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.)