Stellar Evolution: Red Giants Red Giant Evolution: Readings: Schneider & Arny: Unit 64 A star's evolution after the red giant phase depends on its mass. For stars greater than 1 solar mass, but less than 2 solar masses, the hydrogen burning shell eats its way outward leaving behind more helium ash. As the helium piles up, the core becomes more massive and contracts. The contraction heats the core as it becomes more dense. The density of the core increases to the point where the electrons become degenerate. The core begins to act more like a liquid than a gas, and it becomes incompressible and further contraction stops. As the hydrogen shell continues burning, the degenerate core grows hotter and hotter without expanding. Helium, being a larger nuclei than hydrogen, requires more kinetic energy to fuse, which means higher temperatures. At 100 million degrees, helium can be converted to carbon through the triple-α process. Triple-α Process: Energy generation is the key to the lifetime of a star. The proton-proton chain and the CNO cycle are the source of energy for main sequence stars, but as helium builds up in the core, the star must find another source of energy or it will die out.

Fusion can occur between any nuclei if the motion of the nuclei is fast enough to overcome the electrostatic repulsion between the protons. For the waste product of the proton-proton chain or the CNO cycle, helium, there is twice the amount of electric charge (two protons) so it is twice as hard to fuse them compared to protons. In the triple-α process, three He nuclei are fused to form carbon (technically, two He fuse first to form beryllium (Be), then beryllium and another He fuse to form carbon; but this happens so fast that Be only exists for a few microseconds). Carbon is the waste product plus energy in the form of gamma-ray photons. The energy released by the triple-α process continues to heat the core raising its temperature even more. Again, under normal circumstances, the heating would increase the pressure and the core would expand and cool. But, with the core being degenerate, the temperature goes up yet the core does not expand. Higher temperatures means a faster triple-α rate, which means more energy, which means higher temperatures, etc...

When the temperature of the core reaches 300 million degrees, a nearly explosive consumption of the helium takes place called the helium flash. During the very short helium flash (a few minutes), the star emits more energy than 100 times the output of the whole Galaxy. However, this energy never reaches the surface but instead goes into removing the degeneracy of the electrons and expanding the core. For stars more than 2 solar masses, the triple-α process starts before the electrons become degenerate. And, so, there is no helium flash, just a gradual shift to a core helium burning region surrounded by a hydrogen burning shell. Horizontal Branch Stars: After helium burning begins (either explosively with a flash, or gradually for heavier stars), the star has two sources of energy, hydrogen fusion in a shell around the core and helium fusion in the core. Helium burns into carbon, and carbon combines with helium to make oxygen. The core of the star becomes rich in carbon and oxygen nuclei, and the star's surface temperature goes up to become a horizontal branch star. Stars with masses greater than or equal to the Sun become smaller and hotter at a constant luminosity. They evolve to horizontal branch stars by moved across the HR diagram at constant brightness.

Low mass stars at about 10 solar luminosities, high mass stars (10 solar masses) at about 200 solar luminosities. Notice that as they evolve, HB stars cross the instability strip. For a short time, high mass stars will be Cepheid variables and low mass stars will be RR Lyrae stars. Asymptotic Giant Branch Stars: After existing as horizontal branch stars for a few million years, the helium in the core of the star is exhausted (now being mostly carbon and oxygen nuclei) and a helium burning shell will develop underneath the hydrogen burning shell. The electrons and nuclei in the core again become degenerate and the star expands and cools to become an asymptotic giant branch star. Most of the energy is coming from the hydrogen burning shell, the helium burning shell is small at this time. However, the hydrogen shell is dumping helium ash onto the helium shell. After sometime, enough helium is built up so that the helium shell undergoes an explosive event called a thermal pulse. The thermal pulse is barely noticed at the surface of the star, but serves to increase the mass of the carbon/oxygen core, so that the size and luminosity of the star gradually increases with time.

As the star climbs the asymptotic giant branch, a wind develops in the star's envelope which blows the outer layers into space. It is in this wind that dust particles (important for interstellar clouds and proto-solar systems) are formed from carbon material dredged up from the core by convective currents. During this time, a thick dust shell blocks the visible light from the star such that even though it is 10,000 brighter than the Sun, it is only seen in the IR. To summarize the evolution of a stellar core, the following figure shows the changes in a high mass star over time. The stellar wind causes mass loss for AGB stars. This loss is around 10-4 solar masses per year, which means that in 10,000 years the typical star will dissolve, leaving the central, hot core (the central star in a planetary nebula). If the star is larger than 8 solar masses, then the core continues to heat. Carbon and oxygen fuse to form neon, then magnesium, then silicon. All forming into burning shells surrounding an iron ash core. Iron is unusual in that it is extremely stable and resistant to fusion. The temperature of an iron core can reach 3 billion degrees.

When the iron core reaches a critical mass, it collapses, violently, into a supernova explosion. Stars | Science Mission Directorate Science Mission Directorate Exoplanets Stars Stars are the most widely recognized astronomical objects, and represent the most fundamental building blocks of galaxies. The age, distribution, and composition of the stars in a galaxy trace the history, dynamics, and evolution of that galaxy. Moreover, stars are responsible for the manufacture and distribution of heavy elements such as carbon, nitrogen, and oxygen, and their characteristics are intimately tied to the characteristics of the planetary systems that may coalesce about them. Consequently, the study of the birth, life, and death of stars is central to the field of astronomy. Star Formation Stars are born within the clouds of dust and scattered throughout most galaxies. A familiar example of such as a dust cloud is the Orion Nebula. Turbulence deep within these clouds gives rise to knots with sufficient mass that the gas and dust can begin to collapse under its own gravitational attraction. As the cloud collapses, the material at the center begins to heat up. Known as a protostar, it is this hot core at the heart of the collapsing cloud that will one day become a star.

Three-dimensional computer models of star formation predict that the spinning clouds of collapsing gas and dust may break up into two or three blobs; this would explain why the majority the stars in the Milky Way are paired or in groups of multiple stars. The observations of Eta Carinae's light echo are providing new insight into the behavior of powerful massive stars on the brink of detonation. Credit: NOAO, AURA, NSF, and N. Smith (University of Arizona) As the cloud collapses, a dense, hot core forms and begins gathering dust and gas. Not all of this material ends up as part of a star — the remaining dust can become planets, asteroids, or comets or may remain as dust. In some cases, the cloud may not collapse at a steady pace. In January 2004, an amateur astronomer, James McNeil, discovered a small nebula that appeared unexpectedly near the nebula Messier 78, in the constellation of Orion. When observers around the world pointed their instruments at McNeil's Nebula , they found something interesting — its brightness appears to vary.

