Looking for clues to what happens when stars collide, the Hubble Space Telescope has peered into the center of a dense swarm stars called Omega Centauri.
The globular cluster, as it is called, contains several million stars all tied together by gravity and orbiting around the center of the cluster. Omega Centauri is about 17,000 light-years from Earth and is the most luminous and massive globular star cluster in the Milky Way Galaxy.
Hubble was used to spot individual stars in the center of the cluster, where things are so crowded that ground-based telescopes struggle to separate the points of light. Even in the dense center of Omega Centauri, stellar collisions are infrequent. But the cluster is so old that many thousands of collisions have occurred over billions of years.
When stars collide head-on, they probably just merge together and make one bigger star, researchers expect. But if the collision is a near miss, the stars may go into orbit around each other, forming what's known as a close binary star system.
Astronomers found two such binary star systems in the new Hubble image that may have began with a near miss. Both consist of a white dwarf that pulls gas off of its companion star. When the gas falls onto the surface of the white dwarf, it is heated to the point that it emits ultraviolet light. These UV emissions, which Hubble used to generate the image, enabled scientists to pinpoint the two faint binary star systems.
Hubble could not capture the entire cluster. Yet even the small patch contained within the new photograph contains some 50,000 stars, all packed into a region only about 13 light-years wide, astronomers said. For comparison, the same amount of space in our neck of the cosmic woods contains about a half dozen stars, including the Sun.
The vast majority of stars the new picture are yellow-white dwarf stars similar to our Sun. There are also a handful of bright yellow-orange stars, which are actually termed red giants -- stars that have begun to exhaust their nuclear fuel and have expanded to diameters about a hundred times that of the Sun.
A number of faint blue stars are also visible. These are in a brief phase of evolution between the dwarf stage and the red giant stage, during which the surface temperature is high.
The stars in Omega Centauri are all very old, about 12 billion years. Stars with a mass as high as that of our Sun have already completed their evolution and have faded away as white dwarfs, too faint to be seen.
Omega Centauri is one of the few globular clusters that can be seen with the unaided eye. Named by Johann Bayer in 1603 as the 24th brightest object in the constellation Centaurus, it resembles a small cloud in the southern sky and might easily be mistaken for a comet.
This Hubble WFPC2 image was taken in 1997. The science team was led by Adrienne Cool of San Francisco State University. The data are currently being used by Jeff Carlin and Daryl Haggard, two SFSU students, to look for optical counterparts of X-ray sources recently discovered with the Chandra Observatory.
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