Digital Universe Guide - Hayden Planetarium

148 4. THE EXTRAGALACTIC ATLAS
less diverse stellar populations. Some have active, ongoing star formation, and others are relatively
inactive, dead galaxies.
The variety of galactic shapes and colors was categorized by Hubble in the early 1930s. Formulating
three main branches, Hubble identified elliptical galaxies (E0–E5, S0, SB0), spiral galaxies (Sa–Sd) and
barred spiral galaxies (SBa–SBd), along with the separate group of irregular galaxies (Irr). Each has a
range of types within the main branch (noted in parentheses) that refer to the shape or other observed
attributes of the galaxy. For example, an E0 galaxy is rounder, while an E5 galaxy is oblong. An Sa
galaxy has a large bulge with tightly wound spiral arms, while an Sd galaxy has a small bulge with
looser arms.
The Milky Way is among roughly three dozen galaxies called the Local Group. Most of the galaxies
in this group are small dwarf or irregular galaxies, with the exception of three spirals: Andromeda, the
Milky Way, and M33, a face-on spiral in Triangulum. Andromeda is about 2.5 million light-years away
(that’s about 15,000,000,000,000,000,000 miles) and is the farthest object you can see without the use
of binoculars or a telescope. Beyond these neighboring galaxies, it becomes difficult to see the
individual stars that make up the galaxy, even with a telescope. Instead, astronomers see a smooth
distribution of light, bright at the galactic core and dimming toward its edges.
Looking out to 100 million light-years, we begin to see how these galaxies are constructed to form
our Universe. Galaxies do not appear to be haphazardly placed in the Universe. They are organized
into clusters and superclusters. The Milky Way is in the Local Group, and the Local Group is part of the
Virgo Supercluster. Galaxies also form in filaments, connecting galaxy clusters with strands of galaxies.
Large voids occupy space like the inside of a bubble, where space is relatively empty. The clusters,
superclusters, and filaments of galaxies are collectively referred to as the “large-scale structure” of the
Universe, and astronomers have spent much of the past three decades coming to understand it.
4.1.2 Quasi-Stellar Objects (Quasars)
If you look at the Andromeda Galaxy tonight (assuming it’s visible), you are looking at a galaxy that is
2.5 million light-years away. The light that reaches your eye left the galaxy 2.5 million years ago, so in a
sense, you are looking at the galaxy when the Universe was 2.5 million years younger. That’s a short
time on cosmic scales, but what about the farthest objects we see that are billions of light-years away?
At that distance, even galaxies become too dim to see easily.