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eburacum45
2008-Nov-02, 11:09 AM
For the OA project, I'm making a Celestia model of a nearby neutron star, Geminga; here is a movie (2.5 mb) of the work in progress
http://www.orionsarm.com/movies/Geminga.wmv
this movie is slowed down, somewhat; the real star rotates about 4 times a second.
http://en.wikipedia.org/wiki/Geminga

All the models and graphics I've seen of pulsars and neutron stars show the central object as a near-perfect sphere; however, such a fast spinning object should surely be flattened into a highly oblate spheroid.

My question is, should pulsars be modelled as oblate spheroids? In other words is neutronium fluid enough to become distorted by centrifugal force? It would be easy, using Celestia's settings, to model such an object as an M+M shaped spheroid, although I would have to guess the degree of oblateness (unless that information is available somehow).

mugaliens
2008-Nov-02, 12:58 PM
I would imagine that it would be oblate, as Wikipedia (http://en.wikipedia.org/wiki/Neutronium)states, "...all proposed forms of neutron star core material are fluids and are extremely unstable at pressures lower than that found in stellar cores." However, it also states, "...from the last half of the 20th century onward it has been used legitimately to refer o extremely dense phases of matter resembling the neutron-degenerate matter postulated to exist in the cores of neutron stars."

Thus, it appears neutronium isn't consistent throughout a neutron star. Indeed, the Wikipedia entry states that they're about 12 miles in diameter, with a large liquid neutronium core and a solid crust about 1 mile thick. That crust is composed of ordinary atomic nuclei and electrons. Wikipedia says the crust is "extremely hard" and "very smooth" because of the extreme gravitational field.

My arguement would be that if the extreme gravitational field could mold the crust into an "extremely smooth surface," the extreme rotational speed would similarly induce an oblateness.

The question is, what is it's oblateness? That is, for a sphere, a=b=c, and for an oblate spheroid, a=b>c, so what's the a/c ratio?

For a shape that's 12 miles in diameter, a revolution of 4 times per second is significant, but so is the gravitational field.

eburacum45
2008-Nov-02, 03:17 PM
My daughter's boyfriend (bright lad; only 16) suggested to me that gravity would be very much greater than centrifugal force on these objects.

I think he is right; on the Celestia forum a poster has pointed out that the gravity on a neutron star would be about 75 milllion times greater tham the centrifugal force. So that would result in a pretty much perfect sphere, barring any other effects.

grant hutchison
2008-Nov-02, 03:27 PM
My daughter's boyfriend (bright lad; only 16) suggested to me that gravity would be very much greater than centrifugal force on these objects.I was just about to post the same thing.
If we assume reasonable neutron star numbers of 1.5 solar masses and 12km radius, with Geminga's rotation period of 0.237s, then we get surface gravity of 1.4x1012 m.s-2, and equatorial centrifugal pseudoacceleration of 8.4x106 m.s-2. So gravity wins, overwhelmingly.

Grant Hutchison

eburacum45
2008-Nov-02, 04:04 PM
Thanks, Grant.

BigDon
2008-Nov-02, 04:12 PM
So gravity wins, overwhelmingly.

Grant Hutchison


In the end, doesn't it always?

mugaliens
2008-Nov-02, 06:27 PM
Even if gravity is a trillion times grater, you'll still get bullging due to rotation.

eburacum45
2008-Nov-03, 03:27 PM
Yes, but it will be about one part in a million, apparently, and for my purposes, it will not be noticable.

mugaliens
2008-Nov-03, 05:21 PM
Wow - is that all? My "hunch" is that it would have been more. I apologize for not dusting off my calculator! And so much for my hunches...

John Mendenhall
2008-Nov-03, 06:22 PM
I was just about to post the same thing.
If we assume reasonable neutron star numbers of 1.5 solar masses and 12km radius, with Geminga's rotation period of 0.237s, then we get surface gravity of 1.4x1012 m.s-2, and equatorial centrifugal pseudoacceleration of 8.4x106 m.s-2. So gravity wins, overwhelmingly.

Grant Hutchison

Wow. Who woulda thought? I'd have guessed with Mugsy.