Observations with NASA's Chandra X-ray Observatory provided a likely explanation: the interaction between the young star's magnetic field and the surrounding gas causes episodic increases in brightness. Main Sequence Stars A star the size of our Sun requires about 50 million years to mature from the beginning of the collapse to adulthood. Our Sun will stay in this mature phase (on the main sequence as shown in the Hertzsprung-Russell Diagram) for approximately 10 billion years. Stars are fueled by the nuclear fusion of hydrogen to form helium deep in their interiors. The outflow of energy from the central regions of the star provides the pressure necessary to keep the star from collapsing under its own weight, and the energy by which it shines. As shown in the Hertzsprung-Russell Diagram, Main Sequence stars span a wide range of luminosities and colors, and can be classified according to those characteristics. The smallest stars, known as red dwarfs, may contain as little as 10% the mass of the Sun and emit only 0.01% as much energy, glowing feebly at temperatures between 3000-4000K.

Despite their diminutive nature, red dwarfs are by far the most numerous stars in the Universe and have lifespans of tens of billions of years. On the other hand, the most massive stars, known as hypergiants, may be 100 or more times more massive than the Sun, and have surface temperatures of more than 30,000 K. Hypergiants emit hundreds of thousands of times more energy than the Sun, but have lifetimes of only a few million years. Although extreme stars such as these are believed to have been common in the early Universe, today they are extremely rare - the entire Milky Way galaxy contains only a handful of hypergiants. Stars and Their Fates In general, the larger a star, the shorter its life, although all but the most massive stars live for billions of years. When a star has fused all the hydrogen in its core, nuclear reactions cease. Deprived of the energy production needed to support it, the core begins to collapse into itself and becomes much hotter. Hydrogen is still available outside the core, so hydrogen fusion continues in a shell surrounding the core.

The increasingly hot core also pushes the outer layers of the star outward, causing them to expand and cool, transforming the star into a red giant. If the star is sufficiently massive, the collapsing core may become hot enough to support more exotic nuclear reactions that consume helium and produce a variety of heavier elements up to iron. However, such reactions offer only a temporary reprieve. Gradually, the star's internal nuclear fires become increasingly unstable - sometimes burning furiously, other times dying down. These variations cause the star to pulsate and throw off its outer layers, enshrouding itself in a cocoon of gas and dust. What happens next depends on the size of the core. Average Stars Become White Dwarfs For average stars like the Sun, the process of ejecting its outer layers continues until the stellar core is exposed. This dead, but still ferociously hot stellar cinder is called a White Dwarf . White dwarfs, which are roughly the size of our Earth despite containing the mass of a star, once puzzled astronomers - why didn't they collapse further? What force supported the mass of the core? Quantum mechanics provided the explanation. Pressure from fast moving electrons keeps these stars from collapsing.

The more massive the core, the denser the white dwarf that is formed. Thus, the smaller a white dwarf is in diameter, the larger it is in mass! These paradoxical stars are very common - our own Sun will be a white dwarf billions of years from now. White dwarfs are intrinsically very faint because they are so small and, lacking a source of energy production, they fade into oblivion as they gradually cool down. This fate awaits only those stars with a mass up to about 1.4 times the mass of our Sun. Above that mass, electron pressure cannot support the core against further collapse. Such stars suffer a different fate as described below. White Dwarfs May Become Novae If a white dwarf forms in a binary or multiple star system, it may experience a more eventful demise as a nova. Nova is Latin for "new" - novae were once thought to be new stars. Today, we understand that they are in fact, very old stars - white dwarfs. If a white dwarf is close enough to a companion star, its gravity may drag matter - mostly hydrogen - from the outer layers of that star onto itself, building up its surface layer.

When enough hydrogen has accumulated on the surface, a burst of nuclear fusion occurs, causing the white dwarf to brighten substantially and expel the remaining material. Within a few days, the glow subsides and the cycle starts again. Sometimes, particularly massive white dwarfs (those near the 1.4 solar mass limit mentioned above) may accrete so much mass in the manner that they collapse and explode completely, becoming what is known as a supernova. Supernovae Leave Behind Neutron Stars or Black Holes Main sequence stars over eight solar masses are destined to die in a titanic explosion called a supernova . A supernova is not merely a bigger nova. In a nova, only the star's surface explodes. In a supernova, the star's core collapses and then explodes. In massive stars, a complex series of nuclear reactions leads to the production of iron in the core. Having achieved iron, the star has wrung all the energy it can out of nuclear fusion - fusion reactions that form elements heavier than iron actually consume energy rather than produce it. The star no longer has any way to support its own mass, and the iron core collapses.

In just a matter of seconds the core shrinks from roughly 5000 miles across to just a dozen, and the temperature spikes 100 billion degrees or more. The outer layers of the star initially begin to collapse along with the core, but rebound with the enormous release of energy and are thrown violently outward. Supernovae release an almost unimaginable amount of energy. For a period of days to weeks, a supernova may outshine an entire galaxy. Likewise, all the naturally occurring elements and a rich array of subatomic particles are produced in these explosions. On average, a supernova explosion occurs about once every hundred years in the typical galaxy. About 25 to 50 supernovae are discovered each year in other galaxies, but most are too far away to be seen without a telescope. Neutron Stars If the collapsing stellar core at the center of a supernova contains between about 1.4 and 3 solar masses, the collapse continues until electrons and protons combine to form neutrons, producing a neutron star . Neutron stars are incredibly dense - similar to the density of an atomic nucleus.

Because it contains so much mass packed into such a small volume, the gravitation at the surface of a neutron star is immense. Like the White Dwarf stars above, if a neutron star forms in a multiple star system it can accrete gas by stripping it off any nearby companions. The Rossi X-Ray Timing Explorer has captured telltale X-Ray emissions of gas swirling just a few miles from the surface of a neutron star. Neutron stars also have powerful magnetic fields which can accelerate atomic particles around its magnetic poles producing powerful beams of radiation. Those beams sweep around like massive searchlight beams as the star rotates. If such a beam is oriented so that it periodically points toward the Earth, we observe it as regular pulses of radiation that occur whenever the magnetic pole sweeps past the line of sight. In this case, the neutron star is known as a pulsar. Black Holes If the collapsed stellar core is larger than three solar masses, it collapses completely to form a black hole: an infinitely dense object whose gravity is so strong that nothing can escape its immediate proximity, not even light. Since photons are what our instruments are designed to see, black holes can only be detected indirectly.

