Gravitational waves help scientists spot the collision of two neutron stars
My daughter loves hearing about astronomy, as the movement of the planets, the unfathomable scale of the universe, and unanswerable questions like “is the universe contained in something?” really excite her imagination. So with today’s announcement that astronomers had finally observed the collision of two neutron stars, it seemed like the perfect story to share with her. If only we hadn’t gone out for candy-laden frozen yogurt an hour earlier…
Me: So once up on a time, two stars blew up.
Four-year-old: That’s bad.
Eight-year-old: It happens eventually to all stars, right?
Me: Well… many of them? The point is the stars were the right size to use up all their full, supernova and be left as neutron stars.
Eight-year-old: What’s a neutral star?
Me: “Neutron,” but that’s a good connection to make. A neutron star is the remains of a star that’s basically made of neutrons, which is a neutrally-charged part of an atom, as compared to positive protons and negative electrons. Anyway, the important thing here is that a neutron star is incredibly dense. You remember density?
Eight-year-old: That means it’s… hot?
Me: No, it’s not about how much energy it has, but how tightly packed together all of it’s material is. So in this case, imagine something that if you put it on Earth somehow would weigh more than our Sun, but was small enough to fit in a space between San Francisco and the San Francisco Airport.
Kids together: Whooooaa…
Me: Yeah, it’s so packed together that–
Eight-year-old: But is it hot?
Me: Well, it’s not inert. It has some some energy as we’ll see, but I’m not sure about its temperature. [Post-bedtime fact-check: Neutron stars are hotter than Earth, but cooler than most stars.]
So the mass of a neutron star is so dense that a teaspoon full of neutron star would weigh a billion tons. They’re a ton of stuff in a small amount of space. But all that ‘stuff’ means that they have a lot of gravity, which is imporant when these two stars started circling each other. As they drew closer, they started orbiting each other, but also tearing each other apart.
Me: As they spun closer and closer, their immense collective mass started emitting gravitational waves. Do you remember the last time we heard about those?
Eight-year-old, now hanging upside-down off the couch: Uh….
Me: We last heard about this when new sensors detected two black holes crashing into each other. The impact send out waves that were basically warping the universe just a tiny bit, and sensors at two seperate buildings were set to notice when lasers were stretched a tiny bit?
Eight-year-old: Oh right!
Me: Well, those two facilities were called LIGO [Laser Interferometer Gravitational-Wave Observatory], and now a third set of sensors has been set up in Italy, called VIRGO, which is doing the same job. To get back to our neutron stars, we know that 130 million years ago, the two stars finally collided, because the waves from their collision arrived at the Earth, and were picked up by these sensors, around two months ago.
Eight-year-old: Two months ago?!
Me: The collision was very far away- around 130 million light-years. The cool thing was that when the gravitational waves were detected, people were notified to jump into action and start looking for the light that they were expecting to follow.
This kind of collision had been predicted, and the size and shape of the gravitational waves looked like what people expected of crashing neutron stars. So they thought that, unlike a black hole, there’d be some light for telescopes to see. Lots of people at observatories around the world started scanning the sky to find traces of the exploding neutron stars, which is called a kilonova.
Four-year-old: What’s an observatory?
Me: A place with a high-powered telescope.
Eight-year-old: Does that mean it was in the sky the before? Did the constellation [Hydra] change?
Me: They did get to see it, but it wasn’t previously visible.
Many, many teams started working together to look for the colliding neutron stars. One guess is that 25 to 30 percent of all astronomers on Earth helped out with this to get as much information from different telescopes as possible. Finally, someone [Charlie Kilpatrick] from UC Santa Cruz, nearby, found a new bright spot near another star. He told everyone to “look over here!” and you could see a blip appear over time. First it was blue, then red, and then dimmed away to nothing.
It was emitting light, but also something called gamma radiation, which is just a form of energy. We can’t see it, and it usually just flys right through us without doing anything. A lot of it was released in the collision, which is what all the telescopes were really looking for.
We had talked about what it meant to be an “author” on a paper the other day, right? Well, one of the papers about this event has about 3,500 authors on it because so many people helped out.
Eight-year-old: You’re like an author, right Daddy?
Me: Well, not like that kind of author.
Eight-year-old: But you write about science stuff.
Me: But I’m not contributing to experiments or anthing. It’s different…anyway…
Eight-year-old mumbling something…
Me: Because there was so much mass and energy between the two stars, when they smashed together they could essentially make a lot of new atoms. Not making them out of nothing, but recombine material to make new atoms that don’t get created all that often. Most new atoms made by stars are light, like hydrogen, but in this case the neutron star was making heavy metals, meaning silver, gold, platinum, and…
Eight-year-old: Gold?! Oh! Money money money money money…
Me: Uh, yeah. They estimate that there was probably so much gold created in this collision you could ball it up into something the 10 times the size of Earth.
Both kids: Whoaaa….
Eight-year-old: You could be soooo rich!
Me: If you could do something with it, yeah. We use gold for lots of stuff, like some of Mommy’s jewelry, or inside electronics like cell phones, and–
Four-year-old: I want to see Mommy’s jewelry!
Eight-year-old, upside-down again: Inside phones?! Money money money money money…
Me: Ugh… right. So to have any of these metals on Earth means that before the Earth was formed, other neutron stars must have collided and released all these metals, some of which got bundled with the other rocks and dust that eventually clumped together to form the planet. We now dig these things out of the ground to use for all sorts of stuff, like earrings or even dipping strawberries–
Eight-year-old: People do that? What?!
Me: Yeah, we put use gold for all kinds of things, but my point is that none of it was from Earth originally. It all came from these huge explosions in space!
Eight-year-old: …my friend said she bought gold for two dollars. Is that real?
Me, realizing I’ve totally lost my audience: If it was a small amount. Two-dollars worth of gold.
Eight-year-old: So it’s made up? The price?
Me: Prices are only what we decide… look, people predicted this is where these heavy metals came from, and now for the first time we’ve been able to observe that happening. We’ve never seen neutron stars colliding before! This work also helps us learn about the expansion rate of the universe, because we can now compare the speed of the gravitational waves to the speed of the gamma radiation. It’s also an amazing project to have so many people working as a team across the globe in a way that just wasn’t possible before!
Me: You know, platinum is worth more than gold per ounce?
Eight-year-old: Money money money money money…
Me: Bed time?
Source: In a First, Gravitational Waves Linked to Neutron Star Crash by Nadia Drake, National Geographic