2MASS J18082002−5104378 (abbreviated J1808−5104) is an ultra metal-poor (UMP) binary star system, in the constellation Ara, about from Earth, and is a single-lined spectroscopic binary (SB1). Estimated to be about 13.53 billion years old, it is one of the oldest stars known, and possibly one of the first stars in our universe — one 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 [Fe/H] less than , or th of the levels in the Sun. It is a single-lined spectroscopic binary, with radial velocity variations in its spectral absorption lines 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" of the Milky Way, the part of the galaxy 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. Despite its age and low metallicity, 2MASS J18082002−5104378 is still too metal-rich to be considered a population III star.
Primary star The primary component of the binary star system, 2MASS J18082002−5104378 A, is a subgiant, 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, has an orbital period = 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.
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(5 November 2018; Johns Hopkins University)