Indirect observations are possible because the gravitational field of a black hole is so powerful that any nearby material - often the outer layers of a companion star - is caught up and dragged in. As matter spirals into a black hole, it forms a disk that is heated to enormous temperatures, emitting copious quantities of X-rays and Gamma-rays that indicate the presence of the underlying hidden companion. From the Remains, New Stars Arise The dust and debris left behind by novae and supernovae eventually blend with the surrounding interstellar gas and dust, enriching it with the heavy elements and chemical compounds produced during stellar death. Eventually, those materials are recycled, providing the building blocks for a new generation of stars and accompanying planetary systems. Post-Main Sequence Stars Post-Main Sequence Stars What happens when a main sequence star runs out of hydrogen in its core? The answers to this take us along the next stage of stellar evolution. As with most stages in a star's life, the exact post-main sequence is primarily dependent on its mass. We will start by looking at what happens to a a one-solar mass star like our Sun and then explore what happens to higher-mass stars.

Credit: Evolution of Low-Mass Stars Post-Main Sequence One-Solar Post-Main Sequence Evolution. Stars such as our Sun move off the main sequence and up the red giant branch (RGB), fusing hydrogen into helium in hydrogen shell burning. A very short helium flash sees the start of helium core fusion and the star moves along the horizontal branch (HB). Once shell temperature is sufficient, helium shell burning starts and the star moves up into the asymptotic giant branch (AGB). Moving Off the Main Sequence - Red Giant Branch A star remains on the main sequence as long as there is hydrogen in its core that it can fuse into helium. So far we have assumed that a star on the main sequence maintains a constant energy output. In fact, as a main sequence star ages its luminosity increases slightly, resulting in it expanding and its outer layer cooling. This explains why the main sequence is a broad band rather than a narrow line - stars move up and to the right on this band as they age. Eventually the hydrogen fuel in the core runs out and fusion stops, shutting off the outward radiation pressure. Inward gravitational attraction causes the helium core to contract, converting gravitational potential energy into thermal energy.

Although fusion is no longer taking place in the core, the rise in temperature heats up the shell of hydrogen surrounding the core until it is hot enough to start hydrogen fusion, producing more energy than when it was a main sequence star. This so-called shell-burning causes some interesting effects. The new, increased radiation pressure actually causes the outer layers of the star to expand to maintain the pressure gradient. As the gas expands it cools, just as a spray can feels colder after use as the gas has been released. This expansion and cooling causes the effective temperature to drop. Convection transports the energy to the outer layers of the star from the shell-burning region. The star's luminosity eventually increases by a factor of 1000 × or so. During this stage of expansion, the star will move up and to the right on the HR diagram along the Red Giant Branch (RGB). A G (V)-class star may end up as a high-K or low-M luminosity class III giant. A red giant displays extremes of density. The outer envelope is grossly extended and thus at a density below that of a vacuum on Earth. It is only weakly held by gravitational force to the rest of the star and easily ejected.

Mass loss from a giant is typically about 10-7 solar masses per year, compared with only 10-17 solar masses per year currently for the Sun. Whilst the envelope is tenuous and cool, the contracted helium core is incredibly dense. It is only about one-third its original size. Electrons within the core form a degenerate electron gas, they are packed tightly together in a volume governed only by the uncertainty principle. In this state it no longer behaves as an ideal gas. When the Sun becomes a red giant its radius will be approximately 0.5 AU, that is about 100 × its current size. Helium "Burning" and the Helium Flash Hydrogen fusion in the shell produces more helium. This gets dumped onto the core, adding to its mass, causing it to heat up even more. When the core temperature reaches 100 million K, the helium nuclei now have sufficient kinetic energy to overcome the strong coulombic repulsion and fuse together, forming carbon-12 in a two-stage process. As three helium nuclei, also known as alpha particles, are used it is called the triple alpha process. Subsequent fusion with another helium nucleus produces oxygen-16 nuclei.

This process is the main source of the carbon and oxygen found in the Universe, including that in our bodies. The triple alpha process for post-main sequence stars. Two helium nuclei (alpha-particles) fuse to form a beryllium-8 nucleus. This is unstable and normally decays back into two H-4e nuclei within a fraction of a second but given the high number of He-4 nuclei in the core will sometimes collide with one before it has had a chance to decay. This produces a carbon-12 nucleus and releases a gamma photon. The C-12 nucleus in turn may fuse with another He-4 nucleus to produce oxygen-16 and a gamma photon. Neon-20 may also be formed by oxygen nuclei fusing with helium but only negligible amounts are produced. In stars with mass less than about 2-3 solar masses the triple alpha process initiates in a matter of minutes or hours. Once the temperature is hot enough for helium fusion in one part of the core, the reaction quickly spreads throughout it due to the behaviour of the electron degenerate gas. This sudden onset of helium core fusion (or "burning") is called the helium flash.

Post-main sequence evolutionary tracks for 1, 5 and 10 solar mass stars. The Horizontal Branch The energy released by the helium flash raises the core temperature to the point where it is no longer degenerate. It thus starts to behave again as an ideal gas so can expand and cool. Energy transfers result in a hotter outer layer of the star but a smaller overall size. The rise in effective temperature and decrease in surface area are such that the luminosity remains roughly constant. Such a star tracks across to the left along the horizontal branch on the HR diagram. HB stars have helium core-burning and hydrogen shell-burning. A solar-mass star has sufficient helium fuel for core-burning to last for about 100 million years. The Asymptotic Giant Branch Eventually all the helium in the core has fused into carbon and oxygen and so the core contracts again. Carbon and oxygen nuclei have more protons in them than helium does so the coulombic repulsion is greater. The temperature needed to fuse these into heavier nuclei must be even greater than the 100 million K needed for He fusion. In stars of 8 solar masses or less there is insufficient gravitational energy to generate the temperatures required.

