Catalogs can guide your exploration of deep sky objects

Saturday, Mar 13, 2021 4:03 AM

Greetings, stargazers.

It has been several years since I looked up a phone number in a printed book. But a generation ago, a phone book was an essential tool for anyone with a telephone. It had the list of everyone by name and how to contact them. Astronomical catalogs serve much the same purpose. These catalogs can provide the essential details of a celestial object’s location, brightness and type.

If you can think of a class of astronomical object (star, galaxy, etc.), there is probably a catalog for it. Think of the number of different lists of Hollywood movies you can find on the internet, and then compare the number of movies with the number of stars there are. One big difference between astronomical catalogs and movie lists is that most astronomical objects don’t even have a common name and are only identified with a number and location.

Of particular interest to many amateur astronomers are not the lists of stars, but of the many things that are not stars. Diffuse, or spread out, objects can include star clusters, galaxies and glowing interstellar gases, and were historically lumped into the general term nebulae, or fuzzy thing.

Perhaps the most famous list is the one compiled by Charles Messier in the late 18th century. Messier was a comet hunter, and his list was simply a list of things that were not comets. It was never intended as a list of cool things to go look at. The 110 objects in his list include galaxies, open star clusters, globular clusters, diffuse nebulae, planetary nebulae, a supernova remnant and even a double star. These were simply things that might be mistaken for a comet, and he didn’t want you to waste time seeing if they moved. These things are called deep sky objects because they are typically much farther away than most visible stars in the night sky.

Today, many of the favorite targets of backyard astronomers are identified by their number in Messier’s list. Quite a few are visible to the naked eye, and several of those have common names. For example, the Pleiades, or Seven Sisters, is Messier object No. 45, or just M45. The nebula in Orion is M42, and the Andromeda Galaxy is M32.

A much more modern list compiled in 1995 is called the Caldwell catalog. Intended for backyard astronomers, this is a list of the 109 best things to look at that Messier omitted. Because Messier was viewing from France, everything in his list is visible from the Northern Hemisphere. Besides some bright Northern Hemisphere objects, the Caldwell catalog includes things only visible in the Southern Hemisphere.

The New General Catalog of Nebulae and Clusters of Stars, first published in 1888, is a much more complete list of deep space objects, going well beyond Messier’s not-a-comet list. John Dreyer compiled this list of 7,480 objects based on previous compilations by William and Caroline Herschel. Almost all deep space objects visible through an 8-inch telescope will have an NGC number. Within a few decades after it was published, the catalog was expanded with the Index Catalog, which gave IC numbers to thousands of more objects. Revisions, corrections and updates have continued into the 21st century.

This month

From mid-March to mid-April, it is possible to do a Messier Marathon and view all 110 Messier objects in a single night. Many objects would need a telescope to view, but an easier challenge would be to see how many you could find with binoculars.

Mars is still visible in the evening sky, but much dimmer than it was last summer. It is near the Pleiades in the western sky, and slightly dimmer than the nearby red star, Aldebaran.

Jupiter and Saturn are visible in the eastern sky before dawn, and Venus is passing behind the sun, so it won’t be visible until near the end of April.

Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. He can be reached at hakes_c@fortlewis.edu.