White Dwarfs

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White dwarfs form as the outer layers of a low-mass red giant star puff out to make a planetary nebula. Since the lower mass stars make the white dwarfs, this type of remnant is the most common endpoint for stellar evolution. If the remaining mass of the core is less than 1.4 solar masses, the pressure from the degenerate electrons (called electron degeneracy pressure) is enough to prevent further collapse.

what holds degenerate stars up

Because the core has about the mass of the Sun compressed to something the size of the Earth, the density is tremendous: around 106 times denser than water (one sugar cube volume's worth of white dwarf gas has a mass > 1 car)! A higher mass core is compressed to a smaller radius so the densities are even higher. Despite the huge densities and the ``stiff'' electrons, the neutrons and protons have room to move around freely---they are not degenerate.

White dwarfs shine simply from the release of the heat left over from when the star was still producing energy from nuclear reactions. There are no more nuclear reactions occurring so the white dwarf cools off from an initial temperature of about 100,000 K. The white dwarf loses heat quickly at first cooling off to 20,000 K in only about 100 million years, but then the cooling rate slows down: it takes about another 800 million years to cool down to 10,000 K and another 4 to 5 billion years to cool down to the Sun's temperature of 5,800 K.

Their rate of cooling and the distribution of their current temperatures can be used to determine the age of our galaxy or old star clusters that have white dwarfs in them. However, their small size makes them extremely difficult to detect. Because it is above the atmosphere, the Hubble Space Telescope can detect these small dead stars in nearby old star clusters called globular clusters. Analysis of the white dwarfs may provide an independent way of measuring the ages of the globular clusters and provide a verification of their very old ages derived from main sequence fitting. Select the image below to enlarge it (will display in another window).

white dwarfs in a globular cluster
HST image

Novae and Supernovae Type Ia

An isolated white dwarf has a boring future: it simply cools off, dimming to invisibility. White dwarfs in binary systems where the companion is still a main sequence or red giant star can have more interesting futures. If the white dwarf is close enough to its red giant or main sequence companion, gas expelled by the star can fall onto the white dwarf. The hydrogen-rich gas from the star's outer layers builds up on the white dwarf's surface and gets compressed and hot by the white dwarf's gravity.

Eventually the hydrogen gas gets dense and hot enough for nuclear reactions to start. The reactions occur at an explosive rate. The hydrogen gas is blasted outward to form an expanding shell of hot gas. The hot gas shell produces a lot of light suddenly. From the Earth, it looks like a new star has appeared in our sky. Early astronomers called them novae (``new'' in Latin). They are now known to be caused by old, dead stars. The spectra of a nova shows blue-shifted absorption lines showing that a hot dense gas is expanding towards us at a few thousands of kilometers per second. The continuum is from the hot dense gas and the absorption lines are from the lower-density surface of the expanding cloud. After a few days the gas has expanded and thinned out enough to just produce blue-shifted emission lines.

After the nova burst, gas from the regular star begins to build up again on the white dwarf's surface. A binary system can have repeating nova bursts. If enough mass accumulates on the white dwarf to push it over the 1.4 solar mass limit, the degenerate electrons will not be able to stop gravity from collapsing the dead core. The collapse is sudden and heats the carbon and oxygen nuclei left from the dead star's red giant phase to temperatures great enough for nuclear fusion. The carbon and oxygen quickly fuse to form silicon nuclei. The silicon nuclei fuse to create nickel nuclei. A huge amount of energy is released very quickly with such power that the white dwarf blows itself apart. This explosion is called a Type Ia supernova to distinguish them from the other types of supernova that occurs when a massive star's core implodes to form a neutron star or black hole.

Type I supernovae happen in close binary systems and do not show strong hydrogen emission lines. Type I (especially Ia) supernova create most of the iron and nickel found in the interstellar medium. Type II supernovae happen in single star systems (or at least far enough away from any companion star to retain their hydrogen outer layers) and have strong hydrogen emission lines. Type II create most of the oxygen found in the interstellar medium. Type Ia supernovae are several times more luminous than Type Ib, Ic, and Type II supernovae, leave no core remnant behind, and result from when a low-mass star's core remnant (a white dwarf) detonates. They have a strong ionized silicon emission line at 615 nm. Type Ib and Ic supernovae result from the collapse of a massive star's core whose outer hydrogen layers have been transferred to a companion star or blown off from strong winds which is why they do not show hydrogen emission lines. Type Ib have strong helium emission lines and Type Ic do not.

Since the Type Ia supernova form from the collapse of a stellar core of a particular mass, rather than the range of core masses possible for the other types of supernova, the Type Ia supernova are expected to have the same luminosity. The distances to very luminous objects can be derived using the inverse square law of light brightness if their luminosity is known. Because of their huge luminosities, the Type Ia supernovae could be used to measure distances to very distant galaxies. In practice there is a range of luminosities for the Type Ia, but the luminosity can be derived from the rate at which the supernova brightens and then fades—the more luminous ones take longer to brighten and then fade. Astronomers using Type Ia supernova to measure distances to very distant galaxies have come to some surprising conclusions about the history and future of the universe (see the cosmology chapter for more about that).

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last updated: June 6, 2019

Is this page a copy of Strobel's Astronomy Notes?

Author of original content: Nick Strobel