Solved: The mystery of the expansion of the universe

The Earth, solar system, the entire Milky Way and the few thousand galaxies closest to us move in a vast "bubble" that is 250 million light years in diameter, where the average density of matter is half as high as for the ...

A new Einstein cross is discovered

This study, which has combined images from the Hubble Space Telescope with spectroscopic observations from the GTC, has confirmed the existence of a new example of a gravitational lens, a phenomenon predicted by Albert Einstein's ...

Black hole fails to do its job

Astronomers have discovered what can happen when a giant black hole does not intervene in the life of a galaxy cluster. Using NASA's Chandra X-ray Observatory and other telescopes they have shown that passive black hole behavior ...

Astronomers detect galactic space laser

A powerful radio-wave laser, called a "megamaser", has been observed by the MeerKAT telescope in South Africa. The record-breaking find is the most distant megamaser of its kind ever detected, at about five billion light ...

Hubble makes surprising find in the early universe

New results from the NASA/ESA Hubble Space Telescope suggest the formation of the first stars and galaxies in the early Universe took place sooner than previously thought. A European team of astronomers have found no evidence ...

Hubble detects smallest known dark matter clumps

When searching for dark matter, astronomers must go on a sort of "ghost hunt." That's because dark matter is an invisible substance that cannot be seen directly. Yet it makes up the bulk of the universe's mass and forms the ...

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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