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Mouthwash
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How galaxies/nebulae actually look.

09 Nov 2017 19:38

I'm a bit concerned over Space Engine's representation of them... while making them look exactly how they would to the naked eye isn't feasible (or fun), I don't like the idea of seeing totally false colors or shapes. How realistic are the depictions?

1. It seems like nebulae are going to be totally false color in the next version, which is disappointing. Correct me if I'm wrong, but those pictures don't give me an impression of realism.

2. How much of the visible light of galaxies is generated by stars? Seems like the star density in SE above the Milky Way's core is just as thick as in the arms, but that isn't shown in the galaxy model. If this is realistic, then maybe the arms are more visible due to having lots of gas/dust.
 
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Watsisname
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How galaxies/nebulae actually look.

09 Nov 2017 23:32

Mouthwash wrote:
Source of the post 1. It seems like nebulae are going to be totally false color in the next version, which is disappointing. Correct me if I'm wrong, but those pictures don't give me an impression of realism.

False color images of nebulae are pervasive, but false color does not necessarily mean wrong color.  Sometimes astrophotos are taken through specific wavelength filters and then those images are mapped to totally different colors, or they may map non-visible wavelengths to visible colors, which of course will look very unlike how the real thing looks to the eye or even a long exposure with a typical color camera.  But other times the images may be mapped to the same color that the filter transmits, in which case the result is pretty close to true color.

To get an idea of the true colors of a nebulae, a helpful tool is to spread its image out into a spectrum, as in a "slitless spectrograph".  Here's an example for the Cat's Eye Nebula (top) and Ring Nebula (bottom).

Image

The primary colors are red and a greenish turquoise.  Red is from the hydrogen-alpha line, while the turquoise comes from Hydrogen-beta and Oxygen-III.  Which matches up pretty well with the following astrophotographs:

► Show Spoiler


So the true color of nebulae actually is pretty close to what the SE snapshots show.

2)  The majority of a galaxy's luminosity in visible wavelengths is from stars.  Dust dims and reddens the visible light that passes through it and re-emits in infrared.  Clouds of neutral gas are non-luminous (they glow in radio instead).  So the only real visibly luminous feature of galaxies besides stars is ionized gas, which glows with the characteristic redish-pink of hydrogen, and is the main feature of star formation regions.
 
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problemecium
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How galaxies/nebulae actually look.

11 Nov 2017 16:34

Also you can roughly approximate naked-eye appearance for galaxies and nebulae by cranking down the "models lighting" or "GlowMagnLimit" (depending on your version) variable in your config file to 0 or 1. I currently have mine at 2 so I have a balance of realism and visibility.

P.S.: Neat post, Watsisname!
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Mouthwash
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How galaxies/nebulae actually look.

13 Nov 2017 02:15

Watsisname wrote:
Mouthwash wrote:
Source of the post 1. It seems like nebulae are going to be totally false color in the next version, which is disappointing. Correct me if I'm wrong, but those pictures don't give me an impression of realism.

False color images of nebulae are pervasive, but false color does not necessarily mean wrong color.  Sometimes astrophotos are taken through specific wavelength filters and then those images are mapped to totally different colors, or they may map non-visible wavelengths to visible colors, which of course will look very unlike how the real thing looks to the eye or even a long exposure with a typical color camera.  But other times the images may be mapped to the same color that the filter transmits, in which case the result is pretty close to true color.

To get an idea of the true colors of a nebulae, a helpful tool is to spread its image out into a spectrum, as in a "slitless spectrograph".  Here's an example for the Cat's Eye Nebula (top) and Ring Nebula (bottom).

Image

The primary colors are red and a greenish turquoise.  Red is from the hydrogen-alpha line, while the turquoise comes from Hydrogen-beta and Oxygen-III.  Which matches up pretty well with the following astrophotographs:

► Show Spoiler


So the true color of nebulae actually is pretty close to what the SE snapshots show.

So the whole new nebula system is based on this?

Mouthwash wrote:
2)  The majority of a galaxy's luminosity in visible wavelengths is from stars.  Dust dims and reddens the visible light that passes through it and re-emits in infrared.  Clouds of neutral gas are non-luminous (they glow in radio instead).  So the only real visibly luminous feature of galaxies besides stars is ionized gas, which glows with the characteristic redish-pink of hydrogen, and is the main feature of star formation regions.

Then why is there a thick glob of stars extending above and below the galactic core? Is something smothering their light, or is this an inaccuracy in SE?
 
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Watsisname
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How galaxies/nebulae actually look.

13 Nov 2017 08:45

I couldn't say if that's specifically what it is based on. Only that the colors and shapes in the previews look pretty faithful to real nebulae.

As for the light extending above and below the core, perhaps you're seeing the galactic bulge.
 
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Mouthwash
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How galaxies/nebulae actually look.

