GALAXY




What specifically may be a galaxy? shocking because it could sound, astronomers haven't got a solution to the current basic question. there is no agreement on once a set of stars stops being a cluster and starts being one thing a lot of. Now, in AN echo of the recent haggle over Pluto's standing as a planet, a combine of astrophysicists from Australia and Germany need to begin a discussion on the issue—and they need even came upon an online web site for folks to solid their votes.

You might suppose a galaxy is just an outsized cluster of stars, however simply what percentage stars will it take? Astronomers tend to decision 5 approximately stars a "group" and 100 or a lot of a "cluster." At some purpose, a cluster becomes a galaxy—the English language lexicon suggests "millions or billions" of stars is enough—but there has ne'er been a political candidate threshold.

Duncan Forbes of Swinburne University in Australia and Pavel Kroupa of the University of city in Germany entails that the matter gets even a lot of difficult. in a very paper presently to be revealed in Publications of the Astronomical Society of Australia, they draw attention to a sort of star system discovered in 2000 that bridges the gap between what would historically be thought of as a cluster and a galaxy. called AN ultra-compact dwarf, this kind of system has up to a billion stars and may be similar in mass to a galaxy, however it's compact and appears a lot of sort of a star cluster.
"The general question of what defines a galaxy is, of course, a crucial one," says Steve Phillipps, AN uranologist at the University of port within the uk. "In explicit, it's occupied several people WHO study ultra-compact dwarfs, since these appear to share properties with objects on each side of the divide."

Some individual star systems became difficult to pigeonhole for different reasons. bunch, for example, may be a star system placed within the southern sky a touch over fifteen,000 light-years from Earth. it's several options of a galaxy: it's massive and has stars each previous and new. however bunch is wide thought of a star cluster, as a result of its mass is comparatively tiny and it's solely concerning ten million stars.
So wherever will a cluster finish and a galaxy begin? Forbes and Kroupa list many criteria that would be accustomed outline a galaxy, together with the presence of substance and having a radius larger than three hundred light-years. Another key criterion for a galaxy, the researchers believe, would be the absence of star collisions, since galaxies, being a lot of stable than clusters, square measure thought to be largely collisionless. this may exclude objects like bunch, however it might leave in ultra-compact dwarfs.

In a manner the difficulty is comparable to 1 with planets tackled by the International Astronomical Union (IAU) in 2006. many bodies had recently been discovered orbiting within the outer system with a size kind of like Pluto's, culminating in Eris, that perceived to be larger. (Subsequent measurements counsel that Eris could indeed be a smidgeon smaller than Pluto, however it's a lot of large.) baby-faced with the revelation that Pluto was technically no a lot of a planet than Eris, IAU polemically determined to label them each "dwarf planets" in conjunction with many different smaller bodies. once asked by ScienceNOW whether or not IAU ought to currently have a task within the galaxy discussion, Ian Jim Corbett, IAU's general secretary, aforesaid such a discussion would be tough. "The boundaries of a definition square measure as fuzzy because the boundaries of a galaxy."

Some astrophysicists, like archangel Drinkwater of the University of Queensland in Australia, say scientists do not know enough concerning the formation of galaxies to classify them with any certainty. Meanwhile, Wyn Evans of the University of Cambridge within the uk calls the difficulty "pointless." "Science isn't determined by votes and polls," he says. "And though Kroupa and Forbes need to hold out a survey, ... this type of poll is that the worst thanks to eff. it's no applied math validity some."

Nonetheless, Kroupa feels the difficulty deserves attention. "We have a transparent image in uranology of what a star is," he says. "We currently even have a definition for planets. however the excellence between galaxies and star clusters remains blurred."
Forbes plans to publish the end result of the vote at AN physics meeting in Santiago in early April.