A small galaxy, typically called a dwarf galaxy, is one with fewer than a few billion stars and a stellar mass below about 10 billion solar masses. For comparison, the Milky Way holds roughly 100 billion stars. Most dwarf galaxies contain somewhere between a few thousand and a few billion stars, and the smallest confirmed examples have so few stars you could almost count them individually in a telescope image.
How Astronomers Define “Small”
There is no single hard cutoff that separates a small galaxy from a large one, but astronomers generally use stellar mass as the primary yardstick. Galaxies with stellar masses between about 10 million and a few billion times the mass of the Sun land squarely in dwarf territory. Below 10 million solar masses, you enter the realm of ultra-faint dwarf galaxies, the smallest stellar systems known. Above roughly 10 billion solar masses, a galaxy starts to qualify as a standard or large galaxy like the Milky Way or Andromeda.
Physical size tells a similar story. A typical dwarf galaxy spans around 10,000 light-years across, while the Milky Way stretches roughly 100,000 light-years in diameter. Some ultra-faint dwarfs are far smaller still, with their stars packed into a region only a few hundred light-years wide.
Ultra-Faint Dwarfs: The Smallest of the Small
At the extreme low end sit ultra-faint dwarf galaxies, defined as systems with total luminosities less than 100,000 times that of the Sun. That is extraordinarily dim. A single bright star in the Milky Way can outshine an entire ultra-faint dwarf galaxy. These objects have metallicities (the abundance of elements heavier than hydrogen and helium) less than 1 percent of the Sun’s, meaning their stars formed from nearly pristine gas very early in cosmic history.
The least massive galaxy known is Segue 2, with a luminosity of just 900 times that of the Sun and a total mass within its core of no more than about 150,000 solar masses. To put that in perspective, a typical globular star cluster in the Milky Way contains more mass than Segue 2. What makes Segue 2 a galaxy rather than a star cluster is that it sits inside its own clump of dark matter, a defining feature that separates galaxies from simpler stellar groupings.
Why Small Galaxies Are Mostly Dark Matter
One of the most striking things about small galaxies is how little of their mass comes from stars. In the Milky Way’s disk, the ratio of total mass to the light produced by stars is relatively modest. In dwarf spheroidal galaxies, that ratio jumps to 10 to 100 times higher, meaning most of the mass holding the galaxy together is invisible dark matter rather than stars or gas.
Ultra-faint dwarfs push this even further. Their mass-to-light ratios can reach 100 to 1,000, making them the most dark-matter-dominated objects in the known universe. Segue 1, another extreme case with a visible baryon mass of only about 1,000 solar masses, sits inside a dark matter halo that outweighs its stars by a factor of at least 200, and possibly as high as 2,000. These tiny galaxies are essentially small clouds of dark matter with a thin dusting of stars.
Types of Small Galaxies
Small galaxies come in several structural flavors. Dwarf elliptical and dwarf spheroidal galaxies are smooth, rounded collections of older stars with very little gas or dust remaining. Because they have used up or lost their star-forming material, they produce few if any new stars. They tend to be found orbiting larger galaxies, where gravitational interactions have stripped away their gas over billions of years.
Dwarf irregular galaxies, by contrast, have messy, asymmetric shapes and often contain significant reserves of gas and dust. This makes them active star-forming environments. Their irregular shapes sometimes result from gravitational encounters with larger neighbors, which can pull material out of alignment and trigger bursts of new star formation. Irregular dwarfs range from about 100 million to 10 billion solar masses, placing them at the larger end of the dwarf category.
The Large Magellanic Cloud, visible to the naked eye from the Southern Hemisphere at a distance of nearly 200,000 light-years, is classified as a small irregular galaxy and a satellite of the Milky Way. It is one of the more massive dwarf galaxies, sitting near the upper boundary of what counts as “small.”
Small Galaxies Around the Milky Way
The Milky Way is surrounded by dozens of known dwarf galaxies, and new ones are still being discovered. Many of the best-studied examples are dwarf spheroidals orbiting within a few hundred thousand light-years. The Andromeda galaxy has its own swarm of small companions, including NGC 205, NGC 185, NGC 147, and a growing list of ultra-faint systems like Andromeda IX, located about 2.5 million light-years away.
More isolated dwarfs also exist. The Aquarius dwarf galaxy lies roughly 3.3 million light-years from the Milky Way, and the Pegasus dwarf irregular sits about 3 million light-years out. These more distant dwarfs have evolved with less gravitational interference from large neighbors, giving astronomers a cleaner look at how small galaxies develop on their own.
Why So Many Are Hard to Find
Small galaxies are faint, and the faintest ones blend into the background noise of the night sky. Their low surface brightness makes them a serious challenge for traditional detection methods, which rely on software to pick sources out of survey images. If the detection parameters are not tuned carefully, faint dwarfs get missed entirely or their measured sizes come out wrong.
This problem is especially acute for ultra-faint dwarfs with small angular sizes or irregular shapes. Astronomers have historically found them by painstakingly searching data from large sky surveys like the Sloan Digital Sky Survey and the Dark Energy Survey. Newer approaches use deep-learning neural networks trained to recognize the subtle signatures of low-surface-brightness galaxies in telescope images. Upcoming instruments, including the Chinese Space Station Telescope, are expected to dramatically increase the number of known dwarf galaxies by surveying large areas of sky at higher sensitivity.
The current count of known dwarf galaxies around the Milky Way alone has grown from a handful to more than 50 over the past two decades, and simulations predict there could be hundreds more lurking below current detection limits. Every new discovery helps refine our understanding of how galaxies form at the smallest scales and how dark matter behaves in low-mass systems.