While many people would probably be able to describe what a galaxy is, there is actually no clear answer to that question. According to most dictionaries, a galaxy is a large group of stars and other elements that exist in space as independent systems. But, how many stars is exactly “a large group”, or what is exactly the definition of “independent”? Duncan Forbes of Swinburne University in Australia and Pavel Kroupa of the University of Bonn in Germany want to start a debate on the issue, there is even a poll on this website for you to give your opinion.
The problem gets complicated when considering star clusters: where does a star cluster end and a galaxy begin? With more and more surveys, lots of galaxies of various shapes and sizes have been discovered, making any classification more and more difficult. This is what led Forbes and Kroupa to discuss the issue in order to maybe come up with a clear definition of what a galaxy is.
They particularly focus on a star system called ultra-compact dwarf, containing up to a billion stars and sometimes similar in mass to a galaxy, but compact and more like a star cluster.
You might on the other hand think that classifying galaxies according to the results of a poll does not have much to do with science… I personally think we should focus on physical properties and learn more about formation of galaxies before trying to give a clear definition, which is in the end, more about nomenclature than anything else.
If you feel like giving your opinion, you can take the author’s poll here. The outcome of the vote should be published in early April.