So what to do? Today, Duncan Forbes at Swinburne University in Australia and Pavel Kroup at the University of Bonn in Germany put forward a novel solution. They outline the various characteristics that astronomers think about when classifying galaxies.
These include factors such as the presence of stars, so gas clouds can't be defined as galaxies; being gravitationally bound, so materials that has been stripped away by another galaxy wouldn't count; whether the system is stable or not; whether it hosts a good variety of different types of star, which excludes globular clusters which contain only similar stars; and whether it is held together by dark matter, which many galaxies seem to be.
There are other factors too, of course. (Although they do not include the presence of a black hole at the centre of a galaxy as a defining characteristic , which is odd given the growing interest in the link between galactic evolutoin and black holes.)
Forbes and Kroup go on to suggest that the best way to achieve consensus is to crowdsource the problem. In other works, they want to use the wisdom of the crowd to determine what factors are important what aren't.
