Expert: James Gort - 5/4/2011

Question
Hello Sir,
I want to know that what will happen if Two Neutron stars or Two Black holes or a Neutron star and a Black hole come close to each other. Will they Start orbiting each other or There will be a masiive Explosion?
Thank You.

Answer
Hello Ishaan,

If two neutron stars, two black holes, or a neutron star and a black hole came close to each other (but not directly at each other), depending on their relative speed and closest approach, they could capture each other and go into orbit.  Again depending on the size of the orbit, they could eventually lose energy (from tidal distortions) and combine.  If the two objects approached each other "head-on" (a very rare event), they would collide and combine into a more massive object.  The only time there would be an "explosion" might be if both objects were neutron stars.  In that case, the combined object might exceed the "Tolman–Oppenheimer–Volkoff limit", and collapse in a massive explosion (supernova), eventually forming a single black hole.  If only one object was a neutron star, there would be x-rays and gamma-rays as the neutron star approached the black hole, before it is sucked in.  If both objects were black holes, there would be little observational evidence (other than a larger black hole formed).

There are observed cases of orbiting black holes and orbiting black holes and neutron stars (these are often strong x-ray sources).  See http://www.redorbit.com/news/space/1649441/rare_black_holes_found_orbiting_each_...

Hope that helps.

Prof. James Gort