Dwarf galaxies only contain several billion stars. Although
several billion stars seems like quite a lot, large galaxies such as
the Milky Way has two to four hundred billion stars. Most dwarf
galaxies orbit other, larger galaxies. The Large and Small Magellanic
Clouds orbit the Milky Way. The Large Magellanic Cloud is not always
considered to be a dwarf galaxy, as it has 30 billion stars.
Irregular galaxies simply don't fit either
of the other two main shapes for galaxies:
elliptical or spiral.
Most irregular galaxies were originally elliptical or spiral galaxies which
have been distorted by the gravity of larger
neighbouring galaxies, as the Large and Small
Magellanic Clouds were.
Irregular dwarf galaxies usually contain
of metal and large amounts of gas compared to other galaxies, as
astronomers believe the earliest galaxies had. This means that they are
important for studying the history of the Universe.