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Anne’s Image of the Day: Globular Cluster Messier 2
read more at Anne’s Astronomy News http://annesastronomynews.com/
July 27, 2013
Messier 2, a globular cluster in Aquarius
Image Credit: NASA/ESA /HST/STScI, via WikiSky
Messier 2 (also known as NGC 7089) is a globular cluster of about 175 light-years across, located some 37,500 light-years away from Earth in the constellation of Aquarius (the Water-Bearer), five degrees north of the star Beta Aquarii. It is slowly moving toward us at approximately 6.7 kilometers per second.
Globular clusters are gravitationally bound, roughly spherical, collections of hundreds of thousands of stars that orbit in the halos of galaxies. They contain considerably more stars and are much older than the less dense open clusters, which are found in the disks of galaxies. The more than 150 known globular clusters in our Milky Way galaxy contain some of the oldest stars known.
Messier 2 is one of the largest known globular clusters, and its tidal influence is a lot larger than the 175 light-years, around 233 light-years. It has a rich and significantly elliptical concentration of stars with a very dense core. Estimated to be some 13 billion years old, it is also one of the older globulars associated with the Milky Way galaxy.
The cluster contains about 150,000 stars and its brightest stars are red and yellow giants. Because (almost all) the stars in Messier 2 are much older than our Sun, they have very few heavier elements than hydrogen and helium. Compared to the Sun, they have just 2% of its heavy elements, making Earth-like planets (not to mention Earth-like life) nearly certain to be a rarity in this cluster.
Messier 2 is home to 21 known variable stars; most of them are RR Lyrae variables, with short periods of less than a day and three are Cepheids variables.
Cepheids variable stars are very luminous variable stars with a strong direct relationship between their variable’s luminosity and pulsation period. That secures their status as important standard candles for establishing the Galactic and extragalactic distance scales.
RR Lyrae variables are also periodic variable stars. They are pulsating horizontal branch stars (stars in a stage of stellar evolution that immediately follows the red giant phase), with a mass of around half the Sun’s. They are thought to have previously shed mass and consequently, they were once stars with similar or slightly less mass than the Sun, around 0.8 solar masses.
These stars pulse in a manner similar to Cepheid variables, so the mechanism for the pulsation is thought to be similar, but in contrast to Cepheids, RR Lyraes are old, relatively low mass, metal-poor stars. They are much more common than Cepheids, but also much less luminous. Their period is shorter, typically less than one day, sometimes ranging down to seven hours.
The relationship between pulsation period and absolute magnitude of RR Lyraes makes them also good standard candles, especially for relatively near objects within the Milky Way. This type of variable is named after the prototype, the variable star RR Lyrae in the constellation Lyra.
This image was taken with the Hubble Space Telescope.
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Source: http://annesastronomynews.com/annes-image-of-the-day-globular-cluster-messier-2/
