5 ms·

It would glow white, and due to relativity bending light, we’d be able to see the back.

ajross 2y ago
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 https://en.wikipedia.org/wiki/Color_temperature
russdill 2y ago
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 2y ago
  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-perfectly-smooth-metal-surfaces-3_fig1_236582531 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 2y ago
    > 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 2y ago
      Hot. The atmosphere might be only a few mm thick though.
      - dataflow 2y ago
        Ah gosh, that's not what I would call an atmosphere. Cool though! (well, hot)
        adastra22 2y ago
        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.
        deleted 2y ago
        [deleted]
        jiggawatts 2y ago
        I can second that book recommendation! Dragons Egg is one of the best hard science fiction books ever written.
        lazide 2y ago
        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 2y ago
  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 2y ago
Light blue, rather, https://johncarlosbaez.wordpress.com/2022/01/16/the-color-of-infinite-temperature/ https://johncarlosbaez.wordpress.com/2022/01/16/the-color-of... https://news.ycombinator.com/item?id=29962625 https://news.ycombinator.com/item?id=29962625

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 https://en.wikipedia.org/wiki/Color_temperature

russdill 2y ago

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 2y ago
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-perfectly-smooth-metal-surfaces-3_fig1_236582531 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 2y ago
  > 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 2y ago
    Hot. The atmosphere might be only a few mm thick though.
    - dataflow 2y ago
      Ah gosh, that's not what I would call an atmosphere. Cool though! (well, hot)
      - adastra22 2y ago
        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.
        deleted 2y ago
        [deleted]
        jiggawatts 2y ago
        I can second that book recommendation! Dragons Egg is one of the best hard science fiction books ever written.
        lazide 2y ago
        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 2y ago
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 2y ago

Light blue, rather, https://johncarlosbaez.wordpress.com/2022/01/16/the-color-of-infinite-temperature/ https://johncarlosbaez.wordpress.com/2022/01/16/the-color-of... https://news.ycombinator.com/item?id=29962625 https://news.ycombinator.com/item?id=29962625