Astronomy Picture of the Day
APOD: 2002 December 29 - NGC 1818: A Young Globular Cluster
Explanation: Globular clusters once ruled the Milky Way. Back in the old days, back when our Galaxy first formed, perhaps thousands of globular clusters roamed our Galaxy. Today, there are perhaps 200 left. Many globular clusters were destroyed over the eons by repeated fateful encounters with each other or the Galactic center. Surviving relics are older than any Earth fossil, older than any other structures in our Galaxy, and limit the universe itself in raw age. There are few, if any, young globular clusters in our Milky Way Galaxy because conditions are not ripe for more to form. Things are different next door, however, in the neighboring LMC galaxy. Pictured above is a "young" globular cluster residing there: NGC 1818. Observations show it formed only about 40 million years ago - just yesterday compared to the 12 billion year ages of globular clusters in our own Milky Way
APOD: 2002 July 30 - A Star Cluster in Motion
Explanation: Star clusters are a swarm of complex motions. The stars that compose globular clusters and many open clusters all orbit the cluster center, occasionally interacting, gravitationally, with a close-passing star. The orbits of stars around the cluster are typically not as circular as the orbits of planets in our solar system. Cluster stars frequently fall more directly toward the center and many times trace out unusual and complex loops. The vast space inside a cluster results in stars colliding only rarely. The above computer animation, derived from a type of computer code called an N-body simulation, shows 100 identical stars in a time-lapse movie where hundreds of years pass in one second.
APOD: 2004 October 14 - Glimpse of a Globular Star Cluster
Explanation: Not a glimpse of this cluster of stars can be seen in the inset visible light image (lower right). Still, the infrared view from the Spitzer Space Telescope reveals a massive globular star cluster of about 300,000 suns in an apparently empty region of sky in the constellation Aquila. When astronomers used infrared cameras to peer through obscuring dust in the plane of our Milky Way galaxy, they were rewarded with the surprise discovery of the star cluster, likely one of the last such star clusters to be found. Globular star clusters normally roam the halo of the Milky Way, ancient relics of our galaxy's formative years. Yet the Spitzer image shows this otherwise hidden cluster crossing through the middle of the galactic plane some 10,000 light-years away. At that distance, the picture spans only about 20 light-years. In the false color infrared image, the red streak is a dust cloud which seems to lie behind the cluster core.
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