a CAR (a Carinae). Yes, a third magnitude (ok just barely, 3.44,
but see below) star without a Greek letter,
as applied across the board in Argo (the Ship) by Nicolas de Lacaille, not
Bayer. Lacaille continued in each of Argo's parts with lower case
Roman letters, so "a" Car (in Carina, the
Keel of Argo) just barely missed Greek-letter status, as it directly
follows brighter Omega Carinae. It
might be less confusing to use the name from the Bright Star Catalogue, HR
3659, but out of respect for tradition we'll continue with "a Car."
As bright as it is, the star is not a part of the constellation's outline. But that's really
no surprise as it's just 0.7 degree west-northwest of much brighter
second magnitude Aspidiske (Iota Car),
which places it just within the boundaries of the False Cross made of Iota and Epsilon Carinae and Delta and Kappa
Velorum. The False Cross can fool the observer into thinking it's
the Southern Cross, which lies well to the
east. Yet as a blue-white class B (B2) subgiant-dwarf (classed sort of in the
middle) 447 light years away (give or take 16), a Car can easily hold
its own. The star is clearly double, with a short orbital
period of 6.7447 days. But the binary's nature is ambiguous. At the
extremes, the two may be equal, or one star may provide essentially
all the light. In either case, the binary is an "ellipsoidal" variable
that changes between magnitudes 3.20 and 3.56, rotation and tidal
forces distorting the stellar shapes so that as the stars orbit they
present different angular sizes to us. With a temperature of 21,180
Kelvin, and allowing for a slight dimming of 0.21 magnitudes by interstellar dust, it or they shine
with the light of 5060 Suns, most of it in the invisible ultraviolet. If we are seeing
just one star, the radius is 5.3 times solar, and the rotation period
(from an equatorial speed of at least 35 kilometers per second) is
under 7.6 days, which is consistent with the orbital period and tidal
locking (the rotation and orbital periods the same). Theory then
shows that the mass is 8.5 Suns, that the star
is about midway through its dwarf lifetime of around 30 million years,
and that it's near the cusp above which stars explode as supernovae. Even if it
becomes a massive white
dwarf, it might get enough mass flowing from the companion that
it will still collapse and blow up, as from Kepler's Laws, the two
are just 0.14 Astronomical Units apart. Assuming equality between
the two gives a rotation period of under 5.4 days (less than the
orbital period), each then with masses of 7.5 Suns. They would
produce a double white dwarf that if merged through tides and prior
evolution could still cause a blowup. And all this for a seemingly
innocuous star that one hardly notices within the great star clouds
of Carina.
Written byJim Kaler 5/16/14. Return to STARS.