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Color of neutron star?

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d...@temple.edu

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Mar 11, 1998, 3:00:00 AM3/11/98
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As I understand it, it's the electrons that give objects color. So what
color would a neutron star be?

Dan
in Philly

john baez

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Mar 11, 1998, 3:00:00 AM3/11/98
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In article <3506D3...@temple.edu>, <d...@temple.edu> wrote:
>As I understand it, it's the electrons that give objects color. So what
>color would a neutron star be?

Highly conductive things are usually reflective: the electric field must
be perpendicular to the surface of a highly conductive object, so -
thanks to the mathematical wonders of wave equations with boundary
conditions - electromagnetic waves bounce off! This is why most metals
look silvery. (Why do copper and gold look different? For gold,
check out the physics FAQ! - I don't know about copper.) The neutronium
in the interior of neutron star is actually a superconducting superfluid
mixture of neutrons, protons and electrons, so it would probably be
silvery. However, I don't think the crust of a neutron star is made
of neutronium - I believe it's made of elements like iron. I don't know
what color it is. I'd guess something metallic-looking.


Paul Arendt

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Mar 11, 1998, 3:00:00 AM3/11/98
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Dan in Philly wrote:
>As I understand it, it's the electrons that give objects color. So what
>color would a neutron star be?

Short answer: same color as a mirror (reflective).

Longer answer: Neutron stars are not pure neutrons; the outer crust
is probably crystalline iron and other nuclei, while deeper you find
neutrons and protons (in a superfluid state, making the inner neutron
star a superconductor). There are electrons throughout a neutron
star; it would appear metallic.

(It is unclear just what lies at the center of a neutron star; we
don't know how to solve QCD well enough. There are probably copious
strange quarks, but we don't know if the quarks are grouped into
hadrons, or if they make "quark soup.")

Mac Almy

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Mar 11, 1998, 3:00:00 AM3/11/98
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On 11 Mar 1998, john baez wrote:

> In article <3506D3...@temple.edu>, <d...@temple.edu> wrote:
> >As I understand it, it's the electrons that give objects color. So what
> >color would a neutron star be?
>

> Highly conductive things are usually reflective: the electric field must
> be perpendicular to the surface of a highly conductive object, so -
> thanks to the mathematical wonders of wave equations with boundary
> conditions - electromagnetic waves bounce off! This is why most metals
> look silvery. (Why do copper and gold look different? For gold,
> check out the physics FAQ! - I don't know about copper.) The neutronium
> in the interior of neutron star is actually a superconducting superfluid
> mixture of neutrons, protons and electrons, so it would probably be
> silvery. However, I don't think the crust of a neutron star is made
> of neutronium - I believe it's made of elements like iron. I don't know
> what color it is. I'd guess something metallic-looking.
>
>

Current theories also predict that the surface temperature is on the
order of 1 million Kelvin for the first 10,000 years. That's of course
a flea bite out of the life-time, but the text I checked didn't talk about
later times. Anyway, at this temperature the thing may be glowing visibly
as well. It depends on the emissivity. If the surface is shiny, as
supposed above, the emissivity will be quite low. The black-body peak
at 1 million Kelvin is somewhere in the soft X-rays, well above the
visible in energy. If the emissivity is constant in the visible range
(I have no idea how good an assumption this is) the star would be
giving off a nice sullen blue glow.


Mac

Robert Erck

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Mar 11, 1998, 3:00:00 AM3/11/98
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In article
<Pine.ULT.3.91.980311...@wisp4.physics.wisc.edu>, Mac
Almy <al...@wisp4.physics.wisc.edu> wrote:

----------------------

High temperature ceramic superconductors conduct electricity. Why do they
look black instead of silvery?

Steven B. Harris

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Mar 12, 1998, 3:00:00 AM3/12/98
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In <bob_erck-110...@et212pc113.et.anl.gov>
bob_...@qmgate.anl.gov (Robert Erck) writes:

>> Current theories also predict that the surface temperature is on
the
>> order of 1 million Kelvin for the first 10,000 years. That's of
course
>> a flea bite out of the life-time, but the text I checked didn't talk
about
>> later times. Anyway, at this temperature the thing may be glowing
visibly
>> as well. It depends on the emissivity. If the surface is shiny, as
>> supposed above, the emissivity will be quite low. The black-body
peak
>> at 1 million Kelvin is somewhere in the soft X-rays, well above the
>> visible in energy. If the emissivity is constant in the visible
range
>> (I have no idea how good an assumption this is) the star would be
>> giving off a nice sullen blue glow.

