Galaxies are vast gravitationally bound systems that consists of stars and
stellar remnants, an interstellar medium of gas and dust, and an important but
poorly understood component tentatively dubbed dark matter. The name is
from the Greek root galaxias, meaning “milky,” a reference to our own Milky
Way galaxy.
Typical galaxies range from dwarfs with as little as ten million stars up to giants
with over one trillion stars, all orbiting the galaxy’s center of mass. Galaxies
can also contain many multiple star systems, star clusters, and various
interstellar clouds.
There are probably more than 100 billion galaxies in the observable universe.
Most galaxies are 1,000 to 100,000 parsecs in diameter and are usually
separated by distances on the order of millions of parsecs (or megaparsecs).
The majority of galaxies are organized into a hierarchy of associations called
clusters, which, in turn, can form larger groups called superclusters. These
larger structures are generally arranged into sheets and filaments, which
surround immense voids in the universe.
Although it is not yet well understood, dark matter appears to account for upto
90% of the mass of galaxies. Observational data suggests that supermassive
black holes may exist at the center of many, if not all, galaxies. Our very own
Milky Way galaxy appears to harbor at least one such object within its nucleus.
Historically, galaxies have been categorized according to their apparent shape
(usually referred to as their visual morphology). A common form is the elliptical
galaxy, which has an ellipse-shaped light profile. Spiral galaxies are
disk-shaped assemblages with dusty, curving arms. Galaxies with irregular or
unusual shapes are known as peculiar galaxies, and typically result from
disruption by the gravitational pull of neighboring galaxies. Such interactions
between nearby galaxies, which may ultimately result in galaxies merging, may
induce episodes of significantly increased star formation, producing what is
called a starburst galaxy. Small galaxies that lack a coherent structure could
also be referred to as irregular galaxies.
Current cosmological models of the early Universe are based on the Big Bang
theory. About 300,000 years after this event, atoms of hydrogen and helium
began to form, in an event called recombination. Nearly all the hydrogen was
neutral (non-ionized) and readily absorbed light, and no stars had yet formed.
As a result this period has been called the “Dark Ages”. It was from density
fluctuations (or anisotropic irregularities) in this primordial matter that larger
structures began to appear. As a result, masses of baryonic matter started to
condense within cold dark matter halos. These primordial structures would
eventually become the galaxies we see today.