Module 11: Star Lives: How Stars Evolve and Die

Life track after the main sequence

Observations of star clusters show that a star becomes larger, redder, and more luminous after its time on the main sequence is over.

Supernova Remnants and Synchrotron Radiation

Supernova remnants are sources of synchrotron emission. When a particle is accelerated in a magnetic field, the particle releases high-energy photons. Synchrotron sources like supernova remnants are best viewed in any wavelength other than visible light.

Age-dating star clusters

Combining models of stars of similar age but different mass helps us to age-date star clusters. Galactic or open star clusters contain relatively young, bright stars born together. Separated by about 1º, two nice examples are M46 (lower left) 5,400 l-y (1.7 kpc) distant and M47 (upper right) only 1,600 l-y (0.5 kpc) away. At around 300 million years, M46 contains a few hundred stars in a region about 30 l-y across. At 80 million years, M47 is a smaller but looser cluster of about 50 stars spanning 10 l-y.

Radiation Pressure in stars

Considering the role of accretion disks leads to an upper mass limit of 40 M(sun symbol). However, observations show that considerably more massive stars exist (up to 150 M(sun symbol) and more). The formation mechanism of extremely massive stars remains uncertain. Radiation pressure is most important in high-mass stars. Gas pressure is most important in low-mass stars. Gas pressure is most important in low-mass stars. Radiation pressure dominates in high-mass stars. The ratio of radiation pressure to gas pressure (P rad I P gas) for the Sun is 7 x 10<-4. Thus radiation pressure is not important in the center of the Sun. Westerlund 2 is an obscured compact young star cluster in the Milky Way, with an estimated age of about 1 or 2 million years. It contains some of the hottest, brightest, and most massive stars known. he Westerlund 2 cluster resides inside a stellar breeding ground known as Gum 29, located 20,000 l-y (6,132 pc) distant in the constellation Carina. The cluster contains at least a dozen early O stars. All are hotter than 38,000 K and more luminous than 230,000 L☉. Core temperatures are likely 40 million K and above. Because radiation pressure scales as the fourth power of the temperature, at these high core temperatures, radiation pressure becomes significant. There are around 20 further O class stars in the cluster, all main sequence objects implying a very young age for the cluster.

Electron Degeneracy Pressure Prevents White Dwarf Collapse

(A) An analogy for degeneracy pressure is a parking lot. Only one car is allowed per space. When there are many empty spaces, there is very little motion in the parking lot. As an occasional car enters the lot, it is quickly parked. When the parking lot is full, however, the picture changes. There is continual motion as cars move from one row to another while drivers search for a space. The pressure builds to get into position whenever a space is opened up. (B) Extremely dense matter is like a crowded parking lot. All of the low-energy "parking spaces" are taken, so electrons are forced into higher energy states, not because they are hot, but because there is nowhere else to go. This creates a degenerate electron pressure because all the low-energy states are occupied. This pressure is what prevents white dwarf stars from collapsing under their own weight.

Bright stars on and off the Main Sequence

(A) Note that many of the bright, named stars in our night sky are giants and supergiants. Main sequence stars hotter than the Sun also appear bright to us, often because they are relatively close by in distance. (NASA) (B) Fomalhaut (Alpha Piscis Austrini) is a bright spectral type A3 main sequence star 16 times brighter than the Sun that lies 25 l-y (7.7 pc) from Earth. Its lifespan is only 10% of the Sun.

SuperNova 1987A

(A) The supernova explosion of SN 1987A, taken in 1987. This supernova occurred in the Large Magellanic Cloud, a small galaxy about 163,000 l-y (50 kpc) distant. (Right) This image was taken before the supernova was detected. The arrow points to the star that exploded. (NASA/GSFC) (B) A time sequence of Hubble Space Telescope images, taken in the 15 years from 1994 to 2009, showing the collision of the expanding supernova remnant with a ring of dense material ejected by the progenitor star 20,000 years before the supernova.

Flare Star

(A) UV Ceti, the first flare star discovered, in X-rays, observed with the ROSAT High Resolution Imager. (Max-Planck-Institut für extraterrestrische Physik) (B) Flare on UV Ceti, observed by T. Cragg, January 23, 1959. (C) An infrared image of Proxima Centauri, the closest star to Earth (other than the Sun) and a flare star.

SN1987A Evolution

1. A binary stellar system. The more massive (primary) star evolves first. 2. As the primary star becomes a giant, it engulfs its companion. The core of the primary and the companion are in a common envelope. 3. As the companion spirals in, it ejects the envelope, mostly in the orbital plane. The companion merges with the core. 4. A fast wind from the core interacts with the torus around it, forming a ring of denser material. 5. The primary star explodes as a supernova, causing the inner edge of the ring to glow. 6. Ejecta from the explosion starts to move outward. 7. The bubble of ejecta grows, approaching the inner edge of the disk. 8. The ejecta strike and shock the inner ring at an increasing number of sports, which light up on impact. This is one model that explains the evolution of SN1987A. The supernova is 163,000 light-years (50 kpc) away in the Large Magellanic Cloud. It actually blew up about 161,000 BC, but its light arrived here in 1987.

