Galaxy wars, when is a collection of stars no longer a galaxy?

Friday, 28 January, 2011

Following the recent argument over what constitutes a planet – which famously saw Pluto downgraded in status a few years ago – debate is now raging over whether groups of stars should be considered galaxies when they may in-fact be star clusters instead.

Star clusters and galaxies both contain stars bound together by gravity, but while the members of a star cluster are thought to form simultaneously from a collapsing ball of gas, galaxies have richer histories. In the most popular cosmological model, they form along swathes of dark matter and contain enough gas to form many generations of stars. Yet the distinction is not always clear-cut. Take Omega Centauri, a round swarm of stars that orbits the Milky Way and is visible to the naked eye. It has long been classified as a star cluster, but there is now evidence that it contains multiple generations of stars, suggesting it is actually the remnant of a galaxy.

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