File:M2, NGC 7089 (noao-m2).jpg

English: M2 is a globular cluster in the constellation of Aquarius. Its actual position in the galactic halo places it in the southern galactic cap, almost directly beneath the southern pole of our Galaxy, the Milky Way, and at a distance from us of around 47000 light-years. M2 is a compact and fairly dense globular cluster. In fact, it is one of the richer globulars, containing more than 100,000 stars (of which only about 5000 are visible here), and stretching more than 150 light-years across. The brightest stars that can be seen within it are mostly red and yellow giants of absolute magnitude about -3. Overall, the absolute magnitude is -9, but the apparent magnitude (as seen from Earth) is 6.3. This makes it a good candidate for viewing with binoculars and small telescopes. Like other globular clusters, which are the oldest, densest, and most populous star clusters in the Milky Way, M2 is made up of stars that are all the same age. These are amongst the oldest stars in our Galaxy, at an age of about 13 billion years. Because the stars in a globular cluster are all the same age and all at the same distance from Earth, they provide an excellent laboratory in which astronomers can study stellar evolution. The stars of a globular cluster can also be used to understand "standard candles", which is to say, types of stars that all have the same intrinsic brightness, and can therefore be used to measure distances. Although M2 does contain RR-Lyrae variables, one such type, there are fewer of them in M2 than in most other globular clusters. M2 also contains Cepheid variables, but since they are population II Cepheids and not the usual population I type, they cannot be used as standard candles. This picture was created from six images taken in July 1997 at the KPNO 0.9-meter telescope during the summer Research Experiences for Undergraduates (REU) program operated at the Kitt Peak National Observatory and supported by the National Science Foundation. Image size 17.0x15.5 arc minutes.