Life Stages of a Low-Mass Star

1. Molecular cloud 2. Protostar 3. Main Sequence Star 4. Subgiant 5. Red giant star 6. Helium Core-Burning Star 7. Double shell-burning red giant star 8. Planetary Nebula 9. White Dwarf 10. Black Dwarf

Low-Mass Star Summary

1.Main Sequence: H fuses to He in core. 2.Subgiant: Star leaves main sequence. 3.Red Giant: H fuses to He in shell around He core. 4.Helium-Core Burning: He fuses to C in core while H fuses to He in shell. Star briefly returns to equilibrium state. 5.Double-shell Burning: H and He both fuse in shells. Core is non-fusing carbon. 6.Planetary nebula ejects from unstable stellar envelope leaving an inert carbon core. 7.White dwarf forms as inert carbon core contracts under gravity.

High-Mass Star Summary

1.Main Sequence: H fuses to He in core. 2.Red Supergiant: H fuses to He in shell around He core. 3.Helium-Core Burning: He fuses to C in core while H fuses to He in shell. 4.Multiple-Shell Burning: Many elements fuse in shells surrounding the core. 5.Supernova explosion is initiated by iron core collapse that ultimately leaves neutron star behind. 6.Neutron star forms from collapsed iron core that becomes neutron degenerate. If neutron star rotates, it may be observed as a pulsar.

Carbon Stars

A carbon star is typically an asymptotic giant branch (AGB) star, whose atmosphere contains more carbon than oxygen; the two elements combine in the star's upper layers, forming carbon monoxide (CO), which consumes all the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds, giving the star a "sooty" atmosphere and a strikingly ruby red color. Photospheres of carbon stars become especially carbon-rich from thermal pulses that pull carbon from their cores. The low-speed solar winds of carbon stars help create interstellar dust grains (like smoke particles in a fire). This dust is believed to be a significant factor in providing the raw materials for the creation of subsequent generations of stars.

Stars and the Main Sequence

A star remains on the main sequence as long as it can fuse hydrogen into helium in its core. Star clusters M35 and NGC 2158. M35 contains the bluer stars at the top and left and is about 2,600 l-y (0.8 kpc) distant. NGC 2158 is the smaller cluster to the lower right and contains many more redder stars. It is older than M35 and is at a distance of 15,000 l-y (4.6 kpc).

Mass Transfer

A star that is now a subgiant was originally more massive. As that more massive star reached the end of its life and started to expand, it began to transfer mass to its companion —which is known as mass exchange or mass transfer. Now the companion star is more massive and hotter. The Algol system is an example of mass transfer that has occurred over time.

Role of Mass

A star's mass determines its entire life story because mass determines the star's core temperature. Low-mass stars with < 2 M(sun symbol) have long lives, never become hot enough to fuse carbon nuclei, and end their lives as white dwarfs. Intermediate-mass stars can make elements heavier than carbon (but not iron) and also end as white dwarfs. High-mass stars with > 8 M(sun symbol) have short lives, become hot enough to make iron, and end in supernova explosions. Life cycle of a low-massstar. The star ends as a white dwarf and planetary nebula.

What are the four basic life stages of a low-mass star? (That is, name the nuclear processes occurring at each stage.) Then, for each stage, indicate where on the H-R diagram the star sits. (Hint: The first stage begins when the star officially becomes a star.)

A.H fusion in core-->main sequence B.H fusion in shell around contracting core-->red giant branch C.He fusion in core-->horizontal branch D.Double-shell burning-->AGB—asymptotic giant branch

Multiple Shell Burning

Advanced nuclear burning proceeds in a series of nested shells inside massive stars. (slide 58)

Horizontal branch star

After the helium flash, the star is on the horizontal branch of the H-R diagram. At first, He fuses to C in the core, while H fuses to He in a shell around the core. This process lasts about 100 million (10<8) years. Because the core is no longer degenerate, the star becomes smaller and hotter, though its overall luminosity is 1/100th as luminous as it was at the time of the helium flash. The diameter and luminosity of the former red giant are now less than 2% of their former values. For stars of 1 M(sun symbol), the result of the helium flash is a collapse into an orangish-yellow star with 10 times the current solar diameter and 40 times the current solar luminosity. Diagram of the structure of a star on the horizontal branch of the H-R diagram. Though hotter, the star's smaller radius produces less surface area and thus a lower luminosity.

Algol Animation

Algol (β Persei) is a triple-star system (Algol A, B, and C) in the constellation Perseus, in which the large and bright primary Algol A is regularly eclipsed by the dimmer Algol B every 2.87 days. The eclipsing binary pair is separated by only 0.062 AU from each other, a distance so close that Algol A is slowly consuming the less massive Algol B by continually stripping off Algol B's outer layers. This animation was assembled from 55 images of the CHARA interferometer in the near-infrared H-band, sorted according to orbital phase. Because some phases are poorly covered, B jumps at some points along its path. The phase of each image is indicated at the lower left. The images vary in quality, but the best have a resolution of 0.5 milliarcseconds, or approximately 200 times better than the Hubble Space Telescope. (A milliarcsecond is about the size of a quarter atop the Eiffel Tower as seen from New York City.) Tidal distortions of Algol B giving it an elongated appearance are readily apparent. Tidal distortions also result in "gravity darkening" effects, whereby in a significant number of images of Algol B, the edge or "limb" of the image is actually brighter than the center.

