PI-1 UMA (Pi-1 Ursae Majoris, with a nod to 2 UMa.) In far western Ursa Major, about as far as you can get from the Big Dipper, seven degrees north of Omicron UMa and near the border with Camelopardalis, lies a small curve of three stars, from north to south fifth magnitude (5.47) 2 UMa (a Flamsteed designation), sixth magnitude (5.64) Pi-1 UMa, and fifth magnitude (4.60) Pi-2 UMa (which are NOT Flamsteed numbers). Pi-1 and Pi-2 have been tagged with the name "Muscida" (the "muzzle"), which more usually is attached to Omicron. They have nothing physical to do with one another. 2 UMa is 152 (give or take 3) light years away, Pi-1 a neighborly 46.8 light years (with an uncertainty just 0.3 ly), and Pi-2 the most distant at 256 (plus or minus 6) light years. At first, the prize would seem to go to Pi-2, an otherwise ordinary class K (K1) helium-fusing giant, but one with a massive orbiting planet about which little else is known and that might even qualify as a brown dwarf, a "star" with insufficient mass to fuse its core hydrogen to helium. Surprisingly, the real winner is the star in the middle, Pi-2 Ursae Majoris, which appears to be a much sought-after solar clone, a star like 18 Scorpii or 9 Ceti that is very much like our own Sun even though its dwarf classification is slightly different (G1.5 as opposed to the G2 Sun), its temperature of 5850 Kelvin about 70 K warmer, its mass three percent higher, and its radius five percent less. The object of studying solar mass stars is to see how the Sun develops from youth to old age and to test theory, that is, we would like to know both in the short and long terms what is going to happen to us. Pi-1 UMa is on the youthful side, the star roughly 500 million years old, and belongs to the Ursa Major cluster or, "moving group," of about the same age (whose most prominent members are the middle five stars of the Big Dipper). Of the star's parameters, the rotation is among the more interesting. The projected equatorial rotation speed of 10 kilometers per second gives a rotation period of under 5 days, But such a star should be magnetically active with star spots and related spectral characteristics. Periodic variations of a few hundredths of a magnitude give a true rotation period of 4.79 days, which then shows the star to be rotating with its axis perpendicular to us. A decade of data also suggests a long-term activity variation of a dozen years, perhaps one similar to the solar cycle. The rapid rotation also produces X-ray radiation 40 times that from the Sun and violent X-ray flaring. On the other hand, the global magnetic field seems to be about the same as that of the Sun. In addition, though we might expect a strong stellar wind, it appears to flow at only about half the rate of the solar wind, showing we have a long way to go in understanding early solar evolution.

What about 2 UMa, which seems to have been left behind? As a metallic- line Am star it's interesting on its own. Such stars alter their surface chemical compositions as a result of gravitational settling of some elements and radiative lofting of others. This phenomenon can give rise to confusion of the subclass depending on the ions used for classification. While usually given as A2m, 2 UMa might be classed anywhere from A3 to A7. From its distance of 152 light years and temperature of 8050 Kelvin, it radiates at a rate of 11.1 Suns. Theory then shows 2 UMa to be a young dwarf with a mass 1.8 Suns. While the rotation velocity is not well constrained, it's low enough to be consistent with the odd composition, as otherwise the atmospheric gases would be stirred back to normal. (Data on Pi-1 UMa from J.J.Bochanski et al., Information Bulletin on Variable Stars No, 5043, Konkoly Observatory, 2001: B.E. Wood et al., Astrophys. J. Letters 781:L33, 2014; I.G.Ribas et al., Astrophys. J., 622, 680, 2005.)
Written byJim Kaler 10/21/16. Return to STARS.