?? How come? With flat emissivity across the visible spectrum,
wouldn't it be white?

Keith Lynch

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Mar 12, 1998, 3:00:00 AM3/12/98
to

Since radiation increases with the fourth power of temperature, the
million degree star would have to have an emissivity of almost exactly
zero for it not to be blindingly bright.

The fact that neutron stars can be seen at all from thousands of light
years away in small telescopes, when they're small enough that you
could walk from one end to the other in an afternoon (if you didn't
mind the heat, radiation, or gravity), shows that every square
centimeter is radiating fantastic amounts of visible light.

In article <6e7bks$l...@dfw-ixnews8.ix.netcom.com>,


Steven B. Harris <sbha...@ix.netcom.com> wrote:
> ?? How come? With flat emissivity across the visible spectrum,
> wouldn't it be white?

No. The radiation curve of anything much hotter than the sun slopes
so that there's much more blue light than red light. From a distance,
it would appear a very pale ("actinic") blue point, like a welder's arc.

(Seen from close enough that you could see the disc, it wouldn't look
like anything, because you'd be reduced to ashes.)

It would be interesting if the sun was replace by a neutron star with
the same apparent brightness as the sun. It would appear to be a
point, not a disc. If you looked at the ground, you'd see dark and
bright bands chasing each other around, due to variations in air
density -- something normally only seen in the few seconds just before
or after a total solar eclipse. Just don't glance at the sky, or the
intense point of light will burn a line across your retina, as if
you'd been staring into a powerful laser.
--
Keith Lynch, k...@clark.net
http://www.clark.net/pub/kfl/
I boycott all spammers.

Steven B. Harris

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Mar 12, 1998, 3:00:00 AM3/12/98
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In <6e7j3a$abf$1...@clarknet.clark.net> k...@clark.net (Keith Lynch)
writes:


>>Since radiation increases with the fourth power of
temperature, the million degree star would have to have an
emissivity of almost exactly zero for it not to be blindingly
bright. ...The fact that neutron stars can be seen at all from

thousands of light years away in small telescopes, when they're
small enough that you could walk from one end to the other in an
afternoon (if you didn't mind the heat, radiation, or gravity),
shows that every square centimeter is radiating fantastic amounts
of visible light.<<

Comment: Well, I'm not sure emissivity can be anything but
unity at those temps. It's a black box full of a sea of
degenerate matter and semi-free charges, and they're jumping
around with all that thermal energy relatively unattached, and
have to radiate. How could they not? Any crust material
catching that radiation would quickly heat to the same temp,
ionize, then radiate itself.

Let's see: figure a neutron star 21.5 miles in diameter (not
an unphysical size, vs. our Sun at 860,000, just to give a
nice even diameter ratio of 40,000. The neutron star would
have to have a temperature the square root of that, or 200 times
higher, to have the same absolute luminosity. And that's
about right: a million degrees or a tad higher.

Steven B. Harris <sbha...@ix.netcom.com> wrote:

> ?? How come? With flat emissivity across the visible spectr-
um, wouldn't it be white?

KL >>No. The radiation curve of anything much hotter than


the sun slopes so that there's much more blue light than red
light. From a distance, it would appear a very pale ("actinic")
blue point, like a welder's arc.<<

Yeah-- you're right. Glad you made me look this up. Let me
see: using the Planck radiation formula for 10^6 K, and for 3000
A light vs 6000 A, the ratio of powers is basically 2^4 = 16.
Same for higher temps as it's the same high energy slope of the
curve, where the slope is fiercer than I thought (basically that
(wavelength ratio)^4 power). For 6000 K it's .57, due to these
values straddling the peak of the emission curve (lambda max here
is 2897/6000 = 4828 A). You start to get noticeably bluish when
the ratio is greater than 2 or so, I suppose, which happens
around 9,000 K. That would be a high spectral class A, such as
Sirius or Vega. Way up on the `Russell diagram, as PeeWee
Reisfield would say (naturally I thought of Have Spacesuit Will
Travel with your description of blue actinic sunlight, and point
like suns, as ours from Pluto, like welder's arcs).

