Telling Tails

Many constellations have a star that represents its character’s tail. And the names of many of those stars begin with the Arabic word for tail, Deneb. The most famous are the tail of the swan, known simply as Deneb, and the tail of the lion, Denebola.

Two lesser-known tail stars are in the southern sky this evening: Deneb Algedi, in Capricornus, the sea-goat, and Deneb Kaitos, in Cetus, the whale.

Deneb Algedi is a good bit bigger and hotter than the Sun, and much farther along in life. In fact, it’s probably around the end of its “normal” lifetime — the phase when a star “fuses” the hydrogen in its core to make helium. If so, then it’s getting ready to puff up as it prepares to convert the helium to carbon.

Deneb Kaitos is already burning its helium, so it’s even bigger and brighter than Deneb Algedi. That extra light makes the star clearly visible across almost a hundred light-years of space.

The star will continue to consume the helium for another hundred million years or so. When it’s done, it’ll cast its outer layers into space. That will form a bright bubble known as a planetary nebula. When the nebula dissipates, only the star’s hot, dead core will remain.

Capricornus is in the south at nightfall. It’s especially easy to pick out right now because bright orange Mars is passing through. The sea-goat’s tail is at the left end of a wide triangle. The tail of Cetus is well to their lower left.

Script by Damond Benningfield