You are here

Moon and Aldebaran

July 21, 2014

A star is the product of a delicate balancing act between gravity and radiation. They remain in balance for most of a star’s life. But as the star ages, the balance changes, causing big changes in the star itself.

An example of that process is visible at dawn tomorrow. Aldebaran, the bright orange eye of Taurus, stands just below the Moon.

Gravity squeezes a star tightly, smashing its gas together into a compact ball. At the middle of the ball, the gas is heated to millions of degrees — hot enough to ignite nuclear fusion. The fusion produces radiation, which pushes outward, keeping the star in balance.

As the star ages, though, it uses up the hydrogen fuel in its core, leaving an “ash” of helium. The helium doesn’t fuse at the same temperature as hydrogen, though. Without nuclear fusion, there’s no radiation to push outward, so gravity squeezes the core even tighter. As the core shrinks, it gets hotter.

Eventually, it gets hot enough to ignite fusion in the helium. That achieves a new balance — one with a smaller, hotter core. The radiation from the hotter core exerts a stronger outward pressure, causing the surrounding layers of gas to puff outward like a giant balloon.

And that’s what’s happened to Aldebaran. The star has used up its hydrogen, so it’s now beginning to burn its helium. So Aldebaran has puffed up to many times the size of the Sun, causing its outer layers to cool — giving the bull a glowing orange eye.


Script by Damond Benningfield, Copyright 2014

Get Premium Audio

Listen to today's episode of StarDate on the web the same day it airs in high-quality streaming audio without any extra ads or announcements. Choose a $8 one-month pass, or listen every day for a year for just $30.