Capella
What is Capella?
Stars· Distance 42.9 ly · sizes 12× and 9× Sun. The yellow star high in the northern winter sky. It looks like a single star, but it is two yellow giants, each far larger than the Sun, circling each other every 104 days.
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Distance and spectral type
Capella is about 43 light-years away, a red star (spectral type M1:), in the constellation Perseus. Its apparent magnitude is 0.1, visible to the naked eye.
Stellar figures
| Distance | 42.9 ly |
|---|---|
| Magnitude | 0.08 — sixth brightest |
| Members | two yellow giants, 104-day period |
| Constellation | Auriga |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Behind the name
Latin "little she-goat" — Amalthea who nursed Zeus. Actually two yellow giants orbiting every 104 days.
How astronomers told Capella's two hidden giants apart
Capella's two stars sit almost on top of each other in the sky. The angle between them is about a 36,000th of the Moon's width, so an ordinary telescope shows only one point. In 1899 astronomers spread Capella's light into a rainbow-like spectrum and noticed something odd. The dark lines in the spectrum split in two and merged again every 104 days. That was the sign of two stars orbiting each other, one moving toward us while the other moved away.
In 1919 John Anderson at Mount Wilson Observatory in the United States tried a new method. He mounted two mirrors far apart in front of the 2.5-metre telescope and combined the light they collected. A device like this is called an interferometer. The wider apart the mirrors, the closer the pair of points it can separate. With it, Anderson confirmed directly that Capella really is two stars.
Careful measurements later showed the two stars differ in mass by only about 4 percent. Even so, they are at different stages of life. The slightly heavier star is already burning helium in its core, a calm phase. The lighter one has not reached that stage yet, and it spins faster and is more active. Capella shows how even a small difference in mass changes how quickly a star ages.
The star in depth
Capella is the brightest star in Auriga, the Charioteer. At magnitude 0.08 it is the sixth-brightest star in the night sky. It lies 42.9 light-years away, so the light you see tonight set out about 43 years ago. To the eye it is a single yellow point, but it is really two large stars travelling together.
Both stars are giants. A giant is an older star that has used up the hydrogen fuel in its core and swollen up. The two are 12 and 9 times as wide as the Sun, and each has about 2.5 times the Sun's mass. Their surfaces are about as hot as the Sun's, so they shine the same yellow colour. They are closer to each other than Earth is to the Sun, which is why they complete an orbit in just 104 days. Much farther out, a pair of small red stars also belongs to the system, making Capella a family of four.
The name is Latin for 'little she-goat'. Auriga is drawn as a charioteer carrying a goat on his shoulder, and Capella marks the goat. From London it never sets, and from New York it dips below the northern horizon only briefly, so it can be seen almost all year. Around midnight in midwinter it passes nearly overhead, and with the bright stars around Orion and Taurus it forms the Winter Hexagon. In the Sky view, choose a January night and look almost straight up to find the yellow star.
Frequently asked
- How far is Capella from Earth?
- The distance of Capella is 42.9 ly.
- How bright is Capella (apparent magnitude)?
- The magnitude of Capella is 0.08 — sixth brightest.
- Which constellation is Capella in?
- Capella lies in the constellation Perseus.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.