A galaxy is a vast, gravitationally bound system composed of stars, stellar remnants, interstellar gas, dust, and dark matter. Galaxies can vary greatly in size and shape, and they are the fundamental building blocks of the universe. They range from small dwarf galaxies with just a few billion stars to massive galaxies with one hundred trillion stars or more.

They come in various forms, including spiral, elliptical, irregular, lenticular and peculiar types. Studying galaxies helps us understand the large-scale structure of the universe, galaxy formation and evolution, and the distribution of matter in the cosmos. The Milky Way is our home galaxy, providing a reference point for exploring the characteristics and behaviour of other galaxies.

1. 1. Types of Galaxies

• • • Spiral Galaxies: Characterized by a central bulge of stars surrounded by a flat, rotating disk with spiral arms extending outward. They also have a halo of stars and globular clusters. Examples include The Milky Way Galaxy, the Andromeda Galaxy, and the Whirlpool Galaxy.
    • Elliptical Galaxies: Spherical or elongated in shape, with little to no structure in terms of spiral arms. They have a smooth, featureless light distribution and are composed mostly of older stars. Examples include The Sombrero Galaxy and Cygnus A.
    • Irregular Galaxies: Lacking a defined shape, these galaxies appear irregular and chaotic. They often have a lot of gas and dust and are sites of active star formation. Examples include The Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC).
    • Lenticular Galaxies: Intermediate between spiral and elliptical galaxies. They have a central bulge and a disk but lack significant spiral arms. Examples include the Cartwheel Galaxy and NGC 5866.
    • Peculiar Galaxies: A galaxy of unusual size, shape, or composition. Between five and 10 percent of known galaxies are categorised as peculiar. Astronomers have identified two types of peculiar galaxies: interacting galaxies and active galactic nuclei (AGN). Examples are Mayall’s object and Hoag’s object.

1. 2. Components of Galaxies

• • • Stars: The primary visible component, forming clusters and populations within the galaxy.
    • Interstellar Medium: Includes gas (mostly hydrogen) and dust between the stars, playing a key role in star formation.
    • Dark Matter: An invisible form of matter that does not emit light or energy but exerts gravitational effects, influencing the galaxy’s structure and dynamics.
    • Stellar Remnants: Includes white dwarfs, neutron stars, and black holes resulting from the evolution of stars.

1. 3. Galaxy Formation and Evolution

• • • Formation: Galaxies form through the accumulation and gravitational collapse of matter, with ongoing processes of merging and interactions shaping their structure over time.
    • Evolution: Galaxies evolve through internal processes like star formation and interactions with other galaxies, such as collisions and mergers.

1. 4. Galactic Structures

• • • Galactic Halo: A spherical region surrounding the galaxy, containing older stars and globular clusters.
    • Galactic Disk: The flat, rotating region where most of the galaxy’s stars, gas, and dust reside, including spiral arms in spiral galaxies.
    • Galactic Bulge: A dense, spheroidal region at the centre of the galaxy, often containing older stars.