19 CEP (19 Cephei). A trio of little (so they seem) fifth magnitude stars that lie in a curved row in south-central Cepheus lead to the eye to the magnificent eclipsing binary and monster star, VV Cephei. From south to north, 19 Cephei (magnitude 5.11), 20 Cep (5.28), and 18 Cep (5.31) epitomize stellar differences, the two "bookends" providing an almost startling contrast near opposite ends of the stellar temperature scale. Flamsteed 18 is a class M (M5) red giant 450 light years away, whereas 19 is a hot (31,500 Kelvin), blue class O (O9) "class I" supergiant (though a "lesser" "Ib" version) that is so far away that there are no direct distance measures. The middle star, 20 Cep, as a class K (K4) giant 320 light years off, also has middling properties. While 18 Cep is clearly reddish, distant 19 Cep -- our focus here -- is dimmed by over a full magnitude by interstellar dust, which reddens the starlight to white. Were the dust not there, "19" would appear as fourth magnitude (3.96) and nicely outshine its seeming (quite coincidental) mates. The direct parallax of 19 Cep gives a whopping distance of 5400 light years, but the associated error is so large -- almost as big as the measure itself -- that such a distance cannot at all be trusted. Were it true, "19" would have to be a bright "Ia" supergiant and it clearly is not. From the star's traditional membership in the Cepheus OB2 association, the distance is "only" 2400 light years. However, one detailed study removes the star from the association. The supergiant class alone then implies a greater distance of 1700 light years. We just don't know. In either case, the star is very luminous and massive. If at the shorter distance, it is 180,000 times brighter than the Sun (allowing for a lot of ultraviolet light) with a radius of 14 solar and a mass of 30 solar. If at the longer distance, the numbers are 345,000, 20, and 35 solar. Oddly, in either case, 19 Cephei appears to be a very luminous class O dwarf that is still fusing hydrogen into helium in its core rather than a supergiant with a dead helium core. A projected rotation speed of 33 kilometers per second provides little constraint, the star rotating in under a month. Whatever the parameters, 19 Cephei is heading for disaster. Only five or so million years old, it is furiously losing mass at a rate of two millionths of a solar mass per year (millions of times that in the solar wind) at a speed of 2000 kilometers per second. When the core hydrogen runs out, the star will swell into a red supergiant and explode as a supernova. It was long ago assigned a pair of dim 11th magnitude companions at angular separations of 20 and 59 seconds of arc, but they are almost certainly just line-of-sight coincidences. (Thanks to Reginald Quinto, who suggested this star.)
Written by Jim Kaler. Return to STARS.