A ring galaxy is a galaxy

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 word galaxy is derived from the Greek galaxias , literally "milky", a...

 with a ring-like appearance. The ring consists of massive, relatively young blue star

Star

A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s, which are extremely bright. The central region contains relatively little luminous matter

Matter

Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...

. Some astronomers

Astronomy

Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

 believe that ring galaxies are formed when a smaller galaxy passes through the center of a larger galaxy. Because most of a galaxy consists of empty space, this "collision" rarely results in any actual collisions between stars. However, the gravitational disruptions caused by such an event could cause a wave

Wave

In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...

 of star formation to move through the larger galaxy. Others think that rings are formed around some galaxies when external accretion

Accretion (astrophysics)

In astrophysics, the term accretion is used for at least two distinct processes.The first and most common is the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disc. Accretion discs are common around smaller stars or stellar remnants...

 takes place. Star formation would then take place in the accreted material because of the shocks and compressions of the accreted material. There might be a link between ring galaxies and polar-ring galaxies

Polar-ring galaxy

A polar-ring galaxy is a type of galaxy in which an outer ring of gas and stars rotates over the poles of the galaxy. These polar rings are thought to form when two galaxies gravitationally interact with each other. One possibility is that a material is tidally stripped from a passing galaxy to...

.

Hoag's Object

Hoag's Object

Hoag's Object is a non-typical galaxy of the type known as a ring galaxy. The appearance of this object has interested amateur astronomers as much as its uncommon structure has fascinated professionals...

, discovered by Art Hoag in 1950, is an example of a ring galaxy.

External links

• Hoag's Object at Astronomy Picture of the Day
  Astronomy Picture of the Day
  Astronomy Picture of the Day is a website provided by NASA and Michigan Technological University . According to the website, "Each day a different image or photograph of our universe is featured, along with a brief explanation written by a professional astronomer."The photograph is not necessarily...
  
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