Movatterモバイル変換

[0]ホーム

2 UMI (2 Ursae Minoris). More or less opposite the trueNorth Celestial Pole fromPolaris (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 thepole star inUrsaMinor (the Smaller Bear). Too faint to have been included inBayer's atlas of the early 1600s, in which he assignedGreek letters, it goes by a seriesof catalogue names that would begin withFlamsteed "2," which it wasoriginally named -- if in fact it WAS in Ursa Minor, which it isnot, the star's role seeming more to show how confusing star names

Polaris (the jewel in a small semicircleof faint telescopic stars called the "Engagement Ring") is centeredin a six-degree-wide field of view that shows a variety of other"polar stars." 2 Ursae Minoris is the bright star at the loweredge to the right of center.LamdbaUrsae Minoris is the reddish star up and to the right ofPolaris, whileYildun (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 thePolar Project.

can be. The original constellations were patterns in the sky thathad no particular boundaries between them. Celestial map makerseventually surrounded the ancient (and modern as well) figures withdotted lines that varied from one to another. When Flamsteed'sstars (which were the first to have accurate telescopic positions)were numbered, 2 UMi was indeed considered to be part of UrsaMinor. The final formal constellation boundaries (set in the1920s), however, placed the star inCepheus. Consequently, "2 UMi" in a sense no longerexists; rather is no longer recognized as a correct name, the stargoing mostly by "HR 285." That, by the way, is not nearly so bad as a much fainter ninthmagnitude (just barely, 8.54) star called R Cephei. Itinexplicably lies well within the ancient confines of Ursa Minorand, 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 classK bright giant. Its distance of 313 light yearsand temperature of 4400 Kelvin tell of astar with a luminosity 273 times that of theSun, a radius 29 times solar, a mass of abouttriple solar, and that the star is now fusing its core helium intocarbon and oxygen. 2's metal content is pretty much solar. Itssingular 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 admittedlyan upper limit since the tilt of the axis is unknown). Such starsabound. Those that lie outside their parent constellations,however do not, neither 2 UMi nor R Cep fitting in very well atall.

Written byJim Kaler. Return toSTARS.

[8]ページ先頭