The Sing Off

Why is Capella yellow and Sirius blue, while Capella is more massive?

Why is Capella, a cluster of 4 star in which the two biggest are two yellow giants, yellow? The biggest star in Capella is 2.7 times the mass of our Sun. Vega and Sirius are about 2 times the mass of our Sun, but they are white blue stars, and much more hotter than Capella. So why isn't Capella shrinking into a blue main sequence star?

Public Comments

1. Sirius is blue because it is hotter than Capella, which in turn is hotter than red stars like Aldebaran or Betelgeuse. Star color depends only on star temperature as we see it.
2. Basically because it has used up it's supply of hydrogen and is in the process of becoming a red giant. As you said, although it appears to be a single star to the naked eye, it is actually a star system of four stars in two pairs. The stars of the brighter pair were probably of spectral class A during their main-sequence lifetime, similar to Vega, but they are now expanding, cooling, and brightening to become red giants, a process that will take a few million years. It is thought that the more massive star has begun fusing helium to carbon and oxygen at its center, a process that has not yet begun for the less massive star.
3. Capella is twice the age of Sirius, it is further in its evolution and is entering its red giant phase, with a core full of helium. Not only it is older, but it is aging faster due to its higher mass.
4. A matter of age rather than mass.