IC 10
Encyclopedia
IC 10 is an irregular galaxy
in the constellation
Cassiopeia
. It was discovered by Lewis Swift
in 1887. Nicholas U. Mayall was the first to suggest that the object is extragalactic in 1935. Edwin Hubble
suspected it might belong to the Local Group
of galaxies
, but its status remained uncertain for decades. The radial velocity
of IC 10 was measured in 1962, and it was found to be approaching the Milky Way
at approximately
350 km/s, strengthening the evidence for its membership in the Local Group. Its membership in the group was finally confirmed in 1996 by direct measurements of its
distance based on observations of Cepheid
s. Despite its closeness, the galaxy is rather difficult to study because it lies near the plane of the Milky Way
and is therefore
heavily obscured by interstellar matter.
The apparent distance between IC 10 and the Andromeda Galaxy
is about the same as the apparent distance between the Andromeda Galaxy and the Triangulum Galaxy
, which suggests that IC 10 may belong to the M31 subgroup.
IC 10 is the only known starburst galaxy
in the Local Group of galaxies. It has many more Wolf-Rayet stars per square kiloparsec
(5.1 stars/kpc²) than the Large Magellanic Cloud
(2.0 stars/kpc²) or the Small Magellanic Cloud
(0.9 stars/kpc²). Although the galaxy has a luminosity similar to the SMC, it is considerably smaller. Its higher metallicity
compared to the SMC suggests that star formation activity has continued for a longer time period. The evolutionary
status of the Wolf-Rayet stars suggests that they all formed in a relatively short timespan. The ratio between the two types of Wolf-Rayet stars (WC stars and WN stars) in IC 10 is very different from the ratio in other galaxies in the Local Group, which may be somehow due to the starburst nature of the galaxy. Currently the galaxy produces stars at the rate of 0.04–0.08 solar mass
es per year, which means that the gas supply in the galaxy can last for only a few billion years longer.
Observations of IC 10 in the far-infrared show that the dust
in this mild starburst galaxy is deficient in small grains. It is hypothesized that any small grains that formerly existed were destroyed by strong ultraviolet
radiation in the areas around the hot luminous stars that were formed in the galaxy's recent burst of star formation.
The galaxy has a huge envelope of hydrogen
gas, with an apparent size measuring 68′ × 80′, which is far larger than the apparent size of the galaxy in visible light (5.5′ × 7.0′). IC 10 is also unusual in the respect that the visible part of the galaxy seems to rotate in a different direction than the outer envelope. It has a H II nucleus.
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...
in 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....
Cassiopeia
Cassiopeia (constellation)
Cassiopeia is a constellation in the northern sky, named after the vain queen Cassiopeia in Greek mythology, who boasted about her unrivalled beauty. Cassiopea was one of the 48 constellations listed by the 2nd century Greek astronomer Ptolemy, and it remains one of the 88 modern constellations today...
. It was discovered by Lewis Swift
Lewis A. Swift
Lewis A. Swift was an American astronomer.-Discoveries:He discovered or co-discovered a number of comets, including periodic comets 11P/Tempel-Swift-LINEAR, 64P/Swift-Gehrels, and 109P/Swift-Tuttle...
in 1887. Nicholas U. Mayall was the first to suggest that the object is extragalactic in 1935. Edwin Hubble
Edwin Hubble
Edwin Powell Hubble was an American astronomer who profoundly changed the understanding of the universe by confirming the existence of galaxies other than the Milky Way - our own galaxy...
suspected it might belong to the Local Group
Local Group
The Local Group is the group of galaxies that includes Earth's galaxy, the Milky Way. The group comprises more than 30 galaxies , with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy...
of galaxies
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...
, but its status remained uncertain for decades. The radial velocity
Radial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...
of IC 10 was measured in 1962, and it was found to be approaching 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...
at approximately
350 km/s, strengthening the evidence for its membership in the Local Group. Its membership in the group was finally confirmed in 1996 by direct measurements of its
distance based on observations of Cepheid
Cepheid variable
A Cepheid is a member of a class of very luminous variable stars. The strong direct relationship between a Cepheid variable's luminosity and pulsation period, secures for Cepheids their status as important standard candles for establishing the Galactic and extragalactic distance scales.Cepheid...
s. Despite its closeness, the galaxy is rather difficult to study because it lies near the plane of 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...
and is therefore
heavily obscured by interstellar matter.
The apparent distance between IC 10 and the Andromeda Galaxy
Andromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...
is about the same as the apparent distance between the Andromeda Galaxy and the Triangulum Galaxy
Triangulum Galaxy
The Triangulum Galaxy is a spiral galaxy approximately 3 million light years from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC 598, and is sometimes informally referred to as the Pinwheel Galaxy, a nickname it shares with Messier 101...
, which suggests that IC 10 may belong to the M31 subgroup.
IC 10 is the only known starburst galaxy
Starburst galaxy
A starburst galaxy is a galaxy in the process of an exceptionally high rate of star formation, compared to the usual star formation rate seen in most galaxies. Galaxies are often observed to have a burst of star formation after a collision or close encounter between two galaxies...
in the Local Group of galaxies. It has many more Wolf-Rayet stars per square kiloparsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
(5.1 stars/kpc²) than the Large Magellanic Cloud
Large Magellanic Cloud
The Large Magellanic Cloud is a nearby irregular galaxy, and is a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs , the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy lying closer to the center...
(2.0 stars/kpc²) or the Small Magellanic Cloud
Small Magellanic Cloud
The Small Magellanic Cloud is a dwarf galaxy. It has a diameter of about 7,000 light-years and contains several hundred million stars. It has a total mass of approximately 7 billion times the mass of our Sun....
(0.9 stars/kpc²). Although the galaxy has a luminosity similar to the SMC, it is considerably smaller. Its higher metallicity
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
compared to the SMC suggests that star formation activity has continued for a longer time period. The evolutionary
Stellar evolution
Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years .Stellar evolution is not studied by observing the life of a single...
status of the Wolf-Rayet stars suggests that they all formed in a relatively short timespan. The ratio between the two types of Wolf-Rayet stars (WC stars and WN stars) in IC 10 is very different from the ratio in other galaxies in the Local Group, which may be somehow due to the starburst nature of the galaxy. Currently the galaxy produces stars at the rate of 0.04–0.08 solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es per year, which means that the gas supply in the galaxy can last for only a few billion years longer.
Observations of IC 10 in the far-infrared show that the dust
Cosmic dust
Cosmic dust is a type of dust composed of particles in space which are a few molecules to 0.1 µm in size. Cosmic dust can be further distinguished by its astronomical location; for example: intergalactic dust, interstellar dust, interplanetary dust and circumplanetary dust .In our own Solar...
in this mild starburst galaxy is deficient in small grains. It is hypothesized that any small grains that formerly existed were destroyed by strong ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
radiation in the areas around the hot luminous stars that were formed in the galaxy's recent burst of star formation.
The galaxy has a huge envelope of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
gas, with an apparent size measuring 68′ × 80′, which is far larger than the apparent size of the galaxy in visible light (5.5′ × 7.0′). IC 10 is also unusual in the respect that the visible part of the galaxy seems to rotate in a different direction than the outer envelope. It has a H II nucleus.