How big is Capella?

Capella is the sixth-brightest star in the night sky, and it is really two giant stars orbiting each other. The larger of the two, the one the explorer shows, is about 17 million kilometers (10 million miles) across, about 12 times the width of the Sun. That is the width of the star's visible surface.

Size

Capella's two stars are only about 56 thousandths of an arcsecond apart in the sky, where an arcsecond is 1/3,600 of a degree. They were first clearly seen as two in 1919, with an interferometer on the 100-inch telescope at Mount Wilson Observatory. Interferometers combine light collected at different points to see very fine detail, and they can also measure how wide each star looks. The distance then turns each apparent width into a true size.

A 2015 study led by Guillermo Torres found that the larger star's radius is 11.98 times the Sun's, making it about 16.7 million kilometers across. The figure is uncertain by about 5%, because the stars' apparent widths are hard to measure. The smaller star is about 12.3 million kilometers across, about nine times the width of the Sun.

Capella's distance is known much better, to about 0.1%. It comes from the pair's orbit: comparing the orbit's true size with how large it looks in the sky shows how far away it is. The answer is about 42.9 light-years, where a light-year is the distance light travels in one year, about 9.46 trillion kilometers.

Size comparisons

The larger star is about 12 times as wide as the Sun, and about 1,300 times as wide as Earth.
About 1,700 Suns would fit inside the larger star, by volume.
If the larger star took the Sun's place, its surface would reach about one-seventh of the way to the orbit of Mercury.
Alnitak, the next larger star in the explorer, is about 1.7 times as wide as Capella's larger star.
If the Sun were shrunk to the size of a basketball, Capella's two stars would be balls about 2.9 meters (9.5 feet) and 2.1 meters (6.9 feet) across, with their centers 19 meters (62 feet) apart.

Two giants in one orbit

The two stars circle each other every 104 days, in a nearly circular orbit. Their centers stay about 111 million kilometers apart, a little more than the distance between the Sun and Venus. The larger star has 2.57 times the Sun's mass, and the smaller one 2.48 times.

Although it is bigger, the larger star is the cooler of the two. Its surface is about 4,700 degrees Celsius (8,500 degrees Fahrenheit), while the smaller star's is about 5,500 degrees Celsius (9,900 degrees Fahrenheit). The hotter, smaller star is slightly brighter in visible light.

The two stars are at different stages of life, even though they are both about 600 million years old. The larger star is near the end of a stage in which it burns helium in its core. The smaller star is in a short-lived stage, changing quickly on its way to becoming a red giant.

A family of four

Capella has two more companions: a pair of small, cool stars called red dwarfs, named Capella H and Capella L. Capella H is at least 9,500 astronomical units from the bright pair. One astronomical unit is the average distance from Earth to the Sun, about 150 million kilometers. Capella L orbits Capella H, perhaps once every 300 years, which makes Capella a system of four stars.