Pulsars

In the previous episode I told how astronomer Jocelyn Bell discovered radio pulses from the deep universe in 1967. And those radio pulses appear to be ticking very stable with a period of 1.33 seconds, as if it were a clock. Yes, it must come from little green men, right? Because how can nature spontaneously produce a radio station that is also a very accurate clock? It is much more likely that the signal comes from intelligent beings such as ourselves. But then a bit smaller and a bit greener. Or would it secretly be a natural phenomenon? After all, we found several such signals from different places in the universe. And we were able to show that the signal is not coming from a planet.

01:07 - In addition, we know that stars and galaxies emit radio waves. So yes, nature does indeed produce radio transmitters. But what is special about this signal is that it is so perfectly periodic. The accuracy of these alien clocks is comparable to the very best clocks we can produce here on Earth. Those are the atomic clocks. And another mystery is the fact that the period is so short. Something like a second. Let’s start there.

What in the universe can have a period 01:42 - of 1 second? A star that revolves around its axis in 1 second? Our sun also revolves around its own axis. But it has a period of 27 days. In other words 2 million seconds. And we are looking for something that rotates around its axis in 1 second. What would happen if a star like our sun revolves around its axis in 1 second? Suppose you would step on the sun. Never do that! Because the sun has a temperature of 6000 degrees Celsius. So never stand on the sun. But suppose you do it anyway and suppose the sun revolves around its axis once per second.

02:23 - The sun has a circumference of about 4 million kilometers. That means that you would then move in a circle at a speed of 4 million kilometers per second. But yes, that is faster than the speed of light. And Einstein’s theory of relativity says: that is not possible at all. But, even more practical: you can imagine if you were to move at such an enormous speed, then you just fly - woops - into space. And the same thing happens with all matter on the surface of the sun. So if the sun were actually spinning that fast , the sun would just explode. So a star like our sun is not a good candidate to explain our alien clocks. But what if the star were much smaller? Then it might be possible. So what we’re looking for is a really tiny star.

03:23 - And what about that other oddity? Namely that the clocks are so stable. Let’s do an experiment with a basketball and a bowling ball. I’m going to spin them both and then we’ll see which of the two keeps spinning the longest. Let’s start with the basketball. Then I’ll do the bowling ball. Three two one. The bowling ball wins because it is heavier. An object that is heavy slows down less quickly. And the same goes for a very heavy star. If a star has a lot of mass, then the period is very stable. So what we are looking for is a star that is both very small but also very massive. Just a bit of history. In 1933 there were two astronomers named Walter Baade and Fritz Zwicky. And they had the idea of ​​a star that consists almost entirely of neutrons. Neutrons are neutral particles without electric charge and they have the property that you can put a lot of those particles together in a very small space.

05:07 - That way you could squeeze a star like our Sun with a diameter of 1.5 million kilometers into a sphere with a diameter of 20 kilometers. And such a neutron star could be formed when a somewhat heavier star burned out at the end of its life. Then it collapses under gravity. And then something very special happens. Due to this implosion all charged particles, the protons and the electrons, are compressed and they form neutrons. What you are left with is a very small star, but with a lot of mass. And something else is happening.

Namely 05:52 - when such a star implodes and all that mass is drawn in, such a star spins faster and faster. We see the same thing happening with figure skaters who do a pirouette. They pull the arms and legs towards the body, causing them to spin faster and faster. This is called angular momentum conservation. And with such a massive star imploding, that means it can rotate on its axis dozens of times per second. Those are exactly the properties we are looking for to explain those mysterious radio pulses. Except that we don’t yet know why such a star emits radio waves. It turns out that those neutron stars have a super strong magnetic field. And that also arises during the implosion of that original star. So what you have is a neutron star surrounded by the magnetic fields.

Those 06:48 - magnetic fields rotate along with that star. This creates a kind of super powerful dynamo. And that ensures that particles from the surface of the star are thrown into space and those particles emit radio waves. The whole thing looks like this. This is a pulsar. That sphere in the middle is the neutron star with a diameter of about 20 kilometers. And those curves around it are those magnetic fields that rotate with the star. In reality, such a star orbits much faster.

And those red beams 07:27 - are the places where radio waves are sent into space. And when such a beam points toward Earth, we see a flash of radio waves here on Earth. Just like a lighthouse. These are the objects that Jocelyn Bell first observed. And with that, the mystery discovered by Jocelyn Bell is solved. They are not little green men. They are pulsars. And the discovery of pulsars proved to be so important that it was awarded the Nobel Prize in 1974. And that award went to her professor, Anthony Hewish. But Jocelyn Bell was not forgotten. She got a pat on the back. This absolutely unjust and - most likely sexist - decision was only corrected in 2018. Then Jocelyn Bell was awarded the Special Breakthrough Prize for the discovery of pulsars. And it has a prize money of no less than 3 million dollars. And she used that money to set up a fund for women, minorities and refugees who would like to go into science. What a wonderful lady.

08:37 - If you have another question about astronomy, please leave a comment. You can also subscribe to my YouTube channel. Or follow me via my website roysmits.nl. Or via Facebook, Twitter, or Instagram. .