White dwarfs are very strange types of stars. They are the degenerate remains of smaller stars which simply dwindle away as time progresses becoming less and less luminous all the while. There is not enough pressure to blow up the star, and it will not get hot enough to burn elements beyond carbon, so it just dies off in a pretty haze of gas.

The planetary nebula NGC 2440 is pictured here. The white dwarf (the bright spot in the middle of the image) gives off so much energy in different forms that it heats and lights up the gas that it earlier blew off of itself, and creates this beautiful spectacle.	White dwarfs are made up, almost entirely, of ionized carbon and oxygen, and little to no hydrogen. Most are about the same mass as our Sun, but smaller (sometimes several orders of magnitude smaller) than the Earth. Obviously, this means that white dwarfs are incredibly dense. So dense, in fact, that a teaspoon of white dwarf material would weigh over 16 tons on Earth, and almost 500,000 times that on the surface of the star. They range in temperature from 5000-80,000 K, which means that they actually come in many colors, not just blue-white. Since they spin quickly (conservation of angular momentum) and generate a lot of energy, they light up the hydrogen and helium that they have expelled from their outer shells. The result is known as a planetary nebula, even though it has nothing to do with planets. It is just a very diffuse gas, but it is brightly lit by the white dwarf progenitor that blew it away.
It is possible for a white dwarf to have a very dramatic ending to its life, if it happens to be a member of a binary system. This means that there are two stars orbiting one another. About half the stars that we observe are in binary systems, so this is a fairly common state in which to observe stars. There are three different types of novae that can occur if the accretion rate onto the white dwarf is not constant or gets too high. Dwarf Novae Classical Novae Type Ia Supernovae Star survives? yes yes no Increase in brightness 10 times 106 times Up to brightness of an entire galaxy Initial mass 0.85 solar masses 0.85 solar masses 1.2 solar masses A dwarf nova brightens by about 2-6 magnitudes in 5-20 days. There are quiet periods when the star stays around the same brightness which last for 30-300 days. Every so often, when the mass transfer rate onto the star increase by 10-100 times, the star gets brighter, and then dims down again. A classical nova increases in brightness by 7-20 magnitudes (the average is about 11) and then dims again. The rise in luminosity is rapid, then constant just before peaking, then it peaks, and then declines in either a slow or fast fashion. This decline lasts for a couple of weeks up to hundreds of days, and is accompanied by the ejection of gas. A type Ia supernova is a supernova explosion, just like those that occur to massive stars, except that strong Si II lines are observed. Around 1.3 solar masses the star begins to burn carbon, and continues to burn elements until it reaches the iron peak. Type Ia supernova are responsible for the majority of the elements around the iron peak. They are known as standard candles because their absolute luminosity is very similar for all of the explosions, namely -19.6 +/- 0.2 (in the blue). This means that the distance to the supernova can be easily determined so long as the apparent magnitude is accurately measured.

Back

photograph: H. Bond (STSci), R. Ciardullo (PSU), WFPC2, HST, NASA.