One of the most beautiful and scientifically interesting objects in space are the globular clusters, containing hundreds of thousands of stars. What are they and where did they come from?
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I know it’s all aliens this and black holes that. Kugelblitzes, Sea Dragons and the dreaded vacuum decay. But sometimes, you just want to talk about something that might seem a little boring, but in fact, is one of the coolest objects in the night sky.
An enormous cluster of stars, many light years across and containing hundreds of thousands of stars. I’m talking about globular star clusters, of course, and they’re one of the best objects you can view in a telescope, and one of the most useful objects for scientists to study.
Now, before we get into the episode, I need to talk about pronunciation. In the past, I used to call them “globe-u-lar” clusters. But I got all kinds of heat from my astronomer friends, so I half retrained my brain to call them “glob-u-lar” clusters. But then I got dug into the proper pronunciation, and it turns out both methods are fine.
Which means that I’ll probably switch back and forth as we go, making you wince in my humiliation. That’s fine.
If you look out into the night sky with a telescope, there are few different kinds of star clusters. There are the open clusters, loose collections of stars nearby one another. Some examples of this might be the famous Pleiades, or Hyades star clusters. They’re what’s left over from a stellar nursery like the Great Nebula in Orion.
But a globular cluster is a much different object. Instead of a few dozen stars, globular clusters can contain hundreds of thousands of stars, bunched together into a region up to 100 light-years across.
On average within the Milky Way, stars are separated by about 5 light years, or about the distance from the Sun to Alpha Centauri. But within globular clusters, stars are only about a light year apart, even closer down in the core of the cluster. Just imagine what it must be like to stand on a planet orbiting one of these stars, and seeing a night sky alive with bright stars. It would be amazing.
Across the entire Milky Way, there are about 150 known globular clusters. There are probably a few dozen more hiding behind the disk of the galaxy from our vantage point. And as we look out into the Universe, we can see that all other galaxies have them too. Andromeda is much larger than the Milky Way and it has hundreds, while smaller dwarf galaxies like the Large and Small Magellanic clouds might only have just a few. Mighty elliptical galaxies could have tens of thousands.
The closest globular cluster is known as NGC 6397, located just 7,200 light years from Earth. It’s one of the smaller clusters with a mere 400,000 stars, measuring about 75 light-years across. But if you’ve got nice dark skies without light pollution, you can just barely see it with the unaided eye.
The largest cluster in the Milky Way is the Omega Cluster, only visible from more southern skies. This monster is 15.8 thousand light-years away and contains 10 million stars. It’s so big and filled with stars that some astronomers think it’s actually the core of a dwarf galaxy that was consumed a long time ago. You can also see the Omega Cluster with the unaided eye, even though it’s farther. It measures 150 light years across, but from our perspective, it appears like faint circle about as large as the full Moon.
Of course, as a Canadian, I’ve never actually seen it. So, enjoy that you lucky lucky Australians. That and all that other cool stuff you can see from the southern hemisphere, like the Large and Small Magellanic Clouds, and Alpha Centauri.
They’re pretty to look at, but globular clusters are very useful for science because you’ve got an enormous number of stars in a tight region. Mayhem can happen.
One exotic object found in some star clusters are known as blue stragglers. These are hot blue stars found within clusters, which stopped forming new stars billions of years ago. All of the large hot stars should have died as supernova, so blue stars shouldn’t be there.
Recently astronomers realized that these blue stars are actually the result of stellar collisions, where two stars came too close and collided. This added mass and mixed up hydrogen from the collision creates a much hotter star than anything else in the cluster.