Thanks, Grant and Ebura.

grant hutchison
2008-Nov-03, 06:29 PM
Wow. Who woulda thought? I'd have guessed with Mugsy.Feel free to check my working: GM/r plays ωr.
Even a millisecond pulsar would look spherical to the naked eye. (Well, the filtered naked eye. :))

Grant Hutchison

BigDon
2008-Nov-03, 07:13 PM
So as a comparision, what is Earth's surface gravity and equatorial centrifugal pseudoacceleration?

grant hutchison
2008-Nov-03, 07:23 PM
So as a comparision, what is Earth's surface gravity and equatorial centrifugal pseudoacceleration?From Allen's Astrophysical Quantities: gravity is 9.79829 m.s-2; centrifugal pseudoacceleration is 0.0339157 m.s-2. Gravity wins by a factor of ~300, and that's enough to make the Earth look spherical to the naked eye. (The difference between polar and equatorial diameter is only about 0.3%.)

Grant Hutchison

timb
2008-Nov-03, 10:01 PM
I was just about to post the same thing.
If we assume reasonable neutron star numbers of 1.5 solar masses and 12km radius, with Geminga's rotation period of 0.237s, then we get surface gravity of 1.4x1012 m.s-2, and equatorial centrifugal pseudoacceleration of 8.4x106 m.s-2. So gravity wins, overwhelmingly.

Grant Hutchison

Let's try PSR J1748−2446ad, 716Hz. Taking your g as read and using the fact that a∝ω, I get a ratio of about 5.8. I don't think it's a sphere. Would it be an oblate spheroid or a prolate ellipsoid like Haumea?

grant hutchison
2008-Nov-03, 10:36 PM
Let's try PSR J1748−2446ad, 716Hz. Taking your g as read and using the fact that a∝ω, I get a ratio of about 5.8. I don't think it's a sphere. Would it be an oblate spheroid or a prolate ellipsoid like Haumea?Oops. Must've faded a decimal somewhere when I estimated the ratio for millisecond pulsars: I got something just under a hundred.
5.8 is in the region of Saturn, whereas my previous number was putting the gravitational/centrifugal ratio higher than Uranus and Neptune.

Actually getting a value for oblateness out of that would be hard work, I think, since it depends on the mass distribution, as well as all those GR calculations. But it seems the "slow" rotators will be near-perfect spheres, and pulsars in the millisecond range will be oblate spheroids.

Grant Hutchison

timb
2008-Nov-03, 10:54 PM
Oops. Must've faded a decimal somewhere when I estimated the ratio for millisecond pulsars: I got something just under a hundred.
5.8 is in the region of Saturn, whereas my previous number was putting the gravitational/centrifugal ratio higher than Uranus and Neptune.

Actually getting a value for oblateness out of that would be hard work, I think, since it depends on the mass distribution, as well as all those GR calculations. But it seems the "slow" rotators will be near-perfect spheres, and pulsars in the millisecond range will be oblate spheroids.


For Haumea I get a=0.19m/s^2 for the point on the equator furthest from the center. According to Wikipedia the equatorial (where?) surface gravity is 0.44, so the ratio is only 2.3. It seems that as a body spins faster at some point it stops being an oblate spheroid and turns into something more like an egg spinning on its side.

grant hutchison
2008-Nov-03, 11:11 PM
It seems that as a body spins faster at some point it stops being an oblate spheroid and turns into something more like an egg spinning on its side.Yes, that's the transition from Maclaurin spheroid to Jacobi ellipsoid. The oblate spheroid becomes unstable when the polar radius drops below 0.58 of the equatorial radius, according to my copy of Lang's Astrophysical Formulae.
This happens when ω = [0.37423πGρ]1/2. Plugging in a middle-of-the-road neutron star density of 4.5x1017 kg.m-3 gives me ω = 6000 rad.s-1 (please check my working! :)), implying that a true millisecond pulsar would be right on the cusp of transition. Of course, there are all sorts of GR considerations which may make that observation nonsense, but it's interesting.

Grant Hutchison

eburacum45
2008-Nov-04, 02:30 AM
So millisecond pulsars would be oblate or even ovoid, eh? That links up with the 'starquake' theory for pulsar 'glitches'. As it slows down, the neutron star changes shape, and the crust is supposed to 'crack' catastrophically.
http://science.nasa.gov/newhome/headlines/ast19jul99_1.htm

grant hutchison
2008-Nov-04, 10:17 AM
So millisecond pulsars would be oblate or even ovoid, eh?Yeah. Shows that when I've actually bothered to do the sums for one situation carefully, it's pretty dumb to then fire off a throwaway line on the basis of a moment's mental arithmetic. :lol:
A bit of googling reveals that the oblateness of the fast pulsars is a significant consideration: it causes precession, and has predictable effects on the light curves.

