Deep-sky objects, often shortened to DSOs, are any astronomical object that is not an individual star or Solar System object. The classification is used mostly by amateur astronomers to denote visually observed faint naked eye and telescopic objects such as star clusters, nebulae and galaxies. This distinction is practical and technical, implying a variety of instruments and techniques appropriate to observation, and does not distinguish the nature of the object itself.
Th term “Deep-sky object” as an astronomical classification for these objects, has its origins in the modern field of amateur astronomy. The origin of the term is unknown but it was popularized by Sky & Telescope magazine’s “Deep-Sky Wonders” column, which premiered in their first edition in 1941, created by Leland S. Copeland, written for the majority of its run by Walter Scott Houston, and currently penned by Sue French. Houston’s columns, and later book compilations of those columns, helped popularize the term, each month giving the reader a guided tour of a small part of the sky highlighting well known and lesser known objects for binoculars and small telescopes.
Classifying non-stellar astronomical objects began soon after the invention of the telescope. As telescopes improved these faint nebulae would be broken into more descriptive scientific classifications such as interstellar clouds, star clusters, and galaxies.
There are many astronomical object types that come under the description of deep-sky objects; however as the definition is objects that are non-Solar System and non-stellar the list includes:
- Star clusters
- Open clusters
- Globular clusters
- Bright nebulae
- Emission nebulae
- Reflection nebulae
- Dark nebulae
- Planetary nebulae
- Bright nebulae
For further reading on this topic, feel free to see this astronomy.com article on the subject. If you’re looking for a telescope to view these dark sky objects or perform astrophotography, check out our article on helping determine which telescope is right for you.