No more core fusion can thus take place. The core contraction does however generate sufficient heat for the surrounding layer of helium to start fusing, that is helium shell burning starts. Energy from the helium-burning in turn heats up surrounding unused hydrogen which also starts shell burning. The giant star expands again, possibly up to 1.5 AU, equivalent to the orbit of Mars. It is now an asymptotic giant branch star (AGB), occupying the upper-right portion of the HR diagram. A one-solar mass AGB may have a luminosity 10,000 × that of our current Sun. Mira (ο Ceti) is an example of an AGB star. Outer layers of AGB stars are only weakly held by gravity. The helium-burning shell is not dense enough to be degenerate so helium flashes occur with a runaway temperature rise. The resulting increased reaction rate generates a large energy release or thermal pulse for a couple of hundred years. During this phase nuclei within the helium-burning shell can be synthesised into heavier nuclei through the capture of neutrons and radioactive beta decay.

This so-called s-process (s for slow, in comparison with the rapid, r-process that occurs in more massive stars) produces elements as heavy as bismuth with 83 protons. AGB stars may produce a thermal pulse every 10,000 - 100,000 years. Large convection currents in AGB stars carry material produced in the thin helium-burning shell up to the surface. These heavier nuclei are detected in the star's spectrum which thus provides an insight on what is happening deep within the star. As with RGB stars, the radiation pressure tends to blow away much of the tenuously-held outer layer. The rate of mass loss is an order of magnitude higher though starting at about 10-6 solar masses per year. As the star evolves up the AGB branch, pulsations increase the rate of mass loss up to about 10-4 solar masses per year. The ejected material comprises a mixture of elements including carbon and oxygen dredged up from within the star. Carbon-rich molecules form dust and soot particles that tend to shroud the actual star. As the cloud expands it cools but the dust absorbs and re-emits the radiation from the star at longer wavelengths.

AGBs are thus often more luminous in the infrared than visible wavebands. The expanding cloud can also be observed at radio wavebands. AGBs such as Mira are intrinsic variable stars with periods of months or a few years. They can vary by up to 10 magnitudes. Mira's radius differs by a factor of two during its oscillations. What happens to solar-mass stars once fusion is completed is discussed on the next page . High-Mass Post-Main Sequence Evolution. Evolution of high-mass stars off the main sequence is an involved process and one still not fully understood. Such stars are rare and have very short lifespans relative to lower-mass stars. Supergiants such as Betelgeuse, Deneb, Rigel and Antares are some of the most prominent stars in our sky and visible over vast distances due to their extreme luminosities. This section provides a basic outline of the stages. High-mass stars consume their core hydrogen at prodigious rates so may only survive on the main sequence for millions rather than billions of years. Once this fuel is used up, the core contracts due to gravity and heats up. This triggers helium-burning in the core.

Unlike lower-mass stars, this helium fusion (triple-alpha process) starts gradually rather than in a helium flash. In moving off the main sequence, the effective temperature of the star drops as its outer layers expand. The decrease in temperature balances the increased radius so that the overall luminosity remains essentially constant. Evolutionary tracks for these massive stars thus move horizontally across the supergiant region of the HR diagram as shown on the diagram above . The energy liberated by helium fusion in the core raises the temperature of the surrounding hydrogen shell so that it too begins fusing. The size of these supergiants is enormous. Betelgeuse, thought to be between 13 and 17 solar masses, is so large that its envelope would extend beyond the orbit of Jupiter if it replaced our Sun. Its angular size is so large that it can be directly imaged by the HST. Credit: Andrea Dupree (Harvard-Smithsonian CfA), Ronald Gilliland ( STScI ), NASA and ESA Betelgeuse is a red supergiant. The bright yellow spot at the bottom of the star is thought to be a hotspot due to a massive convection cell.

In stars of 5 solar masses or higher, radiation pressure rather than gas pressure is the dominant force in withstanding collapse. The mass is large enough that the gravity acting on the core after helium-burning is sufficient to produce temperatures of 3 × 108 K where fusion of carbon with helium to produce oxygen dominates. A star of 8 solar masses or more can go on to synthesise even heavier elements in the core. Gravitational core contraction after all the core helium is used up generates a temperature of about 5 × 108 K at which point carbon nuclei fuse together to produce sodium, neon and magnesium. Production of magnesium releases a gamma photon, that of sodium releases a proton and neon produces a helium nucleus. Once all the core carbon is consumed, further collapse pushes temperatures to about 109 K. At this temperature, reactions that release gamma photons, such as 16O + 4He → 20Ne + γ, may be reversed by a process called photodisintegration. Helium nuclei released via this process can fuse with other neon nuclei to produce magnesium.

Once the neon is used up, core contraction increases the temperature to 2 × 109 K where two oxygen nuclei fuse to form silicon. This in turn may undergo photodisintegration to form magnesium and helium nuclei that then fuse with other silicon nuclei to produce sulfur. Similar stages of reactions see sulfur produce argon and argon synthesise calcium. Eventually elements such as chromium, manganese, iron, cobalt and nickel may be produced. Ultimately the silicon in the core is converted, as silicon-burning, into iron with final core temperature reaching about 7 × 109 K. The core region of a supergiant thus resembles the layers of an onion with a dense iron core surrounding by shells of silicon and sulfur, oxygen and carbon, helium and an outer shell of hydrogen as shown in the diagram below. The onion-like layers inside a supergiant in the final stages of its life. Successive layers correspond to the different elements produced by fusion, with a dense core of iron at the centre. Nucleosynthesis of elements above helium is less efficient so that each successive reaction produces less energy per unit mass of fuel. This means that the reactions occur at greater rates so that radiation pressure balances gravity.

Whilst a massive star may spend a few million years on the main sequence, its helium core-burning phase may be a few hundred thousand years. The carbon burning phase lasts a few hundred years, neon-burning phase a year, oxygen-burning half a year and the silicon-burning only a day. These massive stars evolve extremely rapidly once they move off the main sequence. Statistically they are very low in numbers as they are less likely to form than lower-mass stars and their lifetimes are so short anyway. As we shall see in a later section , they also make dramatic exits. Low-Mass Post-Main Sequence Evolution As discussed previously, low mass stars consume their core hydrogen at much lower rates than stars such as our Sun. Their main sequence lifespans are tens to hundreds of billions of years. Once they have consumed their core hydrogen, gravitational core collapse causes the core to heat up. For stars with less than 0.5 solar masses however, their is insufficient mass to generate the temperatures need for the helium in the core to start fusing. A brief period of hydrogen-shell burning sees its luminosity rise as with higher-mass stars. Unable to release energy from helium fusion, H-shell burning does not last long.

The star's luminosity quickly decreases and the star cools down. Its evolutionary track crosses back across the main sequence and down. The Evolution of the Sun The Evolution of the Sun A cloud of gas and dust begins to contract under the force of gravity. In regions of star birth , we find gaseous nebulae and molecular clouds. These sites of pre-birth are dark patches called globules. The protosun collapsed. As it did, its temperature rose to about 150,000 degrees and the sun appeared very red. Its radius was about 50 present solar radii. When the central temperature reaches 10 million degrees, nuclear burning of hydrogen into helium commences. The star settles into a stable existence on the Main Sequence, generating energy via hydrogen burning . This is the longest single stage in the evolutionary history of a star, typically lasting 90% of its lifetime. Thermonuclear fusion within the Sun is a stable process, controlled by its internal structure. The hydrogen in the core is completed burned into helium nuclei. Initially, the temperature in the core is not hot enough to ignite helium burning. With no additional fuel in the core, fusion dies out.

The core cannot support itself and contracts; as it shrinks, it heats up. The rising temperature in the core heats up a thin shell around the core until the temperature reaches the point where hydrogen burning ignites in this shell around the core. With the additional energy generation in the H-burning shell, the outer layers of the star expand but their temperature decreases as they get further away from the center of energy generation. This large but cool star is now a red giant, with a surface temperature of 3500 degrees and a radius of about 100 solar radii. The helium core contracts until its temperature reaches about 100 million degrees. At this point, helium burning ignites, as helium is converted into carbon (C) and oxygen (O). However, the core cannot expand as much as required to compensate for the increased energy generation caused by the helium burning. Because the expanion does not compensate, the temperature stays very high, and the helium burning proceeds furiously. With no safety valve, the helium fusion is uncontrolled and a large amount of energy is suddenly produced. This helium flash occurs within a few hours after helium fusion begins. The core explodes, the core temperature falls and the core contracts again, thereby heating up.

When the helium burns now, however, the reactions are more controlled because the explosion has lowered the density enough. Helium nuclei fuse to form carbon, oxygen, etc.. The star wanders around the red giant region, developing its distinct layers, eventually forming a carbon-oxygen core. When the helium in the core is entirely converted into C, O, etc., the core again contracts, and thus heats up again. In a star like the Sun, its temperature never reaches the 600 million degrees required for carbon burning. Instead, the outer layers of the star eventually become so cool that nuclei capture electrons to form neutral atoms (rather than nuclei and free electrons). When atoms are forming by capturing photons in this way, they cause photons to be emitted; these photons then are readily available for absorption by neighboring atoms and eventually this causes the outer layers of the star to heat up. When they heat up, the outer layers expand further and cool, forming more atoms, and releasing more photons, leading to more expansion. In other words, this process feeds itself. The outer envelope of the star blows off into space, exposing the hot, compressed remnant core. This is a planetary nebula .

The core contacts but carbon burning never ignites in a one solar mass star. Contraction is halted when the electrons become degenerate, that is when they can no longer be compressed further. The core remnant as a surface temperature of a hot 10,000 degrees and is now a white dwarf . With neither nuclear fusion nor further gravitational collapse possible, energy generation ceases. The star steadily radiates is energy, cools and eventually fades from view, becoming a black dwarf. With this understanding of how the Sun will evolve, we can follow its evolution on the HR diagram . Suggested readings: "Giants in the Sky: The Fate of the Sun", Kaler, James, B., 1993, Mercury, Mar-Apr, pp. 34-41. Chapter 20, Chapter Review SUMMARY Stars spend most of their lives on the main sequence, in the core-hydrogen burning phase of stellar evolution, stably fusing hydrogen into helium at their centers. Stars leave the main sequence when the hydrogen in their cores is exhausted. For the Sun, which is about halfway through its main-sequence lifetime, this stage will occur about 5 billion years from now.

Low-mass stars evolve much more slowly than the Sun, and high-mass stars evolve much faster. When the central nuclear fires in the interior of a solar-mass star cease, the helium in the stars core is still too cool to fuse into anything heavier. With no internal energy source, the helium core is unable to support itself against its own gravity and begins to shrink. The star at this stage is in the hydrogen-shell burning phase, in which the nonburning helium at the center is surrounded by a layer of burning hydrogen. The energy released by the contracting helium core heats the hydrogen-burning shell, greatly increasing the nuclear reaction rates there. As a result, the star becomes much brighter while the envelope expands and cools. A low-mass star like the Sun moves off the main sequence on the HR diagram first along the subgiant branch , then almost vertically up the red-giant branch . As the helium core contracts, it heats up. Eventually, the core of a star more than 0.25 times the mass of the Sun reaches the point at which helium begins to fuse into carbon.

The net effect of the fusion reactions is that three helium nuclei (or alpha particles) combine to form a nucleus of carbon in the triple-alpha process . In a star like the Sun, conditions at the onset of helium burning are such that the electrons in the core have become degeneratethey can be thought of as tiny, hard spheres that once brought into contact, present stiff resistance to being compressed any further. This electron degeneracy pressure makes the core unable to react to the new energy source, and helium burning begins violently in the helium flash . The flash expands the core and reduces the stars luminosity, sending it onto the horizontal branch of the HR diagram. The star now has a core of burning helium surrounded by a shell of burning hydrogen. As helium burns in the core, it forms an inner core of nonburning carbon. The carbon core shrinks and heats the overlying burning layers, and the star once again becomes a red giant. It reenters the red-giant region of the HR diagram along the asymptotic-giant branch, becoming an extremely luminous red-supergiant star. The core of a low-mass star never becomes hot enough to fuse carbon.

