The closest star to our solar system, alpha Centauri, ranks as the third-brightest star in the heavens and one of the jewels of the deep-southern sky. But this system is off limits north of the tropics. So northern stargazers must make do with the second-closest star, the dim and dull-red Barnard’s Star in the constellation Ophiuchus. It’s not much to look at, but this little star invites inspection because of its historical importance and its rapid apparent motion across the sky.
Barnard’s Star lies about 6.0 light years from Earth, about 1.7 light years further than the Sun-like alpha Centauri. But it appears a dispiriting 6,300 times fainter, a consequence of its small size and low mass. Barnard’s Star has 14% the mass of our Sun, spans just 20% of a solar diameter, and shines just 0.35% as brightly as our Sun. It’s classified as an M4-type main sequence star with a surface temperature of 3100 K.
Despite its diminutive size, Barnard’s Star is no odd-ball. Most stars in the Milky Way are faint red M-type dwarfs like Barnard’s Star. These stars are so faint, we can’t see any without optical aid.
Barnard Star’s main claim to fame is its large proper motion, the angular change of its position across the sky. It moves about 10.4” per year, more than any other star, which is a result of its true motion through the galaxy and its proximity to our solar system. Over an average human lifespan, the star moves about a quarter of a degree or about half the diameter of the full Moon. In astronomical terms, that’s mighty fast.
The proper motion of this star was first measured in 1916 by Edward Emerson Barnard, arguably the world’s greatest self-taught astronomer.
You can find Barnard’s Star in the constellation Ophiuchus, tangled in the small asterism known as Poniatowski’s Bull. The star shines at magnitude 9.5, far beyond the limit of detection of the unaided eye and a challenge in most binoculars. But its dull red glow is easily visible in a telescope. The star sits just 4º north of the celestial equator so it’s visible to observers in the northern and southern hemispheres.
Like all red dwarfs, Barard’s Star is a slow burner. While it has less mass than our Sun, it fuses its hydrogen frugally and has been burning for about 12 billion years, just 1.7 billion years less than the universe. The star likely has enough fuel to burn for many more tens of billions of years, far longer than the expected lifespan of our Sun. From a planet around Barnard’s Star, an observer would one day get a front row seat to see our Sun swell into a boiling red giant, then a beautiful if transient planetary nebula, then finally a dim white dwarf.
Not that any planets have been detected around Barnard’s Star. In the 1960’s and early 1970’s, astronomer Peter van de Kamp claimed detection of wobble in the star’s motion cause by a planet, but his results were a consequence of adjustments in the lens of his telescope. And if any planets exists, life is unlikely to thrive because of the substantial flares that radiate from this little red dwarf.