Distribution of stars in our galaxy

Hello,
Except for the obvious spiral, what other information do we have about the
distribution or structure of stars within our own Milky Way Galaxy?
Do stars usually occur in clusters, such as the Pleiades, or are they just
scattered randomly inside the spiral arms, and such clusters are just the
effects of random chance?

Thank you

"M" == Makhno writes:

M Except for the obvious spiral, what other information do we have
M about the distribution or structure of stars within our own Milky
M Way Galaxy? Do stars usually occur in clusters, such as the
M Pleiades, or are they just scattered randomly inside the spiral
M arms, and such clusters are just the effects of random chance?

The large-scale distribution of stars within the Galaxy is highly
concentrated toward the center. I do not remember off hand the exact
numbers for the Milky Way, but the typical spiral disk has a
distribution that is exponential with distance from the center with a
scale length of order a few kiloparsecs. (More formally, the
distribution scales as exp[-r/R], with R ~ 3 kpc.)

The spiral structure has two components. First, the Milky Way appears
to have at least a weak bar in its center. This can be traced by the
distribution of giant stars, for example. The spiral structure itself
would be most noticable in the distribution of hot, young stars, like
O and B stars.

The O and B stars themselves often occur within clusters, so in that
sense, the presence of O and B stars within clusters is not "random."
The clusters are far more likely to form in giant molecular clouds
(GMC), which themselves are more likely to form in spiral arms, but
the actual location of a GMC, and therefore an O and B cluster, within
a spiral arm is for most purposes "random."

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Thank you for you reply it was most helpful. One query though,

The clusters are far more likely to form in giant molecular clouds
(GMC), which themselves are more likely to form in spiral arms, but
the actual location of a GMC, and therefore an O and B cluster, within
a spiral arm is for most purposes "random."

What is a GMC exactly? A gas cloud, or does the term molecular here refer to
a cloud of a large number of stars?

"M" == Makhno writes:

M Thank you for you reply it was most helpful. One query though,

The clusters are far more likely to form in giant molecular clouds
(GMC), which themselves are more likely to form in spiral arms, but
the actual location of a GMC, and therefore an O and B cluster,
within a spiral arm is for most purposes "random."

M What is a GMC exactly? A gas cloud, or does the term molecular here
M refer to a cloud of a large number of stars?

Nope, "molecular" here refers to the clouds' composition. The clouds
are most often detected by their emission from the CO (carbon
monoxide) molecule. In turn, the CO molecules are thought to trace
the distribution of molecular hydrogen (H_2), which, because of its
atomic structure, does not radiate efficiently and so is quite
difficult to detect. So to answer your original question, GMC are
clouds of gas and dust, within which stars form.

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