Galactography

General physics and astronomy discussions not directly related to Celestia
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Paolo
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Galactography

Post #1by Paolo » 14.06.2006, 19:59

I've searched for a while with google but perhaps I'm missing the right terms.

I would like to know how the subdivision of the galaxy space problem was solved from the astronomical point of view .

E.g. for planets cartography we use latitude, longitude, meridians and parallels.
But for galaxy? Some kind of sectorization is required.
In science fiction this was solved with some approaches.
Does anyone know something about this from a scientifical point of view?

I'm interested in this in order to develop a strategy to split and manage the stars database.

Kind regards
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Post #2by Malenfant » 14.06.2006, 20:48

The only thing I know of is Galactic Longitude and Latitude... but that's measured relative to Sol. Otherwise, there's specific "arms" that stars are located in, but I don't know exactly how those are defined.
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selden
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Post #3by selden » 14.06.2006, 20:58

Professional astronomers don't divide the galaxy like Star Trek does, except when they're trying to be "cute".

There are physical divisions like central bulge, halo, bar, thin disk and thick disk, but they don't have precise boundaries. These divisions tend to contain particular stellar populations (I, II, late II, etc). Also, names have been given to what seem to be specific spiral arms, but, again, their boundaries aren't all that precise.

Positions of bodies sometimes are measured in galactocentric galactic coordinates, similar to the solar-centric galactic coordinates. Sorting by distance from the galactic center may be a reasonable approach.

I have seen infrequent descriptions which mention which geometric quadrant bodies are located in when using galactic coordinates. That'd be *somewhat* similar to the Star Trek convention, but if you use those search terms, you'll see Star Trek stuff at the very top of the list, not astronomical information.
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Post #4by Paolo » 14.06.2006, 23:15

Thank you Selden.

For similitude we can think at bulge and arms as we consider isles and continents on Earth. But this does not solve my problem.

I'm a bit surprised that it is so difficult to find informations about this issue.

If I won't be able to find something better I would like to use the following approach.

I think to a galactocentric coordinate system where latitude is expressed in terms of percentage of the disk radious, and longitude in degrees with zero placed on the opposed side of the galaxy where the conjunction line between sol and galaxy center crosses the disk limit.

The sectorization should be performed using this notation: r d h s

r : distance from the center (0..100)
d : degrees (0..360)
h : height (-100..100)
s : size (0..100)

But images are more explanatory.
Image Image Image Image

Adopting such kind of galactography to Celestia it should be possible to perform stars sectorization in order to split the stars databases.

Moreover this should add a galactic reference system when the observer moves through stars.

Perhaps sectorization should allow to use LOD for star rendering adding enhancement in stars octree culling and should be useful in order to go round the 16k distance limit problem.

With this reference systema it should even be possible to add an ZYX path to stars in order to simulate stars motion around galaxy center.

What do you think?

Kind regards
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Post #5by Malenfant » 14.06.2006, 23:31

But isn't there a big problem here? We only know the positions of the individual stars that are relatively near Sol (on a galactic scale)? So while we could make a version of a 'sectorisation' of our galaxy, in practice we're only actually going to be using the parts of it that are nearest Sol....
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Post #6by Paolo » 15.06.2006, 00:10

You are right we of course use Sol as reference. But I would like to extend the vision. I don't want to think like pre-Copernicans, IMHO the galaxy center is a better reference than Sol.

Moreover we should use sector coordinates as relative reference for stars. This perhaps should allow to avoid the 16k limit.

Sectorization and galactography should be useful also for other things.
One of the things that I miss much in Celestia is the absence of a visual reference system. If you don't use the Selden's grids add-on you are completely lost. If the observer is outside solar system and you play a bit with the mouse, its easy to loose the orientation.

If the Celestia's HUD should display the current galactographic coordinate of the observer or of the selected star it should be useful to understand immediately where is Sun (home) instead than type h-c. Moreover sector grids should be available and should help the orientation. What I would like to see in the HUD is some small images displaying the position of the observer related to the galaxy. The following image simulates this.

Image

There is a different problem. If star motion will be implemented the system will be time dependent so the galatographic reference has to be fixed at one date.

