Celestia Forums

For explaining the optimal adjustment of the standard galaxies in Celestia, let me illustrate the procedure with a single and beautiful example, the famous M 101 galaxy.

Two cases are to be distinguished:

A) the observer is located close to or on the Earth (meaning VERY FAR away from the galaxy)
B) the observer is located NOT VERY FAR away from the galaxy in question

Please make sure that the following settings are met for both case A) and case B):

• Automag = ON ( toggle with key 'CTRL+y')
• Adjust Auto magnitude at 45 degrees: -> 7.5 with '[' and ']' keys
• Show galaxies = ON ( toggle with key 'u')
• Show galaxy labels = ON ( toggle with key 'e')

Case A (Observer located in neighborhood of Earth, i.e. far away from galaxy)
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Here, proper adjustment is actually very simple.
Start a fresh Celestia. Then

• hit ENTER to make the command line appear at the bottom and type m 101 followed by ENTER.
  Then M 101 and further info will appear in the top left of the canvas.
• Hit the key 'c' to center M 101 in the display. At this point you will not yet see M 101 or at best you might see a pale dot!
• The only adjustment that remains to be done is to drag your mouse FORWARD with 'SHIFT+left mouse key' pressed (zoom) until you see M 101 in all it's splendor filling most of the display!
• That's it for case A

The result should about look like this:



Case B (Observer located not very far from galaxy (typically after hitting 'g' (GOTO) key))
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Start a fresh Celestia. Then

• hit ENTER to make the command line appear at the bottom and type m 101 followed by ENTER.
  Then M 101 and further info will appear in the top left of the canvas.
• hit the key 'g' to go to the galaxy M 101. With this key, the observer ends up still in a respectable distance from the galaxy. The galaxy covers ~ half the screen and is quite dim.
• Now you need to do a mix of TWO adjustments for optimal rendering!
• First drag your mouse BACKWARD with 'CTRL+left mouse key' pressed (increase distance of galaxy) until you almost can't see M 101 anymore. Typically stop when the M 101 label starts to fade!
• Then proceed as in case A by dragging your mouse FORWARD with 'SHIFT+left mouse key' pressed (zoom) until you see M 101 in all it's splendor filling most of the display!
• Then you may finally want to modify the galaxy's inclination by moving the mouse with 'right mouse key' pressed.
• That's all for case B!

The result would then look like this


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The reason behind this adjustment maneuver for case B) is that the Celestia galaxy rendering with sprites simply requires a sufficient observer distance to look natural... The effect of being close by must rather be created by zooming in (telescope effect)
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Enjoy,
Fridger

PS: you got more than 10000 galaxies in Celestia for practicing

Ah thank you much! Looks a lot better now.

My problem was I kept looking at them at much closer range without the zoom.

Not bad, Reiko!

Here is my way of displaying the MilkyWay



Fridger

Very nice. Where do you get the labels for the galactic arms?

Reiko wrote:Very nice. Where do you get the labels for the galactic arms?

I placed them by hand with GIMP.

You can also overlay Selden's nice Pulsars, so you can see how accurate the arms are!

face on:

sideways


Ignore the yellow crosses...

Fridger

This is off topic but I've noticed that in many science fiction books the galactic arms are treated as where all the stars are. The spaces in between the arms are treated is galactic deserts with little or no stellar population.
My question is, the arms glow as they do because of illuminated gas clouds, neubula etc.?

Reiko wrote:This is off topic but I've noticed that in many science fiction books the galactic arms are treated as where all the stars are. The spaces in between the arms are treated is galactic deserts with little or no stellar population.
My question is, the arms glow as they do because of illuminated gas clouds, neubula etc.?

Right, ...and due to billions of stars, of course! Galaxies are often so far away that individual stars can not be resolved visually...

Fridger

Yes billions of stars but why the gaps between the arms when you see a galaxy? What I'm wondering is why do the arms glow so bright and the gaps don't?

I would imagine because the "arms" contain the light emitting stars. The gaps between the arms would either contain nothing at all (so no light escapes) or possibly dark material (non-reflective).

- Jeff

Reiko wrote:Yes billions of stars but why the gaps between the arms when you see a galaxy? What I'm wondering is why do the arms glow so bright and the gaps don't?

Reiko,

with the help of "super computers", scientists can meanwhile simulate precisely, how spiral galaxies arise within rotating n-particle systems that are subject to gravitational interaction.

Here is a great video about two colliding and merging spiral galaxies. The simulation includes dark matter, gas, and stellar "particles" (, but only the baryonic component is visualized).

http://www.mpa-garching.mpg.de/galform/ ... galaxy.mpg

Credit: Volker Springel
Simulation code: Gadget

Fridger

Reiko wrote:Yes billions of stars but why the gaps between the arms when you see a galaxy? What I'm wondering is why do the arms glow so bright and the gaps don't?

The arms are where the interstellar matter is concentrated, therefore that's where most of the star formation takes place. Massive stars which are by far the most luminous don't live very long, so they don't manage to leave the spiral arms before they die. Therefore the luminosity is concentrated in the spiral arms.

ajtribick wrote:

Reiko wrote:Yes billions of stars but why the gaps between the arms when you see a galaxy? What I'm wondering is why do the arms glow so bright and the gaps don't?

The arms are where the interstellar matter is concentrated, therefore that's where most of the star formation takes place. Massive stars which are by far the most luminous don't live very long, so they don't manage to leave the spiral arms before they die. Therefore the luminosity is concentrated in the spiral arms.

We know that the life of galaxies starts structureless, typically in form of roughly ellipsoidal clumps of gas and dark matter. Hence, isn't Reiko's question rather why --at late times-- spiral arms tend to form, where most of the "late-time action" (star formation...) takes place? Moreover, don't forget the crucial role of Dark Matter in the whole process...

Any massive constituent of a rotating galaxy (gas, Dark Matter, stars,...globular clusters) is subject to different types of gravitational forces (rotational and non-rotational), and it is a non-trivial question, why a logarithmic spiral type of morphology is an energetically preferred late-time configuration compared to the original, structureless ellipsoidal shape.

Fridger

I believe you will find that the bright stars may be concentrated in the spiral arms, as Ajtribick has indicated; but older stars, including Sun-like stars, will be more evenly distributed, as they have had time to migrate out of the arms and distribute themselves more evenly.

So if you are looking for stars which might support life, they could be found in the darker spaces between the arms, just as frequently as in the arms themselves