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The head of the serpent is slithering into the evening sky this month, with its tail twisting along a bit later.
Serpens is the only constellation that’s split apart. The two halves are separated by Ophiuchus, the serpent bearer. The snake’s head is to the west of Ophiuchus, so it rises first. It’s low in the eastern sky as night falls, marked by a serpentine trail of faint stars. The tail, which is to the east of Ophiuchus, follows a couple of hours later.
The brightest of the snake’s stars is Unukalhai — an Arabic name that means “the serpent’s neck.” It’s also known as Alpha Serpentis, indicating its ranking as the constellation’s leading light.
The star is in the final stages of life. It converted the hydrogen fuel in its core to helium, causing the core to shrink and get hotter. That triggered the next round of nuclear reactions, with the helium being converted to carbon and oxygen.
The changes in the core have also caused the thick layers of gas around the core to puff up like a balloon, making the star about 15 times wider than the Sun. They’ve also made the star’s surface much cooler, so it shines yellow-orange.
In time, all the reactions in the star’s core will stop, and the outer layers will puff out into space. For a while, that expanding cloud will form a colorful bubble. As the bubble cools and dissipates, though, only the star’s dead core will remain — depriving the serpent of its leading light.