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Messier 47, NGC 2423, 2425 and Alessi 17

Description of object:

Messier 47 (also listed as NGC 2422 = NGC 2478) is a bright open star cluster with an apparent diameter of 30 arcminutes in the constellation (Puppis). Its apparent diameter corresponds to that of the full moon seen with the naked eye.

The star cluster contains about 80 stars, many of them bright, blue-white and very hot stars. In between there are also a few bright red giants which have already progressed much further in their short life cycles than the less massive and more long-lived blue stars.

At its approximate distance of 1600 light years to the solar system, 30 minutes of arc correspond to a physical diameter of about 15 light years. The hottest main sequence star in M 47 is of spectral type B2, which suggests that the cluster is about 80 million years old. According to Trumpler, M 47 is classified as type III 2 m.

The history of the discovery of Messier 47 is somewhat confused. The first observation reports around 1654 are from the Italian astronomer Giovanni Battista Hodierna. Charles Messier observed the star cluster on February 19, 1771 and entered it as number 47 in his catalogue. The observations of Hodierna were obviously not known to him. But Messier made the mistake to write down the coordinates incorrectly, so that the star cluster could not be found for many years. So he was later assigned the NGC number 2422. Only in 1959 the Canadian astronomer T. F. Morris found out that NGC 2422 and M47 is the same object.
NGC 2423 NGC 2423 is a loose collection of 50 to 60 faint stars scattered over an apparent diameter of about 20 arcminutes and located about 2500 light years from the Sun. Given the distance and apparent diameter of the cluster, it has a physical diameter of about 15 light years. Its age is estimated to be about 800 million years. The brightest star has a magnitude of 9.0, according to Trumpler it is classified as type IV 2 m. NGC 2423 was discovered on March 19, 1786 by William Herschel.

NGC 2425 NGC 2425 consists of about 50 stars which are located about 11 000 light years away from the sun. Its apparent diameter is about 4 arc minutes, which corresponds to a physical diameter of 13 light years. Investigations of individual stars suggest that it is very old, between 2 and 2.5 billion years. According to Trumpler it is classified as type III 2 p. NGC 2425 was discovered on 8 March 1793 by William Herschel.

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