Pismis 24-1

Pismis 24-1, also known as HD 319718, is the brightest star of the open cluster Pismis 24 within the nebula NGC 6357 about 6,500 light-years away. It was once thought to be the most massive star known, but is composed of at least three individual objects, each still among the most luminous and most massive stars known. Pismis 24-1 was first catalogued as HD 319718, later resolved into both Pismis 24-1 and the fainter Pismis 24-16. The surrounding H II region NGC 6357 is prominent, but the compact 10th-magnitude open cluster Pismis 24 was not identified until 1959. In 1973, Pismis 24 was resolved into 15 components of which 12 were considered member stars. The brightest was numbered first as Pismis 24-1 and tentatively considered a supergiant. It was later resolved into an O3.5 supergiant spectroscopic binary and an O4 giant star separated by approximately 500AU. Pismis 24-1 has been resolved visually into two components, usually labelled as NE and SW from their orientation with each other. Pismis 24-1NE is slightly more luminous and hotter than 24-1SW, but is known to be a spectroscopic binary. This is surprising given the spectral luminosity classes, because it would make the individual supergiant stars less luminous than a single cooler giant star. It could be that the interaction between the components of 24-1NE is confusing its classification, or the O4 giant may also be a close binary. Pismis 24-1 is actually a shallow eclipsing binary with a period of 2.4 days. It is presumed to be the NE component which produces the eclipses, but the separate light curves for the components have not been resolved. The light curve is symmetrical, indicating a near circular orbit, and the two eclipsing stars have very similar masses and temperatures. The Catalog of Components of Double and Multiple Stars lists Pismis 24-1 as having two fainter companions 5.5 and 16.4 arcseconds distant. This is not surprising since it is a member of a rich open cluster only 1.5 arcminutes across.