STELLAR EVOLUTION

1. STELLAR EVOLUTION • Once stars have been created, the time scales and other details of their evolution (and eventual fate) depend primarily on their initial mass, and secondarily on their initial composition. • As discussed previously, the total luminosity of a star increases very rapidly with initial mass (as about M3.5) so high-mass stars have much shorter lifetimes (as M-2.5 ) than do low-mass stars. • However, the details of the energy-generation processes that occur in stars also depend on mass, and to some extent on original composition. • Stars on the “main sequence” of the Hertzsprung-Russell Diagram (lower right to upper left) produce their energy primarily by fusion of hydrogen to produce helium, in their cores. • When the supply of hydrogen for fusion in the central core of a star becomes noticeably depleted, the zone of hydrogen “burning” moves gradually outward.

3. STELLAR EVOLUTION • The central core, now without an internal source of energy, contracts under the force of gravity. • The transition of the energy generation to a hydrogen-burning shell also causes the outer regions of the star to expand, and to become cooler (but with increasing total luminosity), causing the star to become a red giant. • The next stage of stellar evolution begins when the temperature in the core of the star reaches the point where fusion of helium can occur. • This process occurs by a “triple alpha” process, forming first beryllium and then carbon: 4He + 4He  8Be + energy; 8Be + 4He  12C + energy • The initiation of this process results in a “helium flash” which corresponds to the peak luminosity of the star as a red giant. • The star’s luminosity then decreases to a more stable value, on what is called the horizontal branch of the HR diagram.

4. STELLAR EVOLUTION • When the helium in the central core has been converted to carbon, yet another stage of evolution occurs as the shells of hydrogen-burning and helium-burning move outward from the central region. • This causes the star to once again increase in luminosity, along what is known as the asymptotic giant branch (to become a red supergiant). • For all but the most massive stars, carbon is the highest level of thermonuclear fusion that can be reached. • Although the carbon core no longer has an internal source of energy, it cannot contract below a size (comparable to that of Earth) due to electron degeneracy pressure (a result of Pauli’s Exclusion Principle of quantum mechanics). • The end result of stellar evolution (for a low or medium mass star) is that the outer layers of the star are (gradually) blown off, creating a planetary nebula, while the central carbon core becomes a white dwarf.

6. The “Helix” Planetary Nebula in Aquarius

7. STELLAR EVOLUTION • This stage of stellar evolution is characterized by a horizontal transition (in a relatively short time scale) from right to left along the top of the HR diagram. • As the remnant core (white dwarf) cools and the ejected outer layers (planetary nebula) dissipates, the star moves downward and to the right along the “white dwarf” track. • Since the star no longer has any internal energy source, it will gradually cool off to eventually become a “black dwarf” (but this takes millions of years to accomplish, due to the very small size and radiating area of the dead star).

9. EVOLUTION IN STAR CLUSTERS • Since star clusters are groups of stars which formed at about the same time (i.e. in a time period short compared to the lifetime of the cluster), observations of star clusters provide information on the time scales of stellar evolution as a function of the initial mass of the star. • In the Pleiades cluster, we note that the brightest main-sequence stars are of middle B type, indicating that the cluster is relatively young (about 100 million years). • The presence of O- and early B-type stars, as in the Orion Nebula, implies even younger ages (about 10 million years), due to the very short main-sequence lifetimes of these very hot and luminous stars. • On the other hand, globular clusters are implied to be very old (more than 10 billion years in age) by the lack of main-sequence stars earlier than type F, and the large proportion of highly evolved stars (red giants and white dwarfs).

11. EVOLUTION OF HIGH-MASS STARS • Stars with masses greater than about 8 times that of our Sun are not limited, in the late stages of their evolution, to fusion only to carbon in their cores (forming carbon or carbon-oxygen white dwarfs). • Stars in the 8-15 solar mass ranges can fuse carbon to form neon, resulting in neon-oxygen white dwarfs. • Stars more massive than 15 solar masses can suffer a far more catastrophic fate, with fusion to even heavier elements, and eventual core collapse resulting in a supernova explosion. • Supernova explosions can also occur in close binary systems, in which a white dwarf star is fed material by a companion star which is expanding into the giant or supergiant stage of its evolution. • In this latter case, the white dwarf explodes as a result of exceeding the Chandrasekhar limit of 1.4 solar masses.

