11. Stellar End States

    • The destiny of a star depends on its mass. With increasing stellar mass these end states are white dwarf stars, neutron stars, and black holes.

    • A red giant star emits strong winds that blow off its outer atmosphere, forming a planetary nebula. The ejected gas exposes a hot, white dwarf core with a low luminosity and high temperature.

    • A white dwarf star is about the size of the Earth and has a mass comparable to that of the Sun. Its high mass density is confirmed by strong gravitational redshifts and intense magnetic fields.

    • White dwarf stars contain carbon nuclei formed in the cores of former red giant stars. Electrons support a white dwarf star by degenerate electron pressure.

    • A star with a mass larger than about 1.5 solar masses cannot become a white dwarf star because the electrons would have to be moving faster than the speed of light to support the star, which is not possible.

    • Novae, or new stars, are binary stars in which mass from a close companion flows onto a white dwarf star, igniting an explosion in the outer atmosphere of the white dwarf star.

    • Supernovae occur when an entire star explodes, and they come in two varieties, Type I and Type II.

    • Type I supernovae occur in binary star systems in which mass overflow from a close star completely blows up its white dwarf companion.

    • Type II supernovae occur when a very massive, isolated star completely runs out of nuclear fuel and blows up.

    • SN 1987A is a Type II supernova that occurred in the nearby Large Magellanic Cloud.

    • Supernovae can be detected with the unaided eye, but such events are rare, occurring in 1006, 1054, 1181, 1572, 1604 and 1987.

    • We can now observe the expanding remnants of supernovae that exploded thousands of years ago. The most famous supernova remnant is the Crab Nebula, the remnant of a supernova explosion that was observed by the Chinese in 1054.

    • The collapsing core of a star that has exploded as a Type II supernova can become a neutron star or a black hole, depending on its mass.

    • A neutron star is about 10 kilometers in radius, or roughly the size of a city.

    • A radio pulsar emits periodic radio radiation with a period of 1 second or less. It is an isolated, rotating neutron star. The period of a radio pulsar increases as time goes on.

    • X-ray pulsars are emitted from neutron stars in binary star systems. The period of an x-ray pulsar decreases as time goes on.

    • Gravitational waves have been inferred from the changing orbit of a binary radio pulsar.

    • A stellar black hole is so dense, and its gravity so strong, that nothing can escape, not even light.

    • A black hole is inferred from a binary star system with an ordinary star seen in visible light and the x-ray emission of material falling into its black hole companion. The orbital properties of the binary system imply a mass of the unseen companion greater than about 3 solar masses, so it cannot be a white dwarf or neutron star.

    • The event horizon, or Schwarzschild radius, of a black hole is the radius within which no events can be seen. It is also known as the gravitational radius.

Copyright 2010, Professor Kenneth R. Lang, Tufts University