Originally posted by VanderL
@May 14 2004, 08:32 PM
Thanks for the article, but the article is based on the fact that a neutron star (and it's high density) is a reality. Could you explain (or point to links of) what solid evidence exists that these objects are truly as dense as is claimed?
We've talked about this before, and I didn't convince you, and probably won't now.
First, the article is an overview of what we know about neutron stars, and I thought it was pretty fair about including information about what isn't known or fully understood [hence Dr. Manuel has already quoted it to his benefit]. It also was a good place for people who aren't really up on what a neutron star is to see what mainstream astronomers believe [It is not simply a ball of neutrons].
Second, as to the actual measurement of the size of neutron stars, the maximum frequency of the pulses of the millisecond pulsars is usually used as an indicator of the maximum size they can be. We do know what the mass is for a few of them pretty accurately. Your argument last time was that there could be some unknown mechanism other than rotation which could produce the highly precise pulses. I certainly can't say there ISN"T an unknown process that does this. If we throw out the assumption that the pulses are rotational, we still have the theory about how nucleons interact in relativistic gravitation and pressures.
A somewhat indirect method involves taking the Boltzman temperature of the object, knowing the distance and computing the surface brightness. This will tell you the total surface area of the object. Using this method Geminga is about 25 km in diameter. You can claim that an unknown process is causing a Boltzman-like spectrum, or that only a portion of the object is luminous.
There are some possibilities for future efforts to find the size of a neutron star more directly. Mostly this would involve some serious interferometry efforts looking at Geminga. Geminga is about 500 lightyears away [give or take 100 lightyears]. So lets call it 5e18 meters away. If it has a disk about 3e4 meters across we'd need an interferometer about 2e14 wavelengths wide to see it. If were using 80 nm hard uv to look at it, we'd need an interferometer 16,000 kilometers across to resolve the disk. If we look using 1 nm xrays, it'd need to be 200 km across. Perhaps one of these will be possible by the middle of the century. I expect that this imaging would show enough details of the space around it that you could not claim that the object was really much larger and only a small part was luminous.
Forming opinions as we speak