Astronomy:Horizons 10th edition Michael Seeds
Gravity is patient—so patient it can kill stars. • All over the sky, astronomers find beautiful nebulae puffed gently into space by dying stars.
In contrast, astronomers occasionally see a new star appear in the sky, grow brighter, then fade away after a few weeks or a year. • A nova—what seems to be a new star in the sky—is produced by the eruption of a very old, dying star. • A supernova—a particularly luminous and longlasting nova—is caused by the violent, explosive death of a star. • Modern astronomers find a few novae each year, but supernovae are so rare that there are only one or two supernovae each century in our galaxy.
Astronomers know that supernovae occur because they occasionally flare in other galaxies and because telescopes reveal the shattered remains of these titanic explosions.
The mass of a star is critical in determining its fate. • Massive stars use up their nuclear fuel at a furious rate and die after only a few million years—whereas the lowest-mass stars use their fuel sparingly and may be able to live hundreds of billions of years. • Massive stars can die in violent explosions that appear as supernovae—but lower-mass stars die quiet deaths.
To follow the evolution of stars to their graves, you need to sort them into three categories according to their masses: • Low-mass red dwarfs • Medium-mass sunlike stars • Massive upper-main-sequence stars • These stars lead dramatically different lives and die in different ways.
Giant Stars • A main-sequence star generates its energy by nuclear fusion reactions that combine hydrogen to make helium. • The period during which the star fuses hydrogen lasts a long time, and the star remains on the main sequence for 90 percent of its total existence as an energy-generating star.
Giant Stars Giant Stars • When the hydrogen is exhausted, however, the star begins to evolve rapidly. • It swells into a giant star and then begins to fuse helium. • However, it can remain in this giant stage for only about 10 percent of its total lifetime—then, it must die. • The giant-star stage is the first step in the death of a star.
Expansion into a Giant • The nuclear reactions in a main-sequence star’s core fuse hydrogen to produce helium. • As the core is cooler than 100,000,000 K, helium cannot fuse in nuclear reactions. • So, it accumulates at the star’s center—like ashes in a fireplace.
Expansion into a Giant Expansion into a Giant • Initially, this helium ash has little effect on the star. • As hydrogen is exhausted and the stellar core becomes almost pure helium, the star loses the ability to generate the nuclear energy that supports it. • As it is the energy generated at the center that opposes gravity and supports the star, the core begins to contract as soon as energy generation starts to die down.
Expansion into a Giant • Although the core of helium ash cannot generate nuclear energy, it does grow hotter because it contracts and converts gravitational energy into thermal energy. • The rising temperature heats the unprocessed hydrogen just outside the core—hydrogen that was never before hot enough to fuse.
Expansion into a Giant • When the temperature of the surrounding hydrogen becomes high enough, hydrogen fusion begins in a spherical layer or shell around the exhausted core of the star. • Like a grass fire burning outward from an exhausted campfire, the hydrogen-fusion shell creeps outward—leaving helium ash behind and increasing the mass of the helium core.
Expansion into a Giant • The flood of energy produced by the hydrogen-fusion shell pushes toward the surface, heating the outer layers of the star and forcing them to expand dramatically.
Expansion into a Giant • Stars like the sun become giant stars of 10 to 100 times the diameter of the sun. • The most massive stars become supergiants some 1,000 times larger than the sun. • This explains the large diameters and low densities of the giant and supergiant stars that you have learned about. • Now, you understand that these stars were once normal main-sequence stars, but they expanded to large size and low density when hydrogen shell fusion began.
Expansion into a Giant • The expansion of the envelope dramatically changes the star’s location location in the H–R diagram. • Just as contraction heats a star, expansion cools it.
Expansion into a Giant • As the outer layers of gas expand, energy is absorbed in lifting and expanding the gas. • The loss of that energy lowers the temperature of the gas. • Thus, the point that represents the star in the H–R diagram moves quickly to the right (in less than a million years for a star of 5 solar masses).
Expansion into a Giant • A massive star moves to the right across the top of the diagram and becomes a supergiant—whereas a medium-mass star like the sun becomes a red giant. • As the radius of a giant star continues to increase, the enlarging surface area makes the star more luminous, moving its point upward in the diagram.
