Untangling a knot of galaxy clusters

Astronomers have captured a spectacular, ongoing collision between at least three galaxy clusters. Data from NASA's Chandra X-ray Observatory, ESA's (European Space Agency's) XMM-Newton, and a trio of radio telescopes is ...

Starry tail tells the tale of dwarf galaxy evolution

A giant diffuse tail of stars has been discovered emanating from a large, faint dwarf galaxy. The presence of a tail indicates that the galaxy has experienced recent interaction with another galaxy. This is an important clue ...

Astronomers confirm age of most distant galaxy using oxygen

A new study led by a joint team at Nagoya University and the National Astronomical Observatory of Japan has measured the cosmic age of a very distant galaxy. The team used the ALMA radio telescope array to detect a radio ...

Were galaxies much different in the early universe?

An array of 350 radio telescopes in the Karoo desert of South Africa is getting closer to detecting "cosmic dawn"—the era after the Big Bang when stars first ignited and galaxies began to bloom.

Detecting galactic filaments with machine learning

Star formation in galaxies takes place in filaments composed of gas (mainly hydrogen) and small solid particles called interstellar dust, which is mainly carbon. Depending on the location of these filaments and their physical ...

Milky Way found to be too big for its 'cosmological wall'

Is the Milky Way special, or, at least, is it in a special place in the universe? An international team of astronomers has found that the answer to that question is yes, in a way not previously appreciated. A new study shows ...

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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