16.5. IRREGULAR AND PECULIAR GALAXIES 231
e first four galaxies in Figure 16.4 are examples (from left to right and top to bottom), with
classifications from de Vaucouleurs [1963].
• SA(s)ab: A normal s-shaped spiral (no ring) with a rather large bulge and tightly wound
spiral arms, intermediate between Hubble-type Sa and Sb.
• SB(s)b: A barred s-shaped spiral with an intermediate-size bulge—equivalent to Hubble
type SBb.
• SB(r)a: A barred spiral like Hubble type SBa, but with a clear ring connecting the ends
of the bar.
• SAB(rs)c: Like a Hubble-type Sc galaxy, but with a hint of both a bar and a ring.
e difficulty of deciding between the finely tuned categories in this more-complex
scheme is apparent from the lower-left example in Figure 16.4. Is it really midway between
a barred and normal spiral? And is it really in between s-shaped and ring-shaped?
e image at bottom center is categorized as a d-type, barred, s-shaped spiral, but it looks
like an irregular blob. e image on the bottom right is also of the same galaxy, and the barred
spiral shape is more evident. e difference is that the center picture shows the galaxy in blue
light, while that on the bottom right shows it in near-infrared light. is highlights another
complication of galaxy classification—galaxies often appear quite different depending on the
wavelength of the light that is used to image them. Blue light highlights the blue-colored O
and B stars that mark regions of star formation; red light de-emphasizes these stars. But red
light does emphasize HII regions, which are also associated with star formation. Near-infrared
light, on the other hand, tends to suppress both high-luminosity stars and HII regions, better
revealing the more-common, lower-mass stars.
16.5 IRREGULAR AND PECULIAR GALAXIES
A galaxy is irregular if it cannot be fit neatly in to one of the other morphological categories of
elliptical or spiral. But it is not quite so simple as that; the morphology of irregular galaxies is not
entirely random. In particular, some irregular galaxies have a hint of spiral structure, especially
those with a lot of star formation in regions of gas and dust. e Large Magellanic Cloud, visible
from the Southern Hemisphere is a good example (see Figure 2.17). It is often classified as a
barred spiral rather than an irregular galaxy. For this reason, irregulars are often categorized as
a natural extension of the classification scheme for spiral galaxies. ere is no clear evidence,
however, that this observed morphology is related to any particular physical or evolutionary
cause.
A dwarf irregular is a loose aggregation of stars with little central condensation. Some
have fewer stars than the largest globular clusters in our own galaxy, although they are much
larger in volume. Some dwarf irregulars (the Small Magellanic Cloud is an example) have a lot
..................Content has been hidden....................
You can't read the all page of ebook, please click here login for view all page.