Degenerate Matter • Although the hydrogen-fusion shell can force the envelope of the star to expand, it cannot stop the contraction of the helium core. • As the core is not hot enough to fuse helium, gravity squeezes it tighter, and it becomes very small. • If you represent the helium core of a giant star with a baseball, the outer envelope of the star would be about the size of a baseball stadium. • Yet the core would contain about 12 percent of the star’s mass compressed to very high density.
Degenerate Matter • When gas is compressed to such extreme densities, it begins to behave in astonishing ways that can alter the evolution of the star. • To follow the story of stellar evolution, you must consider the behavior of gas at extremely high densities.
Degenerate Matter • Normally, the pressure in a gas depends on its temperature. • The hotter the gas is, the faster its particles move and the more pressure it exerts. • However, the gas inside a star is ionized. • So, there are two kinds of particles—atomic nuclei and free electrons.
Degenerate Matter • When a gas is so dense that the electrons are not free to change their energy, astronomers call it degenerate matter. • Although it is a gas, it has two peculiar properties that can affect the star.
Degenerate Matter • First, the degenerate gas resists compression. • To compress the gas, you must push against the moving electrons—and changing their motion means changing their energy. • That is why degenerate matter, though still a gas, is harder to compress than the toughest hardened steel.
Degenerate Matter • Second, the pressure of degenerate gas does not depend on temperature. • To know why, note that the pressure depends on the speed of the electrons, which cannot be changed without tremendous effort. • The temperature, however, depends on the motion of all the particles in the gas—both electrons and nuclei.
Degenerate Matter • These two properties of degenerate matter become important when stars end their main-sequence lives and approach their final collapse. • Eventually, many stars collapse into white dwarfs—and you will discover that these tiny stars are made of degenerate matter. • Long before that, though, the cores of many giant stars become so dense that they are degenerate—a situation that can produce a cosmic bomb.
Helium Fusion • Hydrogen fusion in main-sequence stars leaves behind helium ash, which cannot begin fusing because the temperature is too low. • Helium nuclei have a positive charge twice that of a proton. • To overcome the repulsion between nuclei, they must collide at a velocity higher than that at which hydrogen fuses.
Helium Fusion • The temperature for hydrogen fusion is too cool to fuse helium. • As a star becomes a giant and begins fusing hydrogen in a shell, the inert core of helium contracts and grows hotter. • It may even become degenerate. • However, when it finally reaches a temperature of 100,000,000 K, it begins to fuse helium nuclei to make carbon.
Helium Fusion • How a star begins helium fusion depends on its mass. • Stars more massive than about 3 solar masses contract rapidly, their helium-rich cores heat up, and helium fusion begins gradually. • Less-massive stars evolve more slowly, and their cores contract so much the gas becomes degenerate. • On Earth, a teaspoon of the gas would weigh more than an automobile.
Helium Fusion • In this degenerate matter, the pressure does not depend on temperature. • That means the pressure–temperature thermostat does not regulate energy production. • When the temperature becomes hot enough, helium fusion begins to make energy and the temperature rises. • However, pressure does not increase because the gas is degenerate.
Helium Fusion • The higher temperature increases the helium fusion even further. • The result is a runaway explosion called the helium flash in which, for a few minutes, the core of the star can generate more energy per second than does an entire galaxy.
Helium Fusion • Although the helium flash is sudden and powerful, it does not destroy the star. • If you were observing a giant star as it experienced the helium flash, you would probably see no outward evidence of the eruption. • The helium core is quite small and all the energy of the explosion is absorbed by the distended envelope.
Helium Fusion • In addition, the helium flash is a very short-lived event in the life of a star. • In a matter of minutes to hours, the core of the star becomes so hot it is no longer degenerate, the pressure–temperature thermostat brings the helium fusion under control, and the star proceeds to fuse helium steadily in its core.
Helium Fusion • The sun will experience a helium flash. • Stars less massive than about 0.4 solar mass never get hot enough to ignite helium. • Stars more massive than 3 solar masses ignite helium before their contracting cores become degenerate.
