Galaxies are vast, gravitationally bound systems composed of stars, stellar remnants, gas, dust, and dark matter. They are the fundamental building blocks of the universe, existing in a variety of shapes, sizes, and compositions. The word “galaxy” comes from the Greek galaxias, meaning “milky,” a nod to our own Milky Way. Galaxies are not isolated; they often exist in groups, clusters, or even superclusters, connected by vast cosmic filaments in the large-scale structure of the universe.

Construction

A typical galaxy can contain anywhere from a few million to several trillion stars, along with massive quantities of interstellar gas and dust that serve as the raw material for new star formation. The structure of a galaxy is shaped largely by its gravitational interactions over billions of years, including mergers with other galaxies. Dark matter, which does not emit or absorb light but exerts gravitational influence, makes up most of a galaxy’s mass, holding it together and affecting its rotation.

Classification

Galaxies are broadly classified into three main types according to the Hubble classification system: spiral, elliptical, and irregular. Spiral galaxies, like the Milky Way, have flat rotating disks with spiral arms winding outward from a central bulge, rich in young, bright stars and active star-forming regions. Elliptical galaxies are more spherical or elongated, contain older star populations, and have little gas or dust, making them less active in forming new stars. Irregular galaxies have no distinct shape, often appearing chaotic, and are typically the result of gravitational interactions or collisions.

Size and Scale

The scale of galaxies is immense. The Milky Way spans about 100,000 light-years in diameter, while giant elliptical galaxies can be ten times larger. Distances between galaxies are measured in millions of light-years. The Andromeda Galaxy, our nearest large galactic neighbor, lies about 2.5 million light-years away and is on a slow collision course with the Milky Way, expected to merge in roughly 4 billion years.

Evolution

Galaxies evolve over cosmic time. In the early universe, they formed from density fluctuations in the primordial gas after the Big Bang, gradually growing through the accumulation of matter and mergers. Starburst galaxies experience intense episodes of star formation, while active galactic nuclei (AGN) are powered by supermassive black holes at the centers of some galaxies, emitting tremendous energy across the electromagnetic spectrum.

Black Holes

The study of galaxies also reveals the presence of supermassive black holes—with masses millions or billions of times that of the Sun—residing at the cores of most, if not all, large galaxies. These objects play a key role in regulating star formation and influencing galactic evolution through energetic outflows and jets.

Observation and Summary

Observing galaxies at different distances allows astronomers to look back in time, tracing the history of the universe. Deep-space surveys with telescopes such as Hubble and James Webb have captured galaxies as they appeared billions of years ago, helping scientists understand how structure emerged from the nearly uniform early cosmos.

In essence, galaxies are the cosmic cities of stars, each with its own history, structure, and population, collectively shaping and illuminating the story of the universe across space and time.