Ch 14.2a: What is a neutron star?

Hello everyone, in this mini-lecture we're going to talk about neutron stars. Neutron stars are the dead remnants left over from the supernovae of stars that were initially 8-20 solar masses. Here's a picture of a dead star, and the white dot at the center right there is the neutron star. The stuff you see around it is gas that was blown off in the supernova. This is a composite image showing gas emitting visible light in red, orange, yellow, and green, and very hot gas emitting X-ray light in blue.

To understand neutron stars, we're going to talk about degeneracy pressure, the musical chairs of subatomic particles, because it shows up again in neutron stars. In the mini-lectures on brown dwarfs and white dwarfs, we mentioned two principles of quantum mechanics, which you don't have to understand in detail, but you should know the general idea. The first is the exclusion principle: it says that two particles in the same position cannot have the same momentum. The other is the uncertainty principle: a particle's momentum and position cannot both be specified with infinite precision. We've already learned that these principles are true for electrons. They give an atom its size, and they even give a white dwarf its size. The fact that these principles also are true for protons and neutrons mean that they determine the size of a neutron star.

It's the degeneracy pressure of neutrons that is supporting a neutron star against gravity and keeping the thing in gravitational equilibrium. As a reminder, when a massive star dies in a supernova explosion, its core collapses, and the electrons there combined with protons in the core to make neutrons. So that's why the electron degeneracy pressure went away, because the electrons themselves went away and left neutrons.

Neutron stars aren't exclusively neutrons, but they're made up of the nuclei of neutron-rich isotopes of very heavy elements, so for now, we can think of them as a ball of neutrons. In this diagram of a neutron star interior, I want to point out that there are some question marks over here next to labels for what could be in the core. Neutron star matter is an active area of research that people at MSU in astronomy & astrophysics and in nuclear physics are investigating. One funny thing is that these super heavy, super neutron-rich nuclei no longer look like spheres, like we’re used to, but under the immense density and pressure inside neutron stars, the nuclei get stretched and squished into what’s called nuclear pasta. The nuclear pasta and the layers of a neutron star aren’t on the quiz, they’re just really cool. And yes, we do actually refer to them as spaghetti, lasagna, and gnocchi. Since neutrons are a lot more massive than electrons, it means that a ball of mostly neutrons, held up by degeneracy pressure, can be a lot smaller than a white dwarf that’s held up by electron degeneracy pressure. Remember how white dwarfs were about planet-sized?

Well, a neutron star is city-sized! This diagram shows how a neutron star is about the size of greater Lansing, about 20-25 kilometers in diameter, which is about 15 miles across. MSU campus is right about here. This size is downright tiny compared to other astronomical objects like planets and stars! What's most impressive is how much mass is crammed in to this 15 mile-diameter sphere. The typical neutron star contains about 1.5 to 2 solar masses. This mass, paired with its size, gives neutron stars their incredible density.

To give you some perspective, one cubic centimeter, so like, the size of a marble, of the core of a neutron star has as much mass as all of Mount Everest, around 10^15 grams. Another way to think about the density of this matter is in terms of people. If you were to take every human being on the planet, and imagine not only squishing them into the same country, or same city, or same room, but down into 1 centimeter cubed, that would have the same density as the core of a neutron star. In the next few videos we'll learn how neutron stars also have magnetic fields thousands of times stronger than anything that's ever been created in a lab on Earth, and how some of them spin around on their axis faster than a blender. So, neutron stars are absolutely wild, more than anything we've ever encountered here on Earth or in the solar system. In the next mini-lecture, we'll talk about how neutron stars were discovered, and in the lecture after that, you'll learn how neutron stars are kind of like zombie stars.