# What happens when a neutron star becomes a black hole?

Suppose you have a neutron star that's as close as can be imagined to the required mass to become a black hole, perhaps just one proton mass away from this limit, when it collides with a dust grain from space.

Is the collapse instantaneous? Is the event horizon formed in the center and moves outward as more mass gets pulled in? Are there gravitational waves as a result of the sudden shift in the spacetime geometry?

Any details that are known about such an event would be greatly appreciated! Thank you

Edit: As requested I'm limiting my question to be more specific to what most interests me - Does the event horizon form at the center of the neutron star? Is there a specific "initial size" that grows?

• Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer.
– Community Bot
Commented Jun 21, 2022 at 6:48
• I will note that you probably mean "apparent horizon" and not "event horizon", as the event horizon is a topological feature whose development is very deeply intertwined with what will happen in the future, and event horizons can even form in flat regions of spacetime. Commented Jun 21, 2022 at 14:55
• The asymptotic horizon first forms where the gravitational potential (time dilation) is extremal, which is at the center. Then the spacelike radius of the horizon remains zero, but its circumference increases. Whether or not you call this "growing" depends. Commented Jun 22, 2022 at 5:07
• @safesphere How do you define the spacelike radius of the horizon?
– JanG
Commented Feb 23 at 7:36
• @JanG The existence of the inner singular spacetime is an assumption. The inner solution only shows what it would have been, if it existed. Like that the sides of my one square meter coffee table would be minus one meter, if negative sides existed in reality, but they don’t. In a realistic collapse, the inner spacetime never forms, so there is nothing inside ever as far as the universe is concerned. And if you jump in, they say you can cross to some weird brief temporal sideway extension of spacetime located beyond the cosmological eternity. Even if unlikely true, this doesn’t affect anything. Commented Feb 25 at 6:10