Algol or Beta Perseï

Algol or β Perseï is a multi-star system 93 l-y away with two main components. (A) A massive, bright, blue-white central star (B8) with 3.6 M(sun symbol_, 2.3X solar radius, and 98X brighter than the Sun. (B) An orange-red subgiant star (K2) with 0.8 M(sun symbol), 3.0X solar radius, and 3.4X solar luminosity. Both stars orbit at a distance of 0.062 AU. (C) A third star, a blue-white A5 with 1.7 M(sun symbol), 0.9Xsolar radius, and 4.1X solar luminosity, orbits the others at ~2.7 AU.

Low-mass star Core and Shell Stages

As a low-mass star moves off the main sequence, it begins to experience a series of stages that differ by what is occurring in and around the core. Cross-section of a low-mass star as it evolves from a main-sequence hydrogen-burning start to an asymptotic giant branch star. The outer envelope always remains as non-burning hydrogen. (slide 24)

Reasons for Life Stages of a Low-Mass Star

As a star forms out of a molecular cloud, its core shrinks and heats until it is hot enough for fusion to be sustained. A main-sequence star is born. Nuclei with larger charge (such as helium or carbon) require a higher temperature for fusion. The core thermostat of the star is broken while core is not hot enough for fusion. However, fusion does take place in a shell around the core (shell burning). Core fusion cannot happen if degeneracy pressure keeps core from shrinking. This occurs in low-mass stars with carbon cores created by the fusion of helium.

Broken Thermostat

As the core contracts, H begins fusing to He in a shell around the core. Luminosity increases because the core thermostat is broken—the increasing fusion rate in the shell does not stop the core from contracting. The Sun's helium core will eventually reach a critical point where the pressure from normal gasses cannot hold up the crushing weight being piled on it. A tiny seed of electron-degenerate matter will begin to grow at the center of the Sun. Theoretical calculations indicate that it will begin when the Sun's inert helium core reaches about 13% of a solar mass (about 140 Jupiters). The evolution of a star from a hydrogen-burning main sequence star to an expanding red giant.

Fusion Rates: P-P vs. CNO

At lower temperatures, the P-P chain dominates, but with rising temperatures (above 17 ×10<6 K) the fusion process quickly favors the CNO cycle, which has an energy production rate that varies strongly with temperature. This temperature dependence explains why the CNO cycle is more important for massive stars: their core temperatures are higher. In the case of the Sun, astrophysical models suggest that it is producing about 98%-99% of its energy from the P-P chain and only about 1% from the CNO cycle. However, if the Sun were just 10%-20% more massive, its energy production would be dominated by the CNO cycle.

Helium Fusion

At some point the, the inert He core reaches a temperature of 100 million K (108 K) and helium fusion begins. Helium fusion process is called the triple-alpha reaction (the He nucleus is called an alpha particle.) The result of helium fusion is energy and carbon. The onset of helium fusion initially heats the core without inflating it because of degeneracy pressure. Very quickly, the rising temperature causes the He fusion rate to rocket upward in what is called a helium flash. At this point, helium core fusion and hydrogen burning in the shell are occurring simultaneously. However, total energy output falls, and the outer layers contract. The triple-alpha reaction is the nuclear process by which helium is converted into energy and carbon. Helium fusion requires a core temperature of 100 million K. Helium fusion employs the triple-alpha process.

Supernova Explosion

Core degeneracy pressure goes away because electrons combine with protons, making neutrons and neutrinos. Neutrons collapse to the center, forming a neutron star. Energy and neutrons released in a supernova explosion enable elements heavier than iron (Fe) to form, including gold (Au) and uranium (U). Slide 67

What happens when a star can no longer fuse hydrogen to helium in its core?

Core shrinks and heats up

Advanced Nuclear Burning

Core temperatures in stars with mass greater than 8 M(sun symbol) permit fusion of elements up to iron. Nuclei of elements already formed can combine to create heavier elements up to iron. The formation of heavier elements takes place in multiple shells surrounding the core of massive stars. Ultimately, the core of such stars is transformed into iron through the fusing of silicon at a temperature of 3.5 billion K. Because iron cannot undergo fusion, its continued build-up in the core will lead to a catastrophic collapse that initiates a supernova explosion.

Algol—An Eclipsing Binary

Eclipsing binary star Algol varies in brightness depending on the orbital position of the larger diameter but less luminous secondary (Algol B) relative to the more luminous primary (Algol A).

Supernova Remnant (SNR)

Energy released by collapse of the core drives outer layers into space. The Crab Nebula is the remnant of the supernova seen in A.D. 1054 by the Chinese. It is 6,500 l-y (2.0 kpc) distant. Tycho Brahe witnessed a supernova in 1572, and Johannes Kepler saw one in 1604.

Evidence for Helium capture

Evidence for helium capture exists in the higher abundances of elements with even numbers of protons. Elements with an even number of protons are present in higher quantities because these elements are easier to make using helium capture.

Finding the Crab Nebula

Finder chart for M1. Also note the location of the Pleiades, Hyades, and M42. Though Taurus is a winter constellation, the Crab Nebula (M1) can be seen from November 5 to May 10 at 9:00 pm.

