Reticulum II (or Reticulum 2) is a

dwarf galaxy A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is ...

in the

Local Group The Local Group is the galaxy group that includes the Milky Way. It has a total diameter of roughly , and a total mass of the order of . It consists of two collections of galaxies in a "dumbbell" shape: the Milky Way and its satellites form ...

. Reticulum II was discovered in 2015 by analysing images from the

Dark Energy Survey The Dark Energy Survey (DES) is an astronomical survey designed to constrain the properties of dark energy. It uses images taken in the near-ultraviolet, Visible spectrum, visible, and near-infrared to measure the expansion of the universe using ...

. It is a satellite of the

Magellanic Cloud The Magellanic Clouds (''Magellanic system'' or ''Nubeculae Magellani'') are two irregular galaxy, irregular dwarf galaxy, dwarf galaxy morphological classification, galaxies in the southern celestial hemisphere. Orbiting the Milky Way galaxy, t ...

s and was probably captured relatively recently. Like other dwarf spheroidal galaxies, its stellar population is old: the galaxy was

quenched In materials science, quenching is the rapid cooling of a workpiece in water, oil, polymer, air, or other fluids to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, such as phas ...

before 11.5 billion years ago. Reticulum II is elongated, having an major/minor axis ratio of 0.6. The size is given by a

half-light radius Galaxy effective radius or half-light radius (R_e) is the radius at which half of the total light of a galaxy is emitted. This assumes the galaxy has either intrinsic circular symmetry, spherical symmetry or is at least circularly symmetric as vie ...

of 15

parsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...

s (pc). This is too large for it to be a

globular cluster A globular cluster is a spheroidal conglomeration of stars. Globular clusters are bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of membe ...

. The

absolute magnitude Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse Logarithmic scale, logarithmic Magnitude (astronomy), astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent mag ...

(M_V) of the galaxy is −2.7. The distance from Earth is about 30 kpc. The galaxy contains some blue horizontal branch stars. Other features visible are a

main sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...

, and a main sequence turn off, and a

red giant branch The red-giant branch (RGB), sometimes called the first giant branch, is the portion of the giant branch before helium ignition occurs in the course of stellar evolution. It is a stage that follows the main sequence for low- to intermediate-mass sta ...

. It has an unusual enhancement of ''r''-process elements, meaning that gold and

europium Europium is a chemical element with the symbol Eu and atomic number 63. Europium is the most reactive lanthanide by far, having to be stored under an inert fluid to protect it from atmospheric oxygen or moisture. Europium is also the softest lanth ...

are enriched in the brightest stars in the galaxy. About 72% of its stars are enriched in ''r''-process elements. The implication of the unusual enrichment in elements heavier than zinc, is that the ''r''-process is very rare, and only happened once in this galaxy, possibly by the collision of two neutron stars.

Gamma ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...

s mostly with energies between 2 and 10 GeV have been detected by the

Fermi satellite The Fermi Gamma-ray Space Telescope (FGST, also FGRST), formerly called the Gamma-ray Large Area Space Telescope (GLAST), is a space observatory being used to perform gamma-ray astronomy observations from low Earth orbit. Its main instrument is ...

. The radiation from Reticulum II is more significant than that of other dwarf galaxy emissions. However this finding has been contested.

References

{{Milky Way Dwarf elliptical galaxies Milky Way Subgroup Astronomical objects discovered in 2015 Reticulum (constellation)