July 2011 - Issue #13
Know what a galaxy is? Think again
Story by Julian Cribb
View articles in related topics: Astrophysics
Despite being the most prominent objects in the visible universe, there is in fact no standard definition of a galaxy – at least, not one that satisfies most astronomers.
So Swinburne University of Technology’s Professor Duncan Forbes and colleague Pavel Kroupa from Bonn University are hastening to the rescue by conducting a worldwide survey with the aim of resolving this taxonomic dilemma.
“Scientists like to classify things – it’s a way of helping us understand nature and its relationships,” Professor Forbes explains. “There are perfectly workable dictionary definitions of 'galaxy', but these don’t quite embrace the prodigious diversity of star systems that the new generation of ultra-powerful telescopes is now revealing to us.”
These include new ‘species’ of star systems such as ultra-compact dwarf (UCD) objects and tidal dwarfs, as well as objects such as Omega Centauri, which has been traditionally classed as a globular cluster but which has stars with a range of ages and chemistry resembling those of a galaxy.
“A good starting point is that all galaxies have to be gravitationally bound, and consist of stars – but we decided to use an online survey to seek the views of colleagues and the interested public around the world, and present them with possible criteria.”
These included the dynamical state of the stars, their size, the presence of complex stellar populations, the presence of dark matter, and whether they host satellite star systems.
When he reported at an international astronomy meeting in Chile in April 2011, more than 1600 people had voted, with 68 per cent favouring “the presence of complex stellar populations” as the additional criterion. This would reclassify Omega Centauri and similar objects as small galaxies.
