What’s Out Tonight?

A general ASTRONOMY site to get you started exploring the night sky

Deep Sky Objects

When new amateurs get “tired” of observing the Moon and Planets and ache for more, they often start to search for fainter objects like clusters of stars, nebulae and galaxies that are collectively called Deep Sky Objects (DSOs) which were first catalogued in the late 1700s by the Frenchman Charles Messier from Paris. Everyone has seen pictures of these objects for they represent the biggest and brightest of the lot.

The six rotating photos to the right represent the six categories of Deep Sky Objects (DSOs) (read more below). These photos are spectacular because they were taken by some of the most advanced professional telescopes so don't expect to see anything like these visually through a telescope. 

What is it with these M numbers? If you see or hear someone say M followed by the number 1 through 110, it refers to the very first catalogue of Deep Sky Objects compiled by Frenchman Charles Messier in the late 1700s. His list of objects are most of the biggest and brightest as seen from the Northern Hemisphere. Messier did not put an M in front of his entries—that is a modern designation to signify his catalogue.

Catalogue of the Messier “M” Objects

Deep Sky Objects (DSOs)  include 1. Open Clusters (like the Pleiades), 2. Globular Clusters, 3. Nebulae, 4. Planetary Nebulae, 5. Supernova Remnants, and 6. Galaxies, that is, distant objects beyond our solar system. Individual stars are not considered deep sky objects. Except for galaxies, all deep sky objects that we normally observe are inside our Milky Way Galaxy.  

Star Clusters  
The term “star cluster” is a general term referring to an open cluster or globular cluster. An Open Cluster represents a group of stars born together out of the same nebula, anywhere from a dozen to a thousand or so. Globular Clusters are different from open clusters, being a collection of several thousand to a million stars compacted into the shape of a ball. Our galaxy has about 200 globular clusters surrounding it in a spherical halo. In small telescopes, open clusters appear better than in many photographs. More than any other heavenly object, globular clusters really come “alive” and are breathtaking in larger telescopes (12-inches in diameter plus). 

The word nebulae is a general term that refers to gaseous hydrogen clouds. There are several types of nebulae: Galactic Clouds, Planetary Nebulae and Supernova Remnants. Galactic Clouds represent birthing places of stars. They can appear dark, like silhouettes or bluish/reddish in color because they either reflect starlight or create their own light. These nebulae are located mostly in the arms of our Milky Way Galaxy.

The best example is the Great Orion Nebula, M42. Planetary Nebulae (the name has nothing to do with the planets, it’s just an old name that stuck because some of these nebulae are roundish, like the planets) represent the remains of outer atmospheres from “large” stars in their death throes. These nebulae are spherical, ringed or have diametrically opposed lobes. The best example is the Ring Nebula. Supernova Remnants are nebulae created from the explosions of very large stars at the ends of their lives. M1, the Crab Nebula is a good example. Supernova Remnants are rare compared to other nebulae. On average, nebulae are faint, so they appear better in larger amateur telescopes. However, there are a handful that are magnificent even in small telescopes. One of the best is the Orion Nebula, M42. 

Galaxies represent groupings of billions of stars. All galaxies lie outside our Milky Way Galaxy at distances in the millions to billions of light years. The most common and distinctive looking galaxies are categorized as “spiral,” which have several curved arms radiating from a bulged center or nucleus. However, there are also ellipticals which resembles balls or elongated balls. Finally, irregularly shaped galaxies have “mixed-up” insides. Galaxies are faint and appear brightest in larger amateur telescopes. The most well-known, and one of the few that can just be seen with the naked eye is the Andromeda Galaxy, M31. Galaxies do not show much detail in small or larger amateur telescopes. They are generally faint and diffused, no matter the size of the telescope. Galaxies appear their best in photographs.

Charles Messier

Compiled the very first Deep Sky Object catalogue in the late 1700s listing 110 objects that is still used today by amateurs and professional astronomers

The Messier Deep Sky Object catalogue represents the cream-of-the-crop deep sky objects that can be seen from the mid-latitudes of the northern hemisphere. It was compiled at the end of the 1700s by Charles Messier from Paris, France, using telescopes around 3 to 4-inches in diameter. This catalogue is historically significant because it is the very first catalogue ever compiled of deep sky objects. And, since it lists the biggest and brightest objects in the sky, it has become a logical “next step” for amateurs wanting to go beyond observing the Moon and Planets. 

An interesting point about this catalogue is that it has at least one example of every type of deep sky object that exists, so it represents a good sample of the objects that can be found in the heavens. 

There is a quirk of nature that allows viewing all of the Messier objects in one night. This can be accomplished around New Moon during March. This event has become known as a Messier Marathon (click for more info) and many astronomy clubs sponsor “parties” to accomplish this dusk to dawn task. 

Charles Messier was born in Badonviller, France in 1730. His father held a mayoral-type position in the town but passed away when Messier was 11. 

Hyacinthe, Charles’s brother, trained Charles as an administer’s assistant and eventually found Charles a job in Paris as an assistant to an astronomer. 

Messier did exceedingly well at his job, advanced, and became, during his time, the leading observational astronomer in the world. He eventually acquired his bosses’ position as Astronomer of the Navy. During his career, he wrote numerous articles that spanned the field of astronomy and were published in the leading scientific journals of the time. One of his most notable life-long achievements was discovering about 20 comets, which lead to his induction into almost every European science academy.

Charles Messier
From Paris, France, compiled the very first catalogue of Deep Sky Objects (DSOs)

Messier never would have believed that his namesake would be defined by his little catalogue—he would have thought it would have been his comet discoveries. He catalogued deep sky objects because he realized that such a catalogue was missing in the field of astronomy (astronomy and most sciences were just starting to get organized during this time in history). To start the catalogue, he used a few short lists of deep sky objects complied by other astronomers but quickly added objects he found exploring the night sky. Three editions of his catalogue were published, each growing in size, with the last published in 1781, listing 103 objects. He stopped adding objects because in 1785, William Herschel, inspired by Messier’s catalogue, published a catalogue listing about 1,000 objects using a 18.7-inch diameter telescope. Messier knew he could not compete! 

Although Messier’s last catalogue listed just 103 objects, seven additional objects have been added—objects that he described in notes or other publications but never listed in his catalogue. 

Messier passed away in 1817 at his residence in the Cluny Hotel, (near the Sorbonne and about a 7 minute walk from Notre Dame) now known as the National Museum of the Middle Ages. His observatory was atop the front tower but no trace remains.

Catalogue of the Messier Objects