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Why are there no green stars?

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20 years 10 months ago #369 by albertw
Why are there no green stars? was created by albertw
Why are there no green stars?

A question that came up on a couse that I'm doing...

Cheers,
~Al

Albert White MSc FRAS
Chairperson, International Dark Sky Association - Irish Section
www.darksky.ie/

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20 years 10 months ago #370 by stepryan
Replied by stepryan on topic Re: Why are there no green stars?
albert,
i would guess that maybe they do but that we cannot see it. cannot you not get filters that show oxygen in nebulae?. i would say that there is not much light emitted at that frequency or that it is absorbed by the interstellar medium so much that we cannot see it. from having a quick browse of the net you can see that the eye is most sensitive to green and yellow light see below

www.erin.utoronto.ca/~astro/ast110/lectures/colours.html

seeing the eye is most sensitive at those frequencies and we can see yellow stars then we should be able to see light. seeing that a stars colour gives it it's colour i would say that maybe it doesn't stay long enough in the temprature region to emit the light but i would guess that this is unlikely even if it was there should be given the odds at least one visible given the number of stars in the universe. if i remember correctly oxygen is the element that emits green light. seeing that in the fusion cycle oxygen is the 3rd element produced after helium and carbon there should be a good amount of it in stars and it emitting green light.

as far as i can see there is no reason that it cannot happen but that seeing that there are very few green stars that there must be something preventing it. the only 2 reasons i can see is 1) the star is not long enought at that temprature because for some reason it is not stable in the temprature range needed to make green light or 2) it is being absorbed or
used up in some way.

it is an interesting question.

david do i qualify for my cosmologists badge yet for either a) being obtuse b) being wrong c) crafting a good theory and still being wrong or d) confusing people by sounding knowledgeable and getting away with it ?
:wink: .

stephen.

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20 years 10 months ago #372 by voyager
Replied by voyager on topic Re: Why are there no green stars?
OK, I had never thought of this but the more I think about it the more obvious the answer is to me.

There can only be one reason we do not see green stars and that is because they do not exist, if they did we would see them with ease.

So that lead me to thinking why can we see yellow and red and blue stars? The reason is that Hydrogen emmits light at those frequencies. Hydrogen does not emit green ligh so anything made of hydrogen will never look green.

We know that oxygen looks green and we can see it in nebuale all over the place so since old large stars contain oxygen you might think that they would shine green. But they don't!

The reason they don't is that the light we see from stars is emitted by the photoshpere which is at the surface of the star. Stars that contain oxygen do not contain it at the surface but in a shell inside the star. At every stage of a stars life the outer shell is still made up of Hydrogen, internally the star will consist of concentric shells of other heavier elements but the outer shell remains Hydrogen.

My Home Page - www.bartbusschots.ie

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20 years 10 months ago #383 by michaeloconnell
Replied by michaeloconnell on topic Re: Why are there no green stars?
Ver good question which I never thought of myself. Here's a few more places which hopefully answer your query.

imagine.gsfc.nasa.gov/docs/ask_astro/answers/970408e.html

curious.astro.cornell.edu/question.php?number=14

www.science.psu.edu/alert/Ciardullo1-1999.htm

Michael

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20 years 10 months ago #387 by albertw
Replied by albertw on topic Re: Why are there no green stars?

Ver good question which I never thought of myself. Here's a few more places which hopefully answer your query.

imagine.gsfc.nasa.gov/docs/ask_astro/answers/970408e.html

curious.astro.cornell.edu/question.php?number=14

www.science.psu.edu/alert/Ciardullo1-1999.htm

Michael


yea they were the URLS that camne up in the course too.

Basically stars colour depends on temperature, not elements in them (since the reactions are nuclear not chemical), and follow a black body curve.

When the peak of the curve is in the green band there is also lots of yellow and orange present. so we see orange. Apparently the Sun is a yellow-green star by its curve, but we dont observe any green.

Cheers,
~Al

Albert White MSc FRAS
Chairperson, International Dark Sky Association - Irish Section
www.darksky.ie/

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20 years 10 months ago #478 by albertw
Replied by albertw on topic Re: Why are there no green stars?
heres another angle....

Assuming stars colours follow temperature, then shouldnt there be stars that are so hot that they only radiate in the ultra-violet? or at the extreme, x-ray and gamma ray? And so be invisible at optical wavelenghts?

I need to go work out the maths I think...

Some objects do radiate at these higher frequencies, but they arnt normal stars.

Cheers,
~Al

Albert White MSc FRAS
Chairperson, International Dark Sky Association - Irish Section
www.darksky.ie/

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