Re: Spiral Arms of the Milky Way
- From: Davoud <star@xxxxxxx>
- Date: Sat, 24 Feb 2007 01:03:00 -0500
lanky_lx5 wrote:
Curious...
Our sun and other stars in the spiral arm we are in, (Orion's Arm),
alternately pass through other spiral arms and inter arm regions . So my
question is, do regions where stars form in, like the Orion Nebula, move
in the same way as the sun?
Roughly speaking, yes. The Galaxy is known to be rotating and we and
Orion are rotating with it in such a way that the Orion Nebula will
appear to be where we see it now for a long time to come.
If the stars are dynamic in position over time, does this mean Orion's
belt and stars like Theta, at the heart of Orion, will someday be part
of another arm within our galaxy?
In the Universe there are motions within motions within motions -- more
localized motions matter in this case. Due to perturbations by other
masses (stars, gas clouds, dark matter (maybe), the local motion of a
given body will likely be less regular than its larger motions
(rotation with the rest of the stars of the Galaxy, movement of the
Galaxy itself within the Local Group, and on up the hierarchy.) The
constellations are ephemeral; Orion has looked the way it does today
for a very long time, but in the distant past a hypothetical observer
on Earth would have seen it differently; indeed, if the observer lived
long enough he would have seen Orion taking shape. This goes for the
future, as well; Orion (and the rest of the sky) will look pretty much
the same for a very long time, but not forever. If our observer
continues his very long life he will see Orion break up.
Possibly the most visible short term (circa 26,000 yrs) change is that
the pole star is inconstant due by precession of the Earth's polar
axis. It has not always been Polaris, and it will not always be
Polaris. Please see "Precession" at
<http://www.seds.org/~spider/spider/ScholarX/coord_ch.html> for a good
explanation of some of this motion. This motion, and the motions of
nearby stars are easily detectable in the course just a few years --
well within a human lifetime (also see "Parallax" at the above URL.)
None of this will require you to adjust your polar alignment in the
course of a night's observing, however...
Davoud
--
usenet *at* davidillig dawt com
.
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