Galaxy

A galaxy is a gravitationally bound large collection of stars, planetary systems, gas nebulae, dust clouds, dark matter, and other astronomical objects with a total mass of typically ¹⁰⁹ to ¹⁰¹³ solar masses (M☉). Their diameter can be several hundred thousand light-years. While large galaxies often form the structure of spirals, dwarf galaxies are mostly of irregular type. In addition, other types and shapes exist. The Milky Way, home galaxy of the solar system, is a barred spiral of about 1.5 trillion M☉ with about 250 billion stars. From Earth, more than 50 billion galaxies can be observed with current technology. Since 2016, research has assumed that there are about one trillion galaxies in the observable universe.

The term derives from the synonymous ancient Greek γαλαξίας galaxías and goes back to an ancient legend, according to which it is the splashed milk (γάλα gála) of the goddess Hera, when she wanted to breastfeed Heracles. In German, galaxy (singular) refers specifically to the Milky Way. In English (generally galaxy, for the Milky Way in addition to the Milky Way Galaxy or the Milky Way also briefly the Galaxy) there is a similar distinction. Alexander von Humboldt used the term "world island".

General

Galaxies vary greatly in appearance (morphology), size and composition. The Milky Way has a diameter of up to 200,000 light years and thus belongs to the larger galaxies. Its nearest neighbour galaxy of comparable size is the Andromeda Galaxy at a distance of about 2.5 million light years. Together with other galaxies of smaller mass, both galaxies form the Local Group. Galaxies often appear in groups or clusters with up to several thousand members.

Dust filament in the elliptical galaxy NGC 4696


The Ultra-Deep-Field shows about 10,000 galaxies in a thirteen millionth part of the sky

Exploration history

Before the power of astronomical telescopes was sufficient to resolve distant galaxies into individual stars, they appeared as "nebulae". For a long time it was unclear whether these "spiral nebulae" belonged to the Galaxy or formed their own star systems. Immanuel Kant already suspected milky way-like star systems in the "nebulous stars", and in 1923 Edwin Hubble succeeded in clarifying this question. He determined the distance to the Andromeda Nebula and found that it is much too far away to belong to the Milky Way, i.e. it represents a galaxy of its own.