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Old 12-15-2017, 07:00 PM
 
Location: Near Luxembourg
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In my mind, stars are black corpses (well hydrogen and helium compressed by gravity, which creates more or less heat regarding the initial mass of the initial cloud) that emit a part of their energy through the form of light.
Some of them are red like rupees, colder because of their low mass or huge size (Mu Ceiphei...), orange (Antares), some of them are white (Vega), extremely hot and blue (Alnilam..), or even with a large part of the spectrum in the purple (Naos) (reaching the limit of heat for some rare cases) . While purple stars are hard to see because we detect the blue much more easily and don't see UVs,

What about the green? It's in the 'center' of the spectrum so green stars are a mix, so we see them 'white'?

So our sun is technically 'a green star'??

I was wondering suddenly, watching through my telescope...
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Old 12-15-2017, 11:27 PM
 
3,426 posts, read 2,791,710 times
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Quote:
Originally Posted by Pokitobounto View Post
In my mind, stars are black corpses (well hydrogen and helium compressed by gravity, which creates more or less heat regarding the initial mass of the initial cloud) that emit a part of their energy through the form of light.
Some of them are red like rupees, colder because of their low mass or huge size (Mu Ceiphei...), orange (Antares), some of them are white (Vega), extremely hot and blue (Alnilam..), or even with a large part of the spectrum in the purple (Naos) (reaching the limit of heat for some rare cases) . While purple stars are hard to see because we detect the blue much more easily and don't see UVs,

What about the green? It's in the 'center' of the spectrum so green stars are a mix, so we see them 'white'?

So our sun is technically 'a green star'??

I was wondering suddenly, watching through my telescope...
Some of your facts are not in line with what we know. Red stars can be huge, like Antares, which is also massive, by the way. However, the vast majority of stars are red dwarfs. Proxima centauri is such a red dwarf. We don't see stars as green because stars that emit green color also emit every other color in the visible spectrum, and so we see them as white. Our sun is technically not a green star. It is a yellow dwarf, a main sequence G2 star.
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Old 12-16-2017, 12:42 AM
 
Location: Somewhere in northern Alabama
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Rubies, not rupees.

I think you are trying to bring up black body radiation. The long answer is here:

https://en.wikipedia.org/wiki/Black-body_radiation

The short answer is as above - a BB radiating at around that temperature is also radiating a number of other colors at the same time.
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Old 12-16-2017, 04:13 AM
 
Location: Near Luxembourg
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Originally Posted by orogenicman View Post
Some of your facts are not in line with what we know. Red stars can be huge, like Antares, which is also massive, by the way. However, the vast majority of stars are red dwarfs. Proxima centauri is such a red dwarf. We don't see stars as green because stars that emit green color also emit every other color in the visible spectrum, and so we see them as white. Our sun is technically not a green star. It is a yellow dwarf, a main sequence G2 star.
Thanks!

I m not talking about the type of star, simply the color, even if the mass and size are linked. I know the more massive and young they are, the more they tend to be hot... Might be a rough shortcut for a pro, but it has a part of truth
Our brave sun is too cold yes (what, 5500-6000 k roughly?), to have its peak of emission in the green... I won't consider it as a 'green star' then .

Then comes the next part of the question :

Why do I see that planetary nebula, NGC 6572 (Emerald nebula), really, really green?
This time it doesn't emit in the rest of the spectrum to see it white??? Why?

Sorry for rupees , I m a Gaulois, I thought it meant rubies... I learned an other stuff lol
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Old 12-16-2017, 04:17 AM
 
Location: Near Luxembourg
1,915 posts, read 1,001,539 times
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Originally Posted by harry chickpea View Post
Rubies, not rupees.

I think you are trying to bring up black body radiation. The long answer is here:

https://en.wikipedia.org/wiki/Black-body_radiation

The short answer is as above - a BB radiating at around that temperature is also radiating a number of other colors at the same time.
Roger, still sad that we see most of the colors, but not the green... I wonder how the sky would look like with stars going from red (well only red (super)giants are visible easily) to purple, including green openly visible lol.
I ll stay with my ngc6572 to see green in the night sky...
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Old 12-16-2017, 06:41 AM
 
3,426 posts, read 2,791,710 times
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Quote:
Originally Posted by Pokitobounto View Post
Thanks!

I m not talking about the type of star, simply the color, even if the mass and size are linked. I know the more massive and young they are, the more they tend to be hot... Might be a rough shortcut for a pro, but it has a part of truth
Our brave sun is too cold yes (what, 5500-6000 k roughly?), to have its peak of emission in the green... I won't consider it as a 'green star' then .

Then comes the next part of the question :

Why do I see that planetary nebula, NGC 6572 (Emerald nebula), really, really green?
This time it doesn't emit in the rest of the spectrum to see it white??? Why?

