Galaxies

All the stars, nebulae, and star clusters you see at night are part of a single galaxy: our Milky Way. But the Milky Way is just one of hundreds of billions of galaxies in the universe. The Hubble Space Telescope and large earth-based telescopes have mapped and catalogued galaxies out to a distance of some 12-13 billion light years. The light from these galaxies left just a couple of billion years after the formation of the universe.  Where stars are the constituents of galaxies, galaxies are the constituents of the universe.

Small telescopes don’t show the most distant galaxies, but hundreds of nearby galaxies lie within reach of a 3” or 4” scope.  A dozen or so can be seen with binoculars.  Though galaxies tend to be dim in small optics, a practiced eye can still detect a surprising amount of detail if the sky is dark and free of light pollution.

Face-on spiral galaxy Messier 101 (image credit: Terry Hancock)

Face-on spiral galaxy Messier 101 (image credit: Terry Hancock)

Galaxies come in a number of shapes and sizes. Our Milky Way is a slightly larger than average spiral galaxy, spread out in a flat plane of stars, with spiral arms winding like a pinwheel from a concentrated nucleus at the center. The spiral arms contain gas and dust out of which new stars are formed, which means diffuse nebulae, dark nebulae, and open star clusters also concentrate in the spiral arms. The image above of the spiral galaxy Messier 101 in Ursa Major shows reddish-pink emission nebulae and bright blue stars along the spiral arms. The Milky Way would look much like this if you could see it from high out of the plane of the spiral arms.

Seen from edge-on, a spiral galaxy reveals the dust lanes of its spiral arms and the bulge of its nucleus, which contains older yellow-red stars left over from the early days of the galaxy’s formation. The image of the galaxy NGC 891 below shows what the Milky Way might look like from the side. In fact, our Milky Way looks much like this in wide-angle photographs, since we see our galaxy edge-on from a point out near its edge, about 25,000 light years from the center.

Edge-on spiral galaxy NGC 891 in the constellation Andromeda

Edge-on spiral galaxy NGC 891 in the constellation Andromeda

To form the spiral shape, a minimum mass seems to be necessary. Most spiral galaxies contain at least 50-100 billion stars and are roughly 100,000 light years across. Two nearby galaxies, the Andromeda galaxy and the Triangulum galaxy, are also spirals and are well within the reach of backyard telescopes and binoculars.

Elliptical galaxies have a much wider range of mass and size, and have a much less complex structure than spiral galaxies. A dwarf elliptical might have a few million stars, not much bigger than a globular cluster. A giant elliptical like M87 might have a trillion stars. Elliptical galaxies may have formed by gobbling up smaller elliptical and spiral galaxies after repeated gravitational encounters over billions of years. Because they lack the gas and dust of a spiral galaxy, elliptical galaxies don’t have much new-star formation going on.

The Magellanic Clouds, two nearby irregular galaxies visible only from the southern hemisphere

The Magellanic Clouds, two nearby irregular galaxies visible only from the southern hemisphere

Irregular galaxies are neither spiral nor elliptical: they have an indeterminate shape. They are usually small galaxies, like the Magellanic Clouds, which are visible from the southern hemisphere, without the gravitational capacity to assume a regular form.  Or they may be a large galaxy like M82 in Ursa Major that’s been mangled by a major gravitational disturbance.