The physics of neutron stars are almost too fantastic to believe. Something the weight of two Suns compacted to a sphere the size of a city. Each teaspoon of its material would weigh billions of tons. If youve done any reading on the topic, youve heard these facts before. But despite the intense interest these extreme objects hold, we are still actively learning lots about them. One of the most pertinent outstanding questions is where is the line between becoming a neutron star and becoming a black hole when a star dies. A new paper by researchers at the HUN-REN Wigner Research Centre for Physics in Hungary describes what they believe to be a definitive answer to that question - between 2.2 and 2.3 solar masses.