A collision of two neutron stars some 130 million light-years away sent out a huge shockwave — first, through the universe, and later, through the internet. Perhaps you saw the headlines whizzing around your news feed, e.g., “First Detection of Gravitational Waves from Neutron-Star Crash Marks New Era of Astronomy,” as Space.com wrote — which sounds pretty dramatic. But if you don’t know what the words in the beginning of that headline mean (gravitational waves? Neutron stars?), you probably won’t understand the end of the headline, and why exactly this marks a "new era."
This is indeed a big deal for astronomy. Science has never directly observed a neutron star merger; particularly, this merger was observed both via gravity waves as well as electromagnetically. And the collaborative nature of the observations is unprecedented: around 4500 scientists, from every continent, will be listed as co-authors in the forthcoming paper. That's around a third of all professional astronomers. So yes, this neutron star merger is an astronomically big deal indeed — for science, for humanity, and for the future of astronomy.
What just happened?
Two neutron stars collided and turned into a black hole. This is called a kilonova, as distinct from a nova, supernova, or hypernova, which are other kinds of stellar explosions that you may have heard of.
Does that… happen often?
I mean, in the history of the universe, probably yes. But since the advent of gravitational wave astronomy on our planet, no.
What is a neutron star?
Okay, you know how atoms have protons, neutrons, and electrons in them? And you know how protons are positively charged, and electrons are negatively charged, and neutrons are neutral?
Yeah, I remember that from watching Bill Nye as a kid.
Totally. Anyway, have you ever wondered why the negatively-charged electrons and the positively-charged protons don’t just merge into each other and form a neutral neutron? I mean, they’re sitting there in the atom’s nucleus pretty close to each other. Like, if you had two magnets that close, they'd stick together immediately.
I guess now that you mention it, yeah, it is weird.
Well, it’s because there’s another force deep in the atom that’s preventing them from merging.
It’s really really strong.
The only way to overcome this force is to have a huge amount of matter in a really hot, dense space — basically shove them into each other until they give up and stick together and become a neutron. This happens in very large stars that have been around for a while — the core collapses, and in the aftermath, the electrons in the star are so close to the protons, and under so much pressure, that they suddenly merge. There’s a big explosion and the outer material of the star is sloughed off.
Okay, so you’re saying under a lot of pressure and in certain conditions, some stars collapse and become big balls of neutrons?
Pretty much, yeah.
So why do the neutrons just stick around in a huge ball? Aren't they neutral? What's keeping them together?
Gravity, mostly. But also the strong nuclear force, that aforementioned weird strong force. This isn't something you'd encounter on a macroscopic scale — the strong force only really works at the type of distances typified by particles in atomic nuclei. And it's different, fundamentally, than the electromagnetic force, which is what makes magnets attract and repel and what makes your hair stick up when you rub a balloon on it.
So these neutrons in a big ball are bound by gravity, but also sticking together by virtue of the strong nuclear force.
So basically, the new ball of neutrons is really small, at least, compared to how heavy it is. That's because the neutrons are all clumped together as if this neutron star is one giant atomic nucleus -- which it kinda is. It’s like a giant atom made only of neutrons. If our sun were a neutron star, it would be less than 20 miles wide. It would also not be something you would ever want to get near.
Got it. That means two giant balls of neutrons that weighed like, more than our sun and were only ten-ish miles wide, suddenly smashed into each other, and in the aftermath created a black hole, and we are just now detecting it on Earth?
Exactly. Pretty weird, no?
So what's all this "nuclear pasta" stuff that's been trending on Twitter? Doesn't that have to do with neutron stars?
In physics, we often like to talk about the differences between the quantum world and the everyday world. They have fundamentally different rules, and merging the rules of behavior of very small things and very large things has been a challenge for physicists for a century. Sometimes we see the effects of quantum mechanics on a macroscopic scale — lasers are a good example — but generally, we define the macroscopic world by more general rules, like Newton's Laws.
So understanding what a big ball of neutrons that is as dense as an atomic nucleus might look like, and how it might behave, involves a lot of computer simulations. Indeed, because of some of the "impurities" in neutron stars -- there are things like protons and electrons that get stuck to the surface — and because of how the star spins, different regions of neutron stars are thought to arrange themselves, on a quantum scale, in very specific lattices that resemble pasta, to put it bluntly. These are shapes that Newton's Laws would never predict, which is why our understanding of the macroscopic world and the quantum world have incongruities.
There's a good overview of nuclear pasta here, if you want to read more.
So what about this gravity wave stuff?
Ah, now we're getting to the weighty stuff (literally).
A century ago, Einstein predicted that there could be energetic waves produced by sudden changes in mass — say, two massive objects merging into one, like two black holes. But since gravity is a comparatively weak force — I mean, you can briefly overcome the entire planet's gravity by jumping — this was very hard to detect directly. Only in the past five years were scientists able to create gravitational wave observatories.
What's a gravitational wave observatory? Like a telescope for looking at gravity?
Basically, but these look nothing like normal observatories -- no big glass lenses or anything.
The most functional gravitational wave observatory on Earth, the Virgo-LIGO collaboration, consists of three “telescopes” that are actually just really long buildings with lasers running through them. A gravitational wave of sufficient strength that passes through the building will make the laser wiggle back and forth very slightly.
