24 UMI AND HR 286 UMI (24 Ursae Minoris and HR 286 Ursae Minoris), a two-for-one special and a study in similarity and contrast. Odd stellar coincidences abound. The first two stars of the Little Dipper in Ursa Minor (the Smaller Bear) are famed Polaris and Yildun (Delta UMi). Next to each, at similar angular separations (about a third of a degree), are much fainter stars, HR 286 Ursae Minoris (the name from the Yale Bright Star Catalogue) and 24 UMi (Flamsteed's 24),
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." 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. The star immediately down and to the right of Polaris is HR 286; the one just down and to the left of Yildun is 24 UMi.

See the full-resolution image and more on polar stars in the Polar Project.
two more polar stars. Both are sixth magnitude (HR 286 at 6.47, 24 UMi at 5.79) and thus technically "naked-eye," though HR 286 is surely right at the edge, if not in fact beyond it. Nevertheless, HR 286 is the closest reasonably bright star to Polaris, quite jumping out through binoculars. At just under a degree from the true North Celestial Pole, HR 286 has the additional distinction of vying for "number 2 pole star," as it is almost exactly at the same angular separation from the Pole as the other candidate, Lambda Ursae Minoris. Both "24" and "286" are similar and common class A hydrogen-fusing dwarf stars ("24" class A2, HR 286 class A3). But there the similarities end. Since the classes are about the same, being the fainter, HR 286 should be the farther away, and it is, lying at a distance of 321 light years, as opposed to 156 for "24," which is not far off Yildun's distance of 183 light years. (Only 27 light years apart, from 24 UMi Yildun would appear as zeroth magnitude.) With a temperature estimated at 8900 Kelvin, HR 286 shines with a luminosity of 21 Suns, its radius 1.9 solar, its mass 2 solar. With a higher temperature of 9200 Kelvin, "24" oddly radiates less, only 10 solar luminosities, its radius just 1.2 solar. The mass is difficult to gauge, as the star falls outside the theoretical boundaries of hydrogen fusing dwarfs. It is either underluminous for its temperature (60 percent that expected) or too hot for its luminosity, or a combination of the two (the best mass estimate around 1.6 solar). As opposed to "normal" HR 286, 24 UMi is also a "metallic line star," one with odd proportions of metals in its spectrum caused by "diffusion," in which some chemical elements fall under the action of gravity while others are lofted upward by the action of radiation (and which is consistent with a fairly peaceful rotation speed of 55 kilometers per second). As such, it is difficult to judge 24 UMi's spectral class and temperature, that and error in distance perhaps the cause of the luminosity anomaly. HR 286 then comes back to the fore with a pair of additional distinctions. Very stable, it makes a fine comparison star for determining accurately the variations of Polaris, a low-amplitude Cepheid. As precession (the 26,000-year wobble in the Earth's axis) brings the North Celestial Pole closer to Polaris, it also approaches HR 286, giving the latter clear ascendancy as the "number-2-pole-star" over Lambda. And in only a bit over a century, it will reach number 1, actually being closer to the Pole than Polaris. Though do not look for a name change, since -- close to seventh magnitude -- "286" is devilishly difficult (if not impossible) to spot without some kind of optical aid.
Written by Jim Kaler. Return to STARS.