>>It would be interesting if the sun was replace by a neutron
star with the same apparent brightness as the sun. It would
appear to be a point, not a disc. If you looked at the ground,
you'd see dark and bright bands chasing each other around, due to
variations in air density -- something normally only seen in the
few seconds just before or after a total solar eclipse. Just
don't glance at the sky, or the intense point of light will burn
a line across your retina, as if you'd been staring into a
powerful laser.<<

Don't worry, unless you're wearing SPF 1000, the terminal
sunburn from all the UV A, B, and even C, will have gotten you
long before you worry about your visual fields. And a parasol
won't help that much, because you'll have all the usual sky
scatter, but worse. Incredible blue skies, and beyond blue. The
problem is that with the welder's hood you don't appreciate them.

And then there's the X-rays. For a million K temp sun, lambda
max is 2897 um/10^6 = 29 A. That's about a tenth the energy of
medical X-ray and might be stopped by air (pretty sunsets with
the ionization auroras, I'll bet). On the other hand there's
plenty of harder stuff just up at 10 and 100 times that energy
that won't be so easy to stop. Probably best to just hide out in
the caves during the daytime. With the mutants.


Steve Harris

Kevin A. Scaldeferri

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Mar 12, 1998, 3:00:00 AM3/12/98
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>On 11 Mar 1998, john baez wrote:
>
>> Highly conductive things are usually reflective: the electric field must
>> be perpendicular to the surface of a highly conductive object, so -
>> thanks to the mathematical wonders of wave equations with boundary
>> conditions - electromagnetic waves bounce off! This is why most metals
>> look silvery. (Why do copper and gold look different? For gold,
>> check out the physics FAQ! - I don't know about copper.)

Copper is orange for the same reason gold is. That is, the
possibility of optical transitions from the d-bands to energy levels
just above the fermi surface. The really interesting question is why
silver is silver (when the other noble metal aren't). Ashcroft and
Mermin mumble about a combination of d-band excitation and plasmon
excitations which push this transition above the optical frequencies,
but don't really explain how this works.

Hmm...I know I've read this entry in the physics FAQ before, but I
can't for the life of me find it in the Table of Contents at the
moment. Am I blind or has it disappeared?


--
======================================================================
Kevin Scaldeferri Calif. Institute of Technology
The INTJ's Prayer:
Lord keep me open to others' ideas, WRONG though they may be.

john baez

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Mar 12, 1998, 3:00:00 AM3/12/98
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In article <6e9m8j$1...@gap.cco.caltech.edu>,

Kevin A. Scaldeferri <ke...@cco.caltech.edu> wrote:

>Hmm...I know I've read this entry in the physics FAQ before, but I
>can't for the life of me find it in the Table of Contents at the
>moment. Am I blind or has it disappeared?

Whoops, the stuff about the color of gold is now in the relativity FAQ.
This is linked to the rest of the physics FAQ. Or just go to:

http://math.ucr.edu/home/baez/physics/gold_color.html

The reason this is in the relativity FAQ is that apparently if you
left out relativistic corrections, you'd predict that gold would
be silvery in color!

Ryan Morris

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Mar 12, 1998, 3:00:00 AM3/12/98
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d...@temple.edu wrote in message <3506D3...@temple.edu>...

>As I understand it, it's the electrons that give objects color. So what
>color would a neutron star be?
>
>Dan
>in Philly

Would a neutron star be giving off gamma rays with a surface temp of tens of
millions of Kelvin? But because of the incredible gravity (according to GR)
the gamma rays would be reduced to mere radio or microwaves? Or would the
gravity have to be stronger to do that much energy change so it would go to
infa-red or higher?

Paul Arendt

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Mar 12, 1998, 3:00:00 AM3/12/98
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Keith Lynch wrote:
>
>The fact that neutron stars can be seen at all from thousands of light
>years away in small telescopes, when they're small enough that you
>could walk from one end to the other in an afternoon (if you didn't
>mind the heat, radiation, or gravity), shows that every square
>centimeter is radiating fantastic amounts of visible light.

Fact? If you're talking about light in the optical portion of the
spectrum, we can only "see" a couple of neutron stars: the Crab
and one in the Large Magellanic Cloud are the ones I know of.
However, in both cases, the light is pulsed: the periods are 33 msec
and 50 msec, respectively. (It's interesting that they have such
similar periods. Most pulsars are visible only in the radio, while
others are visible only in the X-ray and/or gamma-ray portion of the
spectrum.)

