May we have some color please?

[don]

Celestia is a free real-time space simulation that lets you experience our universe in three dimensions.

Personally, I would sure like to see more of both -- space and universe -- in Celestia and in 3-D, even if only guesstimated. Somehow, looking at an add-on spiral galaxy that is only a few pixels thick (edge-on) just doesn't seem right.

There are other objects in space and in the universe besides the objects in our solar system, and contrary to what I've seen written in these forums, galaxies are not "dull gray blobs"! They are FULL of stars. How could they possibly be dull gray? Just look around the web for astrophotography web pages. Here is just one of 175,000 pages returned by Google (http://www.astropix.com/). This is not a grant-funded observatory, but a private individual and his personal telescope!

Here is the way M31 should look (http://www.astropix.com/HTML/SHOWCASE/M31.HTM). And here is M33 (http://www.astropix.com/HTML/SHOWCASE/M33.HTM).

Deep space objects in our own galaxy are not gray blobs either. Wanna see some amazing colors? This photo of the Rho Ophiuchus Nebula Complex looks like a photo of fireworks! (http://www.astropix.com/HTML/D_SUM_S/RHO.HTM)

We live in a VERY COLORFUL Universe

All of the above examples are photographs. They are not "compiled from data" from some satellite (x-ray, gamma ray, etc.). They are not "color enhanced", like HST photos. These are the real thing.

Is there any possibility of adding some color to the Celestia universe, aside from planets in our solar system? Along with some means of allowing deep space objects to be modeled and rendered in some 3-D manner, even though we have only 2-D photographs? It sure would be nice.

-Don G.

[mrzee]

Hi Don,

Whilst I agree that it would look great to have objects like that one of the aims was to see the universe as if from a port hole in a space craft. If you look carefully in the supplied links you will see that the images are formed using long exposure photography, so in effect they are color enhanced as that is not what our eyes would see.
Maybe if we were close enough that's the way they would look but from our distance and with our non astronomicaly evolved eyesight they will look white/grey and sometimes fuzzy.
Having said that I do like to see color in the add-on's even if thats not what the sky would look like to me.
Just my 2 cents worth anyway.

Regards,

[Buzz]

I would like to see more colour&variation too! And more (optional) brightness for galaxies, so you could get a good impression of the 3D distribution of galaxies that the galaxy add-ons provide without switching on their labels. I am curious to know if any plans exist to address these things. Selden has made a nice 3D model of a galaxy by the way.

[granthutchison]

Sadly, even from close up the colourful nebulae of the astronomy magazines would be at best a wispy grey, and perhaps entirely invisible. Remember that they are composed of very rarified glowing gas - they just don't put out enough photons to excite colour vision in our eyes. We can only ever see the colours using long exposure photographs.
As for galaxies, the little grey smudge in the sky you see when you look at Andromeda will get bigger as you get closer to it, but its surface brightness won't increase, so it'll just become a bigger grey smudge. Eventually you'll be able to resolve variations in surface brightness and the presence of dust lanes, but it will always be silver-grey in appearance, just like the Milky Way - the individual stars are too far off to excite colour vision, and all you see is a vague mixture of their light.
Sorry, Don.

Grant

[Buzz]

Should Celestia really be so strict as to only simulate what people can see with the unaided eye? I would not mind if it would simulate what you see with a telescope (as big as necessary to get enough luminosity)

[granthutchison]

Should Celestia really be so strict as to only simulate what people can see with the unaided eye?

Not at all . Fridger has already introduced a neat simulation of a telescopic zoom. I personally find a sky full of brightly coloured extended objects quite irritating, and prefer to see a realistic sky, but I do think it would it be nice to be able to toggle long-exposure views on and off so that people can see and understand the contrast.
It's analogous to the business of limit-of-knowledge and interpretive textures for planets - some of us prefer not to be shown detail that isn't there, some like to see imaginary terrain filling the blanks. By having both options, Celestia becomes maximally informative.
My comments were really just a response to Don's vigorous denial of the (perfectly valid) remark that galaxies are grey blobs.
(Straying OT, I wonder if it's a symptom of our level of light pollution today that most people form their opinion of how the sky looks from astronomical images, rather than from looking at the sky itself. If you've actually seen the Magellanic Clouds, the Andromeda Galaxy and the Orion and North American Nebulae as nothing more than fuzzy grey patches of light, then you do begin to realise that astro images are unrealistic in a very fundamental way.)

Grant

[don]

If what all of you are saying is true, then amateur astronomers are lying when they write in various mailing lists or forums, that they can see (with their eyes) the various colors of some nubula and other things.

We do not have a 12" or 14" deep space telescope and don't know anyone who does , so we are not able to view these items "first hand". Thus, I can only take what I read and see as the truth. Sounds like I've been reading lies. BUMMER!

Are you saying that if a human was within a few lightyears (or whatever) of one of these objects that they would still be gray?

As for seeing these objects from Earth (in Celestia), no, they should be properly sized as they already are. It is only when one gets closer to them, that they should become visible, brighter, and with color if they have any.

-Don G.

[chris]

If what all of you are saying is true, then amateur astronomers are lying when they write in various mailing lists or forums, that they can see (with their eyes) the various colors of some nubula and other things.

Some color is visible when the brighter nebulae are viewed through a large telescope. But it's very subtle--you'll never see the saturated hues that appear in long exposure photographs.

Are you saying that if a human was within a few lightyears (or whatever) of one of these objects that they would still be gray?

Well, we're in the middle of the Milky Way, and it looks gray to me . . . It's a similar story with nebulae--they're huge, diffuse objects with low such low surface brightness that color is barely perceptible (if at all) to our eyes. As you approach a nebula, the apparent size obviously increases, but the brightness per unit area remains constant (well, mostly, because attenuation from the interstellar medium can be a factor).

At some point, I'd like to add an exposure adjustment to Celestia. But, I'm going to wait until some new generation of graphics cards better supports floating point rendering.

--Chris

[granthutchison]

If what all of you are saying is true, then amateur astronomers are lying when they write in various mailing lists or forums, that they can see (with their eyes) the various colors of some nubula and other things.

Dear me; of course they're not. They're benefiting from the increased light-gathering capacity of their telescopes - if you had eyes 12" across then you'd be able to detect some nebular colour, too (but not nearly the amount that appears in long-exposure photographs).

Are you saying that if a human was within a few lightyears (or whatever) of one of these objects that they would still be gray?

Yes indeed. Think of it this way. The Orion Nebula is a faint little grey blob in the sky when seen from Earth. Imagine that you are able to move closer until you've halved your distance from the nebula. By the inverse square law it's going to be four times brighter, yes? But it's also going to be looking twice as wide now, and will therefore cover four times the area of sky it previously did. It's four times brighter overall, but that light is now coming from four times the angular area - so each square arcsecond of nebula is just precisely the same brightness it was when seen from Earth; there are just more square arcseconds visible. So each little patch of your retina that's illuminated by nebular light "sees" just the same dim glow it did on Earth - and that level of dim illumination is inadequate to trigger colour vision, so it looks grey. And the same argument applies for any distance of approach.
Remember, the gas in these emission and reflection nebulae could be mistaken for a good laboratory vacuum on Earth - they just can't put out very much light, unfortunately.

Grant

Edit: this posting overlapped with Chris's - it wasn't my intention to repeat stuff he'd already covered.

[don]

Thanks for your message Chris. Guess humans need to invent a pair of photon enhancment glasses.

Middle of the Milky Way ... from our vantage point, in rural America, on a clear, moonless night, the galactic plane actually looks like moon-lit clouds, which is much brighter than "gray". And, if you sit still long enough, focusing on it, you can start to make out individual stars (naked eye). Can't see it at all from the city though.

I look forward to Celestia having more starlight and colors in the future.

-Don G.

[don]

Hey Grant,

From my previous post, make that a pair of 12"-14" photon enhancement glasses.

Thanks for your message. I understand this gray business (photons and human eyesight) a whole lot better now, as disappointing as it is.

I still think it would be a great idea to have a good way to include photos of these deep space objects in Celestia (with a switch to turn them on/off), even if it is to use something like Seldens billboards and then have the objects always facing the camera. Just to get some color in Celestias Universe, and also a way for non-astromomers like me to relate a name (Andromeda, M33, etc.) with an image.

-Don G.

[Christophe]

Chris is right, just look at the sky, we are in space. The atmosphere doesn't filter visible light, you'd see exactly the same thing looking out the window of a spaceship as you would looking at the sky on a dark night from a good site.

Seen with the bare human eye, space is a very dull place... that's why we need those large light collectors and long exposure times.

[don]

Hi Christophe! Good to hear from you. Haven't seen you around for a while.

Yes, between Grant and Chris, I "got it". The colors of these objects are not perceptible to the unaided human eye.

I don't have any scopes to experiment with, but I am now wondering how much color gain / saturation is gained for every minute of film exposure time? Aside from the dependency on the size of the scope.

I'm also wondering, if we were traveling directly towards one of these objects (at very high speed), would the increased "photon stream" (we are traveling into the stream) then allow human eyes to perceive the colors better? Or would we see a color-shifted doppler effect? I suppose this would be a Fridger / Physics question.

-Don G.

