Author: Silvia DiPaola
Editor: Liane Xu
Artist: Tiffany Chen
In the vast expanse of space lies billions of distant galaxies, all ranging in size, shape, and contents. A galaxy is a large collection of gas, dust, millions of stars, and the respective solar systems that they form. Dwarf galaxies are especially interesting. They are the most abundant galaxy type in the universe, but they are quite difficult to identify. What makes dwarf galaxies so special?
The criteria to be classified as a dwarf galaxy include a low surface brightness and little or no central concentrations of light, as per the Palomar Sky Survey. Dwarf galaxies are difficult for astronomers to spot because of their low luminosity, low mass, and relatively small sizes. Dwarf galaxies contain only a few thousand to a few billion stars, compared to the Milky Way’s estimated 200 to 400 billion stars; the Milky Way is larger than all of these 50 smaller galaxies put together! Typically, we don’t see many dwarf galaxies from Earth because they are relatively small and dim compared to huge ones like ours. Furthermore, Dwarf galaxies are often found near larger galaxies, and they sometimes collide with and merge into their larger neighbors. Approximately 20 dwarf galaxies orbit the Milky Way Galaxy, held in place by the Milky Way’s strong gravity.
Interestingly, Dwarf galaxies can form by branching off of larger galaxies, but the vast majority of them likely formed during the early years of the universe. It has been hypothesized by astronomers that larger galaxies like our Milky Way were formed from the merging of smaller dwarf ones. The Milky way continues this process even today; right now, our galaxy is pulling in and absorbing the Sagittarius Dwarf Spheroidal. As for the recent past, astronomers have evidence that the Milky Way has absorbed up at least 5 and possibly as many as 11 small galaxies in the past few hundred million years. These mergers do not happen because of massive bursts of star formation when two large galaxies come together. Instead, they happen when the Milky Way slowly destroys and absorbs small dwarf galaxies that have been unfortunate enough to come too close to it.
Specific types of dwarf galaxies include dwarf elliptical, spiral, and irregular. Dwarf elliptical galaxies are similar to their normal-sized counterparts in that they have the same global properties seen in normal elliptical galaxies, but they are just on a smaller scale. However, the dwarves have low surface brightness, differentiating their light distributions from that of a normal elliptical galaxy. Dwarf Spiral galaxies are more spiral in shape and smaller in size than elliptical ones. Dwarf Irregular galaxies have similar properties to their larger irregular galaxies. They also contain substantial amounts of gas and dust and support ongoing star formation. One example of a dwarf galaxy near us is the Large Magellanic Cloud, which is located about 160,000 light-years from Earth. It is about 1/10th the mass of the Milky Way, and it has approximately 10% of the Milky Way’s 200-400 billion stars.
Dwarf galaxies are especially interesting due to their small size but high prevalence in the universe. Scientists estimate that there may be trillions of them in the universe, and most of them can be found in the orbit of other large galaxies. Although dwarf galaxies are considered “small” on an astronomical scale, they can still contain billions of stars, which is a mind-bending concept! Scientists emphasize the importance of understanding dwarf galaxies because they may lead to revolutions in galaxy evolution models and for the structure and properties of clusters. One may wonder if a dwarf galaxy near us has a star like our sun that can sustain life!
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