Such a star continues to ascend the asymptotic-giant branch until its envelope is ejected into space as a planetary nebula . At that point the core becomes visible as a hot, faint, and extremely dense white dwarf. The planetary nebula diffuses into space, carrying helium and some carbon into the interstellar medium. The white dwarf cools and fades, eventually becoming a cold black dwarf. Most white dwarfs are composed of carbon and oxygen, although stars in binary systems may give rise to helium white dwarfs, while more massive stars may become neon-oxygen white dwarfs. Evolutionary changes happen more rapidly for high-mass stars than for low-mass stars because larger mass results in higher central temperatures. High-mass stars do not experience a helium flash and attain central temperatures high enough to fuse carbon. They form heavier and heavier elements in their cores, at a more and more rapid pace, and eventually die explosively. The theory of stellar evolution can be tested by observing star clusters, all of whose stars formed at the same time. As time goes by, the most massive stars evolve off the main sequence first, then the intermediate-mass stars, and so on.

At any instant, no stars with masses above the clusters main-sequence turnoff mass remain on the main sequence. Stars below this mass have not yet evolved into giants and so still lie on the main sequence. By comparing a particular clusters main-sequence turnoff mass with theoretical predictions, astronomers can measure the age of the cluster. Stars in binary systems can evolve quite differently from isolated stars because of interactions with their companions. Each star is surrounded by a teardrop-shaped Roche lobe , which defines the region of space within which matter belongs to the star. As a star in a binary evolves into the giant phase it may overflow its Roche lobe, forming a mass-transfer binary as gas flows from the giant onto its companion. If both stars overflow their Roche lobes, a contact binary results. Stellar evolution in binaries can produce states not achievable in single stars. In a sufficiently wide binary, both stars evolve as though they were isolated. SELF-TEST: TRUE OR FALSE? 1. High-mass stars are conventionally taken to have masses greater than that of the Sun. HINT 2. All the red-dwarf stars that ever formed are still on the main sequence today.

HINT 3. Once on the main sequence, gravity is no longer important in determining a stars internal structure. HINT 4. The Sun will get brighter as it begins to run out of fuel in its core. HINT 5. As a star evolves away from the main sequence, it gets larger. HINT 6. As a star evolves away from the main sequence, it gets hotter. HINT 7. As a red giant, the Sun will have a core that is smaller than it was when the Sun was on the main sequence. HINT 8. When helium starts to fuse inside a solar-mass red giant, it does so slowly at first, and the rate of fusion increases gradually over many years. HINT 9. With the onset of helium fusion, a red giant gets brighter. HINT 10. A planetary nebula is the disk of matter around a star that will eventually form a planetary system. HINT 11. For a high-mass star, there is no helium flash. HINT 12. High-mass stars can fuse carbon and oxygen in their cores. HINT 13.

A star cluster with an age of 500 million years will contain many O-type stars. HINT 14. In a binary-star system, it is never possible for the lower-mass star to be more evolved than the higher-mass companion. HINT 15. In a mass-transfer binary, one of the stars has filled its Roche lobe. HINT SELF-TEST: FILL IN THE BLANK 1. A main-sequence star doesnt collapse because of the outward _____ produced by hot gases in the stellar interior. HINT 2. The Sun will leave the main sequence in about _____ years. HINT 3. While a star is on the main sequence _____ is slowly depleted and _____ builds up in the core. HINT 4. A temperature of at least _____ is needed to fuse helium. HINT 5. At the end of its main-sequence lifetime, a stars core starts to _____. HINT 6. When helium fuses, it produces _____ and releases _____. HINT 7. Just before helium fusion begins in the Sun, the cores outward pressure will be provided mainly by electron _____. HINT 8.

As a star ascends the asymptotic-giant branch, its _____ core is shrinking in size and growing in mass. HINT 9. As a red supergiant the Sun will eventually become about ____ times its present size. HINT 10. The various stages of stellar evolution predicted by theory can be tested using observations of stars in _____. HINT 11. By the time the envelope of a red supergiant is ejected, the core has shrunk to a diameter of about _____. HINT 12. A typical white dwarf has the following properties: about half a solar mass, fairly _____ surface temperature, small size, and _____ luminosity. HINT 13. As time goes by, the temperature and the luminosity of a white dwarf both _____. HINT 14. As a star cluster ages, the luminosity of the main-sequence turnoff _____. HINT 15. Whether being a member of a binary-star system will affect the evolution of a star depends largely on the _____ of the two stars in the binary. HINT REVIEW AND DISCUSSION Algorithmic versions of these questions are available in the Practice Problems Module of the Companion Website.

The number of squares preceding each problem indicates the approximate level of difficulty. 1. The Sun will evolve off the main sequence when roughly 10 percent of its hydrogen has been fused into helium. Using the data given in Section 16.5 and Table 16.2, calculate the total amount of mass destroyed (that is, converted into energy) and the total energy released by the fusion of this amount of matter. HINT 2. Use the radiusluminositytemperature relation to calculate the radius of a red supergiant with temperature 3000 K (half the solar value) and luminosity 10,000 solar luminosities. (Sec. 17.3) How many planets of our solar system would this star engulf? HINT 3. What would be the luminosity of the Sun if its surface temperature was 3000 K and its radius was (a) 1 A.U., (b) 5 A.U.? HINT 4.

Use the radiusluminositytemperature relation to calculate the radius of a 12,000 K (twice the temperature of the Sun), 0.0004-solar-luminosity white dwarf. HINT 5. Use the graph in More Precisely 20-1 to estimate the factor by which CNO energy production outstrips protonproton energy production in a 10-solar-mass star with a central temperature of 25 million K. What do you think the factor would be if the abundances of C, N, and O were just one-tenth the solar value? HINT 6. A main-sequence star at a distance of 20 pc is barely visible through a certain telescope. The star subsequently ascends the giant branch, during which time its temperature drops by a factor of three and its radius increases 100-fold. What is the new maximum distance at which the star would still be visible using the same telescope? HINT 7.

A Sunlike star goes through a rapid luminosity change between stages 8 and 9, when the luminosity increases by about a factor of 100 in 105 years. On average, how rapidly does the stars absolute magnitude change, in magnitudes per year? Do you think this change would be noticeable in a distant star within a human lifetime? HINT 8. Calculate the average density of a red-giant core of 0.25 solar-mass and radius 15,000 km. Compare this with the average density of the giants envelope, if it has a 0.5 solar-mass and its radius is 0.5 A.U. Compare each with the central density of the Sun. (Sec. 16.2) HINT 9. How long will it take the Suns planetary nebula, expanding at a speed of 50 km/s, to reach the orbit of Neptune? How long to reach the nearest star? HINT 10. What are the escape speed (in km/s) and surface gravity (relative to Earths gravity) of Sirius B? (See Table 20.2.) HINT 11.

