The Four Types of Observable Deep Sky Objects

Astronomy has become more popular since the pandemic began, as more people are staying at home. It is a fascinating and very rewarding hobby, yes, but the learning curve can be very steep. Stargazing applications have made it much easier to find celestial objects in the night sky, but there are thousands of objects up there, and if you are starting out, choosing what to observe can be overwhelming. In this article, we will describe the four most common deep-sky objects.

What constitutes the “deep sky”?

In astronomy, the deep sky is the area outside of the solar system. The different planets and their satellites, the sun, asteroids, comets and the moon don’t count. Because the deep sky is so far away, some of its different components are small, and therefore not very bright. However, they can be observed with a telescope. In the depth of the sky, you will find star clusters, nebulae and galaxies. 

The star clusters

A star cluster is a group of stars that were formed under similar conditions in a relatively short time, is evolving under the same conditions, and is physically linked by gravity. There are two types of star clusters: open clusters and globular clusters.

The open clusters

These much newer clusters are mainly composed of young blue stars. These stars are generally concentrated in a relatively small space (a few hundred light-years in diameter, about 6 thousand times the size of our solar system!) The Pleiades (M45) is an open cluster of stars that can be observed in the northern hemisphere, in the constellation of Taurus. This cluster is the most famous open cluster in the sky because it can be seen with the naked eye, binoculars, and telescopes even in light-polluted cities.

Check out the complete list of open clusters.

The globular clusters

These lie far beyond our galaxy. A globular cluster is very dense and rich in stars in orbit around the heart of the host galaxy. On average, a globular cluster contains a hundred thousand stars. This large quantity of stars is grouped together in a sphere whose diameter is about twenty to a few hundred light-years. We know of a few more than 150 globular clusters in the Milky Way, our galaxy, but astronomers have also observed them in other galaxies. When it comes to observing globular clusters, the larger the telescope aperture, the better. However, some clusters can be seen with just a pair of binoculars.

Check out the complete list of globular clusters.

Nebulae

Nebulae are collections of interstellar gas and dust clouds. There are two types of nebulae: diffuse nebula and planetary nebulae. They are very popular subjects among astrophotographers, and as a result, you can find many breathtaking nebulae pictures online. 

One very famous deep-sky object is the splendid Orion nebula (M45), which is the brightest nebula in the Northern Hemisphere. It is located in the constellation of the same name and can be observed from late October to early March. The rich colors of this cloud of gas and dust are not discernible with the naked eye but can be admired with a telescope. There are a total of 10 nebulae listed in the Messier catalog, seven in the Herschell 400 Catalog, and 25 in the Caldwell Catalog — These catalogs comprise nearly all of the most spectacular examples of the four types of deep-sky objects. Enough to keep you busy for a while!

Galaxies

Galaxies are made of stars, gas and dust and a black hole in its center. Galaxies can be really large, up to 100,000 light-years in diameter. Each of them can contain about 350 billion stars. Most of them have a circular disc-like shape with spiral arms at the ends.

The closest galaxy to us is Andromeda. It is located at a distance of 2.5 million light-years from us. It is hard to see with the naked eye even though it is actually quite large in the sky, about 5 times the size of the Moon! It’s a popular target for people starting out in astrophotography because it is relatively easy to take a picture of with a DSLR camera.

Stargazing is without a doubt one of the most rewarding hobbies. Being able to observe the wonders of the universe with your own eyes from your backyard is incredible. If you don’t know where to start, I would recommend purchasing two essential items: a telescope and a stargazing book. The best type of telescope for beginners are Dobsonian telescopes, they are very easy to use and the best value for the money. Next, pick up “50 Things to See with a Telescope” by John Read, a fantastic stargazing guide that will help you learn to navigate the night sky and locate any celestial target. Happy stargazing!