Sextans B
Encyclopedia
Sextans B is an irregular
galaxy
that may be part of the Local Group
, or lie just beyond it. Sextans B is 4.44 million light-year
s away from Earth
and thus is one of most distant members of the Local Group, if it is indeed a member. It forms a pair with its neighbouring galaxy Sextans A
. It is a type Ir IV-V galaxy according to the galaxy morphological classification scheme. Sextans B may also be gravitationally associated with the galaxies NGC 3109
and the Antlia Dwarf
.
Sextans B has a uniform stellar population, but the interstellar medium in it may be inhomogeneous. Its mass is estimated to be about 2 × 108 times the mass of the Sun
, of which 5.5 × 107 is in the form of atomic hydrogen
. Star formation in the galaxy seems to have proceeded in distinct periods of low intensity, separated by shorter periods of no activity. The existence of Cepheid variable
s in the galaxy implies that Sextans B contains at least some young stars. The metallicity
of Sextans B is rather low, with a value of approximately Z = 0.001. Sextans B is receding from the Milky Way
with a speed of approximately 300 km/s, and probably lies just outside the edge of the Local Group
, so as its neighbour Sextans A
.
Five planetary nebula
e have been identified in Sextans B. It is one of the smallest galaxies in which planetary nebulae have been observed. These appear point-like and can be identified by their spectral emission lines. It also contains a massive globular cluster
.
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...
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...
that may be part of 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...
, or lie just beyond it. Sextans B is 4.44 million 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 away from 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...
and thus is one of most distant members of the Local Group, if it is indeed a member. It forms a pair with its neighbouring galaxy Sextans A
Sextans A
Sextans A , is a tiny dwarf irregular galaxy. It spans about 5000 light-years across, and is located within the Local Group of galaxies, which includes our Milky Way galaxy. At 4.3 million light-years away from Earth, Sextans A is one of most distant members of the Local Group, and is notable for...
. It is a type Ir IV-V galaxy according to the galaxy morphological classification scheme. Sextans B may also be gravitationally associated with the galaxies NGC 3109
NGC 3109
NGC 3109 is a small spiral or irregular galaxy around 4.2 Mly away in the direction of the constellation of Hydra. It is the most prominent member of a Local Group subgroup. NGC 3109 is believed to be tidally interacting with the dwarf elliptical galaxy, Antlia Dwarf...
and the Antlia Dwarf
Antlia Dwarf
The Antlia Dwarf is a dwarf spheroidal/irregular galaxy. It lies about 1.3 Mpc from Earth in the constellation Antlia. It is the fourth and faintest member of the nearby Antlia Group of galaxies. The galaxy contains stars of all ages, contains significant amounts of gas, and has experienced...
.
Sextans B has a uniform stellar population, but the interstellar medium in it may be inhomogeneous. Its mass is estimated to be about 2 × 108 times the mass of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
, of which 5.5 × 107 is in the form of atomic 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...
. Star formation in the galaxy seems to have proceeded in distinct periods of low intensity, separated by shorter periods of no activity. The existence of Cepheid variable
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 in the galaxy implies that Sextans B contains at least some young stars. The 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...
of Sextans B is rather low, with a value of approximately Z = 0.001. Sextans B is receding from 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...
with a speed of approximately 300 km/s, and probably lies just outside the edge of 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...
, so as its neighbour Sextans A
Sextans A
Sextans A , is a tiny dwarf irregular galaxy. It spans about 5000 light-years across, and is located within the Local Group of galaxies, which includes our Milky Way galaxy. At 4.3 million light-years away from Earth, Sextans A is one of most distant members of the Local Group, and is notable for...
.
Five planetary nebula
Planetary nebula
A planetary nebula is an emission nebula consisting of an expanding glowing shell of ionized gas ejected during the asymptotic giant branch phase of certain types of stars late in their life...
e have been identified in Sextans B. It is one of the smallest galaxies in which planetary nebulae have been observed. These appear point-like and can be identified by their spectral emission lines. It also contains a massive globular cluster
Globular cluster
A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is...
.