Capella | Universe Today

Written by Jerry Coffey

Capella, also known as Alpha Aur, is the brightest star in Auriga constellation. It is the sixth brightest star in the night sky and the third brightest in the Northern Celestial Hemisphere behind Arcturus and Vega. Capella appears as a single star, but is actually a four star system consisting of two binary pairs. The stars of the Capella system are relatively close. The system is only 42.2 light years or 12.9 parsecs from Earth. Capella's northern declination is such that it is actually invisible in southernmost New Zealand, Argentina, Chile, and the Falkland Islands. On the other hand, it is circumpolar north of 44°north: for the whole United Kingdom, Scandinavia, most of Canada, and the northernmost United States, the star never sets.

As I said earlier, Capella is a system of four stars. The first binary pair are two bright, large G-type stars, each has a radius that is around 10 times the Sun's. They are in close orbit around each other. G-type stars are usually yellow and are mid-range as far as temperature goes. Many astronomers think that these two stars are cooling and expanding their way to becoming red giants. The second binary pair is about 10,000 astronomical units from the first. These two stars are faint, small, cool red dwarfs. The stars in the Capella system are members of the Hyades moving group which means that they are part of a group of stars moving in the same direction as the Hyades cluster.

Much more time has been devoted to studying the bright binaries of the Capella system. The system was announced to be binary in 1899 based on the belief that there were only two stars total. These were the first two stars outside of our solar system to be studied by interferometry. Capella also became the first astronomical object to be imaged by a separate element optical interferometer when it was imaged by the Cambridge Optical Aperture Synthesis Telescope in September 1995. The primary star has a surface temperature of 4900 kelvin, a radius of approximately 12 solar, a mass of approximately 2.7 solar, and an approximate luminosity of 79 times that of our Sun. The secondary star has a surface temperature of 5700 kelvin, a radius of 9 solar, a mass of approximately 2.6 solar, and an approximate luminosity of78 times our Sun. The primary is the brighter star when considering radiation at all wavelengths, but it is the fainter when observed in visible light. It has an apparent magnitude of approximately 0.91. The secondary's apparent magnitude of 0.76. The two components orbit each other at a distance of around 100 million kilometers and an orbital period of approximately 104 days.

In 1914 the fainter component of Capella was seen for the first time. It wasn't until 1936 that it was discovered to be a binary pair itself. Although this pair has only been observed to cover approximately 30° of its orbit, a rough, preliminary orbit has been computed, giving an orbital period of approximately 400 years. That sums up the little information known about the fainter pair.

There is a good Wiki about Capella. Here on Universe Today we have a great article about the Auriga constellation and another about the brightest stars in the night sky. Astronomy Cast offers a good episode about the technology used to look at the brightness of a star over different wavelengths.

Filed under: Astronomy