|
|
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 a High-Energy Astronomer
|
