GALAXY SORTING
by Sally Stephens
Background Information on Galaxy Classification
In 1924, astronomer Edwin Hubble proved that galaxies are very distant "island
universes" -- each one a collection of millions or billions of stars bound together by gravity.
Within a few years, he had set up a s ystem to classify them that is still used today, albeit with
some modifications and additions.
Hubble's sys tem divides galaxies into three basic categories: 1) elliptical galaxies, relatively
featureless spherical or ellipsoidal (football-shaped) collections of stars; 2) s piral galaxies, with
their distinctive arms of stars that spiral out from their centers; and 3) irregular galaxies, a catch-
all category for galaxies that don't look like either traditional ellipticals or spirals.
Elliptical galaxies, marked by the letter E, are further subdivided depending on how well-
rounded they appear. A number is added that ranges from 0 to 7, with completely round
ellipticals denoted by 0, and flattened systems (which look like a squashed football) denoted by
7. An E5 galaxy, for example, is not very spherical, appearing twice as long as it is wide.
Spiral galaxies are divided into two main type s: regular spirals, denoted by the letter S, in
which the arms spiral outward from the galaxy center, and barred spirals, marked SB, in which
the arms wind outward from the ends of a straight "bar" of stars that passes through the center.
About two-thirds of all spiral galaxies have some kind of bar. Indeed, astronomers have recently
seen evidence of a bar in our own Galaxy's center, making the Milky Way a barred spiral. Both
regular and barred spiral galaxies are defined by a spherical bulge of stars at their center, which is
surrounded by a thin, rotating disk of stars that contains the spiral arms. You can tell the
direction in which the disk rotates by looking at the spiral arms; they trail behind the direction of
rotation, much like the coattails of a runner or water thrown out by a twirling lawn sprinkler.
In addition, both regular and barred spirals are subdivided according to how tightly wound
the spiral arms are and how prominent the central bulge of stars appears. In Sa galaxies, the spiral
arms are tightly wound and the central bulge appears bright, whereas in Sc galaxies, the arms are
more loosely wound and the central bulge is much less prominent. Sb galaxies fall in between the
two. Similar criteria apply to barred spirals, which are denoted by SBa, SBb and SBc. Irregular
galaxies have no subdivisions.
There is also a class Hubble called S0 galaxies, which have characteristics of both
ellipticals and spirals, showing a large central bulge and a disk, but no obvious dust lanes or spiral
structure.
Regardless of the ty pe, galaxies come in all different sizes. It's impossible to say how
large a galaxy is based solely on its photograph. A ver y large galaxy that is also very distant
might look like it is the same size as a much smaller one that's nearby. You have to know its
distance to know a galaxy's true size. Our own Milky Way Galaxy is roughly 100,000 light years
