Galaxy

A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek word galaxias (γαλαξίας), literally "milky," a reference to the Milky Way galaxy, which contains our Solar System. Galaxies range in size from dwarfs containing as few as tens of millions of stars to giants with trillions of stars, each orbiting the galaxy's center of mass.

Galaxies are broadly classified according to their morphology into three major types:

• Spiral galaxies: Characterized by a central bulge surrounded by a flattened rotating disk containing spiral arms, which are regions of ongoing star formation. They are often further subdivided based on the tightness of their arms and the size of their central bulge (e.g., Sa, Sb, Sc, SBa, SBb, SBc).

• Elliptical galaxies: Characterized by a smooth, featureless appearance and an ellipsoidal shape. They generally contain older stars and very little interstellar gas and dust. They are classified according to their ellipticity, ranging from E0 (almost spherical) to E7 (highly elongated).

• Irregular galaxies: Galaxies that do not fit into either the spiral or elliptical classifications. They often have disturbed shapes, resulting from gravitational interactions with other galaxies or recent mergers.

In addition to these major types, there are also lenticular galaxies (S0), which have a disk but no spiral arms, and dwarf galaxies, which are much smaller and fainter than typical galaxies.

Galaxies are often found in groups and clusters. Our own Milky Way is part of the Local Group, which contains over 50 galaxies, including the Andromeda Galaxy. Larger structures called superclusters contain multiple groups and clusters. The distribution of galaxies in the universe is not uniform; they are organized into a vast cosmic web of filaments and voids.

The study of galaxies, their formation, evolution, and interactions, is a major field of astronomical research. Understanding galaxy evolution provides crucial insights into the formation and evolution of the universe as a whole. Key areas of research include the role of supermassive black holes at the centers of many galaxies, the processes that trigger star formation, and the impact of galaxy mergers on galaxy morphology and star formation history.