[Eric A. Taylor (OP)]

We can only see a neutron star if it's a pulsar and we can only see a pulsar if it's beam happens to strike the Earth. Like the beam of a lighthouse, from a ship at sea it's visible for miles, but from an airplane overhead it would be invisible just feet away.

I just saw a very good Outer Limits ('90's version) Inconstant Moon, where a scientist (they never say weather he's an astronomer or a physicist) sees the moon become much brighter and concludes the sun has suddenly gone nova. I'm wondering what might cause a main sequence solar mass star to go nova and I think a collision with a neutron star might do it. I know it's unlikely that the neutron star would crash directly into the sun but I thinks it's possible. What I'm not sure of is if it's possible for a neutron star to get into the solar system without being detected. It would take several years to pass from say Neptune's orbit to the inner solar system (and may take months to pass from Earth's orbit to the surface of the sun. As it passes each planet's orbit it need not pass close to any planet (also unlikely to pass 8 planets without effecting there orbits) So we may only be able to depend on it's light to see. Neutron stars are really really tiny so even if it's bright it may not give enough light to be seen from afar. Could this happen or am I off base?

Please note I have not given the ending away here. Please follow my lead in replies if you've seen the show. Or right SPOILER WARNING at start of reply.

[LeeE]

The short story called 'Inconstant Moon', with the plot you describe, was written by Larry Niven, and a good story it is too.  I've not seen that particular Outer Limits show but I assume they've kept the same ending (which I'll not give away either).

I think the idea of a neutron star passing through our solar system is possible, but because of the huge scale of astronomical distances I think it's very improbable.  There's also the issue of the neutron star acquiring the real speed necessary to travel from where it formed to our solar system.

Neutron stars are reckoned to be, in general, between 1.35 and 2 x the mass of our sun, so if one did approach our solar system I reckon we'd know about it pretty quickly.  In fact, our whole solar system would start accelerating towards it once it got near enough.

I've no real idea how the sun would behave as a result of colliding with something only ~12 miles in diameter, but with a greater total mass that itself, but I can't imagine it would be very good from our point of view.

I think a lot would depend upon how fast the relative speeds are, but if they were relatively low it's possible that as the neutron star started feeding on Sol it become a pulsar.  If it was approaching at very high speed i.e. a good few percent of 'c' it might just pass through but with a brief and very powerful burst of x-rays.

All just guesswork there though.

[litespeed]

I located a computer collision symulation of a 1.4 mass Neutron Star with a .9 solar mass sub giant star.  Ultracompact X-ray binary formation... http://webpub.allegheny.edu/employee/j/jalombar/movies/

Not much narative, but it looks like a very bad day on planet earth. The only good thing is it would not Sneak Up on us!

[Eric A. Taylor (OP)]

All the stars in the galaxy are moving around. They get closer together and farther apart but in our region stars are on average several light years apart.

How close would a neutron star need to be to be seen from Earth if we don't see the beams?

If we did see once coming, what could we do about it?

[litespeed]

Here is a very good URL on neutron stars.  http://www.astro.umd.edu/~miller/nstar.html