The head of the serpent slithers into the early 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 rises first. It’s in the east and southeast at nightfall, marked by a serpentine trail of faint stars. The tail, which is below Ophiuchus, climbs into view about an hour later.

The brightest of the stars of Serpens is Unukalhai — an Arabic name that means “the serpent’s neck.” The star also is 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 caused the star’s outer layers to puff up like a balloon. That’s made the star about 14 times wider than the Sun. The expansion 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 bright neck.

More about the serpent tomorrow.

Script by Damond Benningfield