Typically, elliptical galaxies contain very little gas and dust because the gas and dust has
already formed into stars. However, some elliptical galaxies, like the one shown inFigure
26.14, contain lots of dust. Astronomers believe that these dusty elliptical galaxies form
when two galaxies of similar size collide.
Figure26.14: Thisellipticalgalaxyprobablyformedwhentwogalaxiesofsimilarsizecollided
with each other. ( 9 )
Irregular Galaxies and Dwarf Galaxies
Look at the galaxy inFigure26.15. Do you think this is a spiral galaxy or an elliptical
galaxy? It is neither one! Galaxies that are not clearly elliptical galaxies or spiral galaxies
are calledirregular galaxies. Most irregular galaxies were once spiral or elliptical galaxies
that were then deformed either by gravitational attraction to a larger galaxy or by a collision
with another galaxy.
Dwarf galaxiesare small galaxies containing “only” a few million to a few billion stars.
Most dwarf galaxies are irregular in shape. However, there are alsodwarf elliptical galaxies
anddwarf spiral galaxies. Dwarf galaxies are the most common type in the universe. How-
ever, because they are relatively small and dim, we don’t see as many dwarf galaxies as we
do their full-sized cousins.