This image is one of the latest views from the Hubble Space Telescope, closely resembling a toy snow globe. In this case, over a hundred thousand glittering stars are the little faux snow flakes and the cluster’s globular structure is the glass sides. The cluster of interest is M13, located over 25,000 light years away, measuring 150 light years across (that’s 0.15% the diameter of our galaxy). Although very pretty (and very festive), there are some interesting things going on inside this little cluster of stars…
Globular clusters are very common, and many are known to surround our Milky Way galaxy. Over 150 ‘small’ clusters have been observed in our galaxy’s halo acting like cosmic artefacts; the stars contained within clusters like M13 are thought to be amongst the oldest known in the Universe. These clusters in the Milky Way halo are thought to have formed well before any stars existed in today’s Milky Way spiral disk. So these small, old clusters have some wisdom to impart on today’s astronomers as to the ancient history of our galaxy (why does Yoda from Star Wars spring to mind?).
The stars within M13 are predominantly old red giants that have expanded well beyond their original diameters and cooled significantly. These stars are stuck, gravitationally bound, orbiting a common point in the centre of the cluster’s mass. However, occasionally, as the centre of M13 is so densely packed, the old stars can stray too close, colliding, and creating a new type of star known as a “blue straggler”.
Blue stragglers are formed when the gas from one star is siphoned off by another. This rejuvenates one of the stars, heating it up significantly. This phenomenon has mystified astronomers for a long time as young, blue stars were observed hiding inside clusters of old, red stars. Only recently has this collision mechanism been put forward as a possible explanation to the appearance of “young”, hot stars inside globular clusters like M13.
Putting snow globes, Yoda and blue stragglers to one side, the stunning image at the top of this post comprises of several archival observations from Hubble’s Wide Field Planetary Camera 2 and Advanced Camera for Surveys. Four separate campaigns from November 1999, April 2000, August 2005 and April 2006 were used.
What a great picture to start getting into the Christmas holiday spirit with…