Capella: four stars that appear to be one

As I mentioned in the February 1st issue article about the constellation Auriga the Charioteer, the name Capella means “female goat” or “little female goat” in Latin. Like many objects visible to us in the night sky, Capella is not just a single star. It consists of two binary pairs. A binary pair is two stars revolving around a common center, somewhat like two ice skaters holding hands while they spin.

Capella has one bright binary pair and one dim binary pair. We can only see the bright pair, and with our unaided eyes they appear as a single object. This pair of yellow giant stars is only about 42.9 light years from Earth, which contributes to the brightness of the pair.  The individual stars in this pair are known as Capella Aa and Capella Ab. They have magnitudes of 0.76 and 0.91, respectively. Please see the April 6, 2020 issue for more information about magnitude. These two individual stars are separated by a distance of about three-quarters of the distance between the Earth and the Sun (0.74 AU), very close to the distance between Venus and the Sun. They revolve around their common center every 104 days, but do not eclipse each other. This results in a tangential speed of between 35 and 40 kilometers per second. The masses of these two stars are both close to 2.5 times that of our Sun. They have both exhausted their hydrogen fuel. Capella Aa has begun to fuse helium into carbon and oxygen, but Capella Ab lags a bit behind, still expanding and cooling. As gravity overcomes the expansion, Capella Ab will also undergo what is called a helium flash, which begins the same helium fusion process presently taking place in Capella Aa.

The stars in the dimmer binary pair are known as Capella H and Capella L. They are both red dwarfs (with no connection to the SciFi sitcom by that name), each a bit larger than half the mass of the Earth and with magnitudes of 10.2 and 13.7, respectively. Capella H was discovered in 1914 (Ragnar Furuhjelm, Astronomische Nachrichten, 1914) and Capella L was discovered in 1936 as what was described as a “protuberance” on Capella H (Carl Stearns, The Astronomical Journal, 1936). It takes a good telescope with good seeing to see this binary pair!

Just as raindrops or a glass prism split white light into a rainbow of colors, we can split the light from stars. The amount of light at each color, or wavelength, tells us what chemical elements are in the star. Hydrogen is important in astronomy because it makes up about 75% of the mass of the universe and is the main nuclear fuel for all young stars. Iron is one of the elements prominent in Capella Aa. Iron is abbreviated “Fe” for the Latin name for iron, ferrum. Iron atoms contain 26 negatively charged electrons surrounding a nucleus with 26 positive charges. It takes energy to remove electrons from an atom to produce what we call a positive ion. The more electrons removed, the greater the amount of energy required. Here on Earth, iron usually loses only 2 or 3 electrons to form the ions we write as Fe2+ or Fe3+. In Capella Aa’s corona, or outer atmosphere, up to 23 electrons have been pulled off the iron atoms to produce Fe23+. This takes a great amount of energy (about 690 electron-volts) and thus requires a high temperature, around 8,000,000 °C. For those of us who have never given up on the Fahrenheit scale, that would be about 14,400,000 °F. If we tried to grill brats at that temperature, we couldn’t move fast enough to get them on and off the grill before they vaporized. This amount of energy is only available in the X‑ray region of the spectrum. Rocket-borne instruments detected X rays in this range coming from the region of Capella in 1960-1962.  In fact, the discovery of X rays from Capella marked the beginning of X-ray astronomy in 1974 (Richard Catura et al., Astrophysical Journal, 1975).

All four of the stars that comprise the Capella system will eventually end their lives as white dwarf stars. These are stars in which fusion no longer takes place because all of their nuclear fuel has been exhausted and they are simply radiating heat into space. The larger Capella Aa and Ab will reach this point sooner than Capella H and Capella L because the human axiom “live fast, die young” applies to most stars their size. That is, the larger the mass of a star, the faster it will burn through its nuclear fuel and the sooner it will reach the end of its life. Capella Aa and Ab may reach this stage in a few billion years. The smaller red dwarfs Capella H and L fuse their fuel much more slowly and may take several trillions of years to reach the white dwarf stage. As I suggested last month, humankind will likely not be around to see this.