Galaxy

Galaxy

A galaxy is a massive, gravitationally bound system consisting of stars, stellar remnants, an interstellar medium of gas and dust, and dark matter. The word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million (10^8) stars to giants with one hundred trillion (10^14) stars, each orbiting their galaxy's center of mass.

Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular. Many galaxies are thought to have black holes at their active centers. The Milky Way's central supermassive black hole, known as Sagittarius A*, has a mass four million times greater than the Sun. Despite the prominence of large elliptical and spiral galaxies, the majority of galaxies in the universe are dwarf galaxies. These are relatively small when compared to other galactic formations, containing only a few billion stars.

Formation and Evolution

Galaxies are formed from gravitational collapses of matter in the early universe. They grow by absorbing and merging with other galaxies through galactic cannibalism. The process of galaxy formation and evolution is a fundamental aspect of the study of cosmology.

Galactic formation begins within extended clouds of dark matter. These clouds undergo gravitational collapse, forming a dark matter halo in which baryonic matter (the elements that make up stars, planets, and life) can accumulate. Over time, this matter cools and forms a galaxy. The detailed process of galaxy formation is governed by the complex interplay of dark matter, gas dynamics, stellar evolution, and feedback mechanisms from supernovae and supermassive black holes.

Types of Galaxies

Spiral Galaxies

Spiral galaxies, such as the Milky Way, consist of a flat, rotating disk containing stars, gas, and dust, and a central concentration of stars known as the bulge. These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters.

Elliptical Galaxies

Elliptical galaxies range from circular in appearance to elongated ellipses. They are generally composed of older, redder stars, with little gas or dust, and no new star formation.

Irregular Galaxies

Irregular galaxies do not fit into the spiral or elliptical categories and often exhibit chaotic appearances, with neither a central bulge nor any trace of spiral arm structure.

Notable Galaxies

Among the most notable galaxies are the Milky Way, the Andromeda Galaxy (M31), the Sombrero Galaxy (M104), and the Whirlpool Galaxy (M51). Each of these galaxies has its unique characteristics and has been the subject of extensive study.

Observing Galaxies

Galaxies can be observed at a range of wavelengths. Optical and ultraviolet light can reveal the distribution of stars, while infrared observations can show the dust and cool gas in galaxies. Radio wavelengths can trace out neutral hydrogen gas, and X-rays can expose the high-energy environments around black holes.

Challenges in Galactic Astronomy

Studying galaxies poses numerous challenges. Their vast distances make them appear very faint, requiring powerful telescopes to observe them in detail. Additionally, the universe is expanding, causing galaxies to move away from us, which leads to the redshift of their light.

See Also

• Cosmology
• Dark matter
• Stellar evolution
• Supermassive black hole