All About Pulsars

Transcript

ERIC: From the Museum of Science, Boston. This is Pulsar, a podcast where we answer questions from you, our audience. I'm your host, Eric and for our very first episode, we wanted to discuss our namesake. Pulsars are deep space objects with an incredible back story of mystery and scientific discovery.

Answering our questions today is Talia Sepersky, the planetarium coordinator at the Museum of Science's Charles Hayden Planetarium. Talia, thanks so much for joining us today.

TALIA: My distinct pleasure.

ERIC: So let's start with the basics. What is a pulsar?

TALIA: So, pulsar is actually short for pulsating quasi-stellar object.

It's a small, very, very dense object called a neutron star that is spinning very quickly. And when I say dense, I'm talking, I read a statistic recently, it's kind of fun, if you were to take every car in the United States and pack them down into the density that you find in a neutron star, all of those cars would be packed into something about the size of a dice.

ERIC: That's awesome. I thought you're gonna say like, building, or house, or something.

TALIA: No, a dice. I read that if you could fill a normal matchbox with neutron star material, it would weigh a couple billion tons.

ERIC: That's incredible. But what separates a pulsar from your average garden variety neutron star?

TALIA: It has two beams of radiation coming out of it, sort of like the beams of light from a lighthouse. And as the pulsar spins, these beams act like the beams of light from the lighthouse. Just like you can only see the lighthouse beam for a moment as it spins toward you and then it appears to go dark, we see pulsars appear to flash on and off as they rotate their radiation beam towards us and then away again.

Their spin times are very precise and some would count amongst the most accurate timekeepers in existence. And since they seem to flash on and off, they look like they are pulsating, so we call them pulsars.

ERIC: Wow, that sounds really intense. How does one of these pulsars even form?

TALIA: Appropriately, in a very intense manner, it forms with the death of a very large star. When very large stars, so larger than the sun, run out of fuel at the end of their lives, their outer layers explode in a supernova. Which is one of the most crazy energetic violent processes that happens in the universe.

But while the outer layers are exploding outward, the cores of these stars are collapsing inward, getting smaller and smaller, and denser and denser. For the very largest stars, this collapse is gonna continue until the core forms a black hole. But for stars that are massive enough to go supernova, but not massive enough to form a black hole, their cores stop collapsing when they're about the size of the Boston area.

This is a neutron star, the remnant of a star with a greater mass than the sun packed into something the size of Boston. When these stars are spinning, and we can see the blinking of their radiation beams, we classify them as pulsars, which as I said before, they're kind of the lighthouses of the universe.

ERIC: So how long have we known about pulsars? When was the first one discovered?

TALIA: So we had been theorizing that neutron stars existed as far back as the 1930s, as astronomers started to think about what would happen when a large star went supernova. But nobody could think of a way to actually detect neutron stars in space.

And then in 1967, a British radio astronomer named Antony Hewish and his postgraduate Jocelyn Bell Burnell observed radio pulses coming from the sky in exactly 1.33 second intervals. And they didn't realize it at the moment, but they had actually discovered the first known pulsar, which was also the first confirmation that neutron stars actually exist.

This discovery led to the 1974 Nobel Prize in Physics, which was the first time in history that this prize was awarded to astronomers. Although notably, Jocelyn Bell Burnell was left off the team that was awarded the prize despite being the one who actually observed those first radio signals.

ERIC: For these astronomers seeing these regularly repeating signals every 1.3 seconds, I'm sure neutron star wasn't really high on the list of their possibilities. What did they think they had discovered when they first saw this weird signal?

TALIA: Well, so the tricky thing was at the time of the discovery, the idea of pulsars didn't exist.

Neutron stars, yes but pulsars, no. And it wasn't until after this discovery that someone thought maybe these neutron stars could rotate very rapidly and create these pulses. But Hewish and Burnell weren't sure what they had found. But since it moved across the sky at the same rate as the stars, they knew it came from space and not from humans.

And they were pretty sure that the pulses had a natural source, but Burnell herself pointed out that they couldn't, at that point, be totally certain that it wasn't coming from an alien civilization. So they called that first signal LGM-1 for little green men. It was the discovery of a second pulsar in a different part of the sky that sort of discounted the whole alien civilization possibility, and confirmed that it was a naturally occurring signal.

And by 1968, sort of the year after their discovery, the idea of rotating neutron stars had made its way into the world.

ERIC: So we've got unimaginably dense stars rotating incredibly fast, blasting out lighthouse beams of radiation. I don't want to find myself anywhere near a pulsar. Are there any in our neighborhood?

TALIA: It can be a little tricky to determine exactly which one is actually the closest, because all of these distances have a little bit of uncertainty in them. So I'm going to tell you about one that is certainly one of, if not the closest one to Earth, it may actually be the closest one.

It's designation is PSR-JO437-4715, because pulsar names are great. I'm gonna call it 4715 for short. It's something around 510 light years away so you don't have anything to worry about. It's in the direction of a constellation that we can't see from Boston called Pictor. 4715 is definitely the closest millisecond pulsar to Earth.

It rotates over 173 times every second, it's spinning very quickly. And its spin time is incredibly stable, which actually makes 4715 one of the most reliable timekeepers known to humanity, even though it's out in space.

ERIC: So we can use a super rapidly rotating neutron star, to keep time better than any way we have here on the earth.

TALIA: Isn't that really, really cool?

ERIC: That is pretty awesome. Talia, thank you so much for giving us a crash course in pulsars here on Pulsar.

TALIA: It was, as I said, my pleasure. Everybody keep looking up.

ERIC: That's it for this episode. If you've got a question you'd like to see answered on Pulsar, send it to us at sciencequestions@mos.org.

Join us again soon.