BASIC OBSERVING SERIES

by Greg Burnett (gburnett)

Galaxy Observing

[This article was first published in Star Stuff, the newsletter of the Ford Amateur Astronomy Club, in May, 1994.]

Galaxy! The word itself conjures up a universe of romantic notions: "The Galactic Federation" "Battlestar Galactica" "A Galaxy of Hollywood Stars" These days we take for granted our understanding of galaxies (even though many of us fail to appreciate their scale). Galileo became the first astronomical observer of galaxies when he trained his telescope on our own Milky Way and wrote, "The galaxy is nothing other than a mass of luminous stars gathered together." For a long time after Galileo galaxies were simply regarded as "spiral nebula." It was Immanuel Kant who first guessed their true nature in 1755, but his speculations passed unnoticed at the time. The work of Heber Curtis and Edwin Hubble in the early 20-th century finally confirmed their true character. These days the nature of galaxies is largely beyond speculation, even although we continue to work at establishing accurate intergalactic distance measurements.

Galaxies are huge agglomerations of stars, and vast quantities of dust and gas, all bound together by gravity. Galaxies occur in a wide range of sizes; the largest may contain trillions of stars. Our galaxy, the Milky Way, is variously estimated to contain 100 to 200 billion stars. There is a likewise "astronomical number" of galaxies in the observable universe. As early as 1957, over one million galaxies had been counted on Harvard Observatory photographs. It's estimated that over a billion are within reach of the 200-inch Hale telescope on Mt. Palomar, and the Hubble Space Telescope promises to reveal many more. Galaxies are very large, with typical diameters of tens to hundreds of thousands of light years. Given these dimensions, it's interesting to contemplate one astronomer's observation that the universe is more densely populated with galaxies than a galaxy is with stars.

Only three galaxies are visible to the naked eye: the Andromeda Galaxy (M31), and the Large and Small Magellanic Clouds, but hundreds are within the capabilities of amateur telescopes, perhaps a couple thousand using the larger amateur "light buckets." They occur in a variety of shapes and sizes. the classifications devised by Hubble in 1926 are still in common use today, and they recognize three categories of galaxies: elliptical, spiral and irregular.

Elliptical galaxies are disk-like or spheroidal assemblages of stars with little or no internal structure. The class is further divided into subgroups according to their apparent flatness, from E0, the roundest, to E7, the flattest.

Spiral galaxies are those exhibiting an overall spiral structure. Those with tightly wound arms and a prominent central bulge are designated Sa, while loosely coiled systems with inconspicuous nuclei are labeled Sc. The Milky Way is an Sb type with intermediate characteristics. Sometimes stars and gas in the inner regions of a spiral are organized into a straight bar that extends diametrically across the nucleus. These barred spirals are designated SBa, SBb, and SBc. Irregular galaxies are those that show no apparent symmetry at all, and are designated simply Ir. They are sometimes further classified as Ir+ if they are resolvable into individual stars and Ir- if they are not.

For they amateur astronomer, galaxies provide some of the most numerous yet elusive targets. The successful galaxy hunter will use a combination of large aperture and dark skies to stalk his quarry. While there are quite a few galaxies bright enough to be seen in smaller amateur scopes (the famous Messier catalog of "M" objects includes most of the best ones), the vast majority are very faint and are observable only with larger apertures under favorable observing conditions.

Sky Catalog 2000.0, Volume 2 (Sky Publishing) lists over 3000 galaxies with brightness down to about magnitude 15. The "New General Catalog of Nebulae and Clusters of Stars" (NGC) lists almost 8000 objects, most of which are galaxies. A true appreciation of galaxies can only come from observatory photographs. The finest compilation of photos and information about galaxies I have seen is Galaxies by Timothy Ferris (Sierra Club Books). It's a relatively expensive "coffee table book" but it's well worth the cost and is suitable for "non-astronomer" types as well.