Dwarf galaxy

A dwarf galaxy is a small galaxy made up of only about 1,000 to several billion stars. This is much fewer stars than the Milky Way, which has between 100 and 400 billion stars! One famous example is the Large Magellanic Cloud, a galaxy that orbits close to the Milky Way and has more than 30 billion stars. Some scientists call it a dwarf galaxy, while others think it is a full-sized galaxy.

The Large Magellanic Cloud, a satellite galaxy of the Milky Way

Dwarf galaxies are interesting to astronomers because their shape and what they are made of can change a lot. Many believe that how dwarf galaxies form and behave is strongly affected by their larger neighbors. Scientists have found many different kinds of dwarf galaxies, and they group them based on how they look and what they contain inside. Studying these small galaxies helps us learn more about how all galaxies, including our own, came to be.

Formation

Dwarf galaxies like NGC 5264 typically possess around a billion stars.

One idea is that dwarf galaxies form from gases that contain metals, linked to something called dark matter. However, a space probe called the Galaxy Evolution Explorer found new dwarf galaxies forming from gases with very few metals. These were found in a cloud of hydrogen and helium called the Leo Ring, near two large galaxies in the constellation Leo.

Because dwarf galaxies are small, they can be pulled toward bigger spiral galaxies. This can stretch out their stars into long lines and may eventually combine them with the larger galaxy. Studies show that many dwarf galaxies in quiet areas have stopped forming new stars, showing how their growth is shaped by nearby big galaxies and other forces.

Local dwarf galaxies

The Phoenix Dwarf Galaxy is a dwarf irregular galaxy, featuring younger stars in its inner regions and older ones at its outskirts.

There are many small galaxies called dwarf galaxies in the Local Group. These galaxies often orbit larger ones like the Milky Way, the Andromeda Galaxy, and the Triangulum Galaxy. Some scientists think these dwarf galaxies were created when larger galaxies pulled on them with their gravity.

Over 20 dwarf galaxies are known to orbit the Milky Way. One big group of stars in the Milky Way, called Omega Centauri, might actually be the center of a dwarf galaxy that the Milky Way pulled in long ago. Studying these close dwarf galaxies helps scientists learn about how galaxies form and change over time.

Common types

UGC 11411 is a galaxy known as an irregular blue compact dwarf (BCD) galaxy.

Dwarf galaxies come in several different forms. One type is the dwarf elliptical galaxy, which is small and round. Another is the dwarf spheroidal galaxy, which is also round but was once thought to be a kind of dwarf elliptical. Dwarf irregular galaxies are small and have an uneven shape, while dwarf spiral galaxies look like tiny versions of larger spiral galaxies, such as the Milky Way. There are also special types like Magellanic type dwarfs, blue compact dwarf galaxies, and ultra-compact dwarf galaxies.

Blue compact dwarf galaxies

Blue compact dwarf PGC 51017.

In astronomy, a blue compact dwarf galaxy (BCD galaxy) is a small galaxy filled with clusters of young, hot stars. These bright blue stars make the galaxy look blue. Most BCD galaxies are irregular or lenticular in shape and lack a uniform form because they are made of star clusters. They use gas intensely, which affects how their stars form.

As BCD galaxies create new stars, they cool over time. Since their stars form at different periods, the galaxies have opportunities to gather more material for new stars, slowly changing their shape over millions of years. Some nearby examples include NGC 1705, NGC 2915, NGC 3353, UGC 6541, and UGCA 281.

Ultra-faint dwarf galaxies

A composite color image by the Hubble Space Telescope of the NGC 1052-DF2

Ultra-faint dwarf galaxies, or UFDs, are very small galaxies that contain only a few hundred to one hundred thousand stars. They are the faintest galaxies known and look similar to globular clusters but have important differences, such as containing a lot of dark matter. These galaxies were first discovered in 2005 using digital sky surveys like the Sloan Digital Sky Survey.

Astronomers think UFDs hold clues about the early Universe because they are very old, forming just a few million years after the Big Bang. Some theories suggest there might be younger UFDs, but they have not been observed yet.

Ultra-compact dwarfs

M59-UCD3 and M85-HCCI are the densest galaxies discovered.

Ultra-compact dwarf galaxies are very small and tightly packed collections of stars discovered in the 2000s. They are about 200 light years wide and contain around 100 million stars. Scientists think these galaxies might have formed in two different ways: either by losing most of their stars while moving through dense areas of other galaxies or by being very large groups of stars themselves. These galaxies have been found in clusters like the Virgo Cluster, Fornax Cluster, Abell 1689, and the Coma Cluster. One famous example, M60-UCD1, is about 54 million light years away and has stars packed much more tightly than those near our solar system.

Partial list

Here are some examples of dwarf galaxies: