* Astronomy

The Hubble Space Telescope has captured the most detailed images to date of the open star clusters NGC 265 and NGC 290 in the Small Magellanic Cloud — two sparkling sets of gemstones in the southern sky.

These images, taken with Hubble's Advanced Camera for Surveys, show a myriad of stars in crystal clear detail. The brilliant open star clusters are located about 200,000 light-years away and are roughly 65 light-years across.
Star clusters can be held together tightly by gravity, as is the case with densely packed crowds of hundreds of thousands of stars, called globular clusters. Or, they can be more loosely bound, irregularly shaped groupings of up to several thousands of stars, like the open clusters shown in this image.


Position (2000): R.A. 00h 47m 11s.1 Dec. -73° 28' 40".1

The stars in these open clusters are all relatively young and were born from the same cloud of interstellar gas. Just as old school-friends drift apart after graduation, the stars in an open cluster will only remain together for a limited time and gradually disperse into space, pulled away by the gravitational tugs of other passing clusters and clouds of gas. Most open clusters dissolve within a few hundred million years, whereas the more tightly bound globular clusters can exist for many billions of years.
Open star clusters make excellent astronomical laboratories. The stars may have different masses, but all are at about the same distance, move in the same general direction, and have approximately the same age and chemical composition. They can be studied and compared to find out more about stellar evolution, the ages of such clusters, and much more.


Position (2000): R.A. 00h 51m 15s.2 Dec. -73° 20' 58".9

The Small Magellanic Cloud, which hosts the two star clusters, is one of the small satellite galaxies of the Milky Way. It can be seen with the unaided eye as a hazy patch in the constellation Tucana (the Toucan) in the Southern Hemisphere. The Small Magellanic Cloud is rich in gas nebulae and star clusters. It is most likely that this irregular galaxy has been disrupted through repeated interactions with the Milky Way, resulting in the vigorous star-forming activity seen throughout the cloud. NGC265 and NGC290 may very well owe their existence to these close encounters with the Milky Way.

The images were taken in October and November 2004 through F435W, F555W, and F814W filters (shown in blue, green, and red, respectively).

The central region of globular cluster 47 Tucanae is closely packed, with stars less than a tenth of a light-year apart.
This Chandra false-colour x-ray view of the centre of 47 Tuc also shows the cluster is a popular neighbourhood for x-ray stars, many of which are "normal" stars co-orbiting with extremely dense neutron stars.
One of the most remarkable of these exotic binary systems is catalogued as 47 Tuc W, a bright source near the centre of this image. The system consists of a low mass star and a neutron star that spins once every 2.35 milliseconds.


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Such neutron stars are known to radio astronomers as millisecond pulsars, believed to be driven to such rapid rotation by material falling from the normal star onto its dense companion. In fact, x-ray observations of the 47 Tuc W system link this spin-up mechanism observed to operate in other x-ray binary stars with fast rotating millisecond pulsars.