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.

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. But 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 move around larger ones like the Milky Way, the Andromeda Galaxy, and the Triangulum Galaxy. Some scientists think these dwarf galaxies were made when larger galaxies pulled on them with their gravity.

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

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. 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 full of young, hot stars. These bright blue stars make the galaxy look blue. Most BCD galaxies have irregular or lenticular shapes and lack a uniform form because they are made of star clusters. They use gas a lot, which affects how their stars are born.

As BCD galaxies create new stars, they cool down over time. Because their stars form at different times, the galaxies can 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. They have only a few hundred to one hundred thousand stars. They are the faintest galaxies known and look like globular clusters. But they are different because they have a lot of dark matter. These galaxies were first found in 2005 using digital sky surveys such as the Sloan Digital Sky Survey.

Astronomers believe UFDs give us clues about the early Universe because they are very old. They formed just a few million years after the Big Bang. Some theories say there might be younger UFDs, but we have not seen them yet.

Ultra-compact dwarfs

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

Ultra-compact dwarf galaxies are very small groups of stars. They were discovered in the 2000s. These galaxies are about 200 light years wide and have about 100 million stars. Scientists think they may have formed in two ways. One way is by losing many stars while moving through crowded areas of bigger galaxies. Another way is by starting as very large groups of stars. We have found these galaxies in clusters such as the Virgo Cluster, Fornax Cluster, Abell 1689, and the Coma Cluster. A well-known example is M60-UCD1. It is 54 million light years away and its stars are packed much closer together than the stars near our solar system.

Partial list

Here are some examples of dwarf galaxies: