structure of Stars • Conduction is the most familiar form of heat flow. • If you hold the bowl of a metal spoon in a candle flame, the handle of the spoon grows warmer. – Heat, in the form of motion among the molecules of the spoon, is conducted from molecule to molecule up the handle—until the molecules under your fingers begin to move faster and you sense heat. CHAPTER 1 The Formation and Energy Transport Structure of Stars • Thus, conduction requires close contact between the molecules. – As the particles (atoms) in most stars are not in close contact, conduction is unimportant. – Conduction is significant, though, in white dwarfs, which have tremendous internal densities. CHAPTER 1 The Formation and Energy Transport Structure of Stars • Energy transport by radiation is another familiar experience. • If you put your hand beside a candle flame, you can feel the heat. – What you feel are infrared photons radiated by the flame. – As photons are packets of energy, your hand grows warm as it absorbs them. CHAPTER 1 The Formation and Energy Transport Structure of Stars • Radiation is the principal means of energy transport in the sun’s interior. – Photons are absorbed and reemitted in random directions over and over as they work their way outward through the radiative zone. CHAPTER 1 The Formation and Energy Transport Structure of Stars • This heat-driven circulation of a fluid is convection, the third way energy can move in a star. – The rising wisp of smoke above a candle flame is carried by convection. – Energy is carried upward in convection currents as rising hot gas. – Energy flows outward as convection in the sun’s outer layers—its convective zone. CHAPTER 1 The Formation and Energy Transport Structure of Stars • Convection is important in stars both because it carries energy and because it mixes the gas. – Convection currents flowing through the layers of a star tend to homogenize the gas—giving it a uniform composition throughout the convective zone. – This mixing constantly refreshes the fuel supply of the nuclear reactions—just as the stirring of a campfire makes it burn more efficiently. CHAPTER 1 The Formation and Energy Transport Structure of Stars • The four laws of stellar structure can inform you of how stars are born, how they live, and how they die. CHAPTER 1 The Formation and Main-Sequence Stars Structure of Stars • When a contracting protostar begins to fuse hydrogen, it stops contracting and becomes a stable main-sequence star. – The most massive stars are so hot they light up the remaining nearby gas in a beautiful nebula— as if announcing their birth. CHAPTER 1 The Formation and Main-Sequence Stars Structure of Stars • However, that gas is quickly blown away—and the stars begin their long, uneventful lives as main-sequence stars. – If you discount the peculiar white dwarfs, then 90 percent of all stars are main-sequence stars. CHAPTER 1 The Formation and The Mass–Luminosity Relation Structure of Stars • Observations of the temperature and luminosity of stars show that main-sequence stars obey a simple rule—the more massive a star is, the more luminous it is. – This rule, the mass–luminosity relation, is the key to understanding the stability of main-sequence stars. – In fact, the mass–luminosity relation is predicted by theories of stellar structure—giving astronomers direct observational confirmation of those theories. CHAPTER 1 The Formation and The Mass–Luminosity Relation Structure of Stars • To understand the mass–luminosity relation, you must consider the law of hydrostatic equilibrium, which states that pressure balances weight, and the pressure–temperature thermostat, which regulates energy production. CHAPTER 1 The Formation and The Mass–Luminosity Relation Structure of Stars • A star that is more massive than the sun has more weight pressing down on its interior. – So, the interior must have a high pressure to balance that weight. – That means the massive star’s automatic pressure–temperature thermostat must keep the gas in its interior hot and the pressure high. CHAPTER 1 The Formation and The Mass–Luminosity Relation Structure of Stars • A star less massive than the sun has less weight on its interior and thus needs less internal pressure. – So, its pressure–temperature thermostat is set lower. – Massive stars are more luminous because they must support more weight by making more energy. CHAPTER 1 The Formation and The Mass–Luminosity Relation Structure of Stars • This shows you why the main sequence must have a lower end. • Stars with masses below about 0.08 solar mass cannot raise their central temperature high enough to begin hydrogen fusion. – Called brown dwarfs, they are only about a dozen times larger than Earth. – Although they are still warm from contraction, they do not generate energy by hydrogen fusion. – They have contracted as far as they can and are slowly cooling off. CHAPTER 1 The Formation and The Mass–Luminosity Relation Structure of Stars • Brown dwarfs fall in the gap between lowmass M stars and massive planets like Jupiter. – They would look red to your eyes. – However, they emit most of their energy in the infrared. • The warmer brown dwarfs fall in spectral class L and the cooler in spectral class T. CHAPTER 1 The Formation and The Mass–Luminosity Relation Structure of Stars • Molecules can form in the atmospheres of red dwarfs. • Brown dwarfs, however, are so cool that solid particles of silicates, metals, and other minerals can condense in cloud layers. – Unlike stars, brown dwarfs appear to have weather. CHAPTER 1 The Formation and The Mass–Luminosity Relation Structure of Stars • As they are so small and cool, brown dwarfs have very low luminosities and are difficult to find. – Nevertheless, a few hundred are known. – They may be as common as M stars. CHAPTER 1 The Formation and The Mass–Luminosity Relation Structure of Stars • The evidence shows that nature does indeed make brown dwarfs when a forming star does not have enough mass to begin hydrogen fusion. • This observational detection of objects at the lower end of the main sequence further confirms the theories of