A Weird Thing About Neutron Stars

The Universe is a strange an wonderful place. When a star goes supernova but doesn’t have enough mass to collapse into a black hole, it can turn into a very dense stellar remnant called a neutron star. A neutron star is only about 10 to 20 kilometers in diameter but has a mass about 1.4 times that of our Sun. This results in a star so dense that to get an equivalent density you could take a pack of 50 million elephants and stuff them into a thimble. This incredible mass causes a large gravitational field for such a small object, giving them quite extraordinary properties.

Of the four fundamental forces, gravity is by far the weakest. It’s a strange thought but every day you defeat the gravity of an entire planet simply by picking up a pen or doing a sit-up. And yet it’s this force we are most familiar with. It’s this force that tugs on us constantly. The force that shaped our planet, our solar system, our universe on the largest of scales.

Einstein described gravity in what may seem like a strange way. Rather than thinking of space an time a separate things, General Relativity describes them as one thing called spacetime. Einstein also showed that anything with mass causes a curve in spacetime. This can be hard to visualize but imagine a bowling ball sitting on a trampoline. Resting in the center, the bowling ball causes the trampoline to curve towards it. This curving is like what anything with mass does to the fabric of spacetime. The more massive the object, the greater the curve.

It’s this curvature that causes a very curious effect called a gravitational lens. Imagine having a soccer ball sitting in front of a ping pong ball. What can you see of the ping pong ball? Nothing. And you might think that it doesn’t matter what the two objects are, the father object will always be obscured by the closer one. But it turns out that if our soccer ball is instead a massive object such as a galaxy or even a cluster of galaxies and the ping pong ball is another celestial object, say a quasar or another galaxy, the light from the farther object will be bent around the closer one and we can see it! The most famous example is Einstein’s Cross (Q2237+030), four images of the same quasar decorate the edges of a galaxy.

So what does this have to do with neutron stars? Well they have such a strong gravitational field, that they act as their own gravitational lens. Light from the far side is actually bent around the star to the near side so an observer could see the north and the south poles and then some at the same time. Has your head exploded yet? Mine has.

Original Neutron Star Gravitational Lens image by Corvin Zahn, Physics education group Kraus, Theoretische Astrophysik Tübingen,Tempolimit Lichtgeschwindigkeit Sourced from Wikipedia