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Object description
NGC 5617 and Trumpler 22 - a double star cluster in the southern sky, Pismis 19 and the small bipolar planetary nebula He 2-111

Description of object:

NGC 5617 is a galactic open star cluster in the constellation Centaurus. According to Trumpler it is classified as Type I 3 m. Our image shows about 80 stars, in total about 175 stars are assigned to the cluster. It is located about 5000 light-years from the solar system and with an apparent diameter of about 10 arcminutes its real diameter is 15 light-years. NGC 5617 was discovered on May 8, 1826 by the Scottish astronomer James Dunlop.

NGC 5617 forms together with Trumpler 22 a double star cluster. Trumpler 22 lies 0.6 degrees southeast of NGC 5617. Both clusters have the same distance to the solar system and have the same radial velocity. The stars in both clusters are the same age, about 70 million years, and have similar metallicity (the relative abundance of iron compared to hydrogen), suggesting that they formed together from a cloud of interstellar dust. The distance between NGC 5617 and Trumpler 22 is about 60 light-years.

Henize 2-111 - - also cataloged as PK 315.0-00.3 - is a small, bipolar planetary nebula, near Alpha Centauri. Its apparent diameter is only about 20 arcseconds and its apparent magnitude is given as 16.7 mag. In its morphology it resembles NGC 6302 in the constellation Scorpio. The intensity maximum of the radiation is at 658.4 nm (N-II), therefore it appears in deep red color in the image.

He 2-111 is embedded in an elongated oval halo of reddish nebulae, probably gas masses ejected from the central star long ago. The planetary nebula is about 11.500 light years away from the solar system.

It was discovered in 1967 by Karl Gordon Henize. Karl Gordon Henize was an astronomer and NASA science astronaut. In 1985, Henize launched as a mission specialist on the space shuttle Challenger for the Spacelab-2 mission (STS-51-F). He died tragically in 1993 while attempting to climb Mount Everest.

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Only little information is available about Pismis 19. The cluster has an apparent diameter of only 2 arcminutes and the brightest stars are about 12th magnitude. The yellowish-reddish color of the stars is striking. It is due to a high interstellar absorption by dust lying in front of it.

Pismis 19 has a distance of about 8000 light-years to the solar system. Its age is given as about 800 million years, so the cluster is about 100 times older than NGC 5617 and Trumpler 22.

Paris Maria Pismis was an astronomer of Armenian origin who worked on open star clusters in Mexico and published a corresponding catalog.

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The bluish brightening on the left side is scattered light from the bright star alpha Cenrauri.

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