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.