Asymptotic Giant Branch Star

For low-mass stars, the last major phase of life is as an asymptotic giant branch (AGB) star. This final phase of hydrogen burning happens after the star has moved from the main sequence, through the red giant phase and past the horizontal branch. At this point they are characterized by an inert carbon-oxygen core, surrounded by two separate nuclear burning layers—an inner layer of helium and an outer layer of hydrogen. These layers are in turn surrounded by a strongly convective outer envelope. Layers of the Asymptotic Giant Branch Star: Starting at inner core: Non-burning C-O Core He Burning shell H Burning shell Non-burning H envelope

What happens when a low-mass star's core runs out of helium?

Helium fuses in a shell around the core.

What happens in a low-mass star when core temperature rises enough for helium fusion to begin?

Helium fusion rises very sharply. Hint: Degeneracy pressure is the main form of pressure in the inert helium core.

Helium-burning Stars

Helium-burning stars neither shrink nor grow because the core thermostat is temporarily fixed. A star fusing helium in its core is in gravitational or hydrostatic equilibrium. However, this period of stability exists for a much shorter span of time than when the star was fusing hydrogen in its core. Sun-like stars undergo helium fusion for only about 100 million (10<8) years.

Helium Capture

High core temperatures in high-mass stars allow helium to fuse with heavier elements. This process of helium capture occurs at temperatures in excess of 108 K (100 million K). As the star evolves, heavier elements tend to form through helium capture rather than by fusion of like nuclei. Elements with nuclear masses of 4 units (helium), 12 units (carbon), 16 units (oxygen), 20 units (neon), 24 units (magnesium), and 28 units (silicon) are more readily made. Each element is built by combining the preceding element and a helium-4 nucleus as the star evolves.

New view of Ring Nebula

In this composite image, visible-light observations by NASA's Hubble Space Telescope are combined with infrared data from the ground-based Large Binocular Telescope in Arizona to produce this detailed image of the Ring Nebula (M57). The nebula is shaped more like a jelly doughnut, because it is filled with material (gas) in the middle. Located in the constellation Lyra, the Ring Nebula is about 2,000 l-y (0.6 kpc) distant and measures roughly 1 l-y across.

High-Mass Stars and CNO Cycle

High-mass stars produce the full array of elements on which life depends. In high-mass stars, carbon, nitrogen, and oxygen acts as catalysts that help hydrogen burning proceed at a higher rate. The process of hydrogen fusion in high-mass stars is thus called the CNO cycle. Greater core temperature enables H nuclei to over-come greater repulsion. The CNO cycle converts hydrogen to helium in the following sequence of reactions: 1. Carbon-12 (<12C) captures a proton (p) and emits a gamma-ray (γ), producing a nitrogen-13 (<13N) isotope atom. 2. <13N is unstable and beta decays (β+) to <13C with a half-life of approximately 10 minutes. 3. <13C captures a proton and emits a gamma-ray to become a <14N. 4. <14N captures another proton and emits a gamma-ray to become <15O. 5. <15O undergoes a beta decay to become a <15N. 6. <15N captures a proton and emits an alpha-particle (α), which is a nucleus of helium, to close the cycle and return to <12C.

Hydrogen Shell Burning

Hydrogen shell burning proceeds at a higher rate than core hydrogen fusion did, resulting in the star becoming even more luminous. Because new thermal energy is trapped within the star, thermal pressure pushes the surface of the star outward, creating a luminous red giant. As fusion progresses, the inert He core and the H burning shell continue to contract even as the upper layers expand outward.

A High-Mass Star Dies by Going Supernova

If the core is comparable to 1 M(sun symbol) or more, then collapse of the core generates a huge amount of energy released all at once: a supernova explosion. The ball of neutrons left behind is called a neutron star. If the remaining mass is large enough, the core continues to collapse until it becomes a black hole. It is now thought that the neutrinos produced from the combining of electrons and protons to form neutrons is what drives the shock wave that blows the outer layers of the star into space. The shock wave also heats the ejected layers of gas so that the exploded star shines with incredible brightness. A supernova may shine briefly with the luminosity of 10 billion Suns. Elements heavier than iron are relatively rare since these are made only by rare fusion reactions shortly before and during a supernova.

Fusion in massive stars

In high-mass stars, the CNO cycle can change carbon (C) into nitrogen (N) and oxygen (O). Helium capture builds carbon (C) into oxygen (O), neon (Ne), magnesium (Mg), and so on. Advanced nuclear burning in stars makes elements like silicon (Si), sulfur (S), calcium (Ca), and ultimately iron (Fe). Iron is the end stage of stellar core fusion.

Low-mass stars-2

In low-mass stars like the Sun radiative diffusion is more effective near the hotter core while convection is more effective in the cooler outer layers the convection zone is the outer one-third of its interior High Mass Star: >1.5M (sunsymbol) Convective Core Sun-like Star: 0.5-1.5M (sunsymbol) Outer third is Convective Low-mass Star: <0.5M (sun symbol) Convective throughout A comparison of convection zones in stars of different masses. In stars like the Sun (1 M), the convection zone occupies the outer one-third of the star. In smaller red dwarfs, the convection zone extends inward to the core. Conversely, high-mass stars have convective cores with the outer layer a radiative zone, which is the opposite of Sun-like stars.

