228 16. THE STRUCTURE OF GALAXIES
Figure 16.2: A selection of spiral galaxies that appear from Earth at different angles of inclina-
tion. Clockwise from upper left: NGC 4565, M 31, M 88, M 74 and M 33. (Images made with
the Aladin sky atlas. DSS2; Bonnarel et al. [2000], Lasker et al. [1996].)
is basic fact about spiral galaxies is reflected in the classification schemes described in Sec-
tions 16.3 and 16.4.
at stars and star clusters in our own Milky Way can be categorized into separate popula-
tions (see Chapter 10) applies to other disk galaxies as well. For most spiral galaxies, the gas and
dust is strongly concentrated to the plane of the disk, and this is where the bulk of star formation
occurs. e halos of disk galaxies, on the other hand, are devoid of the cool, dense clouds of gas
and dust needed for star formation.
e visual appearance of a typical spiral galaxy is misleading. Most of the visible light
comes from the most luminous stars—those of type O and B. A single O star with 30 times
the mass of the Sun can outshine more than 100,000 solar-type stars. ese stars are rare, since
they don’t form as often as lower-mass stars, and they don’t last very long. And so we only see
them in places where star formation is still occurring or only very recently ceased. HII regions,
also associated with ongoing star formation, are highly luminous as well, far outshining their
equivalent weight in low-mass stars.
e spiral arms are much brighter than the regions in between them, but there is only a
small increase in actual density. But even this increased density triggers star formation, and thus
a profound brightening of those slightly denser regions.
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