A 15-solar-mass blue supergiant with a surface temperature of 20,000 K becomes a red supergiant with the same total luminosity and a temperature of 4000 K. By what factor does its radius change? HINT 12. The radius of Betelgeuse varies by about 60 percent within a period of three years. If the stars surface temperature remains constant, by how much does its absolute magnitude change during this time? HINT 13. The Sun will reside on the main sequence for 1010 years. If the luminosity of a main-sequence star is proportional to the fourth power of the stars mass, what mass star is just now leaving the main sequence in a cluster that formed (a) 400 million years ago, (b) 2 billion years ago? HINT 14. In roughly 5 billion years, the Sun will eject its envelope as a planetary nebula. Before then, suppose it loses 20 percent of its mass on the giant branch.

(a) If Jupiter stays in a circular orbit while this mass is being lost and its angular momentum stays constant, what will be the planets orbital radius and period when the Suns mass has fallen to 0.8 solar masses? (b) When the planetary nebula is formed, the Sun loses a further 0.3 solar masses rapidly enough that we can regard the loss as immediate. Jupiters instantaneous velocity is unchanged, but the orbit is no longer circular, due to the Suns smaller mass. The planets location at that moment becomes the perihelion of the new orbit, and the orbital semimajor axis increases (in this case) by a factor of 2.5. What are the eccentricity and period of the new orbit? HINT 15. From the discussions presented in More Precisely 2-3 and More Precisely 15-1 , it may be shown that the angular momentum of a circular binary system, of separation r and component masses m1 and m2, is proportional to m1 m2 (if the total mass is constant).

Such a binary has component masses one and two times the mass of the Sun, respectively, and an orbital period of two years. (a) What is its orbital separation r? (b) Mass transfer moves 0.2 solar masses of material from the more massive to the less massive star, keeping the total mass of the system fixed and conserving angular momentum. If the binary remains circular, calculate its new separation and orbital period. HINT COLLABORATIVE EXERCISES 1. Evolutionary Sequences. As a plot of luminosity vs. temperature, the HR diagram is useful for describing how stars evolve over time even though time is not the label on either axis. As a group, create an imaginary graph of dollars of financial income (vertical axis) vs. weight (horizontal axis) and use it to describe the past and future life cycle of one of your group members. Clearly label your diagram and provide a figure caption clearly explaining each life phase. RESEARCHING ON THE WEB To complete the following exercises, go to the online Destinations Module for Chapter 20 on the Companion Website for Astronomy Today 4/e. 1.

Access the "Story of a Rarely Seen Stellar Explosion" page and describe what a Sakurais object is, where it is located in the sky, and how it relates to the evolutionary future of our Sun. 2. Access the "HertzsprungRussell Diagram" page and define the four luminosity classes shown on the HR Diagram. PROJECTS 1. Can you find the Hyades cluster? It lies about 46 pc away in the constellation Taurus, making up the face of the bull. It appears to surround the very bright star Aldebaran, the Bulls eye, which makes it easy to locate in the sky. Aldebaran is a red giant, probably on the asymptotic-giant branch of its evolution. Despite appearances, it is not part of the Hyades cluster. In fact it lies only about half as far awaysome 20 pc from Earth. 2. Now look for the Double Cluster in Perseus, h and chi Persei. These two young clusters probably formed together, and now move together through space.

They lie about 2500 pc away, and are barely visible to the naked eye just east of the W of Cassiopeia. 3. Find a library that has the Astrophysical Journal. Find an article from the late 1950s and 1960s that gives the photometry of a star cluster like the Pleiades or Hyades. Plot a colormagnitude diagram (V vs. BV; see Section 17.6). Determine the V magnitude of the main-sequence turnoff, and hence estimate the age of the cluster. Compare your age with that given in the article. SKYCHART III PROJECTS The SkyChart III Student Version planetarium program on which these exercises are based is included as a separately executable program on the CD in the back of this text. 1. An HR diagram in Figure 17.14 relates the surface temperature of a star to its luminosity. In this and the following exercise you will generate an HR diagram using spreadsheet software. SkyChart does not give us the temperature, so use the color index (see exercise 2 in Chapter 3) as its proxy.

You will also need to use the absolute magnitude. SkyChart gives the apparent magnitude of stars and it can be converted to an absolute magnitude if you know the distance. Pick a region in the sky with plenty of stars (a field of about 10° should be sufficient). Select 50 stars and record their magnitude, color index (BV), and distance in pc. Some information may be missing on a particular star, so just skip it and use another. The first manipulation will be constructing a graph of distance versus apparent magnitude. Looking at your graph, can you identify a general trend? Is there more than one trend at work? Common sense tells us that stars farther away will appear less bright. In fact, the luminosity will decrease as 1/D2, where D is the distance to the star, so a star twice as far away will appear one-fourth as bright. Does your graph support this? If it does not, can you give reasons why? What other factors besides distance play a role in a stars apparent magnitude? 2. Now generate the HR diagram.

To do this, you will have to convert the apparent magnitude, m, to an absolute magnitude, M, using the following equation: M = 5-5log(D) + m, where D is the distance to the star measured in parsecs, pc. We can easily handle this equation using spreadsheet software (such as Excel). Enter in column A the name of the star, in column B the color index, in column C the distance, and in column D the magnitude. In the cell E2, enter the following expression: = 5-5*LOG10(C2) + D2. Select E2 again and select Copy from the Edit menu. Now highlight E3 through to the end of your data and select Paste from the Edit menu. You have just figured out the absolute magnitudes for all your data points. The last calculation that needs to be done is to determine the luminosities of the stars from their absolute magnitudes according to: L = 2.512(Mo 2 M), where L is the luminosity in solar units, M is the absolute magnitude and Mo is the absolute magnitude of the Sun, which is 4.78.

In cell F2, enter the equation: = 2.512^( 4.78 2 E2 ). Now Copy the formula from G2 and Paste it to cells G3 to the end of the data. Now generate a graph of luminosity versus color index, with the vertical axis on a logarithmic scale. What is the coolest star you recorded? How luminous is it? What is the least luminous star you recorded? Estimate how hot it is. What is the most luminous star you recorded? Estimate how hot it is. How does your HR diagram compare to the various HR diagrams in this chapter? In addition to the Practice Problems and Destinations modules, the Companion Website at provides for each chapter an additional true-false, multiple choice, and labeling quiz, as well as additional annotated images, animations, and links to related Websites. Stellar Evolution — University of Leicester The evolution of a Sun-like star When a Sun-like star settles on the main sequence, it is turning hydrogen into helium; about three quarters of its material is hydrogen, the remainder being helium and very small quantities of other, heavier elements.

