White Dwarfs - Planetary Nebulae

by Jeanette Cain

A white dwarf is a tiny, hot star that is within the center of all planetary nebulae. The original was a red giant, and this is its burned-out core. The white dwarf is made of carbon and oxygen, which was made with the helium burning reactions of the star. White dwarfs are no longer capable of producing energy, which causes them to decrease in size to a very small amount. White dwarfs usually have the mass of our Sun, but are compressed to about the size of Earth. There may be as much as ten percent of white dwarfs living in our galaxy, but being faint may not be visible.

The Chandrasekhar Limit, discovered in 1930 by Subrahmanyan Chandrasekhar, says that white dwarfs cannot have a mass larger than 1.4 times the mass of the Sun. He explained that the heavier a white dwarf is, the more it is crushed by its own gravity. Should the core of a burned out star be heavier than 1.4 solar masses, it would collapse to create a black hole, or neutron star.

Planetary nebulae are similar to a flower in bloom. A swollen red giant dies and the outer layers have been blown away into a cloud that expands and shines for tens of thousands of years. Any star having a mass of up to eight times the mass of the Sun will end in this manner. The blown away layers spread out producing shells and glowing rings. As the nebula gradually fades, the white dwarf is revealed and the remaining core will not disappear until billions of years later when it has cooled.

One of the easiest planetary nebulae to be seen without the aid of a telescope is the Ring Nebula in Lyra, which is southeast of Vega. It resembles a faint smoke ring on a moonless night. The Minkowski 2-9 is a butterfly nebula. It is believed that the white dwarf at the center is pulling material from a larger companion star. Minkowski 2-9 is within the Ophiuchus constellation and is around 2,100 light years away from Earth. It was created when a red giant blew away the outer layers at over 300 kilometers per second, and the disk deflected the materials in two directions.

The Cat's Eye Nebula, around 1,000 years old, is one of the more complex planetary nebulae. William Herschel named it due to the round clouds looking like planets. The Cat's Eye Nebula is the result of a red giant with no helium to burn causing the core to shrink, but the star will expand once again. This latter expansion is so quick that it causes the outer layers to blow off and away into space.

Sources:

1. Couper, Heather and Nigel Henbest. Space Encyclopedia DK Publishing, Inc.: NY 1999

2. Editors. Secrets of the Universe. International Master Publishing: US. 1999