What Is a Galaxy?

by Jon Cartwright

Galaxy dilemma. Ultra-compact dwarfs, highlighted here within the so-called Fornax galaxy cluster, are a type of small star system. But should they be called galaxies?

Credit: M. Hilker, 2.5-Meter du Pont Teleskop (Las Campanas Observatory); (insets) M. Drinkwater, Hubble Space Telescope

What exactly is a galaxy? Surprising as it may sound, astronomers don't have an answer to this fundamental question. There's no agreement on when a collection of stars stops being a cluster and starts being something more. Now, in an echo of the recent wrangling over Pluto's status as a planet, a pair of astrophysicists from Australia and Germany want to start a debate on the issue—and they have even set up a Web site for people to cast their votes.

You might think a galaxy is simply a large group of stars, but just how many stars does it take? Astronomers tend to call five or so stars a "group" and a hundred or more a "cluster." At some point, a cluster becomes a galaxy—the Oxford English Dictionary suggests "millions or billions" of stars is enough—but there has never been an official threshold.

Duncan Forbes of Swinburne University in Australia and Pavel Kroupa of the University of Bonn in Germany point out that the problem gets even more complicated. In a paper soon to be published in Publications of the Astronomical Society of Australia, they draw attention to a type of star system discovered in 2000 that bridges the gap between what would traditionally be thought of as a cluster and a galaxy. Known as an ultra-compact dwarf, this type of system has up to a billion stars and can be similar in mass to a galaxy, but it is compact and looks more like a star cluster.

"The general question of what defines a galaxy is, of course, an important one," says Steve Phillipps, an astronomer at the University of Bristol in the United Kingdom. "In particular, it has occupied many of us who study ultra-compact dwarfs, since these seem to share properties with objects on both sides of the divide."

Some individual star systems have become tricky to pigeonhole for other reasons. Omega Centauri, for instance, is a star system located in the southern sky a little over 15,000 light-years from Earth. It has many features of a galaxy: it's big and has stars both old and new. But Omega Centauri is widely considered a star cluster, because its mass is relatively small and it has only about 10 million stars.

So where does a cluster end and a galaxy begin? Forbes and Kroupa list several criteria that could be used to define a galaxy, including the presence of dark matter and having a radius greater than 300 light-years. Another key criterion for a galaxy, the researchers believe, would be the absence of star collisions, since galaxies, being more stable than clusters, are thought to be mostly collisionless. This would exclude objects like Omega Centauri, but it would leave in ultra-compact dwarfs.

In a way the issue is similar to one with planets tackled by the International Astronomical Union (IAU) in 2006. Several bodies had recently been discovered orbiting in the outer solar system with a size similar to Pluto's, culminating in Eris, which appeared to be bigger. (Subsequent measurements suggest that Eris may in fact be a smidgen smaller than Pluto, but it is more massive.) Faced with the revelation that Pluto was technically no more a planet than Eris, IAU controversially decided to label them both "dwarf planets" along with several other smaller bodies. When asked by ScienceNOW whether IAU should now have a role in the galaxy discussion, Ian Corbett, IAU's general secretary, said such a discussion would be difficult. "The boundaries of a definition are as fuzzy as the boundaries of a galaxy."

Some astrophysicists, such as Michael Drinkwater of the University of Queensland in Australia, say scientists don't know enough about the formation of galaxies to classify them with any certainty. Meanwhile, Wyn Evans of the University of Cambridge in the United Kingdom calls the issue "pointless." "Science is not decided by votes and polls," he says. "And even if Kroupa and Forbes want to carry out a survey, ... this kind of poll is the worst way to do it. It has no statistical validity whatsoever."

Nonetheless, Kroupa feels the issue deserves attention. "We have a clear picture in astrophysics of what a star is," he says. "We now also have a definition for planets. But the distinction between galaxies and star clusters is still blurred."

Forbes plans to publish the outcome of the vote at an astronomy meeting in Santiago in early April.