One of the most beautiful and scientifically interesting objects in space are globular
clusters, containing hundreds of thousands of stars.
What are they and where did they come from?
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.
And as we talked about just a few episodes ago, globular star clusters are one of the
places that astronomers have gone looking for intermediate mass black holes.
The missing puzzle piece between stellar mass and supermassive black holes.
If they’re anywhere, they should be lurking inside globular star clusters.
In fact, these clusters were used by astronomers to figure out how old the Milky Way itself
is.
About a decade ago, astronomers formed a detailed study of NGC 6397, looking for the oldest
stars in the cluster.
Based on their observations, they were able to determine that the Milky Way is 13.6 billion
years old, which is just 200 million years younger than the Universe itself.
In a second, I’m going to talk about how the best theory about the formation of globular
clusters was recently thrown out the window, but first I’d like to thank:
Joseph Dutton Hubert Rady
Zigian
And the rest of our 773 patrons for their generous support.
If you love what we’re doing and want to get in on the action, head over to patreon.com/universetoday.
Bend their minds: One of the most interesting things about them
is that globular star clusters are old.
Pretty much as old as the Universe itself.
How did such massive, compact objects form so quickly, and maintain their structure over
billions of years, especially through the carnage of early galaxy merger and formation?
Most of the stars in globular clusters are roughly the same age, which means that they
formed around the same time.
But in many clusters, there seem to be groups of stars that formed in waves.
One cluster might have a huge group of ancient stars with low amounts of metal, and then
another population of younger stars.
Other clusters might have multiple waves of star formation.
Quick side note, when astronomers talk about metal, they’re actually talking about any
elements on the periodic table beyond the primordial hydrogen, helium and trace elements
left over from the Big Bang.
Stars with high metallicity contain recycled stardust.
Since the clusters are ancient, they should contain many more of the low metal stars than
they do.
The traditional thinking was that the clusters kicked out many of the low metal stars out
into their galaxies, where they were hard to detect from the rest of the galactic population.
But Hubble recently observed dwarf galaxies, with normal-sized globular clusters, but a
fraction of the stars that might be present in a larger spiral like the Milky Way.
If these clusters had been stripped of their stars, they’d be easy to spot in the galaxy,
but they’re not there.
It appears now that the clusters themselves might merge and interact, and the globular
clusters we see today are the result of many clusters coming together.
What I really like about globular clusters is how they look in even the smallest backyard
telescope or pair of astronomical binoculars.
Once you know where to find them in the night sky, you can easily see a fuzzy ball in the
eyepiece.
With a more powerful telescope, it resolves into compact ball of stars.
And with a small telescope and DSLR camera, you can capture amazing images of these objects,
even in relatively light-polluted skies.
If you don’t own a telescope, I highly recommend you take a moment and find out when and where
your closest astronomical society is going to be having an open house.
There’ll be many amateur astronomers out with a variety of telescopes, and they’d
be glad to show you what a globular cluster looks like, so you can see one with your own
eyeballs.
Have you ever seen a globular cluster through a telescope eyepiece?
What else do you like to observe?
Let me know know your experiences in the comments below.
In our next episode, we wonder where comets come from.
What causes them to fall out of the Oort Cloud and visit the inner Solar System.
That’s next time.
Time for a playlist.
Let’s start with a great video about star clusters from Crash Course Astronomy, Deep
Sky videos observes the globular cluster M15, a cool video zooming into a globular star
cluster from the European Southern Observatory, a presentation about black holes in globular
clusters.
And finally, a long lecture about globular clusters from the Institute for Astronomy.
And that starts, right now.