BARNARD'S STAR (V2500 Ophiuchi). At just barely tenth magnitude (9.54), Barnard's Star -- named after Yerkes Observatory's E. E. Barnard (1857-1923), who discovered it in 1916 -- is not close to being visible to the naked eye, even though at a distance of just 6.0 light years it is the second closest star to the Earth (considering the Alpha Centauri system, including Proxima, as a unit). That is just what you would expect from a dim, low mass, class M (M4) dwarf. In northeastern Ophiuchus near the asterism known as "Poniatowski's Bull," the star's fame derives from a variety of properties, chief among them its speed record. Barnard's has the world's greatest "proper motion," the angular annual movement across the line of sight against the distant stellar background, a whopping 10.4 seconds of arc per year. While that may not seem like much, it amounts to an easily- seen quarter of a degree in a human lifetime, roughly the angular diameter of the full Moon. This huge angular displacement derives from a truly high speed of 139 kilometers per second relative to the Sun, both toward us and to the north. Cool (3170 Kelvin, as befits an M star), this dim dwarf has a luminosity a mere 0.0035 times that of the Sun, most of it in the infrared, which shows it to have a diameter only 20 percent that of the Sun (found also from the angular diameter) and a mass a mere 17 percent solar. Far from rare, the great majority of stars fall into the M dwarf category: they are just so faint, like Proxima Centauri, that none is visible to the naked eye. Nature seems to love the lesser. No one really knows why. Barnard's has a metal content only 10 percent that of the Sun. That coupled with its high velocity shows it to be a special, rather rare, kind of star called a "subdwarf" that more belongs to the metal-poor and ancient halo of our Galaxy (the Sun belonging to the disk). It is merely passing through our local neighborhood. (Once thought to be too dim for their temperatures, hence the name, subdwarfs are actually too hot for their luminosities, the result of severely decreased metal content.) Indeed, the star is clearly old (born before exploding stars had enhanced the amount of interstellar metals to that seen today), as attested to by its long rotation period of 130 days (stars slowing down as they age), some five times longer than the Sun's. Nevertheless, Barnard's Star still has some magnetic activity, occasionally popping a flare caused by the release of magnetic-field energy, has an active X-ray corona heated magnetically to two million Kelvin (as does the Sun), and probable starspots (from which the rotation period is inferred). Variations from magnetic effects have given Barnard's its variable-star name of "V2500 Ophiuchi." Part of the star's notoriety is that it was among the first to be announced -- decades ago -- as having an orbiting planet, the result of perceived wobbles in its proper motion. Alas, the "discovery" was an error, as the apparent positional shifts were caused by adjustments in the telescope lens. Were there planets, say an Earth (and we are pretty sure there are no planets of substantial size), the star is so dim that for us to receive the same amount of heat that we do here, we would have to be a mere 0.06 Astronomical Units away, 15 percent Mercury's distance from the Sun. Our year would then be only 13 days long, the seasons whizzing by, with a hypothetical winter of just 3 days. Were such a planet to exist, life would likely be impossible because of the occasional flaring, the star showing us the almost infinite variety of nature. Barnard's low internal temperature and resulting feeble energy-generation rate give it an incredibly long life. Indeed, no class M dwarf ever born in all the history of the Galaxy has ever died. (Barnard's Star is included in Jim Kaler's "Hundred Greatest Stars." Thanks to Tom Mazanec and Paul Nolan for suggesting it.)
Written by Jim Kaler. Return to STARS.