When two dissimilar objects pair up in the night sky, each one serves to highlight the other’s distinctive qualities. Last night Comet Garradd posed less than one degree north of the M15, one of the brighter globular clusters in the sky. Both were visible in the same circle of view through my telescope. The comet looked like smoke or fog compared to the sparkly fireworks appearance of the cluster.At 140 million miles away, Comet Garradd was literally next door compared to M15′s 33,600 light years which translates to 5600 trillion miles.
M15 spans 175 light years across or seven times the distance between our planet and the bright star Vega in the Summer Triangle. Just thinking about that makes me stop to appreciate how big things really are out there. As for the comet, it measured about 1/2 degree from head to tip of tail as seen through the scope.
It was undoubtedly larger than this, because the eye can only see so much. But if we take 1/2 degree as an approximation, that makes the comet the same apparent size as the sun, which also measures 1/2 degree. However since Garradd is presently 1 1/2 times the sun’s distance from Earth, the comet is actually larger than the sun — about 1 1/2 times or nearly 1.3 million miles top to bottom.
Just to keep things in perspective, remember that the comet itself – the little chunk of icy rock in the center of all that fuzziness – is probably a few to a few tens of miles across. The rest is vaporous, rarefied tail and coma, which reflects sunlight with vigor but consists of very little material. Comets pack a lot of bang for the buck.
Globular clusters like M15 are tightly-bound, spherical collections of stars that typically number in the hundreds of thousands. Most orbit the center of the Milky Way galaxy in an enormous spherical halo. Open clusters in contrast are much looser agglomerations of stars generally under 50 light years across that contain a couple dozen up to a few thousand members.The Seven Sisters Cluster (Pleiades) is a classic example.
Because they’re loosely bound, they don’t hang together for as long as globulars, breaking free of each other after several hundred million years. Globulars are the galaxy’s most ancient citizens. Some of them are 13 billion years old! One reason open clusters fall apart is because they’re located in the plane of the galaxy. The plane is where all the stars and gas clouds are, and the clusters are tugged this way and that by the gravity of these objects as they orbit about the galactic center.
There are about 2,500 open clusters known in the Milky Way compared to only about 150 globulars. Wait, strike that. Make that about 2,600 open clusters. Today the European Southern Observatory announced that using 161-inch VISTA infrared survey telescope at the Paranal Observatory in Chile, they uncovered another 96 brand new open clusters. They’re all tiny and faint with only 10-20 stars each, but we welcome them with a happy heart to the family.
No one had seen these before before, because they were blocked by dust in the plane of the galaxy. Scientists aimed the powerful telescope at regions in the galaxy’s disk where you’d expect to see star clusters but which appeared blank — but not to VISTA’s eyes. Its sensitive infrared detectors were able to penetrate the dust to uncover these hidden treasures.