Grant Hutchison

timb
2008-Nov-04, 11:10 AM
Yes, that's the transition from Maclaurin spheroid to Jacobi ellipsoid. The oblate spheroid becomes unstable when the polar radius drops below 0.58 of the equatorial radius, according to my copy of Lang's Astrophysical Formulae.
This happens when ω = [0.37423πGρ]1/2. Plugging in a middle-of-the-road neutron star density of 4.5x1017 kg.m-3 gives me ω = 6000 rad.s-1 (please check my working! :)), implying that a true millisecond pulsar would be right on the cusp of transition. Of course, there are all sorts of GR considerations which may make that observation nonsense, but it's interesting.

Grant Hutchison

Your arithmetic looks good as usual. The cusp pulsar would be a 945Hz pulsar1, which isn't much faster than the current record holder. I'm no general relativist, but I believe a axially asymmetric massive rotating body will emit gravitational radiation (http://en.wikipedia.org/wiki/Gravitational_radiation). This may be one of the mechanisms that keeps neutron stars from rotating that fast.

1. Modulo our assumption about the density. From Computation of Neutron Star Structure Using Modern Equation of State (http://lanl.arxiv.org/abs/0810.3482) a maximum mass neutron star would have average density around 1e18 kg/m3. The same paper states that the minimum neutron star has a mass of 0.1 sun and a radius of about 21km for a density around 5e15 kg/m3. How such a small (uh, large) neutron star could be formed is another question!

grant hutchison
2008-Nov-04, 01:38 PM
I'm no general relativist, but I believe a axially asymmetric massive rotating body will emit gravitational radiation (http://en.wikipedia.org/wiki/Gravitational_radiation). This may be one of the mechanisms that keeps neutron stars from rotating that fast.Good point: the Maclaurin spheroid doesn't generate gravitational waves, but any energy added to spin it up into Jacobi territory would end up being be radiated. I suppose how far the neutron star could stray into Jacobi territory would depend on the rate of addition of angular momentum: eventually it would strike a balance and stop increasing its rotation rate.

Grant Hutchison

neilzero
2008-Nov-07, 12:09 AM
I believe all known neutron stars are point sources, even with our most powerful telescopes, due to their small size and multi light year distance. The very minute oblateness is not observable, but we can calculate. Neil

StupendousMan
2008-Nov-07, 01:24 AM
LIGO and other gravitational-wave detectors are looking for emission from rotating neutron stars, since if a star is significantly non-spherical, and rotating rapidly, it ought to emit gravitational waves. There was a recent paper published which placed upper limits on the ellipticity of one (or a few) known neutron stars; let me track it down.

Ah, there it is!

http://adsabs.harvard.edu/abs/2008arXiv0807.2485S

The only pulsar considered in this paper is the Crab Pulsar, but the constraints on its ellipticity are close to ruling out certain equations of state for neutron star matter. Things will only improve with time.

timb
2008-Nov-07, 11:34 PM
LIGO and other gravitational-wave detectors are looking for emission from rotating neutron stars, since if a star is significantly non-spherical, and rotating rapidly, it ought to emit gravitational waves.
http://adsabs.harvard.edu/abs/2008arXiv0807.2485S


Where does it say that?

StupendousMan
2008-Nov-08, 01:21 AM
I suggested that this paper

http://adsabs.harvard.edu/abs/2008arXiv0807.2485S

told of the efforts of scientists to use LIGO to place limits
on the ellipticity or equation of state of neutron stars.

Timb asked:


Where does it say that?

Well, if you go to that link, then to the full paper stored
on astro-ph

http://arxiv.org/abs/0807.2485 (the abstract and author info)
http://arxiv.org/pdf/0807.2485v1 (the full paper, in PDF)

and read the introduction, you'll see equations 1, 2 and 3
explaining the expected gravitational wave emission from
a non-spherical neutron star.

The rest of the paper goes into much more detail, of course.

timb
2008-Nov-08, 02:16 AM
I suggested that this paper

http://adsabs.harvard.edu/abs/2008arXiv0807.2485S

told of the efforts of scientists to use LIGO to place limits
on the ellipticity or equation of state of neutron stars.

