The Sciences

Runaway star

Bad AstronomyBy Phil PlaitJun 22, 2010 11:24 AM


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"I am constant as the northern star, Of whose true-fix'd and resting quality There is no fellow in the firmament." Julius Caesar (III, i, 60 – 62)

Shakespeare was a decent writer, but an astronomer he wasn't. The North Star isn't fix'd, because the Earth's axis wobbles slowly like a top. You wouldn't see this by eye, since the circuit takes 26,000 years to complete, but astronomers deal with it all the time. But Shakespeare did get something right in that passage: the stars themselves do move. It's slow, but it's there. It's caused by their orbital motion as they circle the center of the Milky Way. Their velocity can be hundreds of kilometers per second, but that apparent motion is dwarfed to a near standstill by their forbidding distance. Of course, that means that closer stars will appear to move faster than ones farther away, just like trees by the side of the road whiz by as you drive, but distant mountains slide along in a much more stately manner. It takes decades, sometimes, to see that stellar movement at all -- astronomers call it proper motion -- but it's not impossible. Greek amateur astronomer Anthony Ayiomamitis (who has been featured on this blog before here and here) knew that very well, and he was able to prove it. Behold, the unfix'd heavens!

These two pictures show the same region of sky, separated in time by six decades. The top, taken in 1950, is from the famous Palomar Sky Survey, a tool still used by astronomers to guide their observations. The marked star is Barnard's Star, a dinky, dim red bulb a mere 6 light years away -- which makes it one of the closest of all the stars in the galaxy. Barnard was a phenomenal astronomer, and inferred that since it was a red dwarf, for it to be seen at all means it must be close. He kept his eye on it over the years, and was able to measure its apparent speed across the sky. It moves a phenomenal 10 arcseconds per year, which is tiny in normal life, but pretty frakkin' fast for a star. In 60 years since the Palomar observations, Ayiomamitis was able to capture it in the lower half of that image, where again its position is marked. Note how far it's moved! In the intervening decades it's traveled about 10 arcminutes, or about 1/3 the size of the Moon on the sky! That's fast. If every star moved that quickly, the constellations would last only a few centuries before being distorted beyond recognition. As it is, we see pretty much the same constellations ancient Sumerians did. Note that the Palomar image is in black and white; Ayiomamitis took color images and you can see the dull red glow of Barnard's Runaway Star. It might help to see the two images superposed; Ayiomamitis did that for me when he alerted me to his observations:

Very cool. Note the number of faint stars; Barnard's star is located in the constellation of Ophiuchus, which is near the galactic center, and is loaded with stars. Imagine trying to find that one faint ember among all those stars, and you start to get a glimpse of how amazing an observer Barnard was. Remember, this was before computers, digital photography, or any of those modern conveniences. He used film -- actually, emulsion sprayed on glass plates -- guided the telescope by hand, developed the plates, and measured them, again by hand. And he found that star among the millions of others. In real terms, the star is moving at about 140 km/sec (90 miles/second) relative to the Sun. Its direction is bringing it closer to us, though it'll never get closer than about 4 light years -- slightly closer than Alpha Centauri is to us now. It'll still be faint; only about twice as bright as it is now, and at the moment you need pretty good binoculars to see it at all! It's shining at about magnitude 9.5, or 1/16th as bright as the faintest star you can see with your unaided eye. Of course, it won't slide past us for about another 9000 years, so don't hold your breath. And even though the age of the star is about 12 billion years, as a red dwarf it hasn't even reached middle age yet. They last a long, long time. I bet over its life it's seen far closer passes to stars like the Sun, and will live to see many more. And finally, back to Shakespeare: even ignoring the Earth's wobble, he still blew it in that passage from Julius Caesar. The North Star moves too. Of course, its proper motion is pretty small because it's a long way off, over 400 light years away. Compared to Barnard's Star, it's hardly moving. Given that then, I suppose, I can give Shakespeare some credit. Perhaps the fault lies in ourselves, and not the stars. Image credit: Anthony Ayiomamitis and the Digitized Sky Survey

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