GMB 1830 UMA (Groombridge 1830 Ursae Majoris, HR 4550 in the Bright Star Catalogue), which lies in southern Ursa Major roughly 17 degrees south of the bowl of the Big Dipper and eight or so degrees southwest of Chara (Beta Canum Venaticorum) in Canes Venatici. It's named after Steven Groombridge (1755-1832), who compiled a catalogue of accurate positions of circumpolar stars as seen from England. Just barely sixth magnitude (6.45, not quite seventh), this remarkable class G (G8) hydrogen-fusing dwarf ranks third in the list of high "proper motions" (angular speeds across the line of sight). Moving at a rate of 7.06 seconds of arc per year against the distant stellar background, it's behind only Barnard's Star in Ophiuchus (10.4 seconds) and Kapteyn's Star in Pictor (8.67). The rapid motion was discovered by the German astronomer F. W. Argelander (1799-1875), father one of the great star catalogues, the Bonner Durchmusterung (Bonn Survey), "Argelander's Star" also BD+38 2285 (the star falling between 38 and 39 degrees north of the celestial equator). One might expect that, like Barnard's, it must be quite nearby to be zipping along so fast, but at a distance of 29.6 light years (good to 0.1) it doesn't even make the list of the nearest 100 stars. Instead, it really IS moving fast, at an amazing 305 kilometers per second across the line of sight relative to the Sun. When combined with a speed toward us (the "radial velocity") of 98 km/s, we find the star to be clipping along at 320 km/s, some 20 times normal, making it among the fastest stars known. Equally remarkable is the chemical composition, which is very low in "metals" (to an astronomer everything but hydrogen and helium), the ratio of iron to hydrogen a mere four or five percent what it is in the Sun. The combination of speed and composition immediately tell that the star is a visitor from the Galaxy's extended and sparsely populated halo, which surrounds the Galactic disk that makes our Milky Way. Disk stars like the Sun have roughly circular orbits about the Galactic center, which gives them low relative velocities. Halo stars on the other hand are in highly elliptical orbits similar to those of the great globular clusters. Like its mates, Gmb 1830 is "falling through" the disk on its own eccentric path while the Sun moves past it in a different direction. Stars like this one help give us a look at how the Galaxy is structured.

With a temperature of 4985 Kelvin, from which we get the amount of infrared radiation, Gmb 1830 shines with only 23 percent the luminosity of the Sun, which gives it a radius of 0.64 times solar. Direct measure of angular size via interferometry gives 0.66 solar radii. More detailed studies that include a mass of 0.61 Suns yield similar results. The low metal content comes from age that, like the other stars of the halo, is some 12-13 billion years. Back then, there had not yet been enough time for supernovae and other evolving stars to build up the metal content of stellar birthplaces with their explosions and winds. When plotted on the HR diagram of absolute magnitude vs. spectral class, the low metal content makes Gmb 1830 fall to the left of, or below, ordinary dwarfs, turning it into a "subdwarf" ("sd"). (Do not confuse such with hot subdwarfs, which are highly evolved stars preparing to become white dwarfs.) Gmb 1830 is the closest of its kind. In 1936, it blasted a "superflare" that brightened it for 18 minutes by up to 0.6 magnitudes (70 percent) in blue light, much like the one that that lit 5 Serpentis and some others. By comparison, even the brightest of solar flares would not be visible to an observer at stellar distances. Given Gmb 1830's age, it's surprising that it can generate so much magnetic energy. The flare was so powerful that it apparently gave rise to a "false companion." Given the luminosity, the "habitable zone" for a planet around Gmb 1830 might be in range of 0.38 to 0.83 Astronomical units. But low metallicity may preclude planets and in any case, given the flares, you sure would not want to live there.

Written byJim Kaler 7/11/14. Return to STARS.