Timb asked:



Well, if you go to that link, then to the full paper stored
on astro-ph

http://arxiv.org/abs/0807.2485 (the abstract and author info)
http://arxiv.org/pdf/0807.2485v1 (the full paper, in PDF)

and read the introduction, you'll see equations 1, 2 and 3
explaining the expected gravitational wave emission from
a non-spherical neutron star.

The rest of the paper goes into much more detail, of course.

No, that's wrong. It does not state that if neutron star is non-spherical it will emit gravitational waves. In fact it states that a non-axisymmetric rotating neutron star will emit gravitational waves.

StupendousMan
2008-Nov-08, 03:58 PM
No, that's wrong. It does not state that if neutron star is non-spherical it will emit gravitational waves. In fact it states that a non-axisymmetric rotating neutron star will emit gravitational waves.

Whoops, Timb is absolutely right. A neutron star which is flattened but axisymmetric -- that is, has an equatorial bulge which is exactly the same size in all directions -- won't emit gravitational waves. If the star is flattened, and has an equatorial diameter which is larger in one direction than the other, then it will emit gravitational radiation as it rotates.

I was misled by the terminology that the researchers in this field use. In the paper I mentioned, the word "ellipticity" occurs frequently. I thought that this referred to the ellipticity of a cross-section taken from the north pole to the south pole of the rotation axis; in the case of the Earth, this cross section would be slightly larger across the equator than it would be from pole-to-pole, and the word "ellipticity" is sometimes used to describe the difference mathematically.

However, in the world of rotating pulsars, researchers (apparently) use the term "ellipticity" to refer to a cross-section taken through the equatorial plane. In the case of the Earth, this cross-section would be circle, with equal diameters everywhere. But if, in a neutron star, this cross-section should be larger in some directions than others, then as the star rotates, it will emit gravitational waves.

Sigh. Live and learn. Thanks for pointing out the mistake, Timb.

grant hutchison
2008-Nov-10, 11:58 AM
Whoops, Timb is absolutely right. A neutron star which is flattened but axisymmetric -- that is, has an equatorial bulge which is exactly the same size in all directions -- won't emit gravitational waves.Unless it's precessing, I think. :)

Grant Hutchison

StupendousMan
2008-Nov-10, 03:47 PM
Unless it's precessing, I think. :)

Grant Hutchison

AAaaaaaaaahhhhhh!

Okay, okay, let me try to make a general statement which isn't incorrect, no matter how much nit-picking people do.

If a body's gravitational quadrupole moment changes with time, it will emit gravitational waves.

There -- does that work?

grant hutchison
2008-Nov-10, 04:55 PM
Okay, okay, let me try to make a general statement which isn't incorrect, no matter how much nit-picking people do.Sorry, wasn't trying to nitpick. :sad:
The matter just cropped up earlier in the thread, when I found that the oblateness of millisecond pulsars was important because (among other things) it can drive precession with effects that are measurable here on Earth.

Grant Hutchison

eburacum45
2008-Nov-10, 05:02 PM
What strikes me is that the magnetic field in a pulsar is generally off-axis, otherwise it wouldn't pulse. That imbalance must cause some sort of acceleration and deceleration- which at those speeds could be transferred into gravity waves.

fsgregs
2008-Dec-11, 02:25 PM
As a high school Astronomy teacher, I want to be sure I teach the "correct" facts. As pointed out here, a neutron star's gravity is ENORMOUS. Given the assumption that a millisecond neutron star would bulge at the equator,it also seems logical to assume that the surface particles (neutrons, nuclei or elections) would not have any ability to pile up in hills and valleys ... there would be no pits, holes, bumps or even pimples of any kind. It would just be perhaps the smoothest object in the universe, with an outer atomic layer of neutrons laid down like a solid film of oil.

Likewise, since the internal core temperature of a supernova going off is reported to be in excess of 500,000,000o C, than the resulting neutron star would also be incredibly HOT ... perhaps in excess of that temperature. As such, would not its surface "color" glow far far above the visible range? It would not just be violet-hot, it would have to be X-ray or gamma hot! Thus, what would a neutron star "look like" through a telescope or by naked eye? It seems to me its surface would not be tinted blue, as drawn by many artists. That is the wavelength given off by objects at about 35,000 K, far, far, far below the theoretical surface temperature of a neutron star.

