EPS UMI (Epsilon Ursae Minoris). Eps UMi (the capitol "M" is correct) is the third star in from the end of the handle of the Little Dipper of Ursa Minor (the Smaller Bear). Only 8 degrees from the north celestial pole, Epsilon is circumpolar and perpetually visible from everywhere north of 8 degrees north latitude. In spite of its constant northern- hemisphere visibility, the star is obscured by contrast with its much brighter and famed neighbor, Polaris, which anchors the Dipper-handle, and also by the front bowl stars Pherkad and Kochab. Yet though only fourth magnitude (4.23), the star still has its own tale to tell. Eps UMi is a class G (G5) giant. From its distance of 345 light years, it radiates with the power of 225 Suns from a surface heated to 5200 Kelvin, a bit cooler than that of the solar surface. It is a fine example of a dying transition star, one that has ceased its core hydrogen fusion, and with a dead helium core is preparing to become a true large red giant considerably brighter than it is today. A fair sample of such stars dot the sky, their brightness rendering their apparent number well out of proportion to their actual space densities (number per million cubic light years). What makes Eps UMi more special is that not only is it a close double (detected with the spectrograph), but that the orbit lies closely enough in the line of sight (within about 20 degrees) to make it an eclipsing double (in which one star gets in front of the other) with a period of 39.4816 days. With its mass of 3.6 solar, and a guess as to the mass of the companion, the separation is 0.36 Astronomical Units, not quite Mercury's distance from the Sun. But do not bother trying to see the eclipse with the eye, as it is only partial, and amounts to a mere 6 percent, well below visual detectability. The closeness of the two seems to make Eps UMi something of an "RS Canum Venaticorum" star, in which the orbital motion causes the star to spin faster than it would and helps generate starspot activity (as witnessed by X-ray radiation). Farther out lies another companion. Too far away (at least 8100 AU) for any orbital motion to be detected, the 11th magnitude star must (from its luminosity) be a class K0 dwarf. That the distant star is truly tied to the inner double (rendering the system triple) is revealed by their common motions through space. From the distant companion, the inner pair would be only 10 or so seconds of arc apart and unresolvable without a telescope. Bright Eps UMi proper (the star we see) would appear as bright as three full Moons, whereas from Eps Umi, the distant "C" companion would appear not much brighter than Venus does in our sky.
Written by Jim Kaler. Return to STARS.