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.
