Hubble just can’t resist taking snaps of the most resplendent parts of the night sky, from an ongoing galactic collision shimmering with star birth to – as the latest image showcases – a cluster of aging stars, which has been likened by some to a “retirement home” for the elderly furnaces above our heads.
What you can see here, NGC 6139, isn’t a galaxy, but a globular cluster. These are nebulous, spherical gatherings of stars, bound around some sort of dense core, linked together by the invisible tendrils of gravity. They tend to be found in the diffuse outer realms of galaxies, including our own, and typically contains hundreds of thousands to several million stars.
According to Space.com, there are around 150 globular clusters in the Milky Way, and most of them contain stars that are at least 10 billion years old, which makes them some of the most ancient stars in the universe. Much like galaxies though, they are collections of stars, so what’s the difference between the two?
It’s actually not that clear-cut. Even “galaxy” lacks a proper definition, so trying to work out what a globular cluster may be isn’t an easy task.
It’s been suggested that the distinction could be made using two factors: the number of stars, and the amount of dark matter that’s thought to be present in them. Globular clusters have fewer stars than small galaxies, and seem to essentially lack dark matter compared to their bigger brothers and sisters.
Definitions aside, few would argue that globular clusters aren’t aesthetically and scientifically stunning objects. NGC 6139 is a great example of one, pictured here in the constellation of Scorpius. As noted by the accompanying NASA/ESA release, this gigantic cluster can be compared with the Pleiades, a far more diffuse family of stars known as an “open star cluster.”
It's worth noting that NGC 6139’s size, and even its age, aren’t that unusual. M13, another sizeable globular cluster, is between 12 and 13 billion years old, which means it’s almost as old as the universe itself – just like plenty of others. Although not the oldest, the Omega Centauri cluster is perhaps the largest known to science.
Back then, huge clouds of molecular hydrogen were more commonplace, which allowed for the spontaneous creation of plenty of stars without a more stelliferous galaxy necessarily forming. Fewer gas clouds mean that this is a far rarer occurrence today.
Despite being pretty much everywhere we look, the role and behavior of star clusters, in relation to the overall evolution of galaxies and the wider universe, remains ambiguous. The abundance of old stars in globular clusters has been linked to the conditions present not too long after the birth of the universe.
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