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Making a White Dwarf

Week of December 30, 1998

Is anyone that reads these pages old enough to remember the group 'The Alan Parson's Project'? In one of their songs, they had the line 'Even the brightest star won't last forever.' As it happens, bright stars (that is, stars that give off the most amount of energy) actually last the shortest amount of time. Even though they are typically more massive than the Sun, they burn their fuel so much faster that their lives are much shorter.

For a long time no one was sure just what happened inside the core of a massive star when it uses up its fuel. The situation is complicated, and the details of just what occurs inside the core aren't important for this particular Snack. What is important is that in a normal star, the core wants to collapse because of its own tremendous gravity. What holds it up are two things: heat, and the fact that like charges repel each other. Heat holds it up because for all its mass, the core of a star is still just a fancy type of gas, and a hot gas wants to expand. That helps counteract gravity. Also, the core is so hot that the electrons in the gas are stripped off their atoms, and those electrons repel each other. That also supports the core against collapse.

As the fuel gets used up in a star like the Sun, the outer layers of the star get blown off. The core becomes exposed to space (where we can see it from Earth), but it no longer has enough fuel to generate heat. The core begins to cool, and the only thing holding it up is that electron pressure. We call these kinds of stars white dwarfs because they are small and hot. At those densities, normal physics doesn't work very well, and you have to start worrying about quantum mechanics. As it turns out, the core of a star has a maximum size it can be and still be a white dwarf, and the physics behind calculating that size is fierce. The first person to work it out was the extraordinarily brilliant Indian astrophysicist Subramanyan Chandrasekhar, who found that the core of a star must be below about 1.4 times the Sun's mass to be a white dwarf (what happens if it's higher than that? Ah, that's another Snack). Even today, many decades later, no white dwarf is known with a mass higher than what is known as the Chandrasekhar Limit.

Chandrasekhar was an amazing person, and his contribution to the physics of astronomy is formidable. He died in 1995, and to honor him NASA has renamed their new satellite -- formerly called Advanced X-ray Astrophysics Facility or AXAF -- the Chandra X-Ray Observatory. 'Chandra' was what his friends called him; if that name sounds familiar it was also the name of the man who invented HAL in the movie/book '2001: A Space Odyssey'. The CXRO satellite is designed to look at astronomical objects that emit X-rays. This type of emission is usually generated by very hot or energetic events, and white dwarfs certainly qualify for that (the hottest one known is at about 200,000 degrees Celsius). It's a fitting tribute to the man who helped open up astronomy to the bizarre universe of quantum mechanics.

©2008 Phil Plait. All Rights Reserved.

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