[Arrual (OP)]

What are the possiblilities of a neutron star destroying earth? A follow up question is Is it possible for three neutron starts to collide??

[RD]

What are the possiblilities of a neutron star destroying earth?

You may be referring to the gamma-ray-burst which precedes the creation of the neutron star ... http://en.wikipedia.org/wiki/Gamma-ray_burst#Rate_of_occurrence_and_potential_effects_on_life_on_Earth

[CliffordK]

There is also the Nemesis Theory, but to date, no large bodies have been found in an orbit outside of our known solar system.

As far as a close supernova, I don't think any stars are believed to be close enough to seriously affect life on Earth.  Some estimates is that a supernova would have to be within about 50 light years to cause serious damage, and there are no stars capable of going supernova that close.

I'm not sure how well defined the stellar orbits are around the Milky Way, so perhaps over millions or billions of years, the relative position of some of the stars could change enough to bring a large star closer to Earth.

[evan_au]

What are the possiblilities of a neutron star destroying earth?

If a neutron star wandered close by our solar system, it could well disrupt the orbits of the planets, potentially throwing Earth out of the habitable zone (too hot or too cold), and destroying plant and animal life on Earth. If a neutron star collided with the Sun, the resulting explosion could well sear the surface of the Earth. If a neutron star actually collided with the Earth, it would definitely destroy the Earth.

When the anomalous rotation of galaxies was discovered, there were theories that this might be due to a large population of free-floating planets or expired stars (a class in which we now include neutrons stars and black holes). Searches were made for the presence of such bodies, looking for occultation or gravitational lensing of stars. Most astronomers now think that there are relatively few such objects in the universe (at least, not enough of them to account for the rotation curves of galaxies).

So a neutron star certainly could destroy the Earth, but the space between stars is large, and the population of neutron stars is not thought to be large. So you have to place it in a similar category to other possible but unlikely events - you are far more likely to be killed in a road accident (or even by a lightning bolt), so watch when you are crossing the road, and don't fly a kite in a thunderstorm.

Meanwhile, data from the Kepler telescope should certainly be processed to detect non-recurring occultations that will put a tighter upper limit on the population of neutron stars in the galaxy. And astronomers should have at least one X-Ray telescope in space that is capable of detecting the X-Rays emitted by neutron stars. 

[evan_au]

Is it possible for three neutron stars to collide?

This is a variant of the classical "3 body problem", where the orbital paths can differ wildly, depending on the precise initial conditions.

One of those possibilities is that the three neutron stars could collide with each other at the same time - but this is so unlikely that you would almost have to set it up.

The most common result of three orbiting bodies with random initial velocities is that one is thrown out of the system, and the other two fall into a close orbit around each other. Closely-spaced neutron stars have been observed to radiate gravitational waves, so two closely-spaced neutron stars will gradually approach each other, and eventually collide.

Depending on the initial conditions, the ejected body may well return, and eventually collide with the large neutron star that was previously formed from the collision of two neutron stars.