stellar structure. CHAPTER 1 The Formation and The Mass–Luminosity Relation Structure of Stars • Now that you know how stars form and how they maintain their stability through the mass–luminosity relation, you can predict the evolution of main-sequence stars. CHAPTER 1 The Formation and The Life of a Main-Sequence Star Structure of Stars • While a star is on the main sequence, it is stable—so you might think its life would be totally uneventful. • However, a main-sequence star balances its gravity by fusing hydrogen. • As the star gradually uses up its fuel, that balance must change. – Thus, even stable main-sequence stars are changing as they consume their hydrogen fuel. CHAPTER 1 The Formation and The Life of a Main-Sequence Star Structure of Stars • You have learned that hydrogen fusion combines four nuclei into one. • Thus, as a main-sequence star fuses its hydrogen, the total number of particles in its interior decreases. – Each newly made helium nucleus can exert the same pressure as a hydrogen nucleus. – However, as the gas contains fewer nuclei, its total pressure is less. – This unbalances the gravity–pressure stability, and gravity squeezes the core of the star more tightly. CHAPTER 1 The Formation and The Life of a Main-Sequence Star Structure of Stars • As the core contracts, its temperature increases and the nuclear reactions run faster—releasing more energy. – This additional energy flowing outward through the envelope forces the outer layers to expand. – As the star becomes larger, it becomes more luminous. – Eventually, the expansion begins to cool the surface. CHAPTER 1 The Formation and The Life of a Main-Sequence Star Structure of Stars • Due to these gradual changes in mainsequence stars, the main sequence is not a sharp line across the H–R diagram, but rather a band. CHAPTER 1 The Formation and The Life of a Main-Sequence Star Structure of Stars • Stars begin their stable lives fusing hydrogen on the lower edge of the band, which is known as the zero-age main sequence (ZAMS). – Gradual changes in luminosity and surface temperature move them upward and slightly to the right. CHAPTER 1 The Formation and The Life of a Main-Sequence Star Structure of Stars – By the time they reach the upper edge of the main sequence, they have exhausted nearly all the hydrogen in their centers. – Thus, you find mainsequence stars scattered throughout the band at various stages of their mainsequence lives. CHAPTER 1 The Formation and The Life of a Main-Sequence Star Structure of Stars • The sun is a typical main-sequence star. • As it undergoes these gradual changes, Earth will suffer. – When the sun began its main-sequence life about 5 billion years ago, it was only about 70 percent as luminous as it is now. – By the time it leaves the main sequence in another 5 billion years, it will have twice its present luminosity. – Long before that, its rising luminosity will raise Earth’s average temperature, melt the polar caps, modify Earth’s climate, and ultimately drive away the oceans and atmosphere. CHAPTER 1 The Formation and The Life of a Main-Sequence Star Structure of Stars • Life on Earth will probably not survive these changes in the sun. – Nevertheless, you have a billion years or more to prepare. CHAPTER 1 The Formation and The Life of a Main-Sequence Star Structure of Stars • The average star spends 90 percent of its life on the main sequence. • This explains why 90 percent of all true stars are main-sequence stars. – You are most likely to see a star during that long, stable period while it is on the main sequence. CHAPTER 1 The Formation and The Life of a Main-Sequence Star Structure of Stars • The number of years a star spends on the main sequence depends on its mass. – Massive stars consume fuel rapidly and live short lives. – For example, a 25-solar-mass star will exhaust its hydrogen and die in only about 7 million years. CHAPTER 1 The Formation and The Life of a Main-Sequence Star Structure of Stars – In contrast, low-mass stars conserve their fuel and shine for billions of years. – The sun has enough fuel to last about 10 billion years. – The red dwarfs, although they have little fuel, use it up very slowly and may be able to survive for 100 billion years or more. CHAPTER 1 The Formation and The Life of a Main-Sequence Star Structure of Stars • Nature makes more low-mass stars than high-mass stars. • This fact, however, is not sufficient to explain the vast numbers of low-mass stars that fill the sky. – The additional factor is stellar lifetime. CHAPTER 1 The Formation and The Life of a Main-Sequence Star Structure of Stars • As low-mass stars live long lives, there are more of them than massive stars. – In the figure, the lower-main-sequence stars are much more common than the massive O and B stars. CHAPTER 1 The Formation and The Life of a Main-Sequence Star Structure of Stars – The main-sequence K and M stars are so faint they are difficult to locate. – However, they are very common. – The O and B stars are luminous and easy to locate. – Due to their fleeting lives, though, there are never more than a few visible on the main sequence at any one time. CHAPTER 1 The Formation and Building Scientific Arguments Structure of Stars • Observational evidence shows that the luminosity of a main-sequence star depends on its mass. – The more massive a star is, the more luminous it is. CHAPTER 1 The Formation and Building Scientific Arguments Structure of Stars • You can understand that if you remember that a star supports itself by being hot inside. – The high temperature produces high internal pressure—which balances the weight of the layers pressing inward. – That is called hydrostatic equilibrium. CHAPTER 1 The Formation and Building Scientific Arguments Structure of Stars • A massive star has a stronger gravitational field and it has more mass to support. – So, the pressure needed to keep it from collapsing is very high. – That means the internal temperature must be high, and that means the star must generate energy rapidly. – All that energy—leaking outward through the surface of the star—makes the star more luminous.