You know why astronomy is cool? Because of things like this: Galaxy clusters are collections of galaxies held together by their own gravity. We see clusters all over the place, and they're among the largest structures in the Universe. We can find them at large distances, which means we see them as they (and the Universe) were young -- it takes light a long time to travel across the cosmos. Astronomers went looking to find extremely distant clusters of galaxies, and found one at a staggering distance: 12.7 billion light years away! Here's an image showing the central part of the cluster:

[Click to bigbangenate.] Each of those circled red dots is a young galaxy, so distant that the light has been on its way here for more than 90% of the current age of the Universe! And they're almost lost among all those other stars and galaxies in the image (though their intense red color helps… as to why they're red, read on). Finding this cluster was a magnificent achievement. The astronomers used the massive 8.2 meter Subaru telescope to look at large swaths of the sky. They looked at the colors of the galaxies they found (PDF); distant objects would be so far away their light is significantly redshifted by the expansion of the Universe itself (I explain how this works here and here). Galaxies are distributed throughout space, so you expect to see them scattered across the sky as well as in redshift (distance). When looking at one part of the sky, however, they found an unusually high concentration of galaxies that were very red. Using a different camera on Subaru, they took spectra of those galaxies -- breaking the light up into very fine divisions of colors, like a rainbow with hundreds of colors in it -- to accurately measure the redshifts of those galaxies. Spectroscopy of objects that faint is no easy task, but Subaru is a big 'scope, and collect a lot of light even from faint objects at the remote reaches of the Universe, The astronomers confirmed that many of the galaxies in their sample were at the same redshift (z = 6 for those in the know -- which is a mighty big redshift). The odds of these galaxies all being at the same distance happening by chance is extremely small: only about one in a billion! So it's pretty clear these galaxies really are physically associated with each other. That is, clustered together. This makes the cluster the most distant ever found that has been confirmed spectroscopically -- one other has been found that might be farther away, but it hasn't been confirmed yet. At 12.7 billion light years away, that means we see this cluster as it was a mere one billion years after the Universe itself formed! That provides key information about conditions in the early Universe, which are critical to understanding how it formed and changed as it aged. The cluster itself is vast -- it's something like 50 million light years across. The team of astronomers used various methods to determine its mass, and their best guess is that its total mass is several thousand times the mass of our entire Milky Way galaxy! The estimation methods they used are fairly fuzzy, so it's not clear how accurate this number really is. Still, the cluster is clearly huge, and massive. If we could see it today, it would probably rank among the largest structures in the Universe. That's not terribly surprising, if you think about it: only the biggest monster clusters can be seen at such a mind-crushing distance. The smaller ones will be harder to detect, so we're likely to find the biggest. Still, holy cow. I have read and written about extremely distant objects many, many times over the years, and have no doubt: I get chills every single time I think about this stuff. It wasn't that long ago when the entire human race couldn't be bothered to look beyond the tip of its collective nose. Now we can look into the fires of the Universe's birth, into that forge itself, and tease out the secrets of how we came to be. That's why astronomy is cool.

Related Posts: - An ultradeep image that’s *full* of galaxies! - Most distant object ever seen… maybe - Another record breaker: ultra-deep image reveals ultra-distant galaxy - Record-breaking galaxy found at the edge of the Universe