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What is a galaxy? The people have spoken.

Date posted: Wednesday 11 May 2011

What is a galaxy? The people have spoken.

Through a popular vote, science-savvy members of the public have helped astronomers answer the question, ‘What is a galaxy?

While astronomers have long understood that a galaxy is a massive, gravitationally bound stellar system, it has never been clear what differentiates a galaxy from a large star cluster.

So Swinburne University astrophysicist Professor Duncan Forbes and Professor Pavel Kroupa from the University of Bonn have set out to solve the conundrum. After publishing a scientific paper on the topic, they launched a website where people could read the paper and then vote for their preferred ‘galaxy definition’.

Four months and 1600 votes later, the researchers have presented their findings at an international conference on the ‘Dynamics of Small Stellar Systems’ held in Chile.

"The resounding majority of voters (68 per cent) believed that as well as being gravitationally bound, a stellar system required the ‘presence of complex stellar populations’ in order to be considered a galaxy," Professor Forbes said. "This means a galaxy should reveal stars with a range of age or chemical properties."

This beat out four other possible definitions which were based on factors such as size, the motion of stars and the presence of dark matter and satellites."

As well as voting on the definitions proposed by the astronomers, respondents were also invited to suggest their own. "This exercise generated valuable discussion on the topic of how to define a galaxy".

As well as receiving a number of constructive technical responses, the astronomers enjoyed comments that ranged from ‘pretty to look at’ to ‘a place far, far away where Star Wars happened’.

One stellar system affected by the new definition is the well-known system Omega Centauri.

"Since it was discovered, some people have referred to Omega Cen as a massive star cluster, however others regard it as a galaxy. According to the popular definition, it has the status of a galaxy."

The original paper by Forbes and Kroupa was published in the Publications of the Astronomical Society of Australia.

People are still able to vote for their preferred galaxy definition at: www.surveymonkey.com/s/wlrjmws  

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