A Globular Cluster turns Galaxy?

Globular Cluster is a group of few hundred thousand to one million old stars tightly bound together by their gravity.  They can be found in the outskirts of many galaxies including our own.

Galaxy is a group of millions to billions to trillions of stars, gas and dust bound together by their gravity.

In simple words, both globular cluster and galaxy are a group of stars held together by gravity.  So where do you draw the line between the two?  Very big globular cluster can look like dwarf galaxy; and dwarf galaxy can look like big globular cluster.

This is what happened to “globular cluster” Omega Centauri or NGC5139.

NGC5139 by CF Kwong

Omega Centauri (NGC5139). 
Credit: CF Kwong (Astrophotography Group of Malaysia).

Omega Centauri is the biggest and brightest globular cluster in our sky, about 17,000 light-years away.  It can be easily visible to the naked eye on a clear dark night, towards the south in the constellation Centarus, near to Crux the Southern Cross.  Now in April, Omega Centauri can be seen rising in the southeast after 9 pm.

Click here for video zooming from Centaurus into the heart of Omega CentauriThe two bright stars embedded in the spring Milky Way Band (below the centre) is alpha and beta Centauri, to the right of the two stars is the constellation Crux.

So what is Omega Centauri?  Nearly 2000 years ago, Ptolemy listed it as a star.  Then in 1677, Omega Centauri appeared fuzzy to Edmond Halley, so he reported it as a nebula.  153 years later, John Herschel realized that this fuzzy ball is actually composed of individual stars, a globular cluster.  And today, new result suggests that Omega Centauri is not a globular cluster at all, but a dwarf galaxy stripped of its outer stars.

NGC5139-HST

Omega Centauri (NGC5139). 
Credit: NASA, ESA, Hubble Heritage Team (STScI/AURA)

Omega Centauri has always been a mystery.  It has several characteristics that distinguish it from other globular clusters: it rotates faster than other globular cluster, its shape is highly flattened and it consists of several generations of stars – more typical globular clusters usually consist of just one generation of old stars.

Moreover, Omega Centauri is about 10 times as massive as other big globular clusters, almost as massive as a small galaxy.  These peculiarities have led astronomers to suggest that Omega Centauri may not be a globular cluster at all, but a dwarf galaxy stripped of its outer stars by an earlier encounter with the our big Milky Way galaxy.

The result obtained by the NASA/ESA Hubble Space Telescope and the Gemini Observatory show that Omega Centauri appears to harbour a 40,000 solar masses intermediate-mass black hole in its centre.  It is unexpected for a globular to harbour a black hole of this mass, but it is normal for a dwarf galaxy.  It seems that we may have to reclassify this jewel in the sky, again.

Source: SpaceTelescope.org News Release

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~ by thChieh on April 4, 2008.

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