15 Nov 2017 20:55

Watsisname wrote:
As for the light extending above and below the core, perhaps you're seeing the galactic bulge.

I'm *not* seeing light. I'm seeing lots of stars, but no indication from a distance that anything is there.
 
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Watsisname
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How galaxies/nebulae actually look.

16 Nov 2017 01:35

Sorry I misunderstood.  But I think that this makes sense:

Reality:

Image

Space Engine:

Image

The stars of the galactic bulge are visible from a distance in a manner quite consistent with reality.  Yes, from up close you can see stars and globular clusters at a greater distance from the core, but think about their density compared to closer to the center.  From a distance, that outer bulge and galactic halo is less visible compared to the more star-rich inner regions and the disk.  Nothing is blocking the light, there just isn't as much of it.
 
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XBrain130
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How galaxies/nebulae actually look.

16 Nov 2017 06:06

I think they're talking about this:

scr00052.png


This is actually somewhat wrong: the spherical blob around the core bulge should envelop the entire galaxy instead :P
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Mouthwash
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How galaxies/nebulae actually look.

20 Nov 2017 23:32

Huh, looks like I was wrong. After looking in the search browser, it appears that the stars above and below the bulge are actually an order of magnitude sparser than the stars in the disk. They looked almost identical when I was flying around them. :?
 
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evildrganymede
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How galaxies/nebulae actually look.

21 Jan 2018 11:23

The problem is that all these photos are long exposure. Our eyes don't do that - when we look at the Andromeda galaxy with the naked eye all we see is a fuzzy patch. 

Also, in reality as we get closer to a nebula they should actually get *dimmer*, because the same amount of light is being spread out over a larger area of sky as we approach.
 
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problemecium
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How galaxies/nebulae actually look.

21 Jan 2018 16:58

The second part isn't entirely right.
The nebula may indeed appear dimmer, as the human eye has its own auto-exposure-like feature that makes point light sources look like white dots, but in terms of the absolute amount of light entering the eye, it would be much brighter.
This is because the amount of light received isn't "the same amount," but rather increases as one approaches based on the inverse-square law. Since the visible area increases at the same rate, you could think of the amount of light per unit visible area always remaining the same; and since the visible area increases greatly, so too does the total received light.
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evildrganymede
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How galaxies/nebulae actually look.

21 Jan 2018 17:59

I don't think that's right. For the light per unit visible area to be the same would require each "unit area" to be as bright as the whole nebula from further away, which makes no sense. If you have a nebula being barely visible when it's half a degree across, then when you get closer to it and it's 10 degrees across that same amount of light has to be distributed over that much bigger area. There aren't more light sources being magically added to the nebula as you approach to keep its brightness constant at all distances.

"The big issue is that nebulae are just too faint for the human eye to see. And while it’s tempting to think that they’d look brighter from up close, in fact this isn’t actually true — they actually look just as bright from any distance! This is a law of optics, known in the jargon as the “conservation of surface brightness”. The key is that there are two competing effects in play. Imagine that you can see a nebula that’s, say, the size of the full moon.

Yes, if you get closer, your eye will receive more total power from the nebula. But the nebula will also look bigger, so that energy will be spread out over a larger visual area (technically: “solid angle”). The physics tells you that the power per solid angle in fact stays exactly the same, and this quantity is precisely the “brightness” of an object. So if nebula are too faint for to see from Earth with the naked eye — and they are — getting up close and personal doesn’t help any."
from: https://www.universetoday.com/99989/in-reality-nebulae-offer-no-place-for-spaceships-to-hide/
 
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Watsisname
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How galaxies/nebulae actually look.

21 Jan 2018 20:40

problemecium is correct.  The total amount of light that you receive from the nebula increases as you get closer (the inverse square law), but the surface brightness stays the same (because the solid angle the object takes up on the sky also follows the inverse square law).  If you move twice as far away, you get 4 times less light, but the area it takes up on the sky is also 4 times less.

This is actually what the material you quoted from universetoday is saying.  
 
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evildrganymede
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How galaxies/nebulae actually look.

22 Jan 2018 00:06

Oh i see! But that means the nebula is as bright as it was when viewed say from Earth - so I guess I should have said "as we get closer, the nebula doesn't get any brighter" rather than "as we get closer, the nebula should actually get dimmer". Either way nebulae certainly shouldn't be as visible in reality as they are in SE.
 
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Watsisname
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22 Jan 2018 00:18

Yeah, this is something that often gets confusing, and can be compounded by sloppy terminology.  By "bright", we should really distinguish whether we mean the "surface brightness" (for example, the magnitudes per square arcminute on the sky), or the "flux" (power per unit area received at your location, which corresponds to the object's apparent magnitude).  Apparent magnitude and flux depend on the distance, whereas surface brightness is a property of the object itself and does not depend on viewing distance.

(I almost said apparent magnitude and flux decrease with distance, but then that would be wrong since the magnitude scale is defined backwards). :P

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