Celestia Forums

If I select the Milky Way. Goto it, then kill Galaxy display (U toggle) the individual stars are visible increasing in number as star brightness is increased(}). Intuitively I would have expected globular clusters to do the same and be very much brighter than individual stars. In fact these clusters remain virtually invisible. I had imagined the galaxy surrounded by a mass of balls of light.
Does the appearance in Celestia represent what would appear in reality?
Regards, David.

Just to clarify what I said previously:- why are globular clusters less bright than individual stars at comparable distances?
Regards, David.

I can give a generic answer: globular clusters aren't drawn as individual stars. They're drawn using a specialized routine which currently doesn't understand the various luminosity commands. One can hope that this will be improved in a future version of Celestia.

DOJOMO wrote:Just to clarify what I said previously:- why are globular clusters less bright than individual stars at comparable distances?
Regards, David.

I coded the globular clusters in Celestia. Their luminosity distribution has been incorporated according to scientific standards.

The nominal brightness (i.e. app. magnitude) of a globular involves integration of the cluster's surface brightness [magnitude/arcsec^2] over an extended area (the cluster size), while for single stars the area is effectively a point.
Globulars consist of very many faint stars, each being of much weaker brightness than a single star with the same apparent magnitude as the entire globular.

If you happen to look at the sky at times, you may well check that a globular of app.mag=5 is much harder to see with naked eyes than a single star with app.mag=5.

I hope you always have the "auto-magnitude" feature switched ON.

@Selden:

They're drawn using a specialized routine which currently doesn't understand the various luminosity commands.

Could you please be more specific about my code?

Fridger

Thanks for the both replies.
I guess that it means that globular clusters are not visible when viewed from a distance encompassing the whole Milky Way. Presumably images from Hubble of nearby galaxies which seem to show bright spots are not globulars but stars in the Milky Way in the line of sight.

Regards, David.

David,

a precise rendering of the visual impression of brightness in Celestia is very hard due to the poor dynamical range of computer monitors etc. The best example of this basic problem is the poor rendering of stars at close distance. In reality they would be so bright that humans would get instantly blind when looking at them...

You may see this generic problem also by looking at a particular star and then by toggling the various star rendering models with CTRL+s. The variation of apparent brightness is very large, due to the varying extent of the star disks...On monitors we can mainly emulate a larger range of star brightness in form of increasing disk sizes of the stars!

So clearly, a direct comparison of a globular and a star suffers from even larger problems. All this is independent of the underlying astrophysics, of course.

In the following image I did what you did above and compared the globular M22 with a bright star at about the same distance. At the observer's position (~ 20 Kpc away from the objects), the app.mag of M22 is 8.0 and that of the star (mu Sgr) is 10.1. Indeed M22 looks somewhat brighter than the star. But all of this is to be taken with the caveats I outlined above.

For completeness, in the next image, I did ONLY increase the zoom value (it's like using a telescope at the original observer position).



Of course now, the globular M22 seems much brighter than mu Sgr, but nevertheless, the ratio of app.magnitudes is still unchanged. So the viewing angle is also an important parameter as we know from using telescopes.


Fridger

Although implemented differently in Celestia, galaxies and globular clusters both are primarily extended starfields. It would be nice if the "light gain" functions ( and ) affected the visibility of globulars similar to the way they affect the visibility of galaxies. Currently they do not.

Thanks for your responses. Fridger's explanation is clear and kindly aimed at a dumbo's limited understanding of these things. I was able to repeat the example although I employed a different star as my catalogue doesn't seem to contain the one mentioned by him.
I agree that the galaxy brightness control would be a bonus, at least for me, in my search for "pretty pictures.

Many thanks, David.

selden wrote:Although implemented differently in Celestia, galaxies and globular clusters both are primarily extended starfields. It would be nice if the "light gain" functions ( and ) affected the visibility of globulars similar to the way they affect the visibility of galaxies. Currently they do not.

Of course, I thought about that (the code is trivial). But coupling the extra "light gain", i.e. the '(' and ')' keys, to BOTH the galaxies and the globulars would have led to various intolerable disadvantages. Hence, the only practicable alternative might have been another key which we simply don't have left over!

I think, we nevertheless have sufficient freedom to adjust globular brightness in a sensible manner: namely via the '[' and ']' keys. After all, these keys are largely for adjusting star limiting mags and globulars are a bunch of stars that appear resolved except in the core (i.e. quite unlike galaxies). This is illustrated here. The two renderings of Omega Cen only differ by adjustment of the '[' and ']' keys!


This assignment allowed to decouple the amount of extra brightness for galaxies and globulars. Since many of my 10000+ galaxies are quite dim (14th to 16.5th apparent magnitude for an observer in the solar system), a direct coupling of the light-gain key for galaxies and globulars would have rendered the latter as featureless white blobs when the light was just appropriate for these dim galaxies! This was intolerable for me, although this could be fixed in principle by reducing the "slope" for the globular light gain relative to the one for galaxies. On the other hand, the typical range of HIP star magnitudes and globular cluster magnitudes is quite similar, which makes a coupling of their brightness adjustment physically and practically more sensible!

As an aside, in Celestia.Sci I am pursuing a much more sophisticated and physically more correct approach to brightness AND color in general. When I find some time I'll present another "white paper" about that in Celestial Matters. Because this topic being of more general interest, I had promised to ChrisL to do it here under the premise that he'll restore the Latex feature at shatters. I do need rather complex math formulae for that! This was last summer and still there is no working Latex.

Fridger

DOJOMO wrote:I agree that the galaxy brightness control would be a bonus, at least for me, in my search for "pretty pictures.

Many thanks, David.

David,

was this a typo? We do have of course a (separate) galaxy brightness control [the '(' and ')' keys]. As I detailed in my previous post, we also have a globular brightness control coupled to that for individual stars [the [ and ] keys].

Good to see that my explanatory efforts above were somewhat useful.

Fridger

Fridger,
It was a sort of typo. I meant linking the galaxy brightness control to globulars as well. I see from your previous post that this may have serious limitations. I was, however, aware of {} controlling the star magnitude and globular brightness.

Again, many thanks for taking so much trouble.
Regards, David.