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PostPosted: Sun, 22-01-12, 19:37 GMT 
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Hi all,

in the course of my ongoing Celestia.Sci development, I also implemented a sophisticated, entirely new simulation of globular clusters. It incorporates the state of the art of understanding the evolution of globular star populations. As soon as I find some more spare time, I'll document here what went in. One main aim was to visualize correctly the subtle star colors in addition to the proper spacial cluster structures. The rendering both of normal (isolated) stars and globular cluster stars is done with the same shader code. Also, the mapping of the effective temperatures (T_eff) to RGB colors is the same for all stars.

++++++++++++++++++++++++++++
Let me report here on a cute "Gedanken experiment":
Imagine our solar system was part of a globular cluster!
++++++++++++++++++++++++++++

What I did, was to copy from globulars.dsc the data of the famous globular 47 Tuc (NGC 104) with a couple of changes:

Code:
Globular "SolGlob 1"
{
    RA                   0.4014    # [hours]
    Dec                -72.0811   # [degrees]
    Distance               15.0   # [ly]
    Radius                109.3   # [ly] from refit to King62 profile
    CoreRadius         496.9   # [arcmin]
    KingConcentration  1.94  #      from refit to King62 profile
    AbsMag               -9.42   # [V mags]
    VminusI               1.14    # V-I colour index
    Axis        [ -0.7429  -0.2364  -0.6263]
    Angle                175.9     # [degrees]
}


I called the new ficticious globular SolGlob 1 and merely adapted the distance and --for consistency-- the angular size of the CoreRadius of 47 Tuc:

The μ25 radius of 47 Tuc is 109 ly. So, after some experimentation, I placed the center of SolGlob 1 just 15 ly from the solar system barycenter! This is quite a difference to the original distance of 14680 ly! For the original distance of 47 Tuc, the angular CoreRadius was just 0.5087 arcmin. Since the distance of SolGlob 1 is now much smaller, the angular size must correspondingly increase a lot in order to retain the same CoreRadius in [ly]. The respective formula is simple:

Code:
CoreRadius_SolGlob 1 [arcmin] = 60 * 2 * atan(d_47 Tuc / d_SolGlob1 *
                                                   tan(CoreRadius_47 Tuc[arcmin] /(60 * 2) ) )


This gives the above value of 496.9 arcmin instead of only 0.5087 arcmin.
It means that just the core of SolGlob 1 now covers an angular region of 16.6 degrees in the sky. This amounts to 32 Full Moons...

So let's have a look at some typical views that we would have from space just behind Earth, from the surface of Earth and from the Moon, say (no atmosphere). Of course, one could as well have placed some ficticious "solar system" with planets into any given globular cluster...

The SolGlob 1 globular cluster simulation involves about 100 000 stars...

View from space, just behind the Earth:

Image

Next, I went to the Earth surface, left atmosphere and clouds on. Then you get this amazing view in the evening twilight:

Image

Finally, from the surface of a body without atmosphere (Moon) the view is also spectacular:
I think this shot provides a feel about what it might be like to live amidst a rich star cluster, just 15 ly away from its center

Image

Enjoy,
Fridger


Last edited by t00fri on Mon, 23-01-12, 23:37 GMT, edited 4 times in total.

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PostPosted: Sun, 22-01-12, 21:52 GMT 
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Very nice images there! Guess there is call to finally sort out the distance issues to PSR B1620-26 and see the pulsar+white dwarf+giant planet system in its proper place in Messier 4.

What are you using for the star colours here? I've always been somewhat dissatisfied with Celestia's default colour sets: the hideous pink M stars need to get sorted out, and the "blackbody" palette seems to be far too garish. I suspect this might be because it is normalised to max(r,g,b)=1 rather than something that preserves constant brightness (perhaps Y=constant would be better?)


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PostPosted: Sun, 22-01-12, 22:26 GMT 
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ajtribick wrote:
Very nice images there! Guess there is call to finally sort out the distance issues to PSR B1620-26 and see the pulsar+white dwarf+giant planet system in its proper place in Messier 4.

What are you using for the star colours here? I've always been somewhat dissatisfied with Celestia's default colour sets: the hideous pink M stars need to get sorted out, and the "blackbody" palette seems to be far too garish. I suspect this might be because it is normalised to max(r,g,b)=1 rather than something that preserves constant brightness (perhaps Y=constant would be better?)


Well thanks. The globular cluster work has been challenging and pretty work-intensive. And it still is ;-) . Soon, I'll also publish a long list of scientific references that I used, as well as some great web site URLs, where you can get the latest grids of isochrones, populated CMD's, integrated colors and much more. The real challenge is to squeeze all this scientific material into very fast computer code :)

So far, it is up to the user to choose between two T_eff -> RGBcolor conversions:

//colorTemp = GetStarColorTable(ColorTable_Enhanced);
colorTemp = GetStarColorTable(ColorTable_Blackbody_D65);

The second one (Blackbody_D65) is what I take mostly for now. It has 401 finely spaced entries for T_eff. Additional tables may always be added. What I find great progress is that now the same colors apply for ALL stars, given T_eff.

At this stage, star colors and shapes have not yet been improved (relative to Celestia.SVN) for short distance viewing. The present shader is not meant for really big star sizes.

But I am quite content so far, since the faintest globular stars (appmag <=25!) match well with really big sized ones. See e.g. this image of the dim globular NGC 2419 that I showed already at shatters:

Image
Note that the two "super bright" stars on the right are only 7.2m and 7.9m, respectively! The not-so-bright one near the bottom is 11.16m. Most visible stars in NGC 2419 are dimmer than 20m!

The display does pretty well in comparison with the SDSS DR7 image of NGC 2419

Image

Also, after lots of work with Nick Risinger's 3000x1500 Milkyway that includes beautifully most nebulosities and H_alpha regions, the M 4 - Antares region now looks pretty well, I think:

Image

Fridger


Last edited by t00fri on Mon, 23-01-12, 11:03 GMT, edited 7 times in total.

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PostPosted: Sun, 22-01-12, 23:32 GMT 
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Oh, in my "Gedanken experiment", above I forgot something important:

Here is my ficticious globular cluster SolGlob 1 and the location of the solar system in its core region, as seen from the Small Magellanic Cloud (SMC) dwarf galaxy some 57 Kpc away from us...

The cluster simulation uses about 100 000 stars as above.

Image

or in a close-up view from Sol towards the core of SolGlob 1

Image

Fridger


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PostPosted: Mon, 23-01-12, 10:47 GMT 
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Sorry, I reordered some images, deleted one and added another one. So I suggest you scroll once more from the beginning...

Enjoy,
Fridger

_________________
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PostPosted: Mon, 23-01-12, 19:59 GMT 
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t00fri wrote:
Also, after lots of work with Nick Risinger's 3000x1500 Milkyway that includes beautifully most nebulosities and H_alpha regions, the M 4 - Antares region now looks pretty well, I think:


Beauuuutiful!

And the Gedankenexperiment, wunderbar.

G


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PostPosted: Mon, 23-01-12, 20:51 GMT 
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abramson wrote:
t00fri wrote:
Also, after lots of work with Nick Risinger's 3000x1500 Milkyway that includes beautifully most nebulosities and H_alpha regions, the M 4 - Antares region now looks pretty well, I think:


Beauuuutiful!

And the Gedankenexperiment, wunderbar.

G


Many thanks, Guillermo! When you wrote these pleasing sentences, I was just busy writing yet another thread with a subject that will surely interest you:

http://forum.celestialmatters.org/viewtopic.php?t=423

Cheers,
Fridger


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PostPosted: Wed, 25-01-12, 6:37 GMT 
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Evening twilight is much beautiful.


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