Celestia Forums

Regarding the subject, I have the sensation that, while the most important goal for Celestia is the scientific accuracy of data and imaging, for some addons this is missing, IMHO.
Let me give an example: if you look at the various black Holes addons, you'll see that the matter falling toward the event horizon has all the possible colors: absurd blues, reds, greens, pinks, yellows and so on, moreover mixed in the same addon.
As final result the accretion disks are dark, threatening, terrible, probably just to give a graphical idea of what is in the center.
I'm not a scientist, so I have accepted this, but never convinced that what I was looking at had any graphical agreement to reality.
Now, in the last month's number of "Noticias", the Astronomy Bulletin of the IAC (Instituto de Astrophisica de Canarias), I found this official drawing made for the IAC, showing the surroundings of a black hole



Now, I think that this drawing is probably based on some new phisical interpretation of what happens to the matter teared off the nearby star, and spiraling toward the Black Hole event horizont.
Is this correct? If yes, I think that the various addons of this type should be heavily modified.
What do you think?
Bye

Andrea

Andrea, problem with your image...

I can see it fine now...

That picture is nice looking. I can't tell if it's very realistic, however.

Yes, I had lot of troubles doing the accretion disks around my various black holes (I guess Andrea was talking about my black holes addons). The colors I used are most probably unrealistic.

Firstly, there's a strong luminosity limitation on emissive models in Celestia, so the accretion disks can't emit much light and cannot look very realistic. This is one strong source of frustration, in black holes creation for Celestia.

Second, very hot matter should emit more light in the blue part of the visible spectrum, so the center of the accrretion disk, close to the black hole, should have a blue tint while the matter on the disk's edge should look reddish. However, very close to the event horizon of the black hole, there's a very strong gravitationnal redshift effect. Matter should looks reddish very close to the center. Coarsely, the accretion disk should have some weird color variations : red -> blue -> red.

Thirdly, some very strange optical illusions should occur around the black hole, because of light behavior in a strong gravitationnal field. Color preception may be very weird for an observer moving around the black hole. See that interesting web page about that subject :

http://casa.colorado.edu/~ajsh/schw.shtml

So there's no way Celestia could simulate a realistic black hole. The only thing I can do, is to select some more "natural" color variations on the accretion disk, so the addon could be more "convincing". However, I'm really not a Photoshop pro user, and it's pretty hard for me to create good textures with "realistic" colors and gradients. You should consider all of my black hole addons as "Celestia experiments", or "artistic representations" or "temporary, clumsy representations" of the real thing.

The animations on that website are really fascinating... good link, Cham!

Malenfant wrote:The animations on that website are really fascinating... good link, Cham!

Yep, it would be really nice to have such a simulation within Celestia...

ElChristou wrote:

Malenfant wrote:The animations on that website are really fascinating... good link, Cham!

Yep, it would be really nice to have such a simulation within Celestia...

Hmmm, don't even think about it !

Integrating the General Relativistic equations for light isn't an easy task. Even for the simplest case of a Schawrzschild black hole. Doing it in real time, well, ... forget it ! Or maybe some very crude approximation. But I prefer that the programmer work on more important things.

Hey guys, I found this page this morning when I was fielding Andrea's question. Check out the accretion disk simulations with a moving angle of viewing. Wow. He has some other neat info on the site as well.
http://jilawww.colorado.edu/~pja/research/theory.html

My blanket reply to this topic is:

The matter infalling gains energy as it spirals in thereby shifitng from it's red (relatively cold, low density) to blue (hot, high density) energy stage. Am I correct? Who knows. I have seen numerous simulations which range from a totally blue disk, a red disk, both, what Andrea has in the above picture, et cetera. I suppose it is open to interpretation. I suppose to our eyes, in every circumstance we would just see bright light and low color, such as with our Sun. Other stars, such as Betelgeuse, which we simulate as red in Celestia would be an unbearable bright white to our eyes from what I understand. In my research for binary accretion disks I have read that half of the energy matter has is released in it's journey out of the L1 pointSo coloration due to temperature in this case, is okay as far as I am concerned.

Here is a quote I had found earlier in my research

http://www-pat.llnl.gov/Research/CataclysmicVariables/mauche_nmCVs.html wrote:Because the effective temperature of the accretion disk ranges from ~ 5000 K at its outer edge to ~ few x 104 K at its inner edge, it radiates over a broad energy range from the optical through the far ultraviolet.

Hello everybody, first of all thank you for your replies.
I wish to clarify that my post is not a complaint of the authors incapacity to render in a realistic way a Black Hole or whatever involves accretion disks.
I'm only saying that perhaps in the last months something changed in our Black Holes knowledge (hello Fridger, where are you?), in such a way that now we can need a different graphical approach to them.
I agree with Cham about the blu-red blu colour change toward the event horizont, may be it's correct, but IMHO the emitted light would probably be so high that should appear visually very close to white light, and therefore close to the drawing I've shown here.
Moreover, as I told, this drawing is coming and under responsibility of an important Scientific Astronomical Office (IAC organizes Izana and Las Palmas International Observatories, among the biggest in the world), so that what they show under their own responsibility should be treated with attention.
IMHO this is not a matter of grahics preferences, its a matter of science.
Nothing more!
Regarding the animations, very interesting, but only didactically, without any approach to the graphic problem i'm submitting to you.
I understand (and know very well, for my experience on the field) that showing to the public a color gleaming greenish pulsar or a dark gorgeous red and blu Black Hole can be more winning than a plain, simple yellowiish one but, as has been told here many times, scientific accuracy is the main goal of Celestia, and I agree with this.
For this I invite who knows scientifically more than me on this subject, to express his own opinion, and correct or agree my thinking.
Thank you.
Bye

Andrea

ANDREA wrote:I agree with Cham about the blu-red blu colour change toward the event horizont, may be it's correct, but IMHO the emitted light would probably be so high that should appear visually very close to white light, and therefore close to the drawing I've shown here.

Hello Andrea,

Have you considered the effects of:
Gravitational Redshift,
http://en.wikipedia.org/wiki/Gravitational_redshift
http://www.astro.ku.dk/~cramer/RelViz/t ... xhib3.html
http://archive.ncsa.uiuc.edu/Cyberia/Nu ... nTest.html

and

Hawking Radiation?
http://casa.colorado.edu/~ajsh/hawk.html
http://en.wikipedia.org/wiki/Hawking_radiation
http://math.ucr.edu/home/baez/physics/R ... wking.html

Chuft-Captain wrote: Hello Andrea, Have you considered the effects of:
Gravitational Redshift,
http://en.wikipedia.org/wiki/Gravitational_redshift
http://www.astro.ku.dk/~cramer/RelViz/t ... xhib3.html
http://archive.ncsa.uiuc.edu/Cyberia/Nu ... nTest.html
and
Hawking Radiation?
http://casa.colorado.edu/~ajsh/hawk.html
http://en.wikipedia.org/wiki/Hawking_radiation
http://math.ucr.edu/home/baez/physics/R ... wking.html

Hello Chuff, thank you for the links, I had some of them.
For what I can understand, they don't clarify my doubt.
The matter incoming to the Black Hole should shift from infrared to ultraviolet color, obviously starting from the original color.
This means that if the matter comes from a nearby cold red star, the light should change along all the rainbow colors.
But if the star is blu? Only blu color, disappearing when reaches the ultraviolet frequency?
Anyhow, there cannot be back changing colors, I mean the red-blu-red variation should not exist, correct?
But this is in scientific, mathematical terms, but what happens practically?
Remember that we are speaking of "visual" appearance, so if we have matter at very high temperatures, what would we see?
Its original but changing color, as matter goes closer and closer to the Black Hole, or due to its luminosity we would only see brightness with only a subtle hint of its color?
This is my doubt, and up to now I have no evidence that this be wrong.
Referring the Hawking radiation, I understand it is very small and invisible to human eyes (and up to now to human unstruments, too, due to the fact that there is only a mathematical evidence, not experimental, and many scientists still don't believe it exists), so doesn't matter in our accretion disks mock-ups.
Do you agree?
Your comments are welcome, thank you.
Bye

Andrea

Andrea,

I agree with you regarding the Hawking Radiation. It's effects (if it exists) are miniscule compared to the other effects.

In 1975 Hawking published a shocking result: if one takes quantum theory into account, it seems that black holes are not quite black! Instead, they should glow slightly with "Hawking radiation", consisting of photons, neutrinos, and to a lesser extent all sorts of massive particles. This has never been observed, since the only black holes we have evidence for are those with lots of hot gas falling into them, whose radiation would completely swamp this tiny effect. Indeed, if the mass of a black hole is M solar masses, Hawking predicted it should glow like a blackbody of temperature (6 x 10-8/M) kelvin, so only for very small black holes would this radiation be significant.
The most drastic consequence is that a black hole, left alone and unfed, should radiate away its mass, slowly at first but then faster and faster as it shrinks, finally dying in a blaze of glory like a hydrogen bomb.

Your comments regarding visual appearance seem to be based on an observer travelling with the light into the black hole as described in one of the pages I referred you to:

http://www.astro.ku.dk/~cramer/RelViz/text/exhib3/exhib3.html wrote:Let's consider a spacecraft flying towards the center of a Black Hole. An unfortunate passenger payed a return-ticket to see the singularity from a first class stable orbit. He thought he could return in two weeks. What will he see from the rear window, while looking at the nice and safe universe where he was born?

If the light from the distant stars is red, he will see it being more and more orange, yellow and blue. In the end, the light becomes ultra-violet so he can't see it (but he should remember his sun glasses and sun-lotion.... If the astronaut not dies because of gravitational tidal forces, he will maybe die because of UV-radiation!

However, I assume that what you really want to know is what a black-hole would look like to a non-relativistic Celestia observer at a safe distance from the event horizon. (a much safer proposition)
(If we look at the red lights on the back of the spacecraft - from a safe orbit - it is the opposite situation: The photons will loose energy when leaving the gravitational field, and become infrared.)

http://archive.ncsa.uiuc.edu/Cyberia/NumRel/EinsteinTest.html wrote:According to General Relativity, the wavelength of light (or any other form of electromagnetic radiation) passing through a gravitational field will be shifted towards redder regions of the spectrum. To understand this gravitational redshift, think of a baseball hit high into the air, slowing as it climbs. Einstein's theory says that as a photon fights its way out of a gravitational field, it loses energy and its color reddens. Gravitational redshifts have been observed in diverse settings.

Chuft-Captain wrote: I agree with you regarding the Hawking Radiation. It's effects (if it exists) are miniscule compared to the other effects. Your comments regarding visual appearance seem to be based on an observer travelling with the light into the black hole as described in one of the pages I referred you to. However, I assume that what you really want to know is what a black-hole would look like to a non-relativistic Celestia observer at a safe distance from the event horizon. (a much safer proposition).

Chuff, if I understand correctly, you agree with me on both themes.
The only thing I'm still missing is:
approaching the accretion disk, how will it appear to my eyes, if the nearby star is yellow, or blu, or red?
Will I see substantial color differencies, variating from red to violet as getting closer to the event horizon, or will I see only a luminous spiral cloud, mainly whitish but with hints of the colors, as due to the gravitational shift?
I think this image can better clarify what I mean:


I think that the lower examples could me more real for an observer that is looking at the accretion disk from a safe distance.
What do you think?
Bye

Andrea

Andrea, I'm afraid it's much more complicated than that. The colors perceived highly depends on the observer's motion, AND on the accretion disk movement. The visual aspect of the acretion disk will not be the same for a stationnary observer than for an orbiting one. It will not even be the same for the static observer standing far away than for the static observer close to the event horizon. Doppler effects are very important here, especially since the orbiting matter is moving very fast around the black hole (relativistic motion). For a static observer, the left side (say) of the accretion disk will be blue shifted while the right side of the disk will look reddish, because the disk is rotating.

Also, don't forget the gravitationnal redshift (this isn't a Doppler effect). Light emitted close to the event horizon will look reddish to a distant observer, even if the matter-source is moving faster there.

So really, the color spectrum of a real black hole could be very weird looking to most observers.

Cham wrote:So really, the color spectrum of a real black hole could be very weird looking to most observers.

I think the closest we're going to get is the approximations that we have now... bear in mind that really the shifting of the light won't matter to observers in practice since the disk will be so darn bright that you probably wouldn't see colours if you looked at it anyway (assuming your eyes didn't get fried in a millisecond by the luminosity anyway)

I don't think people should get too involved in getting it looking right. What with the redshifting, relativistic effects and generally intense luminosity it just won't be possible to do in Celestia.

