2 UMI (2 Ursae Minoris). More or less opposite the true North Celestial Pole from Polaris (though quite a bit farther away) lies modest fourth magnitude (4.25) 2 Ursae Minoris, another of the "polar stars" that lie within a few degrees of the pole star in Ursa Minor (the Smaller Bear). Too faint to have been included in Bayer's atlas of the early 1600s, in which he assigned Greek letters, it goes by a series of catalogue names that would begin with Flamsteed "2," which it was originally named -- if in fact it WAS in Ursa Minor, which it is not, the star's role seeming more to show how confusing star names
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." 2 Ursae Minoris is the bright star at the lower edge to the right of center. 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. 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.
can be. The original constellations were patterns in the sky that had no particular boundaries between them. Celestial map makers eventually surrounded the ancient (and modern as well) figures with dotted lines that varied from one to another. When Flamsteed's stars (which were the first to have accurate telescopic positions) were numbered, 2 UMi was indeed considered to be part of Ursa Minor. The final formal constellation boundaries (set in the 1920s), however, placed the star in Cepheus. Consequently, "2 UMi" in a sense no longer exists; rather is no longer recognized as a correct name, the star going mostly by "HR 285." That, by the way, is not nearly so bad as a much fainter ninth magnitude (just barely, 8.54) star called R Cephei. It inexplicably lies well within the ancient confines of Ursa Minor and, in spite of its variable star name, is not variable at all, but is a solar-type class G2 dwarf 1100 light years off! Physically, 2 UMi (no reason not to use the name here) is a class K bright giant. Its distance of 313 light years and temperature of 4400 Kelvin tell of a star with a luminosity 273 times that of the Sun, a radius 29 times solar, a mass of about triple solar, and that the star is now fusing its core helium into carbon and oxygen. 2's metal content is pretty much solar. Its singular feature, other than its name, is its very slow apparent (projected) rotation speed of only a kilometer or so per second, which gives it a rotation period of four years (which is admittedly an upper limit since the tilt of the axis is unknown). Such stars abound. Those that lie outside their parent constellations, however do not, neither 2 UMi nor R Cep fitting in very well at all.
Written by Jim Kaler. Return to STARS.