UPS AUR (Upsilon Aurigae). As the southern anchor to the small triangle in eastern Auriga, along with Nu and Tau Aurigae, Upsilon helps make the informal figure of the "Little Kids," so named in homage to the ancient "Kids" of Capella, which is made from Almaaz (Epsilon), and the two Haedii (Zeta and Eta Aurigae).
The Little Kids. North is to the left. Theta Aurigae, near bottom center, leads the eye upward to the thin triangle of stars we might call "The Little Kids," made (clockwise from lower left) of Nu, Tau, and Upsilon Aurigae. The figure is remarkably similar (though reversed) to the classic "Kids" near Capella, made of Epsilon, Zeta, and Eta Aurigae. The coolest of the trio, Upsilon takes on a slightly reddish cast.
While all three of the Little Kids are giants, this fifth magnitude (4.74, the faintest of them) class M (M0) red giant stands out as the coolest and perhaps the most interesting. At a goodly distance of 475 light years, it is by far the most distant of them, lying more than twice the distance of the other two. From Upsilon's point of view, Nu and Tau would appear much as they do from Earth, while our seemingly powerful Sun would appear as an insignificant and probably unnoticed 11th magnitude blip. Very little work has been done on the star. From distance and an estimated temperature of 3800 Kelvin (from which we find the amount of invisible infrared radiation), we find am impressive luminosity of 990 times that of the Sun, which leads to a radius of 73 solar, or 0.34 Astronomical Units, just shy of the size of Mercury's orbit around the Sun, and a mass of 2.5 times solar. What makes Upsilon particularly interesting is its evolutionary state. It started its starry life around 760 million years ago as a white class B9 dwarf. After fusing its internal core hydrogen into helium for 585 million years, it then began to swell and cool as the hydrogen-fusion expanded into a shell around the now-dead and gravitationally heating helium core. After cooling into class G, the star then brightened by a factor of seven or so, until it began to fire its internal helium and turn it into carbon and oxygen. Shrinking and dimming some, for the next 150 million years it more or less resembled its mates within the Little Kids, Nu and Tau Aurigae. Upsilon then used up its helium, and is now brightening again with a dead carbon-oxygen core. At some point it will become unstable begin to pulsate like Mira, lose its outer envelope, and turn into a white dwarf with a mass of about 70 percent that of our current Sun. Similar fates await the other two Little Kids, though by that time, stellar motions will have obliterated the little figure, each star having wandered off on its own.
Written by Jim Kaler 4/18/08. Return to STARS.