Globular clusters have always been one of my favorite astronomical objects. These balls of stars — sometimes hundreds of thousands strong — are easy targets through a small telescope and are fun and beautiful to see.
But when you train a big space telescope on them, well, their beauty is magnified spectacularly:
You really want to click that to get the very beefy 4000 x 4000 pixel (11 Mb) version. It’ll knock your socks off!
This Hubble image shows NGC 6934, an ancient ball of stars located about 50,000 light years away. Globular clusters are made of stars that are bound to each other gravitationally and orbiting the center on a myriad different paths — think of it as a beehive except with a hundred thousand bees each a million kilometers across. There are about 150 of these guys orbiting the Milky Way, each a dozen or so light years across and containing upwards of a million stars. NGC 6934 is pretty typical of its class, but its great distance dims it to near-obscurity. If it were as close as M 13 or Omega Centauri — both roughly half as far as NGC 6934 — it would be heralded as a gem of the night sky.
Globulars are old. We think they form all at once, with all the stars being born at the same time. Massive stars, which are blue, don’t live long before exploding as supernovae (leaving behind black holes or dense neutrons stars), so they’re all long gone in these billions-of-years-old objects. In fact, in many globulars even stars like the Sun are gone, having used up their fuel and faded away. All that’s left are low mass stars, which means all that remains are red stars.
So why are there so many blue stars in this picture? Ah, it’s false color! It was taken through two filters, one in the red, and the other in the infrared. In the picture, the red filter image is colored blue, and the infrared one is colored red. Note that the brightest stars in the picture are red (meaning they’re bright in the infrared); this is because these are red giants, stars that are nearing the ends of their lives. They’ve swollen up and cooled off, glowing brilliantly.
Even though they’re not actually blue, the ones that look blue in the image are most likely the "normal" stars that are left in the cluster, that is, stars still fusing hydrogen into helium like the Sun is, and are not yet red giants.
Globular clusters like NGC 6934 are incredibly important to our understanding of our galaxy. Because all their stars formed at once and all from the same cloud of gas, they’re a laboratory experiment in astronomy! We don’t have to correct for age or composition of stars (or at least not very much) allowing us to examine other characteristics. They’re located all over the sky, so they’re always around for viewing, and many are isolated in space, making them easy to examine. Much of what we’ve learned about how stars age and die was gleaned from globulars like NGC 6934.
Globular clusters tell us secrets of the Universe, and all we have to do is pay attention. And when they’re as stunningly beautiful as this one, that’s really easy to do.