# What does a neutron star look like

1. Jun 6, 2005

### gonzo

What does a neutron star "look" like

Hi, I have some questions for a scifi story I'm working on. First, what would a neutron star look like? What color are they, and how bright do they tend to be? By look like, I mean both seen from a theortical planet surface orbiting one (or mabye it would always have to be far enough away that it would just be a point since they are so small?). And from a spaceship flying through the system looking out a window?

In other words, how luminous and what color are they?

The other question is with a binary star system. If you wanted to have a planet in orbit in a binary star stystem that had roughly the same conditions as Earth (size and temperature), would the planet have to orbit around both stars? Is this a assumed to be a general property of binary stars, that sattelites are in a large orbit around both, or would some paths take the bodies in between?

For that matter, would the orbital plane of a solar system around a pair of binary stars likely be in the same plane as both stars? Or would it be around the center of mass between them? Or does EVERYTHING just orbit around one of them, with the second star acting more like a very large planet? Or something else entirely?

And finally, how close are the two stars in a stable binary system generally?

Thanks!

2. Jun 9, 2005

### SpaceTiger

Staff Emeritus
A quiescent neutron star (meaning without pulses or accretion) would be a nearly perfect sphere (much more perfect than the sun) and would appear very blue to the human eye. The ones we observe typically have temperatures of around 106 K, but we can only see the youngest ones (ages < 106 years). Why? Well, if you approximate the neutron star as a blackbody and consider that their radii are typically around 10 km:

$$L=4\pi R^2\sigma T^4=0.18\ (\frac{T}{10^6\ K})^4\ L_{sun}$$

This means that even an extremely hot neutron star is only about a tenth as bright as the sun. To top it off, most of this light output is in the ultraviolet and X-rays, so they're hard to find with optical telescopes.

The reason we only see them when they're young is that they cool over time. In fact, we think that after only 10 million years (a short time on cosmic scales) they'll cool to 105 K. You can see from the above equation that this will bring their luminosity down by about a factor of 10,000.

There should be no real difference between these two scenarios. Neutron stars are dense enough that you would probably see some gravitational lensing effects on the background stars and galaxies, but for the most part, they would just look like tiny, bright spheres.

For simple stability of the planetary orbit, there are a lot of configurations that would work. If you're looking to support life, then you'd probably have a lot more difficulty setting this up. If the distance between the two stars were comparable to the distance between stars and planet, then you would get extreme seasonal variations that I would think would be unsuitable for life. I think your best bet is to put the stars really far apart (like beyond pluto's orbit) and the planet close (like earth distances) to one or the other. I think it would be interesting to devote a new thread to this issue.

Yes, unless one of the stars was captured. If they were formed in the same protostellar cloud, then their angular momentum vector should have the same direction. If one star is much more massive than the other, then the system would be similar to the solar system, with the less massive star orbiting like a giant planet.

A two-body system is always stable. Binaries tend to be distributed evenly in log space, meaning that there are about as many binaries with separations of 0.1 - 1 AU as there are for 1 - 10 AU as there are for 10 - 100 AU, etc.