Star Formation - 6 (Chapter 5 – Universe)

Death of Stars REVIEW

Key Ideas Stellar Evolution: Because stars shine by thermonuclear reactions, they have a finite life span. The theory of stellar evolution describes how stars form and change during that life span. Mass Loss by Protostars: In the final stages of pre–main-sequence contraction, when thermonuclear reactions are about to begin in its core, a protostar may eject large amounts of gas into space. Low-mass stars that vigorously eject gas are called T Tauri stars.

Key Ideas The Main-Sequence Lifetime: The duration of a star’s main-sequence lifetime depends on the amount of hydrogen available to be consumed in the star’s core and the rate at which this hydrogen is consumed. The more massive a star, the shorter its main-sequence lifetime. The Sun has been a main-sequence star for about 4.56 billion years and should remain one for about another 7 billion years.

Key Ideas Becoming a Red Giant: Core hydrogen fusion ceases when the hydrogen has been exhausted in the core of a main-sequence star with mass greater than about 0.4 M. This leaves a core of nearly pure helium surrounded by a shell through which hydrogen fusion works its way outward in the star. The core shrinks and becomes hotter, while the star’s outer layers expand and cool. The result is a red giant star. As a star becomes a red giant, its evolutionary track moves rapidly from the main sequence to the red-giant region of the H-R diagram. The more massive the star, the more rapidly this evolution takes place.

Key Ideas Helium Fusion: When the central temperature of a red giant reaches about 100 million K, helium fusion begins in the core. This process, also called the triple alpha process, converts helium to carbon and oxygen. In a more massive red giant, helium fusion begins gradually; in a less massive red giant, it begins suddenly, in a process called the helium flash. After the helium flash, a low-mass star moves quickly from the red-giant region of the H-R diagram to the horizontal branch.

Key Ideas Late Evolution of Low-Mass Stars: A star of moderately low mass (about 0.4 M to about 4 M) becomes a red giant when shell hydrogen fusion begins, a horizontal-branch star when core helium fusion begins, and an asymptotic giant branch (AGB) star when the helium in the core is exhausted and shell helium fusion begins. Planetary Nebulae and White Dwarfs: Helium shell flashes in an old, moderately low-mass star produce thermal pulses during which more than half the star’s mass may be ejected into space. This exposes the hot carbon-oxygen core of the star. No further nuclear reactions take place within the exposed core. Instead, it becomes a degenerate, dense sphere about the size of the Earth and is called a white dwarf. It glows from thermal radiation; as a white dwarf cools, it becomes dimmer.

Key Ideas Late Evolution of High-Mass Stars: Unlike a moderately low-mass star, a high-mass star (initial mass more than about 4 M) undergoes an extended sequence of thermonuclear reactions in its core and shells. These include carbon fusion, neon fusion, oxygen fusion, and silicon fusion. The Deaths of the Most Massive Stars: A star with an initial mass greater than 8 M dies in a violent cataclysm in which its core collapses and most of its matter is ejected into space at high speeds. The luminosity of the star increases suddenly by a factor of around 108 during this explosion, producing a supernova.

Key Ideas Other Types of Supernovae: An accreting white dwarf in a close binary system can also become a supernova when carbon fusion ignites explosively throughout such a degenerate star. This is called a thermonuclear supernova. A Type Ia supernova is produced by accreting white dwarfs in close binaries. A Type II supernova is the result of the collapse of the core of a massive star, as are supernovae of Type Ib and Type Ic; these latter types occur when the star has lost a substantial part of its outer layers before exploding.

Key Ideas Neutron stars form from supernova of original M-S stars greater than 8 M Neutron Stars: A neutron star is a dense stellar corpse consisting primarily of closely packed degenerate neutrons. A neutron star typically has a diameter of about 20 km, a mass less than 3 M, a magnetic field 1012 times stronger than that of the Sun, and a rotation period of roughly 1 second. Pulsars: A pulsar is a source of periodic pulses of radio radiation. These pulses are produced as beams of radio waves from a neutron star’s magnetic poles sweep past the Earth.

Key Ideas Black Holes: form from supernova of original M-S stars greater than 8 M If a stellar core has a mass greater than about 2 to 3 M, gravitational compression will overwhelm any and all forms of internal pressure The stellar corpse will collapse to such a high density that its escape speed exceeds the speed of light.