Globular cluster 47 Tucanae

Only the latitude of Paris could have kept this one out of the Messier catalog (and we don't have a good home for it elsewhere), so we include this as an honorary Messier object. 47 Tucanae (NGC 104) is one of the most spectacular globular clusters in our sky, located about 6 kpc from the Sun in front of the outskirts of the Small Magellanic Cloud. This location makes for some interesting color-magnitude diagrams if you look too far from the cluster core. The red giants are especially easy to pick out, with their color enhanced by our use of a near-IR I filter for the red band and the relatively high metal abundances in 47 Tuc, which makes the slope of the giant branch in the H-R diagram very strong (and thus the brightest giants are very cool). This cluster is especially attractive for telescopic observers because it is quite centrally concentrated, and thus much more attention-getting in a small field of view than is Omega Centauri. HST observations have been of special interest in picking out white dwarfs in 47 Tucanae, and more recently in a concerted search for large, close-orbiting planets detected by their transits temporarily blocking some of the light of their parent stars. Intriguingly, this massive experiment showed that such "hot Jupiters" must be much less common in 47 Tucanae than around stars in our own neighborhood. This may tell us either that the dense cluster environment is unhealthy for even such close planets, or that planet formation is a different matter today than it was very early in galactic history. Such a difference might be plausible since there was so much less material in the right elements for planets then, but it's always best to get evidence for one's preconceptions...

This color composite was produced starting with B and I exposures (2 minutes and 30 seconds, respectively, and we still saturated a few stars in I) using a Tektronix 2048 CCD at the f/13.5 Cassegrain focus of the 1.5m telescope at Cerro Tololo, by Bill Keel, Ray White III, and Chris Conselice. We started off at this to fill ten minutes while waiting for our next galaxy target to clear the telescope's eastern hour-angle limit. Only every fourth pixel is displayed, with a logarithmic scale to show detail over the wide brightness range from faint to brightest stars. The field covers an area slightly more than 8 arcminutes square.

We knew this picture was a winner, and it's proven popular. It was selected as the Astronomy Picture of the Day three times, on Sept. 19, 1997, November 7, 1998, and April 22, 2001.

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Last changes: 4/2001      © 2001