HR 1107 CEP (HR 1107 Cephei). HR 1107 is another in a series of "polar stars" that accompany Polaris as it circles the North Celestial Pole and that fall into two constellations: Ursa Minor (the Smaller Bear), which contains Polaris and the Pole itself, and Cepheus (the King), whose technical boundary comes within 1.2 degrees of the Pole. Polaris in fact just barely makes it into Ursa Minor. Shift it 3/4 of a degree and it would be in Cepheus, as is HR 1107 (which carries no Greek letter name or Flamsteed number, but instead one from the Yale "Bright Star Catalogue"). The object of the exercise is to take a look at all the naked eye stars within a particular more-or-less random block of the sky, the set so far including Lambda, 2, 24, and HR 286 UMi, OV Cephei, Polaris itself, and the next star in the handle of the Little Dipper, Yildun.
pole Polaris (the jewel in a small semicircle of faint telescopic stars called the "Engagement Ring") is centered in a six-degree-wide field of view that shows a variety of other "polar stars." HR 1107 is the brighter of the two stars down toward the left hand corner. Lamdba Ursae Minoris is the reddish star up and to the right of Polaris, while Yildun (Delta UMi) is the brighter of the two stars at the upper right corner. HR 1107 is closer to the Little Dipper in Ursa Minor than it is to the classical figure of Cepheus. Roughly between Lambda and Polaris lies the North Celestial Pole, around which they all seem to revolve.

See the full-resolution image and more on polar stars in the Polar Project.
HR 1107, as a sixth magnitude (5.84) class F (F5) subgiant 133 light years away, serves best to show the huge differences that can exist among stars. Polaris, while having the same F5 spectral class, is a supergiant that lies over three times more distant and yet at second magnitude appears 34 times brighter in our sky, showing the vast power of such stars. With a temperature of 6715 or so Kelvin, HR 1107 radiates 6.0 times more energy than the Sun, which leads to a radius 1.8 solar and a mass 1.45 solar. Rotating with an equatorial velocity of at least 76 kilometers per second, HR 1107's rotation period must be less than 1.2 days (as opposed to 25 days for the Sun), while its metal content (compared with hydrogen) is about 2/3 solar. The fairly rapid rotation is typical of the class, as warm stars generally spin faster than cooler ones, class F5 marking the average line of demarcation, the "rotation break." The temperature and luminosity show clearly that the star is really a hydrogen-fusing dwarf that is rather in the middle of its life path. By comparison, Polaris is 2200 times more luminous than the Sun and 365 times more luminous than HR 1107, the difference caused by Polaris's much greater mass (about 6 times that of the Sun) and advanced state of evolution.
Written by Jim Kaler. Return to STARS.