Re: Two neutron stars on a collision course?

On 1 m=E4rts, 14:14, Tim Little <t...@[EMAIL PROTECTED]
> wrote:
> On 2008-03-01, Matt Browne SFW <matt.h.bro...@[EMAIL PROTECTED]
> wrote:
>
> > What exactly will happen when two neutron stars are on a collision
> > course? I think a supernova and black hole is inevitable if both
> > neutron stars are very heavy. But what about smaller ones?
>
> I was going to post about how neutron stars have a minimum mass that
> would guarantee a black hole.  But being not entirely sure of that I
> looked up a few recent articles, and to my surprise found one that
> calculated the behaviour of a neutron star barely under the modeled
> minimum stable mass, just 0.189 solar masses!
>
> So it seems neutron stars could at least theoretically be small enough
> to avoid black hole formation upon collision.
>
> > Will they merge? Or be torn into thousands of "pieces" from the
> > impact? Another theory is gamma ray bursts with huge amounts of mass
> > converted into energy. Expert opinion?
>
> I'm certainly not an expert, but with that amount of mass having that
> much kinetic energy upon colliding, I'd expect at least the equivalent
> energy release of a small supernova.
>
> Total fragmentation couldn't happen even theoretically unless they
> were merging at a decent fraction of lightspeed initially - the
> gravitational binding energy is far too great for that.  Actual mass
> conversion to energy is unlikely, but it could dissipate quite some
> fraction of the kinetic energy built up as they fall into each other,
> which would not be a negligible fraction of their rest mass/energy.
>
> I think a gamma ray burst would be inevitable.  I don't know even
> within an order of magnitude what fraction of the total energy it
> would include.
>
If the result is a neutron star, all the energy must eventually be
radiated away. On the other hand, if a black hole forms, it swallows
energy - does any energy escape?

The size of a neutron star is in the order of 10 km. The speeds are on
the order of 100 000 km/s. So, the collisions lasts in the order of
100 microseconds.

The neutrons in the torn-out pieces decay over a period of 1000
seconds. Some beta-minus radioactive nuclei decay faster, but even the
least stable of nuclei short of neutron dripline - like helium 6 or 8
- have halflives in the region of 100 milliseconds. 1000 times longer
than it takes to rip the neutron star apart.