Sunday, October 24, 2021

A jewel box in the skies of the southern hemisphere

This is one of the names by which NGC 4755 is known, a splendid open cluster in the constellation of the Southern Cross, whose stars are reminiscent of beautiful jewels.

The cluster appears to be of fourth magnitude and can therefore be observed with the naked eye under dark skies. Through binoculars we begin to recognize its brightest components, most of which are blue supergiants. However, a supergiant that shines with an intense red color stands out in the center of the image.

The cluster is one of the youngest known, as it is only 14 million years old. It contains approximately 100 stars, enclosed in a volume on the side of about twenty light years. The brightest of these stars (the supergiants mentioned above) are already about to enter the final stages of their life and will explode into supernovae over the next millions of years.

The distance to the cluster is uncertain, as part of its light is obscured by the Coal Sack Nebula. This is one of the largest dark nebulae in the sky, made up of dust and cold gas that absorbs the light of the stars behind it, thus confusing distance measurements. While the nebula is 600 light-years away, the best estimates available to date place the cluster at 6500 light-years from Earth.

Credit: ESO/Y. Beletsky.

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