Chapter 12 Part 2


More on the Life of Stars

Sun-Like Stars

When the Sun reaches the real end of its life, the core is continuing to increase in luminosity as the helium burns. The increased energy output pushes the outer envelope off into space, producing a planetary nebula. Your book has some very good pictures on page 338. By the way, these lovely objects got their name from their tendency to look like planets in small telescopes.

The core, relieved of the outer layers, can expand, cool off some, and fairly quickly stop burning. It begins the process of shrinking into a white dwarf.

The star is now in two parts - the white dwarf in the center and the expanding shell outside. If you look at the top 3 planetary photographs on page 338 you can easily see the white dwarf right in the center.

White Dwarfs

Ordinary Novae

There is a special future in store for some white dwarfs. While single white dwarfs simply cool off very slowly, white dwarfs in binary systems have another possibility. If the two components of the binary are close together AND the other component (normal star) starts its metamorphosis into a red giant, things can get interesting. If the red giant gets large enough for its outer fringes to fall within the gravitational influence of the white dwarf, the dwarf will start stealing hydrogen from its mate. This hydrogen will accumulate on the surface of the dwarf. When the temperature of this hydrogen finally exceeds 10 million K, it ignites and fuses. This is different from a normal star which burns hydrogen in its core; this hydrogen burning is on the surface. The dwarf flares up brightly, increasing its luminosity on the order of 10 magnitudes, or 10,000 times. Such an event is a nova. It does NOT destroy the white dwarf, which lives on and can repeat the process. This produces a recurrent nova, which can repeat many times.

One of these appeared in the summer of 1975 in the constellation Cygnus. It was known as Nova Cygni 1975 and reached apparent magnitude 1. It was very obvious as a bright star that didn't belong. The bright star above and to the right of the nova (at arrow) is the mag 1 star Deneb.

Supernovae - Type I

The same circumstance of a white dwarf in a binary system can produce something even more spectacular. If the white dwarf is one of the larger ones, the extra hydrogen mass could push its mass near 1.4 solar masses. At this mass, gravity compresses the degenerate matter to the point where, somewhere in the body of the dwarf, the carbon atoms begin to fuse (burn). The degenerate material is not behaving as a gas, so it does not expand quickly with the additional energy output. The burning spreads through the dwarf vary fast, initiating the explosion of a 1.4 solar mass carbon bomb. This is the Type Ia supernova. A white dwarf will do this only once, as it is blown to fragments in the process. This type of supernova is also easily recognizable; its spectrum shows no significant amount of hydrogen.

Supernovae are very bright. They can be easily seen when they go off in distant galaxies, like supernova 1994D in the galaxy NGC 4526 at a distance of 20 Mpc.

You will notice that Type Ia supernova is NOT shown in the life cycle of a star. It requires the special circumstance of a close binary system. This is why the Sun will never explode as a Type Ia, even after it has become a white dwarf.

The Type Ia supernova is actually very useful. Since we think that the explosions are similar (same thing every time), that means that the absolute magnitudes should be similar. This has been calibrated by looking at Type Ia events in nearby galaxies where we can find the distances by other means. The supernova therefore serves as a standard candle. This refers to any object whose absolute magnitude (M) can be known. Notice that we said can be known, not can be measured. It means that, once a standard candle object is identifed, the astronomer knows what is absolute maginude is. Combine that with measured apparent magnitude (m) and you can calculate the distance.

This image is a composite from 4 telescopes (Image credit: NASA/ESA/JPL-Caltech/UCLA/CXC/SAO). It shows the remains of a Type Ia supernova that was seen and recorded by Chinese observers in A.D 185. This appears to be the earliest documented observation of a supernova explosion.