Galaxies
Galaxies
As we travel away from our Milky Way, objects in the Digital Universe become less familiar. This is partly because only three objects from the extragalactic realm are visible to the naked eye. In the Northern Hemisphere, the Andromeda Galaxy is visible as a blurry smudge in the night sky. In the Southern Hemisphere, the Large and Small Magellanic Clouds are prominent satellite galaxies visible as dim patches of light that resemble the band of the Milky Way.
Once telescopes were put to use scanning the skies, more faint smudges were seen. Even with a pair of binoculars we can see many extragalactic objects. But are they part of the Milky Way, or do they lie outside our Galaxy? We know the answer to that question today, but it was not until 1923, when Edwin Hubble estimated the distance to what was then called the Andromeda Nebula to be around 1 million light-years away (less than half its actual distance). Astronomers had determined the dimensions of the Milky Way only a few years prior, and this distance was well outside the Galaxy. This put to rest the cosmological debate between those who believed the Milky Way was the entire Universe and those who felt that there was a Universe outside our Galaxy. Hence the Andromeda Nebula was renamed the Andromeda Galaxy and extragalactic astronomy was born.
Astronomers find that galaxies come in a wide variety of shapes, sizes, and colors, from giant, red ellipticals shaped like a watermelon to bluer spirals like our Milky Way. Galaxies that do not fit the elliptical or spiral mold are deemed irregular. These are typically smaller and have bluer stars than spirals or ellipticals. There are even dwarf spiral and dwarf elliptical galaxies. The distinction between objects is not always clear; some astronomers believe a few of the Milky Way's globular clusters could be dwarf galaxies.
Despite this complexity, galaxies, including our Milky Way, have one thing in common: they are composed of stars. Some, like our Galaxy, have hundreds of billions of stars, while others have fewer, 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), spirals (Sa-Sd), and barred spirals (SBa-SBd), along with the separate group of irregular (Irr) galaxies. 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 2.3 million light-years away (that's about 14,000,000,000,000,000,000 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.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott