The Local Group of galaxies is our Milky Way's neighborhood: a few dozen galaxies dominated by our own, as well as Andromeda and a handful of largish ones.
But by far the majority of galaxies in our group -- the majority everywhere, really -- are dwarfs; small collections of stars and gas with only a few million stars. Lurking just 1.6 million light years away, half the distance to Andromeda, is Barnard's galaxy, an irregular dwarf with about 10 million stars. Behold!
[As usual, click to embiggen, or get a ginormous 80 Mb TIF.]
This is perhaps the finest near-true color image of Barnard's galaxy I have ever seen, courtesy of the European Southern Observatory's 2.2 meter telescope in Chile... a far cry from the small 12.5 cm (5 inch) refractor Barnard himself used to discover it. Given that this diminutive galaxy is in the direction of Sagittarius -- toward the center of our galaxy, which is loaded with stars -- it's incredible Barnard found it at all. In fact, most of the stars in this image are inside the Milky Way, between us and the other galaxy.
Barnard's Galaxy is not precisely irregular, since it appears to have a bar-like structure across it. The red bubbles are regions where stars are being born in large numbers; the UV and fierce winds of subatomic particles from the massive stars being formed carve out cavities in the gas, creating what look like smoke rings. The red glow is characteristic of hot hydrogen, and is a sure-fire way to know that gas is being excited by the stars nearby. The sharp edges to the bubbles are real, due to the gas piling up as it rams gas in interstellar space in a cosmic snowplow effect. There are over 150 separate bubbles like these in the galaxy, some of which have been observed using the Hubble Space Telescope.
There is another feature here that's not obvious. On the left and right of the galaxy are faint, thick, blue arcs. This is actually gas that's being blown out by the stars in the center of the galaxy, forming a weak ring surrounding the entire structure. I expect that gas will blow right out of the galaxy entirely; the gravity from the meager number of stars making up Barnard's Galaxy can't possibly be enough to restrain it.
Dwarf galaxies are difficult to observe because they are so faint; they fade with distance rapidly, so only nearby ones can be studied in detail. But since they are the most numerous types of galaxy in the Universe, these tiny smudges are well worth studying, and images like this one of Barnard's Galaxy will help us understand how these galaxies formed, and what they're made of, and how they behave. We also think that through collisions, these galaxies can grow to become larger, more magnificent ones like our Milky Way, so, as usual, when we study the Universe, looking ever outward, we are actually turning our gaze inwards to learn more about ourselves.
Image credit: ESO.