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C H A P T E R 16
e Structure of Galaxies
A galaxy is a collection of its parts—individual stars, star clusters, gas, and dust. Each piece orbits
according to the laws of gravity and motion, moving according to the complex gravitational field
of all of the other parts combined. And so the motion of any one star affects the motions of all
the others. And yet, out of this unimaginable complexity, an ordered structure often arises.
Edwin Hubble was the first to make a practical classification of the variety of galaxy shapes
observed in telescopes. is Hubble Classification Scheme [Hubble, 1982] has been expanded
over the years since Hubble first proposed it, by Alan Sandage, Gerard de Vaucouleurs, and
others [Binney and Merrifield, 1998, Section 4.1.1]. e modern system distinguishes between
several possibilities. Does the galaxy have a disk with a central bulge, is it irregular in shape, or
does it appear smooth and elliptical?
1. If it has a disk with a bulge:
(a) How big is the bulge, relative to the rest of the disk?
(b) Are there spiral arms, showing evidence of star-forming activity, with a layer of dust
in the center of the disk? Or does the galaxy instead look smooth and uniform?
(c) Are their many indistinct, tightly wound spiral arms, or just two loose spiral arms
making something like the letter “S”?
(d) Do the spiral arms spiral into the bulge, or instead do they begin at the ends of a
central bar that passes through the bulge?
(e) Is there a ring surrounding the bulge? If so, is the ring in close to the nucleus, or is it
way out at the edge?
2. If it is elliptical:
(a) Is it tiny, huge, or more like the size of typical spiral galaxies?
(b) How elongated is it? Is it essentially spherical? Or is it stretched out like a long oval?
3. If it is irregular:
(a) Is it smooth and uniform in texture, with no hints of gas or dust?
(b) Does it instead show complex knots of gas, dust, and star-forming activity?
(c) Is it tiny compared to other galaxies?
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