[HN Gopher] An equation of state for dense nuclear matter such a...
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An equation of state for dense nuclear matter such as neutron stars
Author : rbanffy
Score : 40 points
Date : 2025-03-03 12:29 UTC (4 days ago)
(HTM) web link (phys.org)
(TXT) w3m dump (phys.org)
| pavel_lishin wrote:
| > _Neutron stars present other challenges besides a humongous
| density. Their small size makes them impossible to study visually
| with telescopes, as they appear no more than a point. (The
| nearest neutron star to Earth is 400 light-years away.)_
|
| Do we have an idea of what they would look like to the naked
| human eye?
| hyperhello wrote:
| It would glow white, and due to relativity bending light, we'd
| be able to see the back.
| ajross wrote:
| Blackbody radiation (which even neutron stars will
| approximate) trends towards a pastel blue as temperature
| approaches infinity, actually. There is always a slope in the
| spectrum in the visible range as the peak is always to the
| UV/Xray side for very hot objects. "White" light requires
| that the spectrum be flat in visible wavelengths, which
| basically requires that the peak be in the visible, which is
| true for objects at about 6000K.
|
| As it happens, the sun is such an object. And it more or less
| makes sense that we have eyes tuned to pick up the dominant
| wavelength arriving from our local light source.
|
| Edit: the wikipedia page has a great animated chart of this
| effect, showing what the visible-only spectrum looks like as
| temperature changes:
| https://en.wikipedia.org/wiki/Color_temperature
| russdill wrote:
| I think what people are intuitively asking is not what would
| neutron stars at their current stage of stellar evolution
| look like. It's, if you could look at it what would it look
| like.
|
| Far in the future once neutron stars have cooled, what would
| they look like? If you shone light on it, would it absorb it?
| Reflect it? Etc
| jiggawatts wrote:
| Some of the oldest neutron stars may have already cooled to
| ordinary temperatures of just a few hundred Kelvin.
|
| It's not certain what their surface would be like, physics
| just isn't developed enough to provide a definite answer.
|
| The trivial case of an isolated cold neutron star is:
| completely black, other than a very slight far infrared
| glow of whatever their remaining thermal radiation is.
|
| An educated guess is that an externally illuminated cold
| neutron star would look extremely smooth and "dark grey".
| Most current models propose that young and hot neutron
| stars have an atmosphere of metal plasma. Presumably this
| would freeze out into a thin surface layer as the stars
| cool.
|
| A complication is that their surface gravity is so high
| that inbound "white" light would be shifted well into the
| UV range, which would interact with the surface very
| differently to visible light. Reflected light would be
| redshifted back out to the original white, but any light
| that's absorbed and re-emitted would likely take on
| interesting colours.
|
| Here's a chart of metal surface reflectivity by wavelength:
| https://www.researchgate.net/figure/Spectral-reflectivity-
| of...
|
| As you can see, most metals don't reflect much in the UV
| range, but there are peaks and troughs.
|
| Depending on the exact surface composition, you might see
| different colours than what you'd normally expect for
| metals.
|
| You'd also have to take the rotation rate into account. If
| there's any inconsistency in the surface albedo, you'd see
| "horizontal stripes" for fast-spinning stars, or flickering
| for slower spinning ones.
| dataflow wrote:
| > Most current models propose that young and hot neutron
| stars have an atmosphere of metal plasma.
|
| How could any atmosphere exist in gravity that strong?
| adastra22 wrote:
| Hot. The atmosphere might be only a few mm thick though.
| dataflow wrote:
| Ah gosh, that's not what I would call an atmosphere. Cool
| though! (well, hot)
| adastra22 wrote:
| Well, it's a neutron star. Mt Everest on a neutron star
| might be 3 mm tall. Distances don't really mean the same
| thing though as matter is compressed down below the Pauli
| electron repulsion regime into direct nuclear contact.
|
| Dragon's Egg is a great sci-fi book that explores what
| life on such a world might be like.
| jiggawatts wrote:
| I can second that book recommendation! Dragons Egg is one
| of the best hard science fiction books ever written.
| lazide wrote:
| My analogy would be -
|
| Normal matter is like expected personal space in Finland.
| Everyone maintains a respectful distance, and they like
| it that way. Sometimes no one sees another human up close
| for _days_ , let alone actually does anything resembling
| 'touch'.
|
| Neutron stars are like a giant coked up swinger orgy
| pile. Sometimes you can't even tell where one person
| begins, and the next one ends, and certainly no one cares
| much anyway.
| ars wrote:
| It would probably shine like a blackbody based on whatever
| temperature it is.
|
| The surface of a neutron star is covered with a thin shell
| of protons and electrons in essentially random
| configurations, so it would absorb and emit light in all
| sort of random frequencies.
| perihelions wrote:
| Light blue, rather,
|
| https://johncarlosbaez.wordpress.com/2022/01/16/the-color-
| of...
|
| https://news.ycombinator.com/item?id=29962625
| nightfly wrote:
| Wiki says they are very hot so probably glaringly bright white
| dots
| OgsyedIE wrote:
| Answered here AFAICT:
|
| https://www.reddit.com/r/spaceengine/comments/15d3xwd/how_co...
| aeonik wrote:
| One thing that I found out recently that blew my mind, is
| according to gravity calculations, neutron stars bend light
| almost as much as black holes, but because they still emit
| light from their surface, this means you would be able to see
| basically the entire star at once.
|
| I.e. You would be able to see the front and the back of the
| star simultaneously. It looks like a weird morphing Mercator
| projected map.
|
| Here is a random video showing the idea, I have no idea how
| precise the simulation is though.
|
| https://youtu.be/9H4NezwJ_ak?si=MV2V6AaG92tu-17n
| mystified5016 wrote:
| Neutron stars are 4D objects embedded in 3D space /j
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