Sorry for rupees , I m a Gaulois, I thought it meant rubies... I learned an other stuff lol
Many planetary nebulas emit in green because of the presence of triply ionized oxygen.
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Old 12-16-2017, 09:12 AM
 
Location: Near Luxembourg
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Quote:
Originally Posted by orogenicman View Post
Many planetary nebulas emit in green because of the presence of triply ionized oxygen.
Ok

Dudes in Wiki wrote this:

"The spectra of planetary nebulae are fundamentally different from those of stars. Instead of a continuous color from red to blue as in the case of the Sun, the spectra of planetary nebulae are dominated by discrete emission lines emitted by atoms and ions. Unlike stars, whose continuous spectra give them a composite white appearance, planetary nebulae have a rich variety of colors. Some examples of strong emission lines are the red line of hydrogen and the green line of doubly ionized oxygen (O++). These bright emission lines are powered by the central star, which is the source of energy for the entire nebula. Ultraviolet light emitted by the central star is intercepted by atoms in the nebula and converted to visible line radiation."

I found this, that is pretty easy to catch for beginner like me that are wondering why PN are green, and stars not :
Absorption and Emission

good to understand this weird phenomena (IMO lol)

Thank you for your answers that put me on the right path,
Well, I will put filters and try to see everything green lol

I will try to find an other weird question, watching the night sky
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Old 12-18-2017, 01:22 AM
 
3,426 posts, read 2,791,710 times
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Quote:
Originally Posted by Pokitobounto View Post
Thanks!

I m not talking about the type of star, simply the color, even if the mass and size are linked. I know the more massive and young they are, the more they tend to be hot... Might be a rough shortcut for a pro, but it has a part of truth
Our brave sun is too cold yes (what, 5500-6000 k roughly?), to have its peak of emission in the green... I won't consider it as a 'green star' then .

Then comes the next part of the question :

Why do I see that planetary nebula, NGC 6572 (Emerald nebula), really, really green?
This time it doesn't emit in the rest of the spectrum to see it white??? Why?

Sorry for rupees , I m a Gaulois, I thought it meant rubies... I learned an other stuff lol
The type of star directly relates to its color.




As far as planetary nebulae are concerned, as I said before, many planetary nebulas emit green light because of the presence of triply ionized oxygen, ionized by the radiation emitted by the star that formed the nebula.
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Old 12-18-2017, 11:10 AM
 
Location: Near Luxembourg
1,915 posts, read 1,001,539 times
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Quote:
Originally Posted by orogenicman View Post
The type of star directly relates to its color.




As far as planetary nebulae are concerned, as I said before, many planetary nebulas emit green light because of the presence of triply ionized oxygen, ionized by the radiation emitted by the star that formed the nebula.
No you don't understand what I meant... I just wanted to know why a star (of the main sequence if you want, doesn't matter), never shine mostly in the green, and appears white, not green.
The reason is not the mass, the age, the diameter, the metallicity...

No, it's that stars are dense enough to have a continuous spectrum so the green is mixed with other wavelength resulting into a white light... That's what I wanted to know.
I know Mu Ceiphei is colder than Rigel, one is red the other is blue... Idem for red dwarves that are too small and cold to shine in the blue. But doesn't explain the green story

For the planetary nebulas, ok, the ionized hydrogen is green.
Now I don't understand why I filter them with an OIII filter (doubly ionized oxygen with this notation no?? Or I misread... ) and you say the oxygen triply ionized to shine green
I think it is ionized 2 times by the ultra hot core of the dead star, no?

https://en.m.wikipedia.org/wiki/Doubly_ionized_oxygen
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Old 12-21-2017, 10:27 AM
 
3,426 posts, read 2,791,710 times
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Quote:
Originally Posted by Pokitobounto View Post
No you don't understand what I meant... I just wanted to know why a star (of the main sequence if you want, doesn't matter), never shine mostly in the green, and appears white, not green.
The reason is not the mass, the age, the diameter, the metallicity...

No, it's that stars are dense enough to have a continuous spectrum so the green is mixed with other wavelength resulting into a white light... That's what I wanted to know.
I know Mu Ceiphei is colder than Rigel, one is red the other is blue... Idem for red dwarves that are too small and cold to shine in the blue. But doesn't explain the green story

For the planetary nebulas, ok, the ionized hydrogen is green.
Now I don't understand why I filter them with an OIII filter (doubly ionized oxygen with this notation no?? Or I misread... ) and you say the oxygen triply ionized to shine green
I think it is ionized 2 times by the ultra hot core of the dead star, no?

https://en.m.wikipedia.org/wiki/Doubly_ionized_oxygen
As I explained before, the reason you don't see green stars is because stars that emit green color also emit all other colors in the visible spectrum. Black body radiation or a star is never green. It is a matter of color temperature, which is related to its composition, mass and temperature.
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