Space objects are usually big. So big in fact that often we struggle to wrap our heads around them. But sometimes there are exceptions. And it’s not just asteroids and meteors. Even black holes can be small, although they are very massive. Neutron stars too.
The actual size of neutron stars has suffered from significant uncertainty. It was known that they had to be roughly the size of a large city but researchers didn’t have enough data to give a very precise value. Things changed last October. With the detection of gravitational waves from a neutron star collision, researchers could give a more precise estimate.
Using that data, the team thinks that neutron stars have a radius between 12 and 13.5 kilometers (7.5 to 8.4 miles). This is close to previous estimates but a lot more precise. The difficulty in getting this value is to do with gaps in our knowledge of these objects. We don’t know exactly how matter behaves under their huge gravitational forces. The findings are reported in Physical Review Letters.
"An approach of this type is not unusual in theoretical physics," lead author Professor Luciano Rezzolla said in a statement. "By exploring the results for all possible values of the parameters, we can effectively reduce our uncertainties."
"As a result," notes the press release, "the researchers were able to determine the radius of a typical neutron star within a range of only 1.5 kilometers [0.9 miles]: it lies between 12 and 13.5 kilometers, a result that can be further refined by future gravitational wave detections."
Neutron stars are one of the possible end scenarios for supernovae. After a star explodes, if it’s not massive enough to become a black hole, it will leave a dense core made of degenerate matter. This is a neutron star. A spoonful of neutron star weighs more than the Great Pyramid of Giza.
The theoretical scaffolding to explain this extreme state of matter is there but the lack of detailed observations stopped physicists from putting strong constraints on it. The goal is to obtain an equation of state for the matter in the neutron star, just like how we can describe gas, liquids, or solids. Currently, there are multiple equations that can explain what we see.
Another issue is that neutron stars can have twin solutions. The extreme density that neutron stars have can turn ordinary matter into something called quark matter. It’s a phase transition like going from liquid to solid, only weirder (for us). In this state, the mass of the star wouldn’t change, but researchers speculate that the quark matter is more compact, so some neutron stars might be lighter.