Why Massive Stars Have Convective Cores

In massive stars (greater than about 1.5M(sun symbol)), the core temperature is above about 1.8 x 10<7 K, so hydrogen-helium fusion occurs primarily via the CNO cycle. In the CNO cycle, the energy generation rate scales as the temperature to the 17th power, whereas the rate scales as the temperature to the 4th power in the proton-proton chains. Due to the strong temperature sensitivity of the CNO cycle, the temperature gradient in the inner portion of the star is steep enough to make the core connective. In the outer portion of the star, the temperature gradient is shallower but the temperature is high enough that the hydrogen is nearly fully ionized, so the star remains opaque to the ultraviolet radiation. Thus, massive stars have a radiative envelope.

Convection Zone

In very cool, low-mass stars (< 1 M sun symbol), the convection zone can extend to the core. In hot, high-mass stars there is no convection zone near the surface though they have convective cores. Low-mass M stars with deep convection zones and fast spin rates are very active. The most active emit huge flares and are known as flare stars. Suspected flare stars were noted in the 1930s and early 1940s; UV Ceti was the first flare star to be studied intensively beginning in 1948 and is considered the class prototype. (A) A low-mass red dwarf star (spectral class M) contains both a convective core and outer envelope.

High-Mass Stars and Late Stage Nuclear Burning

Intermediate-mass stars (2-8 solar masses) do not go beyond carbon cores and end their lives as white dwarfs. In high-mass stars (over 10 solar masses), the carbon core shrinks until the core temperature reaches 600 million K and carbon fusion can begin. At these late stages, helium capture—the fusing of helium nuclei into progressively heavier elements-keeps the process going. Layer upon layer of different fusing elements builds up in the high-mass star. Finally, iron begins to pile up in the silicon-burning core. Located in the constellation Scorpius 550 l-y distant, Antares (Alpha Scorpii) is a red supergiant star of spectral type MO. It's properties include mass=12.4 M (sun symbol), radius=883 R (sun symbol), and luminosity=57,500 L (sun symbol). Stars such as Antares employ helium capture to produce heavier elements up to iron.

Massive Stars Lead to Supernovae

Iron builds up in the core until degeneracy pressure can no longer resist gravity. The iron core then suddenly collapses, creating a supernova explosion. 1. Iron Ash 2. Silicon, Sulfur fusion 3. Neon, magnesium fusion 4. Oxygen fusion 5. Carbon fusion 6. Helium fusion 7. Nonburning hydrogen Carbon fusion- 600 years Neon fusion- 1 year Oxygen fusion- 6 months Silicon fusion- 1 day--->Inert iron core (A) Supergiant stars undergo multiple shell burning until non-fusible iron builds up in the core. (B) The final stages of fusion in very high-mass supergiant stars proceed rapidly, with silicon fusing into iron in only one day at a temperature of 3.5 billion K.

Iron can't fuse

Iron is the one element from which it is not possible to generate any kind of nuclear energy. Elements heavier than iron can generate energy only through fission, not fusion. Iron has the lowest mass per nuclear particle of all nuclei and therefore cannot release energy by either fusion or fission. In a high-mass star, iron piles up in the core past the point of degeneracy pressure. Electrons disappear by combining with protons to form neutrons. Iron (Fe) is a dead end for fusion because nuclear reactions involving iron do not release energy. Iron has the lowest mass per nuclear particle. (A) Iron has an atomic number of 26 and an atomic mass of 55.85. (B) Schematic of an iron atom, which contains 26 protons and electrons and 30 neutrons. With the lowest mass per nuclear particle, iron can neither fuse or fission to produce energy.

Iron: The most stable element

Iron-56 is probably the most stable isotope, which is due to the amount of energy that must be put into the nucleus to break apart the nucleons. This stability prevents iron from undergoing fusion or fission. Iron (element 56) has the highest binding energy of any element, which makes it the most stable.

What element in the cores of massive stars builds up but ultimately cannot fuse to create more energy?

Iron. So much iron builds up that the atomic structure of the iron atoms cannot withstand the force of gravity. The electrons of the iron atoms are crushed into the protons, forming neutrons—and a neutron star. The implosion of the iron core results in a core bounce that produces a tremendous shock wave that blasts apart the star's outer layers in the most spectacular explosion known—the supernova.