Cham wrote:Andrea, I'm afraid it's much more complicated than that. The colors perceived highly depends on the observer's motion, AND on the accretion disk movement. The visual aspect of the acretion disk will not be the same for a stationnary observer than for an orbiting one. It will not even be the same for the static observer standing far away than for the static observer close to the event horizon. Doppler effects are very important here, especially since the orbiting matter is moving very fast around the black hole (relativistic motion). For a static observer, the left side (say) of the accretion disk will be blue shifted while the right side of the disk will look reddish, because the disk is rotating. Also, don't forget the gravitationnal redshift (this isn't a Doppler effect). Light emitted close to the event horizon will look reddish to a distant observer, even if the matter-source is moving faster there. So really, the color spectrum of a real black hole could be very weird looking to most observers.

Cham, I agree on the complexity of the problem, and the difficulty to find a solution on line with the physics/maths of the Black Hole.
Nevertheless, I don't think that this could be an authorization to an approach to the BH accretion disk graphical rendering of this kind: "OK, we cannot do everything, so let's do as we prefer".
I think we should take for firm at least one factor, e.g. that we are "standing observers" (neither approaching nor orbiting or spiralling), let's say at the same distance of the nearby star, with a telescope that allows us closer wiews of the accretion disk, without getting physically closer, and that for a strange and mysterious phenomenon the Doppler effect is not working in this small portion of Universe (this because we cannot take care graphically of both relativistic motion AND Doppler).
If we agree on this, IMHO the accretion disk becomes a bit easier to be realized, avoiding the actual feverish situation.
As I told, this post is against the average public feeling, they love glowing greens, gleaming reds and gorgeous blues, but this aspect of Celestia IMHO is NOT science, it's only fantasy.
What do you think?
BTW, Cham, thank you for your explanation on Doppler effect, I didn't take it into account.
Bye

Andrea

A part of the problem is we can't have very luminous objects in Celestia, except stars. The emissive option isn't enough. There's no way I can define a very luminous accretion disk close to the black hole.

I may try to do a "dull" white accretion disk for another black hole. But it's not easy to make a good looking texture in Photoshop.

Cham wrote:Andrea, I'm afraid it's much more complicated than that. The colors perceived highly depends on the observer's motion, AND on the accretion disk movement. The visual aspect of the acretion disk will not be the same for a stationnary observer than for an orbiting one. It will not even be the same for the static observer standing far away than for the static observer close to the event horizon. Doppler effects are very important here, especially since the orbiting matter is moving very fast around the black hole (relativistic motion). For a static observer, the left side (say) of the accretion disk will be blue shifted while the right side of the disk will look reddish, because the disk is rotating. Also, don't forget the gravitationnal redshift (this isn't a Doppler effect). Light emitted close to the event horizon will look reddish to a distant observer, even if the matter-source is moving faster there. So really, the color spectrum of a real black hole could be very weird looking to most observers.

Cham, I agree on the complexity of the problem, and the difficulty to find a solution on line with the physics/maths of the Black Hole.
Nevertheless, I don't think that this could be an authorization to an approach to the BH accretion disk graphical rendering of this kind: "OK, we cannot do everything, so let's do as we prefer".
I think we should take for firm at least one factor, e.g. that we are "standing observers" (neither approaching nor orbiting or spiralling), let's say at the same distance of the nearby star, with a telescope that allows us closer wiews of the accretion disk, without getting physically closer, and that for a strange and mysterious phenomenon the Doppler effect is not working in this small portion of Universe (this because we cannot take care graphically of both relativistic motion AND Doppler).
If we agree on this, IMHO the accretion disk becomes a bit easier to be realized, avoiding the actual feverish situation.
As I told, this post is against the average public preferences, they love glowing greens, gleaming reds and gorgeous blues, but this aspect of Celestia IMHO is NOT science, it's only fantasy.
What do you think?
BTW, Cham, thank you for your explanation of Doppler effect, I didn't take it into account.
Bye

Andrea

ANDREA wrote:Chuff, if I understand correctly, you agree with me on both themes.

Actually no. I agree with you 100% regarding the Hawking Radiation, but not about the rest.

Sorry, I haven't got time right now to go into this, but I'll get back to you as soon as I have the time.

Chuft-Captain wrote:

ANDREA wrote:Chuff, if I understand correctly, you agree with me on both themes.

Actually no. I agree with you 100% regarding the Hawking Radiation, but not about the rest.
Sorry, I haven't got time right now to go into this, but I'll get back to you as soon as I have the time.

OK Chuff, take your time, Black Holes can wait.
Bye

Andrea