Neutron Stars
2–14–17, Adil
Today I’d like to take a long overdue adventure back into outer space, where we will visit one of the coolest things in the universe: neutron stars.
Neutron stars are the densest objects in the universe (excluding the center of black holes, which theoretically have infinite density). How dense, you ask? Even more than Davis.
Neutron stars typically have a radius of about 10–20 km, but a mass of about two to three solar masses (2–3x mass of our familiar sun). Quick reminder of how heavy the sun is: about two nonillion kg (yes, that’s a real number, as in between octillion and decillion), or 2 x 10³⁰ kg. That’s the same as about 333,000 Earths. Though solar masses are more scientific for talking about this mass scale, for plebes like me, it’s at least slightly more intuitive to use Earth as a reference because the sun is still too damn big. So, a neutron star can quite reasonably have a mass up to about one million Earths, but in a radius of 10 km.
So you’re thinking “okay, that’s pretty dense, but I still can’t really get a feel for this magnitude.” I agree. As with most space facts, it’s impossible to really appreciate this scale, but here’s another analogy: a cubic centimeter of neutron star matter (related to the idea of “neutronium”) would weigh about one billion tons. This is the same as Mount Everest. Or, in other words, if you took 250 million African elephants and squeezed all their mass into the volume of a sugar cube, you would get close to the density of a neutron star.
Woof. But okay, now that we know neutron stars are dope, let’s take a step back and talk about where they come from. A neutron star is actually part of the star death process for large stars, weighing in at about 8–29 solar masses (this range varies a little across sources). When these stars die, they result in a supernova explosion, but also a gravitational collapse which causes the core of the star to compact into this super dense hoopla. If the post-explosion remnants end up with a mass greater than the limit of about three solar masses, you get a black hole (topic for its own FF) instead of a neutron star. There should be about a billion neutron stars in our galaxy (though astronomers have found <2,000 far).
If you harken back to Physics 101, you’ll appreciate the conservation of angular momentum — the new neutron star has to keep the angular momentum of the OG large star. This has awesome consequences for rotational speed in neutron stars. Just as a figure skater makes her body smaller to spin faster, so does our neutron star — it will make at least 60 and up to 600 full rotations every second. This latter speed happens more easily in binary systems, which we’ll come back to.
Let’s continue smelling the roses on this walk back through elementary physics. We also conserve magnetic flux — if the original star had X magnetic flux over a given surface area, it must keep X magnetic flux even when the surface area dramatically shrinks with the creation of the neutron star. So, you get a ridiculously strong magnetic field as well. On the surface, the magnetic field strength has been estimated at between 10⁴ and 10¹¹ Tesla (admittedly huge range). Compare this to Earth’s paltry 25 to 65 microteslas Medium won’t let me put in a negative exponent for scientific notation, sorry fam). This means that the magnetic field on the surface of a neutron star is at least 100 million times higher than that on Earth’s surface; one source says that the average neutron star’s magnetic field is several trillion times stronger than that of the Sun.
One subset of neutron stars has an even stronger magnetic field, and an even cooler name: the magnetar. A magnetar is defined by this stronger magnetic field and slower rotational field — only completes a rotation once every 1–10 seconds. The magnetic field is so strong on magnetars that it can distort the shapes of atoms themselves. For this reason, magnetars are unstable and don’t last too long — a mere 10,000 year lifespan, which is a blink on the cosmic scale. But while it’s alive, it’s home to one of my favorite cosmic phenomenon, which has a name that sounds like it came straight out a Super Mario video game — the starquake. With the ridiculous gravity and rotational speed on magnetars, the magnetar can actually snap its own crust. This idea is a bit similar to an earthquake, but the repercussions for a starquake are mind-blowing: if the crust of a magnetar moves even a centimeter, it can result in massive explosions, which in turn trigger magnetar flares. These are akin to solar flares, but as you guessed, muuuch more intense… in fact, a few trillion times more intense. In a fraction of a second, the mighty magnetar can release more energy than the Sun can in a quarter of a million years.
In 2004, satellites orbiting Earth were blinded by a huge blast of X-ray radiation, which did indeed come from a magnetar flare. Where did it come from? We pinpointed it to a starquake on magnetar SGR1806–20… halfway across the galaxy (about 50,000 light years away).
Speaking of dope radiation, lets talk about pulsars. Really fast spinning stars like our friend the neutron stars plus their less cool buddies the white dwarfs (approximately 1 solar mass in the volume of Earth) are also known as pulsars because they look like they pulse with electromagnetic radiation. They aren’t really pulsing, though; it just looks like that because of the spin. Imagine a spinning night on a lighthouse at night. Now, imagine that you can’t see the light beam unless the beam directly falls in line with your eye. Effectively, you only see the beam of light for a split second, once every rotation, which creates a “pulsing” effect. This is what happens with pulsars and their narrow beams of radiation, and in fact this is how we detect the presence of neutron stars. This pulse is quite stable and predictable, so pulsars can actually be used as pretty good cosmic clocks. Some pulsars are actually part of binary systems (told you I’d come back to this), meaning that there are 2 stars orbiting each other. Effectively, the stars feed off of each others energy and gravity, causing each other to spin even faster (this is how you get neutron stars spinning hundreds of times per second).
In case you now want to go visit a neutron star, a quick reminder that, even if it were physically possible to stand on the surface of a neutron star (it’s definitely not), you would weigh about 100 billion times more than you do on Earth, so you wouldn’t be going anywhere (with this gravitational strength, the escape velocity is about half the speed of light).
Plus, you would be vaporized by the heat: a newly formed neutron star is a ludicrous 10¹¹ to 10¹² K (the sun is ~5,700 K, lol). Aha! You think you’ll go after a few years when it cools down to a cozy 10⁶ K because of the huge number of neutrinos it emits? Well, even at this temperature, most of the radiation it emits is X-rays, so you’d get zapped by those and your DNA would likely mutate to the point that you become a banana.
Sources:
htps://www.nasa.gov/mission_pages/GLAST/science/neutron_stars.html
