Astronomy Picture of the Day 5-05-02

NASA ^ | 5-05-02 | Robert Nemiroff and Jerry Bonnell

[petuniasevan]

Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2002 May 5

The M7 Open Star Cluster in Scorpius
Credit & Copyright: N. A. Sharp, REU Program, AURA, NOAO, NSF

Explanation: M7 is one of the most prominent open clusters of stars on the sky. The cluster, dominated by bright blue stars, can be seen with the naked eye in a dark sky in the tail of the constellation of Scorpius. M7 contains about 100 stars in total, is about 200 million years old, spans 25 light-years across, and lies about 1000 light-years away. This color picture was taken in 1995 at the Burrell-Schmidt Telescope at Kitt Peak National Observatory in Arizona. The M7 star cluster has been known since ancient times, being noted by Ptolemy in the year 130 AD. Also visible is a dark dust cloud near the bottom of the frame, and literally millions of unrelated stars towards the Galactic center.

[petuniasevan]

The presence of bright, blue stars is a benchmark for star cluster youth. (Relatively speaking, of course). Stars that are hotter are bluer, and their fusion processes run much faster than those of smaller stars (some of the processes differ too, but I digress...). Thus they burn bright, and burn out early.

Other clusters show a greater age by the presence of red giants. The beautiful Jewel Box Cluster contains a lot of blue stars and ONE red giant, like a ruby among sapphires on black velvet. One star has reached the end of its ordinary "fuel" (hydrogen) in its core and now has to convert layers around the core, so it expands and the outer layers of the star expand and cool.

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