Kind regards
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Post #7by Paolo » 15.06.2006, 07:28

Assuming the data reported in http://www.glyphweb.com/esky/concepts/milkyway.html

And assuming for s a default value of 0.2% the optimal sector size should be approx. a cube of 50 Ly in size.

I would like to be able to show in Celestia something like this
http://www.anzwers.org/free/universe/index.html

http://www.anzwers.org/free/universe/50lys.html

BTW Celestia contains almost all the data necessary to show this.
A GUI suitable to show this data and some kind of such schematic view are missing.
I think that these info are quite understandable and valuable. Ok are already available on internet, but why not live 3D in Celestia?

Kind regards
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Post #8by Chuft-Captain » 15.06.2006, 08:38

selden wrote:... but if you use those search terms, you'll see Star Trek stuff at the very top of the list, not astronomical information.


Paolo,

You can get rid of the Sci-Fi by searching only on Google Scholar... http://scholar.google.com

eg. This search
http://scholar.google.com/scholar?q=gal ... tnG=Search
has this on the first page: http://adsabs.harvard.edu/abs/1960MNRAS.121..123B

Have fun! :)
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Post #9by Paolo » 15.06.2006, 15:40

Googled for a while but found only sol-centric cartography and catalogues. :?

Well, it is not very important indeed. :wink:
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Post #10by Puffin Magician » 19.06.2006, 17:34

I've thought a great deal about this topic myself, but more for galactic cartography itself rather than it's applications within Celestia.

Although I agree with selden that "Sectorization" is a staple of scifi with little current scientific use, I see it as a natural progression and colloquialization as our knowledge of the galaxy grows and we expand outwards into it.

Anyone that's looked at a map of Earth knows what and where the Prime Meridian is, but people never refer to their location as x kilometers from Greenwich and really only GPS satellites are interested in your Lat/Long coordinates.

At least one imaginary line needs to be drawn in order for this to work [in addition to using the Core and Galactic Plane/Ecliptic], and one might as well pass through Sol since we know where it is - Copernicus would be proud! :P

In everyday speech we'll say what political "section" we're in - be it a nation, province, or municipality. Many boundaries are either arbitrary lines or based around rivers - splitting the Galaxy into manageable sections that are themselves segmented by seemingly random stars and lines doesn't seem all that far-fetched to me.

Basing sectors around well-known interstellar objects [Nebulae are a good example] would be just as "normal" as saying one lives near the Suez Canal, Paris, or the Hawaiian Islands.

Terms like "Galactic Bulge", "Scutum-Crux Arm" or "Halo regions" are simply too massive to be used for any meaningful spatial division of this manner; about as [in]accurate as writing "Europe" as your return address on an envelope. :wink:

The only problem with selecting a far-flung point of reference like the Core is that for it to be accurate we need to be fairly good at knowing where everything else is as well - and we're barely started with the Solar System for now. How could we accurately determine where the Galactic Plane is in order to measure H from? Are there certain stars we know that are on/extremely close to this line to plot it from?

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Post #11by eburacum45 » 20.06.2006, 13:10

I find that Selden's galactic coordinate add-on (the green graticule to be specific) is perfectly adequate for mapping the local part of the Milky Way.
The top image on this page shows it in action
http://www.orionsarm.com/galactography/maps.html

To map the galactic plane in 3-d you can perhaps imagine an extension of this grid north and south of the plane for two or three thousand light years, so that the galaxy is divided up into a series of cubes a kilolightyear on a side.

I expect professional astronomers would want to use parsecs...

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As an artist..

Post #12by Startyger » 26.06.2006, 15:15

As an artist, i've always wondered why scientists haven't made greater attempts at... "realism" in this respect. If we know the Galactic center of Andromeda.. and have the power to view galaxies even more distant.. then.. IMHO we should be able to map Andromeda.. to some degree.. allowing us to 'grow' our understanding of the surrounding universe.

I hold this as a possibity. Just as the Illustrators of Dinosaur books began to solidify the idea of horizontal walking dinosaurs before much of the world accepted the theories.... Celestia may be a catalyst of change for understanding the universe as we know it... Stellarcartography.. Astrometrics... whatevery you wa nt to call it.... This could attract some heavy thinking and begin a new era of Human understanding.
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