Galaxies are large collections of many millions or
billions of stars, along with dust and gas,
that are held together by their mutual
gravitational attraction. They can be grouped into
three main categories: spiral galaxies, like our own Milky
Way, which contain both young and old stars as well as gas and dust;
early-type galaxies including ellipticals, which contain
mostly older stars, and little gas or dust, and which
are round or elliptical in shape; and irregular galaxies, whose shapes do
not fit into the scheme described above. Some irregular galaxies are
as their name implies, are generally smaller in size and less
luminous. Another class of galaxies is peculiar galaxies, which
are thought to be distorted normal galaxies. All galaxies are thought
to contain significant amounts of "dark matter" which contributes to
the gravitational force but which has yet to be detected by other
means. Galaxies may be clustered into groups
containing a few to a dozen members, clusters of galaxies which may
contain hundreds or thousands of members,
or into superclusters that contain tens of
thousands of galaxies.
Many galaxies are "active", meaning that they contain
central sources of intense optical, radio, and/or X-ray emission.
Images of such galaxies are catalogued separately
in the class of
Active Galactic Nuclei.
Click on any Sub-category name below to view all
the images in
that sub-category, or click any thumbnail to view the details of
that image. Other images of the atomic hydrogen in galaxies
can be found in the . Visit non-NRAO
website for more information about galaxies.