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.
Written by Jim Kaler. Return to STARS.