constellation question

In message , Paul Schlyter
writes
In article ,
Tim923 wrote:

I assumed that stars in a constellation weren't actually close to each
other in space, that they only appeared close in our 2D viewing of
them, but I have a galaxy picture that would suggest otherwise.
Clarification?

Your first assumption is correct: stars in the same constellation
merely happen to be positioned in approximately the same direction
from us. They can be at any distance from us. And their distances
do vary widely -- consider for instance the Summer Triangle, where
Deneb is approx. 100 times farther away than Vega or Altair. OK
the Summer Triangle is not a constellation, it is a large asterism,
but we use it frequently to orient ourselves in the sky, so the
same principle applies.

There are exceptions, of course. For instance, all but two of the stars
in the Plough are part of a cluster (it's not a modern constellation,
but it used to be. I was amused to read that the Arabs originally
described the region as a bier but later adopted the bear interpretation
of many other cultures !)
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In article ,
Jonathan Silverlight wrote:

In message , Paul Schlyter
writes
In article ,
Tim923 wrote:

I assumed that stars in a constellation weren't actually close to each
other in space, that they only appeared close in our 2D viewing of
them, but I have a galaxy picture that would suggest otherwise.
Clarification?

Your first assumption is correct: stars in the same constellation
merely happen to be positioned in approximately the same direction
from us. They can be at any distance from us. And their distances
do vary widely -- consider for instance the Summer Triangle, where
Deneb is approx. 100 times farther away than Vega or Altair. OK
the Summer Triangle is not a constellation, it is a large asterism,
but we use it frequently to orient ourselves in the sky, so the
same principle applies.

There are exceptions, of course. For instance, all but two of the stars
in the Plough are part of a cluster (it's not a modern constellation,
but it used to be.

That's not an exception of "the rule", which merely says that stars
within one constellation CAN be at widely different distances. But
that's not a requirement -- stars within the same constellation can
be at more or less the same distance to us as well, and can even be
physically related, such as the Ursa Major stream you refer to here.
Other examples are the Pleiades, the Hyades, the Coma cluster, and
the Alfa Persei association. But the Ursa Major stream is the cluster
which is closest to us -- and some stars belonging to it are even
situated in other constellations, such as Sirius and also Alfa Coronae
Borealis. It has about 100 members, and our Sun is situated in the
outer parts of this "cluster", but it's not a member of it.

The Ursa Major stream even has a catalog number: Collinder 285.

http://www.seds.org/messier/xtra/ngc/uma-cl.html
http://observers.org/tac.mailing.lis...uary/0358.html


I was amused to read that the Arabs originally described the region as
a bier but later adopted the bear interpretation of many other cultures !)

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