The Moon goes nose to nose with the bull this evening. It stands just below the V-shaped pattern of stars that outlines the head of Taurus the bull. The brightest member of that pattern is Aldebaran, the bull’s orange eye. The other members of the V, including the one at the nose, all belong to the Hyades, one of the most prominent star clusters in the sky.

Aldebaran looks so impressive because it’s a stellar giant. It’s at the end of its life, so it’s puffed up to many times its original size. It’s dozens of times wider than the Sun, and hundreds of times brighter.

Several of the stars of the Hyades are also giants. They don’t look as impressive as Aldebaran, though, in part because they’re more than twice as far away.

One of those giants is Gamma Tauri, the star that marks the bull’s nose. It’s heavier than Aldebaran, but not quite as big or bright. That may be because the two stars are in different phases of their “gianthood.”

Aldebaran has converted the original hydrogen fuel in its core to helium. Now, it’s getting ready to burn the helium to make even heavier elements. Changes in the star during this transition have caused it to expand and brighten.

When it starts burning the helium, it’ll get a little smaller and fainter — and that’s the stage that Gamma Tauri is already in. It’s still big and bright, but not as big and bright as it once was. But it’ll puff up again when the helium is gone — giving the bull a big, bright nose.

Script by Damond Benningfield