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How do you distinguish a wandflower from a mere cluster of stars ? That ’s easy , right-hand ? A extragalactic nebula is a expectant collection of gazillion or one thousand million of champion , while a star cluster only has a thousand or so . Well , that kind of mentation wo n’t get you a Ph.D. in uranology ! Seriously , though , the line of descent between galaxy and star cluster is n’t always clear . Case in degree , UMa3 / U1 .
It ’s easy to distinguish galaxies such as Andromeda and theMilky Way . They are enceinte , gravitationally confine , and dominated bydark matter . It ’s also easy to distinguish star clusters such as the Pleiades . They are loosely adhere star groupings without dark thing . But for a character of pocket-size dwarf coltsfoot know as Ultra - Faint Dwarfs ( UFDs ) the dividing line gets fuzzy .
UFDs are master by dark affair . The stack of theMilky Way , for example , is about 85 % dark matter . An ultrafaint dwarf galaxy , however , can have a thousand times more dark matter than aglow topic . This is why they are so light-headed . Since UFDs often contain some of the sometime stars in the Universe , uranologist have intercourse to study them for clues on the origins of galaxies . Which brings us to UMa3 / U1 .
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Even its name tell us that there is a trouble . If , in fact , the target is a dwarf galaxy then its name should be Ursa Major III , as it is a satellite beetleweed in the constellation Ursa Major . If it is an ancient genius cluster , then it should be named UNIONS 1 , since it was discovered by the Ultraviolet Near Infrared Optical Northern Survey ( UNIONS ) . If it is a galaxy , then it is the small and most disconsolate - matter dominate coltsfoot yet discovered . If it is a star bunch , then with an historic period of about 11 billion year , it is the one-time star cluster yet discovered .
UMa3 / U1 is downright tiny . It is only 20 light - eld across , contains only about 60 stars , and has a visible spate of just 16 Suns . In comparing , the Pleiades has about the same diam , but contains more than 1,000 mavin and 800 solar masses . So the real question for UMa3 / U1 is whether it is dominated by obscure issue .
In a late work , the team looked at several tests to distinguish sensation cluster and dwarf extragalactic nebula . Their first approach was to look at the dynamics of the visible stars , assuming it is a ace cluster . Based on their know motions , the squad simulated how long it would take for the stars to break gratis , a process known asevaporation . base on their model , the clustering could come through for another 2 - 3 billion years . That ’s a secure fraction of the estimated 11 billion year age , which intimate U1 is plainly a stable star cluster .
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The second trial the team use is what is experience as the spate function . This is a game of the way the the great unwashed of the clustering varies with aloofness . If it is a cluster , then the good deal should be more equally distributed , but if it is a galaxy , star should be clump toward the heart . Here the information is less conclusive . The distribution of visible stars is a decent match to the cluster model , but for a Galax urceolata the central stars would be mostly bloodless gnome and neutron stars , which are too obtuse to distinguish with current watching .
Overall , the grounds list towards UMa3 / U1 being a star cluster , but the squad mark that it will take more observations of other UFDs to be conclusive . fortuitously , upcoming telescopes such as the Vera Rubin Observatory will find out many more feeble dwarfs in sentence .
Theoriginal versionof this clause was published onUniverse Today .
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