Image: Hubble reveals a river of star formation

This newly revised NASA Hubble Space Telescope image of the Hickson Compact Group 31 (HCG 31) of galaxies highlights streams of star-formation as four dwarf galaxies interact. The bright, distorted clump of young blue-white ...

Image: Giant elliptical galaxy UGC 10143

This new NASA Hubble Space Telescope image spotlights the giant elliptical galaxy, UGC 10143, at the heart of galaxy cluster Abell 2147, about 486 million light-years away in the head of the constellation Serpens. UGC 10143 ...

Black hole winds are no longer as they used to be

During the first billion years of the universe, winds blown by supermassive black holes at the centers of galaxies were much more frequent and more powerful than those observed in today's galaxies, some 13 billion years later. ...

Video: Gaia, the billion star surveyor

ESA's Gaia space telescope revolutionizes our understanding of the Milky Way. It scans the sky to measure the position, movement, distance, and characteristics of billions of stars.

Image: A small sombrero for Hubble

NASA's Hubble Space Telescope has allowed astronomers to view galaxies of all shapes and sizes from nearly every angle. When a galaxy is seen edge-on, the mesmerizing perspective reveals a dazzling slice of the universe. ...

Astronomers find 'gold standard' star in Milky Way

In our sun's neighborhood of the Milky Way Galaxy is a relatively bright star, and in it, astronomers have been able to identify the widest range of elements in a star beyond our solar system yet.

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Galaxy

A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The name is from the Greek root galaxias [γαλαξίας], meaning "milky," a reference to the Milky Way galaxy. Typical galaxies range from dwarfs with as few as ten million (107) stars up to giants with one trillion (1012) stars, all orbiting the galaxy's center of mass. Galaxies can also contain many multiple star systems, star clusters, and various interstellar clouds. The Sun is one of the stars in the Milky Way galaxy; the Solar System includes the Earth and all the other objects that orbit the Sun.

Historically, galaxies have been categorized according to their apparent shape (usually referred to as their visual morphology). A common form is the elliptical galaxy, which has an ellipse-shaped light profile. Spiral galaxies are disk-shaped assemblages with curving, dusty arms. Galaxies with irregular or unusual shapes are known as peculiar galaxies, and typically result from disruption by the gravitational pull of neighboring galaxies. Such interactions between nearby galaxies, which may ultimately result in galaxies merging, may induce episodes of significantly increased star formation, producing what is called a starburst galaxy. Small galaxies that lack a coherent structure could also be referred to as irregular galaxies.

There are probably more than 100 billion (1011) galaxies in the observable universe. Most galaxies are 1,000 to 100,000 parsecs in diameter and are usually separated by distances on the order of millions of parsecs (or megaparsecs). Intergalactic space (the space between galaxies) is filled with a tenuous gas of an average density less than one atom per cubic meter. The majority of galaxies are organized into a hierarchy of associations called clusters, which, in turn, can form larger groups called superclusters. These larger structures are generally arranged into sheets and filaments, which surround immense voids in the universe.

Although it is not yet well understood, dark matter appears to account for around 90% of the mass of most galaxies. Observational data suggests that supermassive black holes may exist at the center of many, if not all, galaxies. They are proposed to be the primary cause of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy appears to harbor at least one such object within its nucleus.

This text uses material from Wikipedia, licensed under CC BY-SA