Dominating the Local Group

When astronomers look to the night sky, whether they happen to be professionals or amateurs, they see a universe filled with galaxies. The galaxies that stand out most, not surprisingly, are the big, bright ones. In fact, once you look more than a few tens of millions of light-years away, those are about the only ones to be seen. Distant, faint galaxies simply fade into the background sky.

Astronomers call those galaxies that nearly disappear into the background "low-surface-brightness galaxies," or LSB galaxies. There is no hard-and-fast rule as to what constitutes an LSB galaxy, in large part because observational techniques have improved so much over the years. A reasonable starting point would be a surface brightness for the galaxy's central region of around magnitude 24 or 25 per square arcsecond or fainter. On the photographic plates of the original Palomar Observatory Sky Survey, such galaxies show up as no more than faint puffs. (However, they show up fairly well on the second-generation Palomar Sky Survey.)

To get a good sample of what the cosmos contains, it's best to look close to home, where most LSB galaxies can be discerned. In the Local Group — the collection of some 40 galaxies spanning roughly 6 million light-years to which the Milky Way belongs — only two galaxies qualify as big and bright: the Milky Way and Andromeda galaxies. And only three others definitely rise above the level of an LSB galaxy. These are the Pinwheel Galaxy in Triangulum and the Milky Way's two brightest satellites, the Large and Small Magellanic Clouds.

That leaves some 35 members of the Local Group — nearly 90 percent — as LSB galaxies. They may not look like much, but these seemingly insignificant wisps account for the vast majority of galaxies. Although they rank among the more challenging objects to see through a backyard telescope, a few of the Local Group's LSB galaxies can be glimpsed by patient observers with large scopes.