White Dwarfs 
When Alvan Graham Clark discovered a faint companion to Sirius 150 years ago, the newly found star was quite a puzzler.
The companion, known as Sirius B, hadn’t been seen before because it was hidden in the glare of Sirius itself, which is the brightest star in the night sky. Sirius B was only one ten-thousandth as bright as Sirius A, but it took decades to figure out why: although Sirius B is as massive as the Sun, it’s only as big as Earth.
Today, objects like Sirius B are known as white dwarfs. They’re the final stage of life for stars that are up to a few times the mass of the Sun.
For most of its life, a battle rages in the star’s core — a battle between pressure produced by the heat from nuclear reactions, which pushes outward, and gravity, which pulls inward. When the star exhausts its supply of fuel, though, the nuclear reactions are extinguished. Without the outward pressure provided by these reactions, gravity squeezes the core tighter and tighter, making it smaller and smaller.
But there’s a limit to how much the core can be squeezed. The negatively charged particles known as electrons exert a pressure of their own. As they get closer together, this pressure halts the core’s collapse. Even so, the white dwarf is quite dense. In the case of Sirius B, for example, a chunk of matter the size of a sugar cube would weigh about five tons.
We’ll have more about white dwarfs tomorrow.
Script by Damond Benningfield, Copyright 2011