LAMBDA UMI (Lambda Ursae Minoris). Our focus in the northern hemisphere is ultimately on Polaris, the North Star, the luminary of Ursa Minor, which closely marks the North Celestial Pole, the point of the sky's apparent rotation. Though other stars surround the pole, they are so overwhelmed by Polaris's fame and brilliance that they go rather unnoticed (the problem added to by the occasional difficulty of using telescopes near the pole). They are also a rather strange bunch. So begins a continuing tale of "polar stars," those within a few degrees of the pole. If you define the limit to naked-eye vision as 6.5 (at the bottom of generic "sixth magnitude" and admittedly on the faint side), then Lambda Ursae Minoris (about halfway between Yildun and Polaris) just makes it over the mark at faint sixth (6.38), which
pole Polaris is centered in a six-degree-wide field of view that shows a variety of "polar stars." Lamdba Ursae Minoris is the reddish star up and to the right of Polaris. Roughly between them lies the North Celestial Pole, around which they all seem to revolve. Yildun (Delta UMi) is the brighter of the pair at the upper right corner.

See the full-resolution image and more on polar stars in the Polar Project.
gives it some distinction as the closest naked-eye star to the Pole other than Polaris itself. (Number 3 in both brightness and position is HR 286, which is only a third of a degree from Polaris, and at magnitude of 6.47 just barely makes it as well.) Lambda is just under a degree from the true pole, which lies more or less between it and Polaris (which is slightly closer). As a result of the Earth's axial precession (the 26,000-year axial wobble), a couple hundred years ago, Lambda was in fact the better pole star. The pole will actually pass between Lambda and Polaris about the year 2060. While the Greek letter name looks "Bayerish," it is not, as the star does not appear on Bayer's Uranometria, nor was it so named by the late 1700s, but was applied later. Physically it is a relatively rare class M (M1) giant 880 or so light years away. Seriously neglected, over the past 20 years it has been mentioned in a mere dozen research papers, and in none has it been the focus. There is no measured temperature, the spectral class giving 3800 Kelvin. With that to account for infrared radiation, Lambda UMi shines with the luminosity of 600 Suns, and is a swollen red giant with a radius 57 times solar, about 69 percent the size of Mercury's orbit. With a mass somewhere around 1 1/2 to 2 solar masses, it is either still brightening with a dead helium core or has begun to fuse helium and is fading some as it readjusts itself. Though Lambda is rated as double, the companion is a 14th magnitude star a hefty 55 seconds of arc away. Typical of the "polar stars," it has not been observed for a century, so it is not possible to tell whether it is a real companion or not. If it is, it has the luminosity of a class K dwarf, is at least 15,000 Astronomical Units away from Lambda proper, and orbits with a period greater than a million years. From there, the giant would appear as a brilliant reddish dot shining the light of our full Moon.
Written by Jim Kaler. Return to STARS.