Omega Centauri or NGC 5139, is a globular cluster in the constellation of Centaurus that was discovered by Edmond Halley in 1677, although in 150 A.D., the Greco-Roman writer and astronomer Ptolemy catalogued this object. Located at a distance of 15,800 light-years, it is the largest globular cluster in the Milky Way galaxy at a diameter of roughly 230 light-years.
It is estimated to contain approximately 10 million stars and a total mass equivalent to 5 million suns, and with an apparent visual magnitude of 3.9, it is visible to southern observers with the unaided eye.
The stars in the core of Omega Centauri, are so crowded that they are estimated to average only 0.1 light years away from each other. The members of this cluster are orbiting the center of mass with a peak velocity dispersion of 7.9 km/sec. The multi-colour top snapshot, taken with the Wide Field Camera 3 aboard the NASA/ESA Hubble Space Telescope, captures the central region of the giant globular cluster. All the stars in the image are moving in random directions, like a swarm of bees. Astronomers used Hubble’s exquisite resolving power to measure positions for stars in 2002 and 2006. The lower illustration charts the future positions of the stars highlighted by the white box in the top image. Each streak represents the motion of the stars over the next 600 years.
NASA’s Hubble Space Telescope also captured this image of 100,000 stars residing in the crowded core of the nebula. The majority of the stars in the image are yellow-white, like our Sun. These are adult stars that are shining by hydrogen fusion. The late-life stars are the orange dots in the image. These stars continue to cool down and expand in size, becoming red giants, shedding their gaseous envelopes. After ejecting most of their mass and exhausting much of their hydrogen fuel, the stars appear brilliant blue. Only a thin layer of material covers their super-hot cores as they desperately try to extend their lives by fusing helium in their cores. At this stage, they emit much of their light at ultraviolet wavelengths. When the helium runs out, the stars reach the end of their lives. Only their burned-out cores remain, and they are called white dwarfs; these are the faint blue dots in the image. They will continue to cool and grow dimmer for many billions of years until they become dark cinders. Other stars that appear in the image are so-called “blue stragglers.” They are older stars that acquire a new lease on life when they collide and merge with other stars, boosting their energy-production rate, making them appear bluer.
A 2008 study presented evidence for an intermediate-mass black hole of 4.0 x 104 solar masses at the center of Omega Centauri, based on observations made by the Hubble Space Telescope and Gemini Observatory on Cerro Pachon in Chile. E. Noyola and colleagues found that stars closer to the core are moving faster than stars farther away. This measurement was interpreted to mean that unseen matter at the core is interacting gravitationally with nearby stars. Subsequent studies have put the size of the black hole in contention
Credits: European Southern Observatory, Hubble, NASA, The Daily Galaxy, Through My Looking Glass, Wikipedia.