Page 29 - Curriculum Visions Dynamic Book

White dwarfs and black dwarfs
When the gas and dust blow away from the planetary nebula, the outer part of the star disappears, leaving behind just the core, which is now called a White dWarf.
A white dwarf star is a faint star that represents the remains of a much brighter star. It has a mass about the same size as the Sun but a size about the same as the Earth, giving it a density about a 1,000 times that of water.
In general, the more massive a white dwarf is, the smaller it is. That is because the greater mass causes a greater gravity to develop, and thus the squashing power of the star is higher. This intense gravity can only be counteracted by the gas left inside the star when it has become very dense. So, the bigger the mass, the smaller the star becomes before the gases can balance gravity.
In the core of a white dwarf is a mixture of carbon and oxygen, and around it is a shell of helium and hydrogen.
A white dwarf is only poorly luminous because its fuel is spent. So, what you see is
the light sent out by the cooling star. No new radiation is being produced, and so the star will get fainter and fainter. Because of the tremendous store of heat and the fact that the outer shell helps insulate the core from heat loss, this does, however, take billions of years. But in the end the star stops sending out any radiation and is called a black dWarf.
Because the amount of light sent out by a white dwarf is low compared with the amount of light at any other stage of the star’s life, white dwarfs can only be seen if they are relatively close to us, perhaps less than a thousand light- years away. Black dwarfs do not shine and are almost invisible.
 White dwarf stars (ringed).
black dWarf A degenerate star that has cooled
so that it is now not visible.
density A measure of the amount of matter in a space.
gravity The force of attraction between bodies. mass The amount of matter in an object.
planetary nebUla A compact ring or oval nebula that is made of material thrown out of a hot star.
radiation The transfer of energy in the form of waves (such as light and heat) or particles (such as from radioactive decay of a material).
White Any star originally of low mass that has reached the end of its life.
29