RHO AUR (Rho Aurigae). Auriga's
prominent pentagon, made of Alpha (Capella), Beta , Theta, and Iota
Aur plus Beta Tauri (which does dual
duty as Gamma Aur), is home to a good stretch of the galactic equator and a variety of
celestial sights. Four degrees south of Capella and just east of the
Charioteer's "Kids" is a hot class B (B3) dwarf that deserves more
attention than it's gotten, fifth magnitude (5.23) Rho Aurigae. The
temperature, needed to allow for quite a lot of ultraviolet radiation, is not at
all well known. One listing gives 13,500 Kelvin, which is way too low
and more appropriate for class B6. At B3 we would expect more like
18,000, which we'll adopt. At a distance of 530 light years (give or
take 31), the star is dimmed by just 0.22 magnitudes (a bit over 20
percent) by interstellar dust.
After these corrections, we look at a star that shines with the light
of 955 Suns, from which we find a radius of 3.2
solar radii. The projected equatorial rotation velocity is also badly
known, the observations running from 55 to 102 kilometers per second.
Picking the later one (55 km/s), the star must be rotating in under
three days. Theory gives a mass of 5.5 Suns and shows that Rho Aur
is quite young as compared to its 68 million-year hydrogen-fusing
lifetime. (If for the sake of argument we adopt the lower temperature
we get 500 solar luminosities and a mass of 4.3 Suns, still pretty
hefty.) Of particular interest is a close spectroscopic companion that shifts
the bright star's spectrum back and forth as the two orbit each other.
The companion, however, is not bright enough to register on the hot
star's spectrum, so we probably need not make any corrections to the
above calculations of luminosity and mass. The orbital period is well
established at 35.5 days. Given inconsequential mass for the
companion, the orbital radius must be just 0.2 Astronomical Units, half
Mercury's distance from the Sun. Unlike many other stars of Rho's
class, there is no evidence for any additional partners. Even if Rho-B
is small it may well have significant consequences for its brighter
mate. When Rho Aurigae proper runs out of core hydrogen fuel, it will
expand mightily and encompass Rho B. But it will also lose most of its
mass. Rho B may then stir the ejecta enough that, once Rho A's hot
core is exposed, it will light up a highly structured bi-polar planetary nebula. Indeed, it's thought by some
that close binaries are needed for the very creation of planetary
nebulae. In any case, Rho B, as massive as it is now, will someday
be reduced to a white dwarf
with a mass of about 90 percent that of the Sun. If Rho-B survives
Rho-A's giant stages the pair might be brought sufficiently close
together by being within a common envelope that tides raised in "B"
by the white dwarf may cause mass to flow onto the white dwarf's
surface, resulting in a nova.
Better yet, if someday the white dwarf exceeds the theoretical mass
limit of 1.4 solar masses, it will collapse and annihilate itself in
a supernova of the sort visible
across most of the known Universe.
Written byJim Kaler 2/26/16. Return to STARS.