Welcome back!

I hope you are all safe and doing well. 

Last time, we wrapped up our discussion about stars as a whole. We discussed them from start to finish, both literally and metaphorically: from their fiery birth, to their (sometimes) fiery ends, as well as the tremendous journey that it took to reach there.

Now, we also discussed the end of the most massive stars. We discussed how they essentially concluded their lives in one of two manners; neutron stars or black holes. And as we have already discussed the latter (you can find that blog post here), it’s about time that we discussed the former, which happen to be some of the densest (and most interesting!) objects in the universe.

As always, feel free to comment with any questions or any topics that you would like to discuss. And now, without further ado, let us delve into our discussion of neutron stars!

…

Neutron stars are among the unsung heroes of the cosmos. Although they are as interesting as black holes, they get significantly less attention. Ask any passerby to describe black holes, and they will happily rattle off some variation of “interstellar vacuum cleaner” (read this to understand why that isn’t the case). Ask any passerby about neutron stars, however, and you are more likely to be met with blank stares.

Credit: Facts Legend

The face of the matter is, however, that neutron stars are quite similar to black holes. They are both endgames for supermassive stars; they are both tremendously dense; they both have incredibly large gravitational spheres of influence.

I will go on to say that neutron stars can be even more interesting than black holes. Their physics is absolutely fascinating, and they even have more variety than black holes; pulsars and magnetars are both types of neutron stars. They have even played a large role in the discovery of gravitational waves, and, therefore, a significant part of Einstein’s famous theory of relativity.

So let us delve deeper into these fascinating objects, and find out what makes them what they are!

Neutron stars are typically about 1.4-2 solar masses. They originate from stars that are about 29 solar masses. Imagine the density! A good way to visualize the density of neutron stars is to think of crushing the entire Empire State Building into an area the size of a grain of sand. That’s about how dense neutron stars are.

Something interesting about neutron stars is the physics behind their makeup. As I’m sure you can tell by the name, neutron stars are primarily made up of—well, neutrons! Now, given the density, I’m sure you are all wondering why the neutron stars don’t collapse on themselves? The answer is a fascinating phenomenon known as neutron degeneracy pressure.

Now, if that sounds really complicated, don’t worry: I’m here to break it down. 

To understand neutron degeneracy pressure, we must first discuss electron degeneracy pressure (commonly found in white dwarfs). This essentially means that the energy levels of atoms in a star have filled up completely with electrons, preventing further compression due to the fact that more electrons simply cannot be added to the atoms; additionally, as two electrons cannot exist in the same state, the compression cannot increase further! White dwarves live quite happily in this state, which happens to be less than or equal to 1.4 solar masses. Incidentally, 1.4 solar masses is known in astronomy as the Chandrasekhar Limit, or the largest size that stable white dwarfs can reach.

But what happens if the star’s size exceeds 1.4 solar masses?

This is where neutron degeneracy pressure comes into play. Past 1.4 solar masses, the compression is too great for the electron degeneracy pressure to resist it. As a result, through inverse beta decay, the electrons combine with protons to form neutrons. Now, neutrons are much larger than electrons, and, therefore, more massive, resulting in the density to increase considerably. However, the anti-compression pressure generated by these neutrons holds, and they are able to sustain quite happily as neutron stars.

But what happens if the star gets bigger?

Well, if the star exceeds 2.3-2.7 solar masses—which, incidentally, is called the Tolman-Oppenheimer-Volkoff limit—then the degeneracy pressure generated by the neutrons cannot sustain the intense compression, and the neutron star becomes a black hole. But that’s a discussion for another time.

Whew! That was a lot of science. But hopefully the makeup of neutron stars (and how they can exist in the first place) makes more sense.

Now, neutron stars not only have a large gravitational field, they also have a tremendously strong magnetic field. This can lead to two interesting types of stars (well, subtypes of neutron stars) to form: pulsars and magnetars.

Let’s start with the first. A pulsar is basically a rapidly spinning neutron star, with electromagnetic radiation emanating from the poles. 

 
Credit: NASA

But why is it spinning so fast? You may ask. And that is because of another fascinating physics concept: conservation of angular momentum.

Think of it this way. You are watching figure skating, and the skater is making graceful turns. Suddenly, the skater stretches out a leg and starts spinning. The skater pulls in a leg; suddenly, the spinning gets much faster.

 Credit:  Tenor

But why?

Well, conservation of angular momentum essentially states that initial angular momentum must equal final angular momentum. Angular momentum is calculated by multiplying mass, radius squared, and angular velocity. So when the skater brings their leg in, the radius decreases considerably, and in order to maintain momentum, the speed must increase.

Pulsars follow the same logic. They originate from slowly spinning, massive stars. But when the stars push out their outer layers, their radius becomes much smaller, and, therefore, their angular velocity increases, resulting in a swiftly spinning pulsar!

Pulsars have extremely precise periods between each “pulse”, or burst of electromagnetic radiation observed on Earth. This can make them important tools for establishing gravitational radiation, or even keeping time better than some atomic clocks; by measuring a simplified aspect of the frequency, one can determine a fixed time interval!

Magnetars are not as decorated as pulsars, but they are no less fascinating; they are neutron stars with immense gravitational fields, often exceeding a trillion times the magnitude of the Earth’s magnetic field. And even the most powerful magnetic fields ever generated in laboratories fall short, being 100 million times less powerful than the fields generated by magnetars. Here is a diagram that shows these types:

 Credit: NASA | JPL

Now, for the last item on our docket for today, let us discuss the role that neutron stars have had in proving the existence of gravitational waves, or waves through the space-time continuum, predicted by Einstein’s theory of relativity.

Essentially, the immense density of neutron stars is so immense that when they collide, they cause a tremendous emanation of gravitational waves that spread in all directions throughout the cosmos. One such collision was picked up by LIGO—the Laser Interferometer Gravitational-Wave Observatory—adding to the substantial proof of the existence of these waves. Here’s a quick gif that shows how gravitational waves are generated by colliding neutron stars:

Credit: University of Texas

Whew! That was a lot of information. But hopefully I’ve been able to foster a new appreciation for neutron stars, the long-lost and oft-overlooked siblings of black holes.

Stay safe, and clear skies!