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The Question

(Submitted April 11, 1998)

My question is, are stars fixed or stationary?

The Answer

All stars move, and for a variety of reasons. One reason is that all stars in our Galaxy are revolving about the center of the Galaxy. Our Sun makes one such revolution every 2 hundred million years or so. They may also move, if an explosion gives them an extra "kick." Or a star may be in a binary system, orbiting about another star. Those are just several examples.

There are two major ways of observing the motion of stars. One method, for stars close by, is to actually observe the movement in the sky , against a fixed background of stars which are known not to move much over long periods of time. Note, that if the star was heading straight in our direction, we wouldn't observe any motion at all. This is comparable to observing an airplane at night which is heading into your line of sight. You will just see it as a light getting brighter, but it won't actually seem to be moving with respect to distant background objects. So, though this is a good method, its somewhat limited. Another method is to observe the Doppler shifting of spectral lines. Lines in the spectrum of a star will shift slightly into the blue region of the spectrum if the star is moving towards us, and red if its moving away. If the object happens to be moving perpendicular to us, and is too far away to use the first method, then we probably cant get much information on the stars velocity. Fortunately, few objects move exactly in our line of sight or exactly perpendicular, so we can usually get some velocity information for a star, if we look at it enough.

Steve Bloom
for Ask an Astrophysicist

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