Antares is the brightest star in the constellation of Scorpius. It has the Bayer designation α Scorpii, which is Latinised to Alpha Scorpii. Often referred to as "the heart of the scorpion", Antares is flanked by σ Scorpii and τ Scorpii near the center of the constellation. Distinctly reddish when viewed with the naked eye, Antares is a slow irregular variable star that ranges in brightness from an apparent visual magnitude of +0.6 down to +1.6. It is on average the fifteenth-brightest star in the night sky. Classified as spectral type M1.5Iab-Ib, Antares is a red supergiant, a large evolved massive star and one of the largest stars visible to the naked eye. Its exact size remains uncertain, but if placed at the center of the Solar System, it would extend out to somewhere between the orbits of Mars and Jupiter. Its mass is calculated to be around 12 times that of the Sun. Antares appears as a single star when viewed with the naked eye, but it is actually a binary star system, with its two components called α Scorpii A and α Scorpii B. The brighter of the pair is the red supergiant, while the fainter is a hot main sequence star of magnitude 5.5. They have a projected separation of about . Antares is the brightest and most evolved stellar member of the Scorpius–Centaurus association, the nearest OB association to the Sun. It is a member of the Upper Scorpius subgroup of the association, which contains thousands of stars with a mean age of 11 million years. Antares is located about from Earth at the rim of the Upper Scorpius, and is illuminating the Rho Ophiuchi cloud complex in its foreground Nomenclature α Scorpii (Latinised to Alpha Scorpii) is the star's Bayer designation. Antares has the Flamsteed designation 21 Scorpii, as well as catalogue designations such as HR 6134 in the Bright Star Catalogue and HD 148478 in the Henry Draper Catalogue. As a prominent infrared source, it appears in the Two Micron All-Sky Survey catalogue as 2MASS J16292443-2625549 and the Infrared Astronomical Satellite (IRAS) Sky Survey Atlas catalogue as IRAS 16262–2619. It is also catalogued as a double star WDS J16294-2626 and CCDM J16294-2626. Antares is a variable star and is listed in the General Catalogue of Variable Stars, but as a Bayer-designated star it does not have a separate variable star designation. Its traditional name