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Moon and Aldebaran
For most of its long lifetime, a star like the Sun is pretty stable. It steadily “fuses” the hydrogen in its core to make helium, releasing energy in the process. That keeps it shining steadily for billions of years.
As it uses up the hydrogen, though, things change.
Aldebaran, the bright orange eye of Taurus, is going through the first of those changes right now. The star rises to the lower left of the Moon before midnight, and is closer to the Moon at first light tomorrow.
Aldebaran has already converted the hydrogen in its core to helium. But the core isn’t hot enough for the helium to fuse together to make heavier elements. Instead, fusion probably is taking place in a thin layer of hydrogen around the core. That’s pushed outward on the surrounding layers of gas, causing Aldebaran to puff up to more than 40 times the diameter of the Sun. That’s caused the surface to cool, so it glows reddish-orange.
Over the next few million years, Aldebaran will get even bigger and brighter. At the same time, its core will get smaller and hotter. Eventually, it’ll get so hot that the helium will ignite. That will begin a new round of fusion reactions, which will create carbon and oxygen.
The star will burn through the helium quickly. When it’s gone, fusion will shut down for good. Aldebaran’s outer layers will puff out into space, leaving only its hot but dead core, which will spend many billions of years cooling and fading from sight.