For many amateur astronomers with small telescopes, spring is galaxy season. At any moderately dark site, which includes most of the Four Corners, you can see scores of these faint, fuzzy, blotches. Unfortunately, fuzzy blotches are all that one can typically see without either photographic assistance, or a bigger scope. With practice, however, you can learn to distinguish all the subtle variations in the shapes of these objects.
Galaxies have historically been categorized into three general groups – elliptical, spiral and irregular.
Many people are familiar with the classic spiral galaxy shape – a flat disk that includes a spiral arm structure, and a central bulge. The Milky Way is a spiral galaxy, but it is in a sub-class called a barred spiral because the central bulge is stretched out into a bar going across the disk. The details of the Milky Way’s shape are less well-known than most other galaxies because it is the one galaxy we can’t photograph from the outside.
The stars in the spiral arms are called Population I stars. They are generally young, hot and blue, and are associated with active star forming regions. In photographs, the arms are very colorful. They have the blue Population I stars, red emission nebulae and dark dust clouds. Stars in the disk move in an orderly fashion in the disk that rotates around the core. Population II stars in the core are typically older and cooler, and their motions are very random around that central region. In photographs, the Population II stars will have the yellowish appearance of a low-wattage incandescent light.
Elliptical galaxies, as the name implies, are elliptical in shape. The ellipses can range from elongated football shapes, to ones that are almost perfectly spherical. This other common galaxy type includes galaxies that can be much more massive than spirals. Ellipticals have no active star-forming regions like those found in the arms of the spiral galaxies. In many respects, they are just giant galaxy cores with no disk. Because they have mostly older Population II stars, elliptical galaxies appear yellower in photographs.
Irregular galaxies are the leftovers that are not obviously either elliptical or spiral. Some of them have star-forming regions with Population I stars, but some don’t. All galaxies have at least two things that you won’t get to see. They each have a supermassive black hole at their center. And extending far out beyond the visible region is a halo of mysterious dark matter.
For completeness, I should mention Population III stars, but those aren’t in any nearby galaxy. Those are stars that formed in the earliest years of the universe and have almost no elements heavier than helium.
This monthThe constellation Leo is high in the southern sky. Leo can be found most easily by looking for the backward question mark, called the sickle, with the bright star Regulus as the dot underneath. If you are familiar with the pointer stars in the Big Dipper – the two stars at the end of the dipper that point towards Polaris – you can use these same two stars but go in the opposite direction to get to Leo. Algieba, a little higher up in the sickle from Regulus is a good double star for a small telescope. The two stars in the double are about one magnitude apart in brightness, and can provide a pleasing view.
If you have an available telescope, there are two nice groupings of galaxies to look for. M95, M96 and M105 are set below the Lion’s belly. They are all within 3 degrees of each other but may not be quite close enough together to be in the same field of view. Below the tail of Leo, M66, M65 and NGC 3628 make up the Leo Trio, a much tighter grouping.
Following Leo in a large region to the east, is the Virgo cluster of galaxies. A small telescope can make out scores of these galaxies, but unless you have a detailed chart, or lots of practice, it is difficult to tell which is which. But even if you can’t tell them apart, it is fun to see something so distant.
The Lyrid meteor shower peaks on the night of April 22.
The full moon on April 27 counts as a supermoon because it occurs when the moon is near perigee.
Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. Reach him at [email protected]