Starship Asterisk*

47 Tuc Near the Small Magellanic Cloud

Explanation: Globular star cluster 47 Tucanae is a jewel of the southern sky. Also known as NGC 104, it roams the halo of our Milky Way Galaxy along with around 200 other globular star clusters. The second brightest globular cluster (after Omega Centauri) as seen from planet Earth, it lies about 13,000 light-years away and can be spotted naked-eye near the Small Magellanic Cloud (SMC) in the constellation of the Toucan. Of course, the SMC is some 210,000 light-years distant, a satellite galaxy of our Milky Way and not physically close to 47 Tuc. Stars on the outskirts of the SMC are seen at the upper left of this broad southern skyscape. Toward the lower right with about the same apparent diameter as a Full Moon, dense cluster 47 Tuc is made up of several million stars in a volume only about 120 light-years across. Away from the bright cluster core, the red giants of 47 Tuc are easy to pick out as yellowish tinted stars. Globular cluster 47 Tuc is also home to exotic x-ray binary star systems.

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There is an undescribable quality about this image, as if looking at it is to look back to the primordial soup. It has a certain sparkly and magical feel as if it quite possibly isn't real, it makes me feel something amazing in my heart!!! This is my favourite southern globular cluster and it's great to see it on APOD! In fact it makes me want to hug someone so tight and never let go!!!

APOD Robot wrote:... 47 Tuc is made up of several million stars in a volume only about 120 light-years across. ...

A sphere with the radius 60 light years has about 905,000 cubic ly. So there are something in the vicinity of 4 stars per cubic light year on average (assuming "several million" means roughly 3.5 million) but many more than that nearer the center of the cluster.

Our own neighbourhood , out to a similar radius, has perhaps 3000 stars in it, which works out to an average of .003 per cubic light year.

Think of 47 Tuc as innner city density, while we live out in the fresh air.

I can see what look like very small clusters in the image. I can't tell how close they are, but I'm guessing they're companions of the SMC. I've attached an image with lines pointing to them.
Rob

The one just below center is NGC121, and is indeed a globular cluster of the SMC. One can bring both in the same field of view of a telescope, and it is interesting to have this depth, two objects, maybe about the same size, one 15000 ly, the other 220000. NGC121 and the globular clusters in both magellanic clouds are the only one so easy to resolve (the ones in M31 are "suspected" not to be stellar in most cases).
Alain

Love the colors of the stars and the SMC here, this image makes me long to see the southern hemisphere's night sky in person. Some day...

This is a very beautiful and fascinating APOD, but the colors of the Small Magellanic Cloud and 47 Tuc confuse me.

The Small Magellanic Cloud is a blue galaxy. Its B-V, according to my software, is +0.45. 47 Tuc, on the other hand, is red as globulars go. Its B-V, according to Sky Catalogue 2000.0 part 2, is +0.89. So the Small Magellanic Cloud is clearly bluer than 47 Tuc, as can be seen here in a picture from Capella Observatory. The reason for the red color of 47 Tuc is that although this globular is much more metal-poor than the Sun, it is still metal-rich for a globular. Its [Fe/H] value is -0.71, again according to Sky Catalogue, while a typical "blue globular" like M13 in Hercules has a [Fe/H] value of -1.65.

The relatively metal-rich nature of 47 Tuc means that this globular has no blue horizontal stars. This diagram from aanda.org shows a short little "wing" stretching to the left about midway up the diagram. The "wing" is a red horizontal branch. It's red, because the bluest stars here are no bluer than the Sun, and the reddest stars on this "wing" are considerably redder. Compare the red horizontal branch of 47 Tuc with a blue horizontal branch in a more metal-poor globular cluster. The blue horizontal branch is in the upper left of this diagram.

So 47 Tuc actually contains no blue stars, apart from relatively faint blue stragglers and extremely faint white dwarfs. But in today's APOD, a swarm of blue stars seem to form a halo around 47 Tuc. By contrast, blue galaxy SMC, which is full of young blue stars, seems to contain a large number of red stars. It does, of course. Interestingly, the SMC is quite metal-poor as galaxies go, and it would be fascinating if it had about the same metallicity as 47 Tuc. Indeed, according to The Galaxies of the Local Group (2000) by Sidney van den Bergh, young clusters in the SMC has a [Fe/H] value of +0.74, almost exactly the same as 47 Tuc! An interesting aspect of metal-poor galaxies is that they contain a disproportionate number of very red carbon stars (or rather, low-luminosity galaxies in the Local Group contain disproportionate numbers of red carbon stars, according to Sidney van den Bergh). In any case, the SMC has a lot of carbon stars, and some of the very red stars in today's APOD may be such carbon stars.

I'm trying to say that I would have expected the SMC to look bluer than 47 Tuc, but it doesn't in this image. Of course, we only see a rather small part of the SMC, which may not be typical of the overall color of this galaxy. In any case, this is certainly very fascinating and beautiful APOD!

Ann