So, the optical light that we see is bright (but not bright enough
that it needs to be coherently emitted), but only over a small
region of the neutron star. The lack of observed optical/X-ray
emission of radio pulsars is used to constrain radio emission models,
since some of them imply significant heating of the stellar surface
(and therefore thermal emission).

Jason Kodish

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Mar 12, 1998, 3:00:00 AM3/12/98
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In article <6e7j3a$abf$1...@clarknet.clark.net> k...@clark.net writes:
>
>
>The fact that neutron stars can be seen at all from thousands of light
>years away in small telescopes, when they're small enough that you

It's not the light from the surface you're seeing, but the interaction
between the magnetic field and surrounding gasses.

--
Jason Kodish
Thirring Institute for Applied Gravitational Research
http://www.freenet.edmonton.ab.ca/thirring
-----------------------------------------------------
"when I help the poor, they call me a saint. But when I ask why
there are poor, they call me a communist."-bishop in Brazil.

me...@cars3.uchicago.edu

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Mar 13, 1998, 3:00:00 AM3/13/98
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In article <6e9sml$3gn$1...@newshost.nmt.edu>, par...@nmt.edu (Paul Arendt) writes:

>Keith Lynch wrote:
>>
>>The fact that neutron stars can be seen at all from thousands of light
>>years away in small telescopes, when they're small enough that you
>>could walk from one end to the other in an afternoon (if you didn't
>>mind the heat, radiation, or gravity), shows that every square
>>centimeter is radiating fantastic amounts of visible light.
>
>Fact? If you're talking about light in the optical portion of the
>spectrum, we can only "see" a couple of neutron stars: the Crab
>and one in the Large Magellanic Cloud are the ones I know of.
>However, in both cases, the light is pulsed: the periods are 33 msec
>and 50 msec, respectively. (It's interesting that they have such
>similar periods. Most pulsars are visible only in the radio, while
>others are visible only in the X-ray and/or gamma-ray portion of the
>spectrum.)
>
AFAIK, you don't see the neutron star itself, only radiation emitted
from electons trapped in its magnetic field.

Mati Meron | "When you argue with a fool,
me...@cars.uchicago.edu | chances are he is doing just the same"

Keith Lynch

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Mar 13, 1998, 3:00:00 AM3/13/98
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In article <6e83ae$d...@dfw-ixnews8.ix.netcom.com>,

Steven B. Harris <sbha...@ix.netcom.com> wrote:
> And then there's the X-rays. ...

Good point. I hadn't thought past the fact that the atmosphere would
stop them. Since most of the energy is in the form of X-rays, if you
wanted your planet to be a reasonable temperature, you'd have to be at
a distance where the neutron star looked a lot dimmer than the sun.

Probably most of the light wouldn't be coming directly from the star
at all, but from the X-ray induced fluorescence of the air! It
wouldn't look like an aurora, since X-rays aren't charged and hence
aren't channelled by the planet's magnetic field. I'm not sure what
color this fluorescence would be.

Weren't one or two atomic bombs detonated at extreme altitudes? That
should have caused a fair amout of X-ray induced fluorescence of the
air. What color was it?

> That's about a tenth the energy of medical X-ray and might be

> stopped by air ...

Definitely. Where I used to work, we used a medical X-ray machine
to measure air density over a ~10 meter path. (It worked to measure
rapid changes in air density, but only after dividing the signal by
an unattenuated copy of the signal -- those machines give really
crappy waveforms.)

> On the other hand there's plenty of harder stuff just up at 10 and
> 100 times that energy that won't be so easy to stop.

I don't think any of that would get through the atmosphere, at least
not until you get up into the primary cosmic ray energy range. Those
get through indirectly, via cosmic ray showers. But at 1 million K,
there shouldn't be *any* photons in that range.

You do still have to worry about UV, though. Perhaps the planet would
have an especially effective ozone layer, since ozone doesn't just
stop UV but is caused by it?

If there are clouds of ionized gas around the star, you might get a
wild strobe light effect. And you might get quantities of radio waves
sufficient that you could pull useful amounts of electric power out of
thin air with a moderate sized antenna.

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