NU CEP (Nu Cephei). Talk about an underappreciated star! Nu
Cephei (of no proper name, in Cepheus,
the King) lies more or less on a line between Alderamin (Alpha Cephei) and the
constellation's most famous star, variable Delta, which is the main focus of
attention. While an amazingly luminous class A (A2) supergiant (with a temperature
of 8400 Kelvin, though there are measures up to 9100), this fourth
magnitude (4.29) star is also lost beside other grand supergiants
(Mu and VV Cep, to name two) such that
much less honor is paid to what is really due. The last of Nu's
problems is that it is significantly dimmed by interstellar dust in
the Milky Way, which if it were not in
the way would bring it to bright third magnitude (2.82). Class A
supergiants such as this one are quite rare (the brightest by far
being Deneb in Cygnus). As such all are far away, which compromises
distance measurement. Nu Cep's formal parallax gives a distance of
5100 light years, but with a huge uncertainty of some 75 percent.
That distance is certainly much too high. Factoring in the formal
uncertainty, the star should however be at least 2900 light years
distant. From its rough distance and motions, Nu is a member of
the sprawling, expanding (gravitationally unbound) association of
O and B stars known as Cepheus OB2 (which was once thought to
contain Mu as well), which
from the analysis of all the prominent members yields a much lower
distance of 2000 light years. The parallax limit would then place
the star outside the association, suggesting that it too is wrong.
If nothing else, the whole affair shows that there is still a lot
of work to do in the distance game. If at 2000 light years (which
seems the more likely), the star shines with a luminosity 22,000
times that of the Sun, its diameter 70 times solar, which would
make it just shy of the size of Mercury's orbit. If at 2900 light
years, the figures would go up to nearly 50,000 solar luminosities
and about 100 solar diameters (about half the size of Earth's
orbit). Grand as it is, however, Nu pales beside Deneb, which
radiates some 160,000 Suns into space (explaining why Nu Cep is
sometimes downgraded to a "lesser supergiant" status).
Nevertheless, our star has a mass of between 12 and 16 times that
of the Sun and is only about 15 million years old. It may be
making a transition to becoming a larger and redder supergiant with
a now-quiet helium core, or it may be a bit older and now fusing
its helium into carbon. From its surface blows a 150 km/s wind
with a mass-loss rate of a tenth of a millionth of a solar mass per
year. Like most supergiants of its class, Nu's most likely fate is
to grow an iron core and someday explode.