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It is the brightest, most massive, and most evolved stellar member of the nearest OB association (the Scorpius–Centaurus Association).Antares is a member of the Upper Scorpius subgroup of the Scorpius–Centaurus Association, which contains thousands of stars with mean age 11 million years at a distance of approximately 170 parsecs (550 ly).Defining an effective temperature is difficult due to spectral lines being generated at different depths within the atmosphere, and linear measurements produce different results depending on the wavelength observed.The diameter of Antares can be measured most accurately using interferometry or observing lunar occultations events.Antares has an extended shell which radiates strongly at those particular wavelengths.Antares is a type LC slow irregular variable star, whose apparent magnitude slowly varies between extremes of 0.6 and 1.6, although usually near magnitude 1.0.An apparent diameter from occultations 41.3 ± 0.1 milliarcseconds has been published.
Notably, like all red giants, the true size of Antares is uncertain based on the uncertainties like the star's distance and luminosity, so the 300 million kilometre radius shown here tends towards minimum of the range of published values. The brightness of Antares at visual wavelengths is about 10,000 times that of the Sun, but because the star radiates a considerable part of its energy in the infrared part of the spectrum, the true bolometric luminosity is around 100,000 times that of the Sun.It was first observed by Scottish astronomer James William Grant FRSE while in India on 23 July 1844.The last is equal to a projected separation of about 529 astronomical units (au) at the estimated distance of Antares, giving a minimum value for the separation of the pair.For approximately two to three weeks on either side of November 30, Antares is not visible in the night sky, because it is near conjunction with the Sun; this period of invisibility is longer in the Northern Hemisphere than in the Southern Hemisphere, since the star's declination is significantly south of the celestial equator.Antares is one of the four first magnitude stars that lies within 6.5° of the ecliptic (the others are Spica, Regulus and Aldebaran) and therefore can be occulted by the Moon. The event was visible in much of southern Asia and the Middle East.