Planetary Nebulae

Just before a low-mass star dies, the star ejects its outer layers into space in the form of a huge shell of gas. The exposed carbon core is very hot, and the intense UV radiation ionizes the gas in the expanding shell. This brightly glowing shell of gas is what we call a planetary nebula. (Note: Planetary nebulae have nothing to do with planets.) Most planetary nebulae last between 50,000 and 100,000 years before dissipating into space. Double-shell burning ends with a pulse that ejects the H and He into space as a planetary nebula. The inert carbon core left behind becomes a white dwarf. In 1966, astronomers George O. Abell and Peter Goldreich presented the first coherent evidence that planetary nebulae evolve from red giants. M57, the Ring Nebula, imaged by the Hubble Space Telescope. This planetary nebula's appearance is thought to be due to perspective: our view from Earth looks straight into what is actually a barrel-shaped cloud of gas ejected by a dying central star. Hot blue gas near the central star gives way to progressively cooler green and yellow gas at greater distances with the coolest red gas along the outer boundary. (A) The Eskimo Nebula (NGC 2392) is a bipolar double-shell planetary nebula discovered William Herschel in 1787. NGC 2392 lies more than 2,870 light-years (0.9 kpc) distant in the constellation of Gemini. (NASA/ESA/STScI) (B) The Spirograph Nebula (IC 418) is a planetary nebula in the constellation Lepus (the Hare) that lies about 2,000 light-years (0.6 kpc) away and spans 0.3 light-years across. (A) The Engraved Hourglass Nebula (MyCn 18) is a planetary nebula in the southern constellation Musca (the Fly) about 8,000 l-y (2.5 kpc) distant. (Hubble image, NASA/JPL) (B) The Butterfly Nebula (NGC 6302) is about 3,000 l-y (0.9 kpc) away in Scorpius. The ejected gas is moving outward at 268 km/s (167 mi/s) and is at a temperature of 20,255 K (36,000 ºF). The white dwarf star (shrouded by dust ) is very hot at 222,480 K (400,000 ºF).

Star Quakes and Red Giants

Kepler spacecraft data have now revealed that red giants in all evolutionary states show "star quakes." Using asteroseismology, researchers have been able to place red giant stars into two clear groups: hydrogen-shell-burning stars (period spacing about 50 s) and those that are also burning helium (period spacing of 100-300 s). Comparison of the size of the Sun to the smallest and largest red-giant for which oscillations have been detected with Kepler. Never before have star quakes been discovered in such a wide variety of red-giant stars.

Life-Stages of High-Mass Stars

Late life stages of high-mass stars are similar to those of low-mass stars. High-mass stars do not experience helium flash. Hydrogen core fusion (main sequence) Hydrogen shell burning (supergiant) Helium core fusion (supergiant) The successive burning of a massive star, which is 18 M(sun symbol_, is shown at the hydrogen burning stage (top). It emits energy continuously which comes at the expense of its mass. Once most of the hydrogen has been fused into helium, the star has gotten hotter and helium burning starts to produce carbon. Then carbon burning starts. At each stage, the temperature rises, mass of the star is transformed into energy, and the next stage of fusion (of the next heavier element) proceeds. The very last stage fuses sulfur and silicon into iron (Fe). After this stage, the fusion reactions do not produce energy at an expense of mass. Once the star is depleted of its fusion-sustaining fuel, death comes to the star.

Future of stars

Low-mass and intermediate-mass stars swell into red giants near the ends of their lives and ultimately become white dwarfs. For small stars less than 1 M(sun symbol), there has not yet been enough time for these stars to move off the main sequence, become a red giant, eject a planetary nebula, and shrink to a white dwarf. High-mass stars will also become red and large in their late stages, but their lives will end more violently. Stars are born of interstellar gas and return much of that gas to interstellar space when they die.

Low-Mass stars-1

Low-mass stars (0.3-1.5 M, sun symbol) like the Sun fuse hydrogen into helium in their cores via the proton-proton chain move energy outward through radiative diffusion and convection use radiative diffusion to transport energy through random collisions of photons from one electron to another use convection to transport energy by the rising of hot plasma and the falling of cool plasma The proton-proton chain reaction does not establish a steep temperature gradient. Thus, radiation dominates in the inner portion of solar mass stars. The outer portion of solar mass stars is cool enough that hydrogen is neutral and thus transparent to gamma-ray photons, so convection dominates. Therefore, solar mass stars have radiative cores with convective envelopes in the outer portion of the star. The proton-proton chain is the nuclear process used by low-mass stars like the Sun.

Three basic groups of stars

Low-mass stars: born with < 2 solar masses(Some astronomers classify low-mass stars as those having 1 solar mass or less.) Intermediate-mass stars: born with 2-8 solar masses High-mass stars: born with > 8 solar masses Smallest to largest (mass): M, K, G, F, A, B, O

M27, Dumbbell Nebula

M27 is called a planetary nebula (though it has nothing to do with planets). Planetary nebulas are formed when Sun-like stars at the end of their lives eject large shells of gas. These shells of glowing gas are short-lived at between 50,000 and 100,000 years. (B) M27 in color. M27, the Dumbbell Nebula (also known as Apple Core Nebula or NGC 6853), is a planetary nebula in the constellation Vulpecula (the Fox), at a distance of about 1,360 l-y (0.4 kpc). M27 was the first planetary nebula to be discovered by Charles Messier in 1764. At a visual magnitude of 7.5 and a diameter of about 8 arcminutes, it is easily visible in binoculars and small telescopes. The Dumbbell Nebula is best viewed from July to November.

Life track after helium flash

Models show that a red giant should shrink and become less luminous after helium fusion begins in the core. Core helium in a low-mass star will run out in about 100 million years (10<8 y).

