Canis Major Dwarf Galaxy
Encyclopedia
The Canis Major Dwarf Galaxy is located in the same part of the sky as the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....

 Canis Major
Canis Major
Canis Major is one of the 88 modern constellations, and was included in the 2nd-century astronomer Ptolemy's 48 constellations. Its name is Latin for 'greater dog', and is commonly represented as one of the dogs following Orion the hunter...

. The 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...

 contains a relatively high percentage of red giant
Red giant
A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...

 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, and is thought to contain an estimated one billion
1000000000 (number)
1,000,000,000 is the natural number following 999,999,999 and preceding 1,000,000,001.In scientific notation, it is written as 109....

 stars in all.

The Canis Major Dwarf Galaxy
Dwarf galaxy
A dwarf galaxy is a small galaxy composed of up to several billion stars, a small number compared to our own Milky Way's 200-400 billion stars...

 is classified as an irregular galaxy
Irregular galaxy
An irregular galaxy is a galaxy that does not have a distinct regular shape, like a spiral or an elliptical galaxy. The shape of an irregular galaxy is uncommon – they do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a...

 and is now thought to be the closest neighbouring galaxy to our location in the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

, being located about 25,000 light-years away from our Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 and 42,000 light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...

s from the Galactic Center
Galactic Center
The Galactic Center is the rotational center of the Milky Way galaxy. It is located at a distance of 8.33±0.35 kpc from the Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius where the Milky Way appears brightest...

. It has a roughly elliptical shape and is thought to contain as many stars as the Sagittarius Dwarf Elliptical Galaxy
Sagittarius Dwarf Elliptical Galaxy
The Sagittarius Dwarf Elliptical Galaxy is an elliptical loop-shaped satellite galaxy of the Milky Way Galaxy. It consists of four globular clusters, the main cluster being discovered in 1994...

, the previous contender for closest galaxy to our location in the Milky Way.

Discovery

The galaxy was first discovered in November 2003 by an international team of French, Italian, British and Australian astronomer
Astronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...

s. Although closer to the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 than the centre of the galaxy itself, the Canis Major Dwarf Galaxy was difficult to detect as it is located behind the plane of the Milky Way, where concentrations of stars, gas and dust are densest. This, along with its small size, explains why it was not discovered sooner.

The team of astronomers that discovered it were collaborating on analysis of data from the Two-Micron All Sky Survey
2MASS
Observations for the Two Micron All-Sky Survey began in 1997 and were completed in 2001 at two telescopes located one each in the northern and southern hemispheres to ensure coverage of the entire sky...

 (2MASS), a comprehensive survey of the sky in infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 light, which is not blocked by gas and dust as severely as visible light. Because of this technique, scientists were able to detect a very significant over-density of class M
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...

 giant stars in a part of the sky occupied by the Canis Major constellation, along with several other related structures composed of this type of star, two of which form broad, faint arcs.

Characteristics

Astronomers believe that the Canis Major Dwarf Galaxy is in the process of being pulled apart by the gravitational field
Gravitational field
The gravitational field is a model used in physics to explain the existence of gravity. In its original concept, gravity was a force between point masses...

 of the more massive Milky Way galaxy. The main body of the galaxy is extremely degraded. Tidal disruption
Tidal force
The tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force per unit mass exerted on one body by a second body is not constant across its diameter, the side nearest to the second being more attracted by it than the side...

 causes a long filament of stars to trail behind it as it orbits the Milky Way, forming a complex ringlike structure sometimes referred to as the Monoceros Ring
Monoceros Ring
The Monoceros Ring is a ring of stars around the Milky Way which is proposed to consist of a stellar stream torn from the Canis Major Dwarf Galaxy as it merges with the Milky Way over the course of billions of years...

, which wraps around our galaxy three times. The stream of stars was first discovered in the early 21st century by astronomers conducting the Sloan Digital Sky Survey
Sloan Digital Sky Survey
The Sloan Digital Sky Survey or SDSS is a major multi-filter imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project was named after the Alfred P...

. It was in the course of investigating this ring of stars, and a closely spaced group of globular clusters similar to those associated with the Sagittarius Dwarf Elliptical Galaxy, that the Canis Major Dwarf Galaxy was discovered.

Globular clusters thought to be associated with the Canis Major Dwarf galaxy include NGC 1851
NGC 1851
NGC 1851 is a globular cluster located in the constellation Columba.- External links :*...

, NGC 1904, NGC 2298 and NGC 2808
NGC 2808
NGC 2808 is a globular cluster in the constellation Carina. The cluster belongs to the Milky Way, and is one of our home galaxy's most massive clusters, containing more than a million stars...

, all of which are likely to be remnants of the galaxy's globular cluster system before its 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...

, or swallowing, into the Milky Way. NGC 1261
NGC 1261
-External links:****...

 is another nearby cluster, but its velocity is different enough from that of the others to make its relation to the system unclear. The Canis Major Dwarf Galaxy may also have associated open cluster
Open cluster
An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist...

s, including Dol 25 and H18, and possibly AM 2. It is thought that the open clusters may have formed due to the dwarf galaxy's gravity perturbing material in the galactic disk and stimulating star formation
Star formation
Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young...

.

The discovery of the Canis Major Dwarf Galaxy and subsequent analysis of the stars associated with it has provided some support for the current theory that galaxies may grow in size by swallowing their smaller neighbors. Martin et al. believe that the preponderance of evidence points to the accretion of a small satellite galaxy
Satellite galaxy
A satellite galaxy orbits a larger galaxy due to gravitational attraction. Although a galaxy is made of a large number of objects which are not connected to each other, it has a center of mass, which represents a weighted average of the positions of each component object...

 of the Milky Way which was orbiting roughly in the plane of the galactic disk.

A new study by Yazan Momany using 2MASS data casts doubts on the nature of the dwarf galaxy. The data suggests that the trail of stars is actually part of the warped galactic disc. This conclusion, however, is still being challenged and the true nature of the overdensity in Canis Major
Canis Major
Canis Major is one of the 88 modern constellations, and was included in the 2nd-century astronomer Ptolemy's 48 constellations. Its name is Latin for 'greater dog', and is commonly represented as one of the dogs following Orion the hunter...

remains unknown.

External links

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