What Are DSOs?

Deep Space Object (DSO) is a term used for the most part by amateur astronomers to denote visually observable objects such as nebulae, star clusters and galaxies.


Named after the Latin word for “cloud”, nebulae are massive clouds of dust, hydrogen and helium gas, and plasma.  They are also often the place where stars are created.  Most nebulae are vast in size, measuring up to hundreds of light years in diameter. 

The two main types of nebulae are Emission Nebulae and Reflection Nebulae.  An emission nebula is a cloud of high temperature gas where nearby stars are bombarding the gas with ultraviolet radiation.  This radiation excites the atoms of gas causing the nebula to glow.  The process is similar to that of a neon light.  Emission nebulae tend to be red in colour because of the abundance of hydrogen. Additional colours, such as blue and green, can be produced by the atoms of other elements, but hydrogen is almost always the most abundant. Here is an example of an Emission Nebula.

A reflection nebula is a cloud of gas and dust which does not emit its own light (like the emission nebula), but instead shines by reflecting the light from nearby stars. The brightest reflection nebulae are places where new stars are being formed where the gas and dust is thick.

A reflection nebula is created when light from a nearby star is scattered or reflected off a neighbouring dust cloud. The dust tends to filter out the red frequencies of the spectrum leaving the characteristic blue colour of these nebulae. Here is an example of a Reflection Nebula.

There is a third type of nebulae called a Planetary Nebulae.  This name is a bit misleading as they actually have nothing to do with planets.  A planetary nebula is a shell of gas surrounding a low-mass star entering the final stage of its life. The outer shell of gas is created when a dying star’s solar wind expels its outer layers.  When the star has lost enough material, its temperature increases and the ultra violet radiation it emits ionizes the surrounding material causing it to glow. Here is an example of a Planetary Nebula.

Star Clusters

Some of the stars in the universe are part of multiple star systems known as star clusters. Most appear to be part of a binary system where two stars orbit a common center of gravity. A few are even part of a triple star system. But some stars are also part of a larger group. They can be found together in associations known as star clusters. They vary greatly in size and shape as well as the number of stars. They also vary in age from just thousands of years to billions of years old.

There are two types of star clusters, Open Clusters and Globular Clusters. Open clusters usually contain somewhere between a dozen and a thousand stars. They are held together by mutual gravitational attraction and have a common center of mass. Open star clusters are composed of hot, relatively young stars and it is estimated that there are about 20,000 open star clusters in our galaxy. Here is an example of an Open Cluster.

Globular clusters are densely packed collections of ancient stars. Roughly spherical in shape, they contain hundreds of thousands, and sometimes millions, of stars. Here is an example of a Globular Cluster.


Galaxies are defined as large groupings of stars, dust and gas all held together by gravity. Most of the objects we know of in space are contained within galaxies and they can vary greatly in size and shape. They typically contain stars, planets, moons, comets, asteroids, nebulae, dust  and black holes. Since most of the space between galaxies is thought to be empty, a galaxy is essentially an oasis in space. Our own solar system is located in the Milky Way galaxy, about 3/4 of the way out from the center of one of its spiral arms and our Sun is only one of over 100 billion stars in the Milky Way. All of the stars we can see in the night sky are part of the Milky Way. Here is an example of a Galaxy.

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