He-Burning Stars Are Found Along the Horizontal Branch

Observations of star clusters agree with this model of stellar evolution. Helium-burning stars are found in a horizontal branch on the H-R diagram. H-R diagram showing globular cluster M3, its turn-off point, and the location of helium-burning stars that fall along what is known as the horizontal branch. (The "RR Lyrae gap" in the horizontal branch between about B−V = 0.1 and B−V = 0.4 is not real; stars exist in the cluster that would be plotted in this region, but they are variable stars which were inadequately characterized to be included in the tables from which this figure was made.)

R Leporis: Hind's Crimson Star

One of the reddest carbon stars known is R Leporis (in Lepus, the Hare) discovered by J. R. Hind in 1845 and commonly known as Hind's Crimson Star. R Leporis also has the distinction of being a "long period variable" (period = 432 days) like Mira. Its radius is 480-535 times that of the Sun, or between 2.2 and 2.5 AUs. Were it our star, it would extend nearly halfway to Jupiter, well into the asteroid belt. Like many carbon stars, its mass is estimated at between 2.5 and 5 M(sun symbol), meaning R Leporis began life as a hot class B star. The star is ~1,100 l-y (0.34 kpc) distant. A) R Leporis (aka Hind's Crimson star) is located in the constellation of Lepus (the Hare)

Star clusters and stellar lives

Our knowledge of the life stories of stars comes from comparing mathematical models of stars with observations. Star clusters are particularly useful because they contain stars of different mass that were born about the same time. The Quintuplet Cluster is a 4-million-year-old cluster that has stars on the verge of blowing up as supernovae. It is the home of the brightest star seen in the Milky Way Galaxy, called the Pistol Star. The cluster is a dense grouping of massive young stars about 100 l-y from the galactic center, which is 26,000 l-y (8.0 kpc) from Earth. Its name comes from the fact it has five prominent infrared sources residing in it. Along with the Arches cluster, it is one of two in the region near the galactic center. The trapezium of four bright red stars just below center, plus one to the left, are the original Quintuplet. Due to heavy extinction by dust, it is invisible to optical observation and must be studied in the X-ray, radio, and infrared bands.

Radiation pressure in stars

Photons streaming outward from a high-mass star create a pressure called radiation pressure. Stars with masses > 20 M(sun symbol) are still accreting when they reach the main sequence. The huge luminosity drives a large radiation pressure force on dust grains in the incoming gas and sets an upper mass limit on the star's final mass. Spherically symmetric calculations found limits of 20-40 M(sun symbol). Radiation pressure: As a consequence of nuclear fusion, radiation pushes outward from a star. Always present gravity pulls inward.

SN 1987A Ring Lights Up

Shown are the evolving images of hot spots from Supernova 1987A in visible light from the Hubble Telescope (left) alongside images taken at approximately the same time from the Chandra X-ray Observatory (middle), and the Australia Telescope Compact Array (ATCA) radio observatory (right). The X-ray images show an expanding ring of gas, hotter than a million degrees, that has evidently reached the optical ring at the same time as the hot spots appeared. The radio images show a similar expanding ring of radio emission—synchrotron radiation—caused by electrons moving through magnetized matter at nearly the speed of light.

Algol and Mass Transfer

Stars in Algol (β perseï) are close enough that matter can flow from the subgiant onto the blue-white main-sequence star. (A) A schematic of the mass transfer in the Algol system. (B)The Algol system as it appeared on 12 August 2009. This is not an artistic representation, but rather is a true two-dimensional image with ½ milli-arcsecond resolution in the near-infrared H-band, reconstructed from data of the CHARA interferometer. The elongated appearance of Algol B and the round appearance of Algol A are real. The form of Algol C, however, is an artifact. (Fabien Baron/U. Michigan) (C) An animation of an eclipsing binary system undergoing mass transfer.

When and how will our Sun die?

Sun: The Sun has been shining for 4.5 billion years since it began fusing hydrogen into helium. It will continue in this fashion for the next 5.5-6 billion years Red Giant: As the nuclear fuel becomes depleated, the core contracts, but the outer layers expand. As a red giant, the Sun's radius will nearly reach the Earth's orbit. This stage will last for 1 billion years. Planetary Nebula: Near the end of the red giant stage, the Sun will become very unstable. Pulsating violently, it will finally blow off it's outer layers, which will glow from the heat of the contracting core. White Dwarf: With fusion stopped in its core, gravity will rapidly shrink what is left of the Sun to the diameter of the Earth. Incredibily dense and hot, the white dwarf Sun will slowly cool for billions of years. Black Dwarf: Following a long cooling process of perhaps 10-20 billion years, the Sun will end its days as a dark, dead lump of carbon.

Elements and Nuclear Fusion

The Big Bang made ~76% H / 24% He—stars make everything else. Stars begin by converting hydrogen into helium in their cores. Helium fusion (triple alpha process) can make carbon (C) in low-mass stars. For low-mass stars, carbon is the end stage.

CNO Cycle

The CNO cycle (for carbon-nitrogen-oxygen) is a fusion reaction for stars that operates at a higher temperature than those that use the proton-proton chain. Theoretical models show that the CNO cycle is the dominant source of energy in stars heavier than about 1.5 times the mass of the Sun. In the CNO cycle, 4 protons fuse using carbon, nitrogen, and oxygen isotopes as a catalyst to produce 1 alpha particle, 2 positrons and 2 electron neutrinos. The positrons almost instantly annihilate the electrons, releasing energy in the form of gamma rays. The neutrinos escape from the star carrying away some energy. The carbon, nitrogen, and oxygen isotopes are in effect one nucleus that goes through a number of transformations in an endless loop.

