“What’s that star?
It’s the Death Star.
What does it do?
It does Death. It does Death, buddy. Get out of my way!” –Eddie Izzard
Like it was for many people, the original, very first Star Wars movie was one of my favorites as a child. And while there was a lot to be in awe of, the idea of jetting around the Universe in your own private, gargantuan structure, free from planets, Solar Systems, and even the rest of the galaxy was simply the most amazing idea to me.
That’s what I wanted: a Death Star. Of course, you know what happens to the Death Star, don’t you? At least in the version I remember, Darth Vader shoots down Keith Hernandez, Han Solo saves Luke, Luke blows up the Death Star, and then goes home and reunites with Leia, whom he calls Carrie. (Watch it!)
This image — of the blown-up Death Star — was the one that stuck with me. And it wasn’t until I was in graduate school, learning about the structures that form in the Universe, that actual astrophysics made me think about the Death Star once again.
This object, Messier 22, is known as a globular cluster. A collection of somewhere around a hundred thousand stars in a sphere that’s maybe 100 light years across, globular clusters exist in great abundance around — but not in — our galaxy.
Consider that the nearest single star to us is still over four light years away to get an appreciation of how tightly packed these stars are! The Hubble Space Telescope — taking a deep look inside — can show you better than I could ever describe on my own.
This 2009 Hubble image is of globular cluster Omega Centauri, which lives some 16,000 light years away from us. All the stars in this image belong to Omega Centauri’s core, and the width of the image is 6.3 light years. For comparison, know how many star systems there are within 6.3 light years of us?
Three. The Alpha Centauri trinary star system at 4.3 light-years distant, Barnard’s Star, barely making the cut at a distance of 6.0 light years, and… the Sun itself. That’s what a globular cluster is. Isolated but full of riches all its own, traveling throughout the galaxy.
Looking around the vicinity of our Milky Way galaxy, there are well over 100 globular clusters — dense collections of hundreds of thousands of stars — orbiting and plunging through our galactic plane. Over the entire history of the Universe, each globular cluster has had time to make a mere ten-to-twenty passes through the galaxy, and spend nearly all their time well outside the galaxy itself.
These objects — globular clusters — are what I think of as Death Stars. Isolated objects, And as far as we can tell, the Milky Way is awfully typical among galaxies for having a little over a hundred of these “Death Stars.”
Andromeda, as a careful observer can find in a night, has over 100 globular clusters as well! These object range from old — like, many billions of years old — to the very old. In the case of Messier 22, the first globular cluster I showed you, it’s almost as old as the Universe itself, with an estimated age of over 12 billion years! (Not bad, considering the Universe itself has only been around for 13.7 billion years.) Based on what we know about structure formation, we can understand their ages, their distributions in and around galaxies, and their masses. All of that makes sense within our picture of how the Universe works.
But there is a problem with globular clusters, one that has troubled theorists like me. You see, knowing what we know about the Universe — a Universe that started with the Big Bang, and that contains the measured amounts of radiation, normal matter, dark matter, and dark energy — there shouldn’t just be a couple of hundred globular clusters for every large galaxy. When we do our simulations of structure formation, we get… well, let’s just call it a different answer.
Instead of hundreds, our simulations of structure formation predict tens of thousands of globular clusters for each isolated galaxy. And you don’t have to be Einstein to realize that that’s wrong. But the question, of course, is why that’s wrong.
In other words, who destroys all these Death Stars, and how?
Well, if you came by this site for Valentine’s Day, you might recall something interesting.
This is the Rosette Nebula, one of the largest star-forming regions in our galaxy, with a total mass of about 10,000 Suns. The central region has the hottest, brightest, youngest stars, and — as you can also see — the least amount of hot, pink, star-forming gas. Why is that?
Because these ultra-hot, young stars emit great stellar winds, blowing the gas and dust out of the region where these stars live! And this nebula is in our galaxy. Our quiet, boring, low-rate-of-star-formation galaxy. What would happen if we took two similarly-sized galaxies and — as structure in the Universe is wont do to over billions of years — allowed them to merge together?
Instead of star-forming regions containing the mass of thousands of Suns, colliding galaxies (like the Antennae Galaxies, above), have star forming regions containing the mass of billions of Suns! That’s right: billions. We even have a special name for galaxies that are doing this right now: Starburst galaxies.
So you can imagine how powerful the stellar winds are in galaxies like this. And in the early Universe, where mergers between similar-sized objects were how galaxies like the Milky Way got so big in the first place, it is conceivable that — during this intense period of star formation — the vast majority of globular clusters were blown apart!
This is all just theory, of course. But if we can put this starbursting into our simulation, we should be able to see — for the first time — whether the globular clusters come out right! Let’s go to the video.
What the simulation shows is that nearly all of the globular clusters get destroyed due to the merger-induced starburst! What you can’t see so obviously is that it’s the most isolated, largest globular clusters that survive intact. As the researchers from Germany and the Netherlands say themselves:
It is ironic to see that starbursts may produce many young stellar clusters, but at the same time also destroy the majority of them. This occurs not only in galaxy collisions, but should be expected in any starburst environment. In the early Universe, starbursts were commonplace – it therefore makes perfect sense that all globular clusters have approximately the same number of stars. Their smaller brothers and sisters that didn’t contain as many stars were doomed to be destroyed.
So that’s how the Universe does it! Create a large enough star forming region that the vast majority of your globular clusters are blasted apart; that’s how the Universe destroys its Death Stars!
What the researchers don’t say is that this may help explain another mystery of globular clusters: blue stragglers.
Inside some globular clusters, there are stars that are hotter, bluer, more massive, and younger than all the other stars found in that cluster. Where did they come from? There are a few ideas, of course, but now there’s actually some good evidence pointing towards the simplest idea: that passing through a star-forming region may help form these blue straggler stars, but your cluster needs to be large enough to hold together or be destroyed.
In other words, globular clusters with blue stragglers may be the Universe’s failed attempts to destroy that Death Star. And that’s how the Universe does it!