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Public asked to define a galaxy
The public is being asked to help settle a question of astronomical proportions: what is a galaxy?

While the term galaxy is bandied around regularly by astronomers, and the broader community, no clear definition exists.

But what defines a galaxy is the question is being posed by Swinburne University astrophysicist Professor Duncan Forbes and his colleague Professor Pavel Kroupa from the University of Bonn.

Forbes says he was motivated by the decision to strip Pluto of its planet status during the International Astronomical Union General Assembly in Prague in 2006.

"The decision of how to define a small planet was quite divisive, Pluto was effectively kicked out of the planetary club," says Forbes. "Astronomers need to take a more democratic approach to galactic classification."
What's large?

Forbes says there's no consensus as to how big a system has to be in order for it to be considered a galaxy, or what differentiates a small galaxy from a large star cluster.

"At the moment there's a general understanding that a galaxy is a massive, gravitationally bound stellar system," says Forbes.

He says objects called 'ultra compact dwarf galaxies' share many properties in common with globular star clusters.

"Then there's the recent discovery of ultra-faint dwarf spheroidal galaxies', which contain very few stars."

Forbes says the current definition currently states a galaxy is a "gravitationally-bound stellar system".

"But to separate a dwarf galaxy from a globular cluster means we need to look at other things such as a minimum size, the gravitational affect of the galactic centre compared to that of other bodies, the presence of complex stellar populations and the issue of dark matter," he says.

According to Forbes there are implications for each of these definitions, as well as ambiguous and special case objects.

He says one stellar system that may be impacted by a new definition is Omega Centauri, which when viewed with the unaided eye appears as large as the full Moon.

"Although many amateur astronomers know Omega Centauri as a massive star cluster, some professional astronomers regard it as a galaxy. This is a stellar system that could be upgraded or downgraded by this exercise, depending on your point of view."
Citizen science in action

Forbes hopes the spirit of 'collective wisdom' will help create a definition.

"There's a discussion paper on the issue on the pre-press website arXiv.org and a website where people can submit ideas and cast a vote," he says.

The paper has also been accepted for publication by the Astronomical Society of Australia.

Forbes says they came up with the idea of a citizen science based survey because of the success of other similar projects.

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