12. NOVAS AND SUPERNOVAS • A nova, or “new star”, is a sudden flaring-up of a white dwarf star (increasing its brightness by a factor of 10,000 or more) as a result of its being fed gas by a binary companion. • The donation of gas results from the companion star’s evolution from a main-sequence star to a giant star, allowing some of its expanded atmosphere to cross over into the zone of gravitational influence of the white dwarf star. • The feeding of gas is not direct, but by way of an “accretion disk” centered on the white dwarf star. • Supernovas are quite different (and much brighter) than novas, and are of two types, of quite different nature: • Type I supernovas result from the white dwarf in a binary system accreting mass to in excess of the Chandrasekhar Limit of 1.4 solar masses (for an object not supported by internal-energy generation). • This results in the collapse of the core of the (mostly carbon) white dwarf, and instantaneous fusion of the outer carbon layers (also known as a carbon detonation supernova).

13. NOVAS AND SUPERNOVAS • Type II supernovas are quite different from Type I, in that they involve massive supergiant stars which have depleted all core resources for thermonuclear energy generation (their cores consist of iron, which cannot yield energy by either fusion or fission processes). • The collapse of the iron core (under the weight of the outer layers of the star) results in its conversion to neutrons and neutrinos (the former collapse to form a much smaller “neutron degenerate” core, and latter can escape the star altogether). • This, in turn, allows the outer regions of the star to collapse, and then (under the influence of an outwardly-directed shock wave) to create a fusion “detonation wave” which releases as much energy (in a few months) as our Sun produces in its entire lifetime! • Both Type I and Type II supernovas have peak brightnesses of more than 109 that of our Sun, but have differing light curves. • Type I (exploding white dwarf) supernovas do not show hydrogen in their spectra, whereas Type II (exploding supergiant stars) do.

14. Supernova 1987a in the Large Magellanic Cloud Before After

15. Crab Nebula Supernova Remnant

16. The Cygnus Loop is the expanding shell (plus shock-excited interstestellar gas) resulting from a supernova explosion.

17. NEUTRON STARS AND BLACK HOLES • Type II supernovas may leave behind compact objects of even higher density than white dwarf stars (which consist of carbon, oxygen, and neon, and are in an electron-degenerate state). • White dwarf stars cannot be more massive than about 1.4 solar masses (the Chandrasekhar Limit). • A neutron star is the most compact object which can be directly detected by its emission of electromagnetic radiation, and consists of neutrons in their degenerate energy state. • A neutron star is much smaller than a white dwarf (electron degenerate) star, packing more mass than that of the Sun in a volume only about 20 km in diameter. • The maximum mass that a neutron star can have is about 3 solar masses. • In comparison to white dwarf stars, neutron stars are much smaller in size but can be of much higher temperature (of order of a million K) when first formed.

18. NEUTRON STARS AND BLACK HOLES • Neutron stars can also have much higher rotation speeds (many times per second!) in comparison to white dwarf stars. • This gives rise to the observation of radio-emitting objects known as pulsars, which emit radio (and in the case of the Crab Nebula supernova remnant, visible light) pulses at a frequency of many cycles per second. • The neutron star remnant of the Crab Nebula supernova was first identified by its radio pulsations, and later by its visible light and X-ray pulsations. • The Hubble Space Telescope has also imaged a rapidly-moving neutron star (presumed remnant of a supernova explosion). • Black holes, supernova remnant cores with greater than 3 solar masses, cannot be detected by any form of electromagnetic radiation emission, because the escape velocity from these objects exceeds the speed of light! • Black holes can be detected only by their gravitational influences on companion stars.