The star will remain in this phase for around 10 billion years (our own Sun is roughly half way through this period). As more and more of the heavier helium is produced in the core of the star, the central regions begin to contract, and the temperature in the centre of the star increases. The effect of this is to cause the star to increase in brightness or luminosity. Eventually the centre of the star is made entirely of helium, and the nuclear reactions which give the star energy by converting four atoms of hydrogen into one of helium occur in a shell surrounding the helium core. As more helium is made, the central helium region gets larger and the shell moves closer and closer to the surface of the star. The energy given off by this shell pushes the very outer layers of the star outward, and these layers cool. The star expands and becomes a Red Giant. At this stage the star can be hundreds of times larger than it was when it first entered the main sequence. During this process, the helium core has beome more and more dense as material is added to it, and so the temperature has increased. Eventually the temperature becomes so great that the star can begin new reactions, turning helium into carbon.

The shell of hydrogen which pushed the outer atmosphere of the star away gradually becomes weaker and the star shrinks slightly - it is no longer a red giant. The star continues to obtain its energy by turning helium into carbon, and in the same way that hydrogen burning produced a helium core and a hydrogen-burning shell, this helium burning produces a carbon core and a helium burning shell; once again the star becomes a red giant. However, this time the situation is different, because the temperature in the very dense central carbon region never becomes great enough to begin a new phase of nuclear reactions. Around one million years after the star becomes a red giant for the second time, nuclear reactions finally end in the core. Only the shells of hydrogen and helium burning closer to the surface now produce energy for the star. Over a period of thousands of years, the star's central region shrinks and heats up, blowing the outer regions off. Astronomers can see the effects of this process; many objects are seen which are made up of a very small, dense central star surrounded by a shell of gas which appears to be expanding. These objects are called planetary nebulae, because the round shape of the gas cloud can look like a planet as seen through a telescope.

The image shown here (courtesy of the Mount Palomar Observatory) shows the "Ring Nebula" found in the constellation of Lyra. The central star responsible for the surrounding ring of gas is clearly visible. This is one of the most spectacular planetary nebulae to be seen in the night sky. When the outer layers of the star's atmosphere are blown away to form the nebula, the object seen at the centre of the gas cloud is the core of the original star. It is still very hot - perhaps as high as 100,000K. But the material gradually cools and contracts, to become a tiny dim object called a white dwarf, which will, over a period of billions of years, cool to become a black dwarf. This, then, is the fate of our Sun and the stars like it - ending its life as a dense body not much larger than the Earth. But before the sun reaches this stage, the Earth and all the inner planets will have been consumed in the fiercely hot atmosphere of the red giant which our star will first become. The evolution of stars with several times the Sun's mass In general, the more massive the star, the more rapid its sequence of evolutionary stages become.

However, that is not the only difference between "light" and "heavy" stars; more massive stars go through stages which their less weighty counterparts do not. The more massive star will enter the main sequence with a higher temperature (typically over 10,000K for a star of over 4 times the mass of our Sun) and greater luminosity than the Sun like star. The central core regions of these stars are also far hotter than in the previous class, and this causes the star to "age" more rapidly since, although the larger star has more fuel, it uses this fuel much quicker. For example, a star with five times as much mass as the Sun will evolve into a red giant one hundred times quicker ! The star completes its hydrogen and helium burning stages in the same way as the Sun-sized star described above. However, because the star is hotter, it can burn carbon when the helium supply becomes scarce. At this point, helium is burned around a carbon core, sending the star into a red giant stage. Carbon is then burned, then neon, and each successive element is burned to exhaustion.

At each stage when the star switches from burning one element to another, it again goes through the red giant phase, before returning to the main sequence for another period. However, when the star gets so far into this process that it develops a core of iron, no further reactions are possible because iron cannot be converted into the next element without huge amounts of energy being consumed. The death of stars more massive than the sun So far we have looked at the entire evolutionary path of stars like the Sun, and we've looked at the way stars with several times the Sun's mass expend their nuclear fuel. What next? In fact, the last section applies not only to stars a few times more massive than the Sun, but to the most massive stars in the galaxy. The next major difference in behaviour comes when we consider how stars more massive than the Sun end their lives. The mass of a star at the end of its nuclear burning phase determines what happens to it next. As we've already seen, stars like the Sun will end their days as a white dwarf.

In fact, stars which are slightly more massive than the sun also reach this point; the only difference is that since these stars managed to produce elements other than helium and carbon, the resulting white dwarf will be made of a different mixture of materials - but a white dwarf it is, nonetheless. If, on the other hand, a star is so massive that even at the end of its main sequence lifetime, it still has more than 1.4 times our Sun's current mass, then the situation changes dramatically. In such cases, the star suffers a supernova explosion. The cores of these stars contain iron, which cannot be used to produce more energy. As more and more material is added to the core, the density gets higher, until it equals that found in white dwarfs. Eventually the supply of nuclear fuel runs out, and in a few seconds the star's atmosphere falls rapidly towards the centre under the influence of the powerful gravitational field. The central region is compressed to form an incredibly dense object called a neutron star, and the infalling atmosphere "bounces" off this object, and the star explodes; for a brief moment in time, the star can outshine all the other stars in the galaxy put together.

The result of such a supernova explosion is the creation of the neutron star, and a rapidly expanding cloud of gas - once the stars atmosphere - called a supernova remnant. This image (taken with the Anglo Australian Telescope) shows the supernova which appeared in the Large Magellanic cloud (visible from the southern hemisphere) in 1987. Comparison of the left hand image with the one on the right showing the same star before the explosion, illustrates just how powerful a supernova event is. This image shows an artist's impression of the death of a star in a supernova explosion, seen from within the star's planetary system. The planets in the foreground will be destroyed by the immense amount of energy from the event - but there can be no life on the planets to witness the event, since the surfaces of these worlds were baked as the star passed through its red giant phase. (Courtesy of SEDS) Share this page:

Question: A red giant star typically evolves when its central hydrogen has converted to what gas?
Answer: Helium