We've discussed observing Capella in the context of the sky and the seasons, and its name and lore in the context of history and story. Now let's turn to the topic of Capella's nature in space. In this respect, it turns out to be one of the most distinctive and interesting of all the brightest stars.
First, the vast majority of the point of light we call Capella is coming from not one but two stars. That's not unusual among the brightest stars. More distinctive (but still not unique among the brilliant stars) is how similar Capella A and B are to each other. They are truly twins in size—each about ten times the width of the Sun. They orbit each other at a distance of about 60 million miles apart (closer together than the Sun and Venus) in a period of only 104 days. This helps us calculate that the mass of each of the Capella pair must be approximately 2.5 times that of the Sun—but the brighter (Capella A) must be a little more massive. Capella A shines with eighty times the luminosity of the Sun, B with fifty. At Capella's distance of 42 light-years, that works out to magnitude 0.6 and 1.1 for A and B, respectively. The composite magnitude of the two is +0.08 (only 0.05 dimmer than Vega), making it the sixth brightest of all night's stars. But if you already know a little bit about astronomy, you may know that Capella A and B do not make a visual
Relative sizes of Capella A, Capella B, and the Sun (top diagram); relative sizes and separations of Capella A and B versus the Sun and Earth (Earth not drawn to scale—it would be much smaller).
pairing—the duplicity of Capella was discovered spectroscopically. That is common among the brightest stars. But what is unique about Capella in this respect is how close it is to being visually splittable. The separation of Capella A and B is 0.04", which theoretically could be split with about a 12-inch telescope. The problem is that the seeing in Earth's unsteady atmosphere is almost never good enough to allow such a split anywhere in the world. Still, I have to wonder whether Capella could just be split by a large telescope at the best locations on Earth on the best nights. Astronomers have been able to obtain photographs of the pair separated from each other by using inter-ferometry.
Like so many of our bright stars, Capella also includes a distant red-dwarf member in its system—in this case, a 10th-magnitude object located about 12' to the southeast of Capella AB. It is about 11,000 AU away but shares a common proper motion through space. Since the letters C through G were assigned to faint unrelated field stars, this red dwarf is commonly known as Capella H. Capella H, however, turns out to be really a rather close pair of red dwarfs, Ha and Hb. These two are, respectively, about one-half and one-quarter the diameter of the Sun, but even their combined light is only about
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