advertisement
Reference Terms
from Wikipedia, the free encyclopedia

1

2

Galaxy

A galaxy is a massive, gravitationally bound system consisting of stars, an interstellar medium of gas and dust, and dark matter.

Typical galaxies range from dwarfs with as few as ten million stars up to giants with one trillion stars, all orbiting a common center of mass.

Galaxies can also contain many multiple star systems, star clusters, and various interstellar clouds.

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 neighbouring 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 one hundred billion 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 metre.

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, home of Earth and the solar system, appears to harbor at least one such object within its nucleus.

Note:   The above text is excerpted from the Wikipedia article "Galaxy", which has been released under the GNU Free Documentation License.
Related Stories
 

1

2


Space & Time News
August 8, 2020

A large number of the valley networks scarring Mars's surface were carved by water melting beneath glacial ice, not by free-flowing rivers as previously thought, according to ...
Scientists have developed a new model that successfully predicted seven of the Sun's biggest flares from the last solar cycle, out of a set of nine. With more development, ...
NASA's Mars 2020 Perseverance rover mission is on its way to the Red Planet to search for signs of ancient life and collect samples to send back ...
Using known distances of 50 galaxies from Earth to refine calculations in Hubble's constant, astronomers estimates the age of the universe at 12.6 ...
Latest Headlines
updated 12:56 pm ET