The reason there are multiple gravitational wave observatories is to help confirm that signals are not just noise and to figure out the direction they’re coming from. In other words, a signal from a black hole merger might hit the Virgo observatory first (that's the one in Italy), and a fraction of a second later, hit the LIGO observatories (those are in Washington state and Louisiana). Or maybe vice versa. So having multiple ones helps accurately figure out the location of the signal.
(If you were wondering, "LIGO" stands for Laser Interferometer Gravitational-wave Observatory.)
This year, the Nobel Prize in Physics went to the scientists who helped detect gravitational waves for the first time. And while there have been many black hole mergers detected — that seems to be a common signal that sets off the observatory — this is the first time a neutron star merger was picked up by the telescopes.
So if we've detected gravitational waves on Earth before, what makes this event so special?
To understand that, you have to understand something called multi-messenger astronomy. It's basically the holy grail of astronomical observation.
You may remember a thing called the electromagnetic spectrum — that is, the range of different frequencies of light, or photons. Humans only see a very narrow band of this spectrum, what we call visible light. But some animals see higher frequency light, like ultraviolet -- bees, for instance -- and some animals see lower frequency light like infrared light -- cats, for instance. The electromagnetic spectrum goes all the way up to things like gamma-rays and x-rays, which are very energetic and don't make it too far through Earth's atmosphere before getting absorbed or broken up into less energetic photons. And at the bottom of the spectrum, there are radio waves, which do travel readily through our atmosphere.
Earth astronomers now have a complement of telescopes in different wavelengths of light. These observatories can look quite different depending on their function. Radio wave telescopes generally resemble giant dishes: think of the Arecibo Observatory in Puerto Rico, featured in a famous fight scene in the climax of the James Bond film "Goldeneye." X-ray telescopes have to be in space, as the atmosphere blocks x-rays from penetrating to the surface. And gamma-ray telescopes typically only detect gamma-rays indirectly, by observing the constituent lower-energy photons that they break up and turn into upon hitting the atmosphere. When this kilonova explosion happened, observatories spanning all wavelengths from all over Earth -- and in space -- set their sights on the location of the merger, producing a wealth of data across the electromagnetic spectrum.
But when big, energetic events happen in the universe, often they release energy in other forms besides photons — such as gravitational waves, and sometimes tiny particles like the elusive neutrino. And so having a gravitational wave observatory to complement electromagnetic observatories is a boon for astronomers.
This was actually the first time scientists had witnessed an astronomical event that made its mark on Earth in both the electromagnetic spectrum and in producing gravitational waves. That's because previous gravitational wave events were generally of things like black hole mergers, which didn't emit any light or anything else that was detectable by other observatories. But this neutron star merger was the first multi-messenger event that used both gravitational wave observations in concert with electromagnetic waves.
You mentioned neutrinos briefly. Did the neutrino observatories pick up anything?
True, the third prong in multi-messenger astronomy is neutrino observatories. The reports from the IceCube neutrino observatory at the South Pole indicate that this neutron star merger actually didn't produce any neutrinos -- at least, not any that were pointed in our direction that were detectable on Earth. Still, the lack of detection is equally interesting, as it indicates to physicists that the physics happening in the kilonova did not create a huge blast of neutrinos the way that, say, supernovae do.
I read something about how neutron star mergers produce heavy metals like gold and platinum. How does that work?
To answer this question, it is important to understand that the composition of Earth is very different from the composition of most of the universe. Of the "normal" matter in the universe (meaning, not dark matter or dark energy), most of it consists of hydrogen and helium. Only a tiny fraction of elemental matter in the universe consists of things heavier than the two lightest elements, which means that much of Earth's composition — elements like carbon, iron, phosphorus, uranium, you name it — are comparatively rare. We live on an exceptional planet.
Stars are responsible for producing much of the universe's non-hydrogen elements. Through the process of nuclear fusion, smaller nuclei combine to become larger ones. Stars are basically stellar element factories: they convert hydrogen to helium and then higher elements, and produce heat and energy in the process. But there is a limit to the size of the atoms they can produce: iron is actually the heaviest isotope that is directly produced by fusion. By the time a star's core is dense enough that it's making iron, it has only a short amount of time to live before inevitably exploding or imploding in some fashion.
So where do elements heavier than iron come from? As you may recall, iron is only the 26th element on the periodic table, meaning there are about 60 heavier stable elements, including things like silver, gold, lead, uranium, etc. The answer is complicated, but some of these elements are generated in hypernova explosions, when the matter in stars is compressed against other matter in ongoing shockwaves and nuclei are shoved into each other to produce small quantities of really heavy elements.
Similar things happen in kilonova explosions: the cascading waves of extremely dense particles collide and form really heavy isotopes of elements like gold and platinum. UC Berkeley astronomers estimated that this neutron star merger produced a "yield of gold [of] around 200 Earth masses," and "500 Earth masses" of platinum, per a press release.
To make a crude analogy: imagine if you had a bunch of little magnets spread across the floor, a few feet apart from each other. Imagine each one is a proton or neutron. Individually, they are only faintly attracted or repellant to each other. But if the floor suddenly tilted, they would all smash together and form one giant ball of magnets. Vaguely, that's what happens in these big stellar explosions that bind together smaller nuclei into bigger, heavier ones, creating elements like gold and platinum.