Helium Fusion • Whether a star experiences a helium flash or not, the ignition of helium in the core changes the structure of the star. • The star now makes energy in its helium-fusion core and in its hydrogen fusion shell. • The energy flowing outward from the core halts its contraction of the core. • The distended envelope of the star contracts and grows hotter.
Helium Fusion • Thus, the point that represents the star in the H–R diagram moves downward and back to the left toward the hot side of the diagram.
Helium Fusion • Helium fusion produces carbon and oxygen atoms in the core, atoms that require much higher temperatures to fuse. • Thus, as the helium fuel is used up, carbon and oxygen atoms accumulate in an inert core. • Once again, the core contracts and heats up, and a helium-fusion shell ignites below the hydrogen-fusion shell.
Helium Fusion • The star now makes energy in two fusion shells, it quickly expands, and its surface cools once again. • The point that represents the star in the H–R diagram moves back to the right, completing a loop.
Star Clusters: Evidence of Evolution • The stars in a star cluster all formed at about the same time and from the same cloud of gas. • So, they must be about the same age and composition. • The differences you see among stars in a cluster must arise from differences in mass. • That makes stellar evolution visible.
Star Clusters: Evidence of Evolution • There are three points to note about star clusters. • One, there are two kinds of star clusters. • However, they are similar in the way their stars evolve.
Star Clusters: Evidence of Evolution • You can estimate the age of a cluster by observing the distribution of the points that represent its stars in the H–R diagram.
Star Clusters: Evidence of Evolution • Finally, the shape of a cluster’s H–R diagram is governed by the evolutionary path the stars take. • By comparing clusters of different ages, you can visualize how stars evolve almost as if you were watching a film of a star cluster evolving over billions of years.
Star Clusters: Evidence of Evolution • Were it not for star clusters, astronomers would have little confidence in the theories of stellar evolution. • Star clusters make evolution visible and assure astronomers that they really do understand how stars are born, live, and die.
Building Scientific Arguments • How are giant stars different from main-sequence stars?
Building Scientific Arguments • A giant star has used up the hydrogen in its core, and it can no longer maintain main-sequence stability. • Hydrogen fusion continues in a shell—but the core of helium contracts while the envelope of the star expands and cools. • Later, helium fusion will begin in the core of the star and will produce an inert carbon-oxygen core and a helium-fusion shell below the hydrogen-fusion shell. • Thus, the internal structure of a giant star is dramatically different from the simple structure of a main-sequence star.
Building Scientific Arguments • Consider a 5-solar-mass star and the sun. • They are both spheres of hot gas held together by gravity and supported by internal pressure. • They look quite similar, and both will become giant stars.
Deaths of Lower-Main-Sequence Stars • Contracting stars heat up by converting gravitational energy into thermal energy. • Low-mass stars have little gravitational energy. • So, when they contract, they can’t get very hot. • This limits the fuels they can ignite.
Deaths of Lower-Main-Sequence Stars • You have learned that protostars less massive than 0.08 solar mass cannot even get hot enough to ignite hydrogen. • This section will concentrate on stars more massive than 0.08 solar mass but no more than a few times the mass of the sun.
Deaths of Lower-Main-Sequence Stars • Structural differences divide the lower-main-sequence stars into two subgroups: • Very-low-mass red dwarfs • Medium-mass stars such as the sun
Deaths of Lower-Main-Sequence Stars • The critical difference between the two groups is the extent of interior convection. • If the star is convective, fuel is constantly mixed. • The resulting evolution is drastically altered.
Red Dwarfs • Stars less massive than about 0.4 solar mass—the red dwarfs— have two advantages over more massive stars. • First, they have very small masses. • Thus, they have very little weight to support. • Their pressure–temperature thermostats are set low, and they consume their hydrogen fuel very slowly.
Red Dwarfs • Second, they are totally convective—that is, they are stirred by circulating currents of hot gas rising from the interior and cool gas sinking inward. • This means the stars are mixed like a pot of soup that is constantly stirred as it cooks. • Hydrogen is consumed and helium accumulates uniformly throughout the star. • Thus, the star is not limited by the fuel in its core—it can use all its hydrogen to prolong its life on the main sequence.