The colour of a neutron star?

Algr

IMHO, all this stuff about black body radiation and surrounding matter is misreading the question. If I asked you "what color is iron" you would not say "Orange" even though orange light does emit from iron at certain high temperatures. "Fresh iron surfaces appear lustrous silvery-gray"- Wikipedia.

The original post asks "What is the colour of pure neutrons confined together?". In other words, Neutronium. As far as I understand it, "Transparent" seems to be the most favored answer here.

chill_factor

It would not be transparent. The surface is not purely neutron; the pressure is lower at the surface than at the interior. Instead it is made up of nuclei and electrons. In fact throughout the entire neutron star there are still electrons and nuclei, just getting less and less as you get closer to the core. A likely color is gray or black since that's what nuclei + free electrons in an actual material looks like. Most solids are after all just this.

ImaLooser

Well, if you lived somewhere where it was normal for iron to be at that temperature then you would say "orange." The surface of a young neutron star is typically a million degrees, so the natural answer would be bluish white, with lots of X rays, and gamma rays from infalling matter. To me it seems forced to wonder what the surface of the star would look like at "room temperature," but if you want to know....

The astronomers tell us that there is an atmosphere of carbon, probably only a few millimeters thick. There could be an "ocean" of liquid carbon too, though possibly very shallow. So if that was cooled down to ambient Earth temperature it would be ... well, it would depend. If there was the huge pressure of the star you would get diamond. If this were done on Earth you would have black graphite.

So a very old neutron star -- they last forever, as far as anyone knows -- might cool down enough to have a thin surface of diamond. Cool, huh? Or the nuclei might fuse. I dunno.

Algr

If you lived somewhere where it was normal for iron to be at that temperature then you would say "Aaaaa!" and then die.

All materials have the same black body radiation at a given temperature so it makes no sense to describe black body radiation as being a property of the material.

Dmitry67

Lets get rid of the complicated but unrelated stuff like high temperature, iron crust etc.

Assume that strange matter hypotesis is true, so small macroscopic chunk of strange matter is stable. What color reflects that chunk (if it is very cold - 20C)?

Jarfi

Yeah, seems like 70% of the responses here didn't read my question... or they keep saying, durr well yea there are electrons too, yeah I aknowledged that, that was the 1st question, the second was how pure neutronium theoretically behaves alone, in great pressure, and how it interacts with light and such.

Vanadium 50

There is no such thing as pure neutronium. If you could somehow get some, it would quickly produce protons and electrons through neutron decay. So this becomes a bit like "what color would gold be if it weren't gold".

Additionally, there seems to be a misconception that light cannot interact with a neutron. That's not true - it can interact magnetically.

Jarfi

hmm, there was stated above in this thread, that neutronium exists pressurized in the core of the neutron star, and is mostly neutrons, also if you'd shine a light on it just before it's half life ensue

Algr

I disagree. For an object to have a definable color, it need only exist long enough to reflect a lone wavelength of red light. (The longest visible to humans, and thus defining "color") Free neutrons have a half life of about 10 minutes. Roentgenium has a similar half life and a predicted color. (Silver)

Edit: Also, is it safe to assume that the half life of neutronium is the same as that of a free neutron?

ImaLooser

Neutrons in an atomic nucleus have a much longer half life than that. The core of a neutron star is like a huge atomic nucleus, so the half life should be much more than a free neutron.