MU VEL (Mu Velorum). With most of the world's population in the northern hemisphere, the southern stars do not get the respect they deserve. Bright, and just over the border into third magnitude (2.69), Mu Velorum -- the trailing eastern star of Vela the Sails (of Argo) -- sadly carries no proper name, as do few others of such southern status, the star not even visible north of 40 degrees north latitude. A pity too, as it provides a fine way to locate another star that seems much like the Sun, and gives a sense of what it would be like to live in a double star system. Mu Vel itself is a class G (G5) yellow-white giant with a middling temperature of 4900 Kelvin (a bit cool for its class) that has begun its death process. From a distance of 116 light years, it shines with the luminosity of almost exactly 115 Suns. Beginning life as a 3.0 solar mass class B star 360 million years ago, it has a dead helium core and has nearly completed making the transition to the true beginning of red gianthood, wherein it will cool at the surface and become over five times brighter than it is today. This transition is quick, rendering stars such as Mu Vel rather rare. Its expansion as a giant, to a radius 15 times that of the Sun, seems to have slowed its rotation speed from the high value it probably had as a class B star to only 6 kilometers per second (a lower limit), giving a rotation period under 117 days. Tucked in next to it, only 2.1 seconds of arc away, is an ordinary class G2 dwarf star that is still quietly fusing its core hydrogen. Its luminosity of thrice solar tells that it is just a bit more massive (by 20%) than our Sun. The orbit is long, 138 years, and somewhat uncertain, the measured average separation 51 Astronomical Units. A high eccentricity takes the two from 93 AU to 8 AU and back again. It seems unlikely that any planets could exist in such an environment. Orbital analysis gives a total mass to the system of 6.8 solar masses, far too high, revealing errors in the period, separation, or both. Ultraviolet observations also suggest that the companion is hotter than previously believed, and that it may be a 1.25 solar mass mid-class F star. By far the most remarkable feature of Mu Vel is its magnetic activity, which results in X-rays from a hot corona in spite of the fact that the star's expansion as a giant has slowed it down (rotation and atmospheric convection acting as a magnetic dynamo). When solar magnetic fields collapse, the Sun can pop bright flares that last for several minutes. In March of 1998, Mu Velorum proper -- the giant star -- was seen to burst out with a huge flare that brightened it by a factor of two in ultraviolet light (as observed by Earth-satellite) and that lasted for an amazing two days before normalcy returned. The star may have produced a major coronal mass ejection (of the kind that makes our northern and southern lights). The event must have been quite a sight from the sunlike companion.
Written by Jim Kaler. Return to STARS.