Monday, November 14, 2011

Globular Cluster Photo Series (Part 14): M71

Today's picture is of the globular cluster Messier 71 in the tiny constellation Sagitta, the Arrow (not to be confused with the much larger and more familiar constellation Sagittarius, the Archer). M71 is an unusual globular cluster between 12-13,000 light-years away with a diameter of about 27 light years, fairly small for a globular cluster. It has a small apparent diameter of only 7.2 arcminutes (less than a third the width of the full Moon).

Messier 71 in Sagitta.
Sagitta is located in the plane of the Milky Way from our point of view, which explains the high stellar density in the background of this image. M71 was for a long time (up until the 1970's, in fact) thought to be a dense open cluster rather than what it actually is, a loose globular cluster. One reason was that the stars in M71 are younger than is typical for globular clusters, although that fact simply turned out to mean that M71 is a relatively young globular cluster. M71 also lacks a particular kind of variable star called RR Lyrae stars (after the prototype RR Lyrae) that are common in globular clusters, which turned out to be related to its age: its stars are too young to have become RR Lyrae-type variables yet. In fact, M71 contains only 8 known variable stars, though one of them is an interesting irregular variable.

This lack of RR Lyrae stars is one reason the distance to M71 is known to no better than a thousand light-years. RR Lyrae stars make good standard candles within our Galaxy, as the relation between their periods and their luminosities is well-known. They are also much more common than the other type of variable star commonly used as standard candles, Cepheid variables. RR Lyrae stars can be found at all angles in the sky (in contrast to Cepheids which is are strongly associated with the galactic plane), and consequently make up 90% of the variable stars found in globular clusters.

Anyway, that's it for tonight, I need to get some sleep. A hui hou!

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