Galaxies: Types, Distribution, Clusters and Super Clusters

A galaxy is a vast collection of stars, gas, dust, and dark matter that forms a cohesive gravitational unit in the universe. In a way, galaxies are to the universe what cells are to the human body: each galaxy has its own identity, and it ages and evolves on its own, but it also interacts with other galaxies in the cosmos.

Within the observable universe alone, there exist an estimated 50 to 100 billion galaxies.

Types of Galaxies

Galaxies are of various kinds mainly divided into four types viz.  elliptical, normal spirals, barred spirals and irregular.

Hubble Sequence and Tuning Fork Diagram

Hubble had proposed a way to classify galaxies based on their shapes. He proposed a “sequence” of galaxy types: from E0 (sphere-shaped elliptical galaxies) to E7 (cigar-shaped ellipticals), S0 (lenticular galaxies) to Sa and SBa (spiral galaxies with large bulges and bars), Sb and SBb (spirals with medium-sized bulges and bars), and Sc and SBc (spirals with small bulges and bars). The sequence is known as the Hubble sequence, and it is often shown visually as a Hubble “tuning fork diagram.”

Further, an irregular galaxy is a galaxy that does not fit well into the standard categories of elliptical, spiral, or barred spiral galaxies. Two examples of irregular galaxies are the Large Magellanic Cloud and Small Magellanic Cloud, which are visible from Earth’s southern hemisphere.

Size of Galaxies

Galaxies range greatly in size and mass. The smallest galaxies contain perhaps 10 to 100 million stars, whereas the largest galaxies contain trillions of stars. There are many more small galaxies than large ones. Our galaxy Milky Way has at least 100 billion stars and is a large galaxy. Its disk spreads around 100,000 light-years across.

Dwarf Galaxy

Dwarf galaxies have the least mass and fewest stars. The Large Magellanic Cloud, a galaxy that orbits the Milky Way, is considered a large dwarf galaxy; it contains, at most, about one billion stars.

Distribution of Galaxies

Galaxies are distributed unevenly throughout the universe. Majority of galaxies are collected along vast filamentary and sheet like structures many millions of light years long.

These filaments and sheets connect at dense nodes—clusters and super clusters— of galaxies, and the net result is a three-dimensional weblike distribution of matter in the universe. This is known as Cosmic Web.

Between the filaments and sheets are large pockets of space with relatively few galaxies; these sparse regions are called voids.

Group of Galaxies

Group of Galaxies contains two or more galaxies of bigger size and a dozen or more smaller galaxies. The Milky Way and Andromeda galaxies are the two large galaxies in the Local Group. There are a few dozen smaller galaxies in the group, including the Magellanic Clouds, the dwarf elliptical Messier 32, the small spiral galaxy Messier 33, and many small dwarf galaxies. The Local Group of galaxies is a few million light-years across.

Cluster of Galaxies

A cluster of galaxies is a large collection of galaxies in a single gravitational field. Rich clusters of galaxies usually contain at least a dozen large galaxies as massive as the Milky Way, along with hundreds of smaller galaxies. At the center of large clusters of galaxies there is usually a group of elliptical galaxies called “cD” galaxies. Clusters of galaxies are usually about ten million light-years across. The Milky Way galaxy is near, but not in, the Virgo cluster, which itself is near the center of the Virgo supercluster.

Supercluster of Galaxies

Superclusters are the largest collections of massive structures. There are usually many clusters of galaxies in a supercluster, or a single very large cluster at its center, along with many other groups and collections of galaxies that are collected in the supercluster’s central gravitational field. Superclusters contain many thousands—and sometimes millions—of galaxies.  The Milky Way galaxy is located on the outskirts of the Virgo supercluster.