A star that’s moving into the final stages of life arcs high across the south tonight. Although the star is bigger and heavier than the Sun, its fate is the same: It will end its life as a hot cosmic cinder called a white dwarf.
The star is Rasalhague, the brightest star in Ophiuchus, the serpent bearer. It climbs into view in the east shortly after darkness falls, but climbs high across the sky during the night.
Rasalhague is actually a binary — two stars that are bound by their mutual gravitational pull. But only one of the stars is visible to the unaided eye. It’s more than twice as massive as the Sun, and about 25 times brighter. It’s much hotter than the Sun as well.
From this profile, astronomers deduce that the star is moving into the penultimate phase of its life: It’s becoming a giant. The star has fused the hydrogen fuel in its core to make helium, so the core is shrinking and getting hotter. Right now, it’s fusing the hydrogen in a thin layer around the core. As part of that process, the star’s outer layers are puffing up, making Rasalhague many times larger — a stellar giant.
When the giant phase ends, the star will cast its outer layers into space, leaving only its dense core — a white dwarf. Rasalhague will no longer produce energy, but it will still shine faintly because it’s extremely hot — the remains of a once giant star.
The Sun will go through the same series of changes — in several billion years.
Script by Damond Benningfield