[Paul]

Seen with the bare human eye, space is a very dull place...

Exactly, it looks dull. Do we want Celestia to look dull, though? Just for the sake of accuracy? I certainly don't.

Cheers,
Paul

[Ynjevi]

Read Bad Astronomer's article The Making of "Under Alien Skies". There he describes how Orion Nebula would really look like when viewed near.

About star colors, red stars look IMHO ugly pinky. They should be orange like here.

[don]

Read Bad Astronomer's article The Making of "Under Alien Skies". There he describes how Orion Nebula would really look like when viewed near.

Nice article, and a great web site too. Thanks!

[mrzee]

I don't have any scopes to experiment with, but I am now wondering how much color gain / saturation is gained for every minute of film exposure time? Aside from the dependency on the size of the scope.

I don't have a scope either but as far as I'm aware, please correct me if I'm wrong here, each film has its own sensitivity for both intensity and frequency(color). This is the starting point for the relevant film which should include a speed rating that references it to some traceable standard. If an object requires 1sec exposure at 400 speed film to be correctly exposed then we could also use a 2sec exposure of 200 speed film or a 4sec exposure of 100 speed film. Since it's cheaper and more practical to increase the exposure time than increase the film sensitivity this is whats done. N.B. this assumes all else being equal in the films apart from speed. Tricks for astro photography include using films that are specturaly blind in certain areas (eg like sodium vapor lamp emissions etc) or enhanced (eg IR) so the object can be seen more clearly or so invisible (to us) bands are made visible.

I'm also wondering, if we were traveling directly towards one of these objects (at very high speed), would the increased "photon stream" (we are traveling into the stream) then allow human eyes to perceive the colors better? Or would we see a color-shifted doppler effect? I suppose this would be a Fridger / Physics question.

I'd imagine both to occur to some degree, it's effects if noticable would be very dependant upon the speed you are traveling. The doppler effect would change the real color towards the blue end of the spectrum depending on your speed. The intensity would vary as more photons would be received per second by your eye. Just how much more is I'll leave to someone with more experience to answer. If I had to quess it would be a linear increase at non-relativistic speeds as expressed as a ratio of the speed of light. At relativistic speeds I have no idea as I've always had trouble understanding how the speed of light is a constant to begin with:) (For the fussy reader, replace speed with velocity in the above description, I used speed as the question implied travel toward the light source)

Regards,

[don]

... each film has its own sensitivity for both intensity and frequency(color).

Howdy mrzee,

Yes. The reason I was asking is because some of the astro photos are 40 to 60 minute exposures. That's a LONG time, which is why I wondered out loud about how much gain / saturation is obtained per minute (or 10 minutes, etc.)

Thank you for your answer about traveling towards the light also. Yes, that is a difficult one, and due to the constant speed of light, it would seem that the brightness would stay the same but maybe be doppler shifted.

-Don G.

[selden]

A complication with photographs on film is what's called "reciprocity failure". The exposure times are not linear. You need to more than double the exposure time in order to double the apparent brightness, and this exposure difference is different for the different colors and for different brands of film. People have used many different techniques in order to try to improve the situation, including chemical baths and low temperature units.

[granthutchison]

I'm also wondering, if we were traveling directly towards one of these objects (at very high speed), would the increased "photon stream" (we are traveling into the stream) then allow human eyes to perceive the colors better? Or would we see a color-shifted doppler effect?

Complicated question to answer!
There's a factor called "eta" in special relativity, which is calculated from your velocity, and tells you by how much the frequency of incoming light is shifted. For the scenario you describe, eta crops up in several ways:
1) The frequency (and therefore the energy) of each incoming photon is increased by a factor of eta.
2) Because of your motion, you run into eta times more photons in a given time period than you would if you were standing still.
3) The apparent diameter of an extended object appears to shrink by a factor of 1/eta: so its area shrinks by 1/eta^2
So you get eta^2 more energy from 1/eta^2 the angular area: the total energy output of the object per unit surface area increases by a factor of eta^4. In terms of overall energy output, things ahead of your spaceship become very much brighter indeed as you approach them at high velocity.
Unfortunately, the blue shift of the emitted wavelengths makes the overall effect unpredictable: an object radiating a lot of infrared might flare up and become very bright as its energy output shifted into the visible range. A hot B-type star would actually become dimmer as its spectrum shifted into the ultraviolet. For an emission nebula, you'd need to know in detail how much energy it radiated at its various emission frequencies before you could predict what sort of effect the blue shift would have.
So some objects might well get bright enough for you to see colours if you approached them at relativistic velocities - but the colours you saw would bear no resemblance to the ones in astronomical photographs, because of the effects of blue shift.

Grant