A galaxy is a collection of stars, dust and gas. All these materials bound together gravitationally to form a galaxy. Gas and dust found in galaxy in between the stars. It is called interstellar gas, which is mainly hydrogen in its atomic form(H) and to some extent, in molecular form(H2).
Interstellar gas in its atomic form can be neutral or ionic. Neutral, if it has an equal number of electrons ans protons in its atom or ionic if one or more electron is removed from atom. Let's take example of hydrogen; the most abundant interstellar gas. Hydrogen atom has one proton in its nucleus and one electron revolving around the nucleus, and spinning on its own axis at the same time. Since electron has negative charge and proton has positive charge, both cancel out each other and atom is electrically neutral. If we remove electron from atom, then its net electrical will be positive, since there will be no negative charge(or electron) to cancel proton's positive charge. Atom, in this state is called ion. Gas in galaxy is both in neutral and ionic form.
In neutral atomic form, interstellar gas is mainly hydrogen. In ionic form, some other elements like oxygen, carbon, helium, iron, neon, sulphur and some other elements are present along with ionic hydrogen.
Another constituent of galaxy is dust. Dust can be in small or large particle sizes. Molecular chains of Polycyclic Aromatic Hyrocarbons are the smallest particles of dust. It is believed that most dust particles are made of carbon or some silicate type.
Stars are the third and final constituents of galaxies. Number of stars in a galaxy ranges from 10 million (107) to more than a trillion (1012). Even with this high number, stars are not the greatest contributer to the mass of galaxy. Mass of gas is the greatest contributer to the mass of galaxy.Some galaxies have both old and young stars. These galaxies have been forming stars at an invariable rate since formation of these galaxies. These galaxies also contain active star forming regions. Our own galaxy (Milky Way) is one such galaxy.
Some galaxies contain only old stars. For example, if a galaxy ceased to form stars 10 mllion years ago, then main sequence stars in this type of galaxy will be older than 10 million years. In these galaxies there are no active star forming regions and not enough material for producing stars.
Like stars, galaxies are of different sizes. Some are small with a few stars. Some are medium sized with a moderate number of stars and some are large with a large number of stars.
Smallest galaxies are hardly larger than star clusters and contain all the constituents of a galaxy. Possibly, some objects that are called dwarf spheroidal galaxies can be runaway giant globular clusters. Astronomers discovered smallest galaxies from a galaxy cluster which is known as Fornax cluster, and named them ultra compact dwarf galaxies.
These ultra compact dwarf galaxies contains a few million stars and are only a few tens of parsecs accross. These are about 103 times less bright than average sized galaxies. Astronomers are trying to understand that whether these are building blocks of large galaxies or remains of small galaxies that got separated from their discs. Current theory suggests that there should be more small galaxies than larger ones as astronomers believe that small galaxies join to form large galaxies.
Average or medium sized galaxy contains about a billion (109) stars with gas and dust. Some galaxies are disc dominated, i.e; gas and dust in these galaxies is located only in disc and stars are scattered in whole galaxy. Other galaxies are not disc dominated, i.e; gas and dust is not limited to disc, but scattered in whole galaxy. Lumiousities of avearge sized galaxies are about 1040 erg/s or 1034 Watts.
Large galaxies can have trillion (1012) of stars. Largest galaxies are also called giant elliptical galaxies. These are found in center of dense galaxy clusters. Giant elliptical galaxies are believed to be created by merging of smaller galaxies. These galaxies contain very little gas and dust and this shows that these galaxies produced stars at a rapid rate and thus exhausted all the fuel for star formation. These galaxies contain stars of all ages.
Giant elliptical galaxies are about 10 times brighter than average galaxies and about 2 million (107) parsecs accross. These galaxies have a regular shape and do not include disc.