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Moon and Aldebaran

July 11, 2015

When a star like the Sun reaches the end of its “normal” lifetime, its core takes a little nap. But the nap doesn’t last long. And it ends in a flash — a helium flash.

One star that’s already gone through that process — or is just about to, depending on which source you read — is in good view at dawn tomorrow. Aldebaran stands close to the lower left of the Moon. It’s the bright orange eye of Taurus, the bull.

For most of their long lifetimes, stars like Aldebaran and the Sun are powered by the fusion of hydrogen atoms in their core. The atoms stick together to form heavier helium atoms, releasing a lot of energy in the process.

When the hydrogen in the core has all been converted to helium, the fusion stops. Without the radiation from the fusion reactions, though, gravity takes over and squeezes the core tighter and tighter, making it even hotter. Eventually, it gets hot enough to ignite the helium, starting a new series of nuclear reactions.

That ignition point is known as the helium flash. It races through the entire core in just a few seconds. If the core were exposed to view, it would briefly outshine all the other stars in the galaxy combined. But the core is surrounded by thick layers of hydrogen, which absorb that surge of energy and hide it from view.

After the flash, the core expands a bit, with the helium continuing to burn at a more leisurely rate — a process that may continue for hundreds of millions of years.

Script by Damond Benningfield, Copyright 2015

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