Question: what is the smallest star in existence?

Evan Keane answered on 21 Jun 2011:

That’s a great question! The official answer is that neutron stars are the smallest stars in existence and they are about 20 kilometres in diameter (black holes are smaller but aren’t stars). BUT there might exist a smaller type of star – which is called a quark star. These would be more like 2 kilometres in diameter. I spoke before about “neutron degeneracy pressure” (here: http://ias.im/44.383) and how you can squeeze neutrons very tightly together but it is possible to squeeze them too tight. If you do this then the quarks which are inside the neutrons (each proton and neutron is made up of 3 smaller particles called quarks) will MAYBE (we don’t know for sure because we can’t do it in labs on Earth) break out and we will get a much smaller star made out of “quark matter”. If this does not happen and quark matter is not possible then squishing a neutron star will just give you a black hole.

We have never found a quark star but maybe we will. For example if we found a pulsar which was spinning more than 1000 times per second it could not be a neutron star as neutron stars would break apart if they spun this fast. The fastest pulsar we know about spins 716 times per second so we don’t need to look that deeper to be able to search the speeds of where quark stars would exist!

So the answer is that neutron stars are the smallest stars in existence that we know about, but there is a possibility that there are things called quark stars which might exist, and if they do they would be the smallest!

Very advanced question! 😀