Stellar life cycle chemistry

The entire cycle of the birth and death of stars promotes chemical reactions. Nuclear fusion in stars produces the elements—H, C, O, N—necessary to make life as we know it.

Why are high-mass stars able to make elements that are heavier than the elements made in low-mass stars?

The higher masses of big stars produce higher core temperatures that enable fusion of heavier elements.

Stellar mass and fusion

The mass of a main sequence star determines its core pressure and temperature. All main-sequence stars are in gravitational equilibrium. Stars of higher mass have higher core temperature and more rapid fusion, making those stars both more luminous and shorter-lived. Stars of lower mass have cooler cores and slower fusion rates, giving them smaller luminosities and much longer lifetimes.

End of Fusion

The naked, inert, hot cores of low-mass stars are called white dwarfs. Eventually, a white dwarf will cool into a black dwarf and disappear from view. Fusion progresses no further in a low-mass star because the core temperature never grows hot enough for the fusion of heavier elements (some helium fuses to carbon to make oxygen). Degeneracy pressure supports the white dwarf against gravity. (A) A white dwarf is composed primarily of carbon and oxygen, held up by electron degeneracy pressure.

Fusion process continues

The process repeats with an inert C core, He-burning shell next, and a H-burning shell on top. As the star expands again, luminosity of the double-shelled star then grows greater than ever; the star moves upward again on the H-R diagram. For a 1 M(sun symbol), the H- and He-burning shells last at best a few million years. Helium burning at this stage is not very stable, and the star's fusion rate will proceed in a series of violent thermal pulses. Because carbon burning requires a core temperature of 600 million degrees, most low-mass stars never get beyond this point. (A) Low-mass stars cannot proceed to carbon burning in the core because they lack the mass to create higher temperatures. (B) A high-mass star can proceed past carbon burning to create an oxygen-neon-magnesium core.

Helium Flash

The thermostat is broken in a low-mass red giant because degeneracy pressure supports the core. Core temperature rises rapidly when helium fusion begins. The helium fusion rate then skyrockets until thermal pressure takes over and expands the core again.

SN1987A Debris Ring

This image shows the entire region around Supernova 1987A. The most prominent feature in the image is a ring with dozens of bright spots. A shock wave of material unleashed by the stellar blast is slamming into regions along the ring's inner regions, heating them up, and causing them to glow. The ring, about a light-year across, was probably shed by the star about 20,000 years before it exploded. Astronomers detected the first bright spot in 1997, but now they see dozens of spots around the ring. The pink object in the center of the ring is debris from the supernova blast. The glowing debris is being heated by radioactive elements, principally titanium 44, created in the explosion. The debris will continue to glow for many decades. The origin of a pair of faint outer red rings, located above and below the doomed star, is a mystery. The two bright objects that look like car headlights are a pair of stars in the Large Magellanic Cloud. The supernova is located 163,000 light-years (50 kpc) away in the Large Magellanic Cloud.

M57, Ring Nebula

This planetary nebula's simple, graceful appearance is thought to be due to perspective—our view from planet Earth looking straight into what is actually a barrel-shaped cloud of gas shrugged off by a dying central star. M57 is 2,000 light- years (0.6 kpc) away from Earth. (B) M57 as it will appear to the visual observer. At 9:00 pm, M57 can be seen from about June 1 to December 1. (C) M57 imaged in color by the Hubble Space Telescope. At the center of M27 is a white dwarf star whose energy makes the nebula glow.

Fate of Earth and Sun

Ultimately, the Sun's luminosity will rise to 1,000 times its current level —too hot for life on Earth. But, long before the Sun leaves the main sequence and becomes a red giant—in about 1.0-1.5 billion years—the luminosity of the Sun will have increased enough to render Earth's surface too hot for life as we know it. Near the end of the Sun's life, its radius will grow to near the current radius of Earth's orbit. Though Mercury and Venus will not survive, Earth may be spared total destruction. At the very least, our planet will become a seared ball of rock.

From Main Sequence to Red Giant

When core hydrogen depletes, fusion stops, the core shrinks, and the outer layers expand outward. The star moves to the right on the H-R diagram, becomes a subgiant and then a red giant, and its luminosity increases. For a 1 M(sun symbol) star, this process takes a billion years (1 × 10<9 y). The star's radius will increase 100X, and its luminosity grows by an even greater factor. Though the core is now helium "ash," the surrounding layers—shell—is still hydrogen, which soon becomes hot enough to sustain fusion. A) HD 149026 is a yellow subgiant star (spectral class G0IV) approximately 257 l-y (78.8 pc) distant in the constellation of Hercules. In 2005, an extrasolar planet was confirmed to be orbiting the star. (CDS/ SIMBAD) (B) Aldebaran, the "eye" of Taurus the Bull, is a KIII red giant star44 times the Sun's diameter, 1.5 times the Sun's mass, and about 500 times the Sun's luminosity. Aldebaran is 65.3 l-y (20 pc distant).

High-mass stars have...

hotter cores faster rates greater luminosities shorter lifetimes