I have read that it takes billions of years for neutron stars and white dwarfs to cool down. If this is true, then would we see any neutron star at all in visible light? Would they not be a kind of X-ray glowing, rapidly spinning, whirling demon of death, giving off deadly radiation far above the visible range ... emitting crushing gravity and whopping magnetic fields ... just waiting for the unsuspecting to come by :(

There is also the question of texture. If it is that hot, would it appear granular, as many scientific artists also sketch? I thought neutron stars are made of neutrons ... a kind of super-super hot spherical solid ball that would probably appear as smooth as a ball of glowing glass.

Your opinions would be much appreciated. :)

eburacum45
2008-Dec-11, 02:53 PM
Well, before any experts get here, perhaps I can give my unscientific impressions , and wait for them to be shot down in flames. Yes, a neutron star will be very hot, but it will cool slowly, so it will gradually reach slightly less insane temperatures. But I think that at no point will it be black, or even dark, simply because it is emitting most of its energy in the far UV spectrum. A hot object emits according to a black body curve; even for a hot body which emits 99.999..% of its energy in the ultraviolet and beyond, there will still be a tail of that curve in the visible spectrum. And because the neutron stars are small, that tail will be very bright- they are trying desperately to emit energy as fast as their little surfaces will permit.

What colour will that visible light emission be? Well, if the curve of the black body has a peak well outside the visible range, we will only get the tiny, nearly flat tail-end of the emission. So I'd expect the colour to be nearly white. Perhaps with a slight bias to the blue end.

Will the surface be granular? I understand that the surface of a neutron star would be made of normal matter- very compressed normal matter. It would indeed be very smooth, but you might be able to detect the scars of former starquakes. Or not, as the case may be.

But in truth, you would not ever be able to see the surface of a neutron star with your naked eye- as soon as you got close enough to see it as a disk, the invisible luminosity of the star would fry your eyes (and you) to a crisp.

m1omg
2008-Dec-11, 05:45 PM
As a high school Astronomy teacher, I want to be sure I teach the "correct" facts. As pointed out here, a neutron star's gravity is ENORMOUS. Given the assumption that a millisecond neutron star would bulge at the equator,it also seems logical to assume that the surface particles (neutrons, nuclei or elections) would not have any ability to pile up in hills and valleys ... there would be no pits, holes, bumps or even pimples of any kind. It would just be perhaps the smoothest object in the universe, with an outer atomic layer of neutrons laid down like a solid film of oil.

Likewise, since the internal core temperature of a supernova going off is reported to be in excess of 500,000,000o C, than the resulting neutron star would also be incredibly HOT ... perhaps in excess of that temperature. As such, would not its surface "color" glow far far above the visible range? It would not just be violet-hot, it would have to be X-ray or gamma hot! Thus, what would a neutron star "look like" through a telescope or by naked eye? It seems to me its surface would not be tinted blue, as drawn by many artists. That is the wavelength given off by objects at about 35,000 K, far, far, far below the theoretical surface temperature of a neutron star.

I have read that it takes billions of years for neutron stars and white dwarfs to cool down. If this is true, then would we see any neutron star at all in visible light? Would they not be a kind of X-ray glowing, rapidly spinning, whirling demon of death, giving off deadly radiation far above the visible range ... emitting crushing gravity and whopping magnetic fields ... just waiting for the unsuspecting to come by :(

There is also the question of texture. If it is that hot, would it appear granular, as many scientific artists also sketch? I thought neutron stars are made of neutrons ... a kind of super-super hot spherical solid ball that would probably appear as smooth as a ball of glowing glass.

Your opinions would be much appreciated. :)

It would radiate very "much" in the visible light too through, because altough its blackbody spectrum peaks in the extreme UV or X-rays or even Gamma rays the emmision in the lower wavelenghts would be still much brighter than if it was colder, because the power still increases everywhere even if it peaks in the shorter wavelenghts.

Some neutron stars were even imaged optically by Hubble telescope, they're extremely dim apart from pulsed radio/X-rays/Gamma rays through, simply because they're the size of a city, so even at millions of degress they are VERY dim outside the pulsar beam.

And on the surface they're not hotter than core during the supernova simply because most of the energy is carried away during a few years after the supernova explosion in neutrinos so they cool the neutron stars surface to around just a million degress Celsius.

