78 UMa (78 Ursae Majoris). With nothing but Flamsteed and catalogue numbers, 78 Ursae Majoris may look anonymous. But if you've looked carefully at Ursa Major's Big Dipper, it may have caught your attention. Look first at the second star in from the handle, Mizar, which has Alcor as an outlier. Then to the next star in, fifth magnitude (4.93) Alioth (Epsilon UMa), which is remarkably similar, except that its "companion," 78 UMa, is angularly just farther away. Along with many others, 78 and Alioth are both part of the Ursa Major cluster, the nearest such cluster to the Earth, our star 78 UMa at 81 light years, the average at 80. However, while Mizar and Alcor are probably true orbiting companions, Alioth and 78 are most likely not: see below. No matter, as 78 UMa is a fine double in its own right, one component of which is a lesser version of the Sun.
78 UMa 78 Ursae Majoris B goes around 78 UMa A (the brighter and more massive of the two, placed at the cross) with a period of 106 years at an average separation of 31 1/4 Astronomical Units. Observations (the colored points) are missing where the stars were too close together. Tilted through a 50 degree angle to the plane of the sky, the orbit is quite noticeably foreshortened, as can be seen by the off-center displacement of the true ellipse's major axis (the dot-dash line). The arrow at lower right shows the direction of motion. Double star observers measure the position of the fainter star relative to the brighter rather than the actual motions of both stars around the common center of mass that lies between them. North is down, as would be seen in a telescope. The scales around the edges are in seconds of arc. (From an article by M. Scardia et al. in the Monthly Notices of the Royal Astronomical Socity, vol, 357, p. 1255, 2005, image courtesy of W. I. Hartkopf.)
The main star, 78 UMa A, is a class F (F2) dwarf at 7000 Kelvin coupled with a G-type (G6) 5500-Kelvin dwarf that have respective luminosities of 4.6 and 0.87 times that of the Sun, radii of 1.5 and 0.9 solar, and (from theory) masses of 1.5 and just under 1.0 solar (perhaps as low as 0.8 solar). Rotating quickly, more than 92 kilometers per second at the equator, 78 UMa A spins in less than 0.8 days. The orbital period of the pair of 106.4 years coupled with an average separation of 31.25 Astronomical Units (1.25 seconds of arc on the sky) yields a combined mass of 2.7 times that of the Sun, somewhat above that found from evolutionary theory. A significant eccentricity of 0.41 takes the stars between 49 and 18 AU apart. They were last closest in 1921 and will be again in 2027. The stars are both quite young, consistent with the 250 million-year age of the UMa cluster. Can 78 and Alioth be a real pair? Given their masses and a separation of at least 1.4 light years, the orbital period would be more than 10 million years. The two are so far apart, however, that the influence of the UMa cluster would prevent any stable orbit. Mizar and Alcor alone, at a distance of five light years, would have roughly a third of the gravitational influence of Alioth. But 78's sky would be glorious. Alioth would glow with the light 7.5 times that of our Venus at its brightest. Even Mizar would rival the planet. From Alioth, 78 UMa might at times in its orbit appear as a close near-naked-eye double, the two a bit over a minute of arc apart, the brighter almost another Venus, the fainter about equal to our Sirius. (Thanks to Jerry Diekmann, who suggested this star.)
Written by Jim Kaler 5/02/08. Return to STARS.