Barnard’s Star
Published on Jul 24, 2006 at 5:27 am.
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Late in the evenings, in the northeast part of the constellation Ophiuchus, there is a star with a designation BD+04º 3561a. That is its designation in the Bonner Durchmusterung star catalog. At magnitude 9.5, it is far too dim to have a Bayer Designation or a Flamsteed Number. These are the ways that most any star that you can go outside and see are designated. But, magnitude 9.5 is far too dim to see with the naked eye. In fact, this rather stretches the limits of binoculars. You even want to be in dark skies to see it with an amateur telescope. This star appears othewise dim and insignificant. So, what is so special about it that I’d blog about it? More to the point, why does such a faint little star get the fancy name of Barnard’s Star?Barnard’s Star has one claim to fame that makes it different from other stars in the sky. It has the fastest proper motion of any star in the sky. From ancient times, people have looked at the heavens and seen the same patterns of stars. However, in 1718 the astronomer Edmund Halley (of Halley’s Comet fame) observed that several bright stars were in slightly different positions than had been recorded by ancient observers. From this, he infered that stars do in fact slowly move in the sky, just very slowly. This motion is called proper motion. Well, Barnard’s Star turns out to hold the current record for highest stellar proper motion. Even though it is the fastest moving star, it moves at only 10.4 arcseconds per year. At this rate, it moves about the width of the Full Moon through the sky every 17 years or so. Barnard’s Star isn’t just moving to the side, either. It is heading sort of towards the Sun, and will be only 3.8 lightyears away, closer than any other star, in about 10,000 years. Even so, it still will be too dim to see with the naked eye.
Barnard’s Star is what we call a red dwarf star. It is very small and dim, only about 15% the size of the Sun, and about 0.0004 times as bright. Stars are composed mostly of hydrogen and helium, but with a tiny percentage of other elements. Barnard’s Star has perhaps 20% (various studies have suggested from 10% to 30%) of these other elements compared with the Sun. This tells us that Barnard’s Star is about 10 billion years old. That means that it formed very early in the history of our galaxy. Its low metallicity (astronomers call anything other than hydrogen and helium metals) and its motion suggest that Barnard’s Star is a sort of halo star, one of the stars orbiting in a cloud around the galaxy rather than in its disk.
Like a great many red dwarf stars, Barnard’s Star is magnetically very active. This magnetic activity makes it a variable star. It varies somewhat in brightness due to rotation bringing very large starspots into view and taking them away again. It is also susceptable to massive stellar flares. As a variable star, Barnard’s Star has a variable star designation of V2500 Oph.
For a number of years prior to the first confirmed extrasolar planets, some astronomers had suggested that Barnard’s Star might have a slight wobble to its motion, suggesting the presence of a planet. Later, much more careful studies, show no indicatio at all of a wobble, so Barnard’s Star is unlikely to have planets.
And, as a final bit of Barnard’s Star trivia, it is currently the second closest star to the Sun, other than the Alpha Centauri triple star system.
-Astroprof