A fresh, just a few years old NS can be from hundred million to even trillion (!) degrees Celsius hot however, but the neutrino cooling will cool it soon to million degrees range, then it will cool at normal tempo for billions of years.

Btw, according to Wikipedia they'll be white, simply because at these temperatures the visible light energy is radiated basically equally in all wavelenghts.

And the gamma radiation is strong enough near them to erode matter but not strong enough to prevent planets from existing at orbits outside the pulsar beam as in these systems;
http://en.wikipedia.org/wiki/Pulsar_planets
http://www.extrasolar.net/starlisttour.asp?StarcatID=pulsar

Here you have it even with calculated temperatures and speculations about how they might look like, how they formed etc..

grant hutchison
2008-Dec-11, 06:00 PM
The bluewards tail of the black-body radiation curve is essentially exponential, and as such is self-similar: if you magnify and shift it, as happens with increasing temperature, you get the same shape of curve.
So all extremly hot black bodies glow a desaturated shade of blue, becoming very slightly bluer with increasing temperature but tending towards a limit. Once you're up around a million Kelvin, you don't get much of a change in colour with increasing temperature. You continue to get an increase in visual luminosity with each increase in temperature, but the increments taper off, because the luminous efficacy steadily declines as the temperature increases: most of the energy increase takes place at very short wavelengths.
There's also the matter of gravitational redshift, which is significant for neutron stars. That will decrease the apparent temperature a little, but not enough to be visually significant when we're dealing with such hot bodies.

Grant Hutchison

grant hutchison
2008-Dec-11, 07:33 PM
Just a little more detail on the above.
Here (http://www.cs.umt.edu/CS/COURSES/CS486/color/blackbody.gif) are black body curves for various temperatures, plotted on a logarithmic scale. Notice how the slope towards high frequency is always the same, just shifted upwards a little with increasing temperature. This means that at temperatures above about 100000K, the difference in log luminosity across the visual spectrum is always pretty much the same, implying that the ratio of luminosities is likewise the same. The blue:red ratio in W.m-2.m-1 settles towards about 9.4.
Here's how it goes:
105K: 8.65
106K: 9.31
107K: 9.37
108K: 9.38

So the colour creeps ever more slowly towards a more saturated blue with increasing temperature.
Here (http://upload.wikimedia.org/wikipedia/commons/thumb/b/ba/PlanckianLocus.png/303px-PlanckianLocus.png) is a chromaticity diagram, showing how the black body colour varies with increasing temperature. The entire range from 10000K to infinity is contained within the "pale blue" area.

Grant Hutchison

Spaceman Spiff
2008-Dec-11, 07:41 PM
fyi -- the effective surface temperatures of white dwarf stars are cooler than within their nearly isothermal interiors (isothermal due to high electron conductivities of degenerate electrons, just as with electrons in the conduction bands of metals at room temperature). There is a thin boundary layer just beneath the surface in which convection carries the energy, and then radiation does so within a thin atmosphere.

Such a boundary layer exists for neutron stars as well. As I recollect, we observe neutron stars with typical surface temperatures of ~1 million K, much cooler than the interior with temps of a few 100 million K (having cooled very rapidly from ~100 billion K to ~10 billion K in minutes, ~1 billion in hours, and to 10^8K in several 1000 years).

grant hutchison
2008-Dec-15, 05:03 PM
Some more on the colours of black bodies at extreme temperatures, which might be useful for folk who are planning on modelling neutron stars in Celestia.
According to Yves le Grand's Light, Colour and Vision, the theoretical colour for an object with infinite temperature lies at XYZ chromaticity coordinates x=0.2399 y=0.2342. Using the sRGB standard (http://www.w3.org/Graphics/Color/sRGB.html), that converts to R=149, G=178, B=255: type these into the colour selector of your favourite image editing program.

That RGB point has already been reached when the temperature reaches 106K: although the colour continues to shift chromaticity coordinates very slowly with increasing temperature above 106K, it never manages to creep away from that RGB locus.
For comparison, at 105K, the sRGB values are R=152, G=180, B=255; just a tad less saturated than the infinity point. I really can't detect the difference, visually.

Grant Hutchison

eburacum45
2008-Dec-15, 05:26 PM
They are both surprisingly blue, and very similar.

On the other hand, if you had a car sprayed in the one colour, you probably wouldn't want to touch up the paintwork using the other...