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Object description
Pismis 24 - a very young open star cluster in NGC 6357

Short objekt description:

Our image shows the open star cluster Pismis 24. The raw sum images were only exposed briefly for this image to prevent overexposure of the bright stars of the star cluster. In addition, this also avoided overexposure of the bright nebulae in which the star cluster is embedded.

The star cluster Pismis 24 is located in the central region of the large H-II emission nebula NGC 6357 (also popularly called the Lobster Nebula), which is located in the constellation of Scorpio. Among others, the star Pismis 24-1 (HDE 319 718 A) is located in the central region. It has the spectral class O3 and its age is estimated at just under 6 million years. With an estimated mass of 200 to 300 solar masses, it was long considered the "heaviest and largest" star in our Milky Way. The distance from Pismis 24 to the solar system is about 8,000 light years.

The problem: according to the theory of star formation, such large and heavy stars should not be able to form at all. Detailed investigations with the Hubble Space Telescope in 2006 showed that Pismis 24-1 is a double star, perhaps even a triple star system.
Despite dissolving into two individual stars, the two stars of spectral class O with a mass of 70 to 100 solar masses are among the heaviest known stars. With the star Pismis 24-17, another star with about 100 solar masses belongs to this extremely rare class of giant stars.

The intense and hot ultraviolet radiation of these giant stars creates a bubble in the H-II region and ionises the surrounding hydrogen gas, causing it to glow. Pismis 24 was discovered by the Armenian astronomer Paris Pismis in 1959.

A detailed technical article about Pismis 24 can be found as a pdf download here. A picture taken with the Hubble Telescope can be seen here.

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