2MASS J18082002−5104378 (abbreviated J1808−5104) is an ultra metal-poor (UMP)

binary star system A binary star or binary star system is a Star system, system of two stars that are gravity, gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved ...

, in the constellation Ara, about from

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

, and is a single-lined spectroscopic binary (SB1). It is one of the oldest stars known, about 13.53 billion years old, possibly one of the first stars, a star made almost entirely of materials released from the Big Bang. A tiny unseen companion, a low-mass UMP star, is particularly unusual.

System

J1808−5104 is an ultra metal-poor (UMP) star, one that has a logarithmic

metallicity In astronomy, metallicity is the Abundance of the chemical elements, abundance of Chemical element, elements present in an object that are heavier than hydrogen and helium. Most of the normal currently detectable (i.e. non-Dark matter, dark) matt ...

e/Hless than , or th of the levels in the Sun. It is a single-lined spectroscopic binary, with

radial velocity The radial velocity or line-of-sight velocity of a target with respect to an observer is the rate of change of the vector displacement between the two points. It is formulated as the vector projection of the target-observer relative velocity ...

variations in its spectral

absorption line Absorption spectroscopy is spectroscopy that involves techniques that measure the absorption (electromagnetic radiation), absorption of electromagnetic radiation, as a function of frequency or wavelength, due to its interaction with a sample. Th ...

s interpreted as orbital motion of the visible star. The companion is invisible, but inferred from the orbit. J1808−5104 is the brightest UMP star, as a binary system, known, and is part of the "

thin disk The thin disk is a structural component of spiral and S0-type galaxies, composed of stars, gas and dust. It is the main non-centre (e.g. galactic bulge) density of such matter. That of the Milky Way is thought to have a scale height of around i ...

" of the

Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

, the part of the

galaxy A galaxy is a Physical system, system of stars, stellar remnants, interstellar medium, interstellar gas, cosmic dust, dust, and dark matter bound together by gravity. The word is derived from the Ancient Greek, Greek ' (), literally 'milky', ...

in which the Sun is located, but unusual for such a metal-poor and old star. At , the star is the oldest known thin-disk star, and several billion years older than most estimates for the age of the Milky Way's thin disk.

Primary star

The primary component of the binary star system, 2MASS J18082002−5104378 A, is a

subgiant A subgiant is a star that is brighter than a normal main-sequence star of the same spectral class, but not as bright as giant stars. The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution ...

, cooler than the Sun, but larger and more luminous.

Secondary star

The secondary unseen companion, 2MASS J18082002−5104378 B, thought to be a

red dwarf A red dwarf is the smallest kind of star on the main sequence. Red dwarfs are by far the most common type of fusing star in the Milky Way, at least in the neighborhood of the Sun. However, due to their low luminosity, individual red dwarfs are ...

, has an

orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...

= days and a mass of . It is the first low-mass UMP star to be discovered, and one of the oldest stars in the universe, about 13.53 billion years old. It is possibly one of the first stars, a star made almost entirely of materials released from the

Big Bang The Big Bang is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models based on the Big Bang concept explain a broad range of phenomena, including th ...

References

External links

* * * (5 November 2018;

Johns Hopkins University The Johns Hopkins University (often abbreviated as Johns Hopkins, Hopkins, or JHU) is a private university, private research university in Baltimore, Maryland, United States. Founded in 1876 based on the European research institution model, J ...

) {{DEFAULTSORT:2MASS J18082002-5104378 J18082002−5104378 Ara (constellation) 20181101 November 2018 Spectroscopic binaries TIC objects