K-Tec

Why are there no green stars?

  • albertw
  • albertw's Avatar Topic Author
  • Offline
  • IFAS Secretary
  • IFAS Secretary
  • Posts: 4173
  • Thank you received: 181

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/
19 years 5 months ago #369

Please Log in or Create an account to join the conversation.

  • Posts: 744
  • Thank you received: 25

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.
19 years 5 months ago #370

Please Log in or Create an account to join the conversation.

  • Posts: 3663
  • Thank you received: 2

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
19 years 5 months ago #372

Please Log in or Create an account to join the conversation.

  • michaeloconnell
  • michaeloconnell's Avatar
  • Offline
  • IFAS Astronomer of the Year 2009
  • IFAS Astronomer of the Year 2009
  • Posts: 6319
  • Thank you received: 298

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
19 years 5 months ago #383

Please Log in or Create an account to join the conversation.

  • albertw
  • albertw's Avatar Topic Author
  • Offline
  • IFAS Secretary
  • IFAS Secretary
  • Posts: 4173
  • Thank you received: 181

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/
19 years 5 months ago #387

Please Log in or Create an account to join the conversation.

  • albertw
  • albertw's Avatar Topic Author
  • Offline
  • IFAS Secretary
  • IFAS Secretary
  • Posts: 4173
  • Thank you received: 181

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/
19 years 5 months ago #478

Please Log in or Create an account to join the conversation.

  • Posts: 3663
  • Thank you received: 2

Replied by voyager on topic Re: Why are there no green stars?

Stars do NOT radiate as black bodies because the reactions happen deep in the core of the star, NOT in the photosphere so the make up of the star WILL affect the light it radiates.

This we know for a FACT because we used the spectrum of the sun to show that it is made of Hydrogen!

Bart.
My Home Page - www.bartbusschots.ie
19 years 5 months ago #479

Please Log in or Create an account to join the conversation.

  • albertw
  • albertw's Avatar Topic Author
  • Offline
  • IFAS Secretary
  • IFAS Secretary
  • Posts: 4173
  • Thank you received: 181

Replied by albertw on topic Re: Why are there no green stars?

ok perhaps I'm completely lost but I thought:
A Stars spectra was basically a black body curve.
The presence of H (or Fe, O etc.) in the corona causes absorption lines in that spectum, and that was how we know the elements in the corona.

If it wasnt then wouldnt we see just an emission spectra for the sun at Hydrogen wavelengths?

Have I got the end of the wrong stick here?

Cheers,
~Al
Albert White MSc FRAS
Chairperson, International Dark Sky Association - Irish Section
www.darksky.ie/
19 years 5 months ago #481

Please Log in or Create an account to join the conversation.

  • Posts: 3663
  • Thank you received: 2

Replied by voyager on topic Re: Why are there no green stars?

ok perhaps I'm completely lost but I thought:
A Stars spectra was basically a black body curve.
The presence of H (or Fe, O etc.) in the corona causes absorption lines in that spectum, and that was how we know the elements in the corona.

If it wasnt then wouldnt we see just an emission spectra for the sun at Hydrogen wavelengths?

Have I got the end of the wrong stick here?

Cheers,
~Al


OK, I can never remeber whether we look at emission or absorption spectra from the sun. However the colour is a direct result of the spectrum so it IS affected by the materials in the photosphere. In other words the material in the photosphere directly affects the colour we see a star as. We either see Everything except Hydrogen and co or ONLY Hydrogen and co.

I'm off to do some more research ... post more later.

Bart.
My Home Page - www.bartbusschots.ie
19 years 5 months ago #482

Please Log in or Create an account to join the conversation.

  • Posts: 3663
  • Thank you received: 2

Replied by voyager on topic Re: Why are there no green stars?

Well that didn't take long.

*Bart eats his hat*

OK, the sun IS approximately a black body, the spectrum we get from it is an absorption spectrum. Basically the Hydrogen and Helium in the sun are obsorbing some specific wavelengths, hence preventing us from seeing them but allowing trough the vast majority.

This means that for all intents and purposes the light we get from a star follows the black body curve for an object at what ever temperature the star is at. This curve is quite wide at the top so we always see a large range of wavelengths. We only consider a very small part of the spectrum to be green so I would suggest that we donlt see green stars because any star that is emitting green is also emitting other colours the drown out the green. Pretty much all visible stars radiate strongly in the wavelengths at the middle of the spectrum however cold stars will also radiate strongly towards the red end of the spectrum but weakly towards the violet end of te spectrum while hot stars radiate strongly in both the middle and indigo ends of thes pectrum but not in the red end.

This causes cold stars to appear redish as they contain red, orange and green light, medium stars to appear yellowis as they contain yellow green and blue light and hot stars to appear blueis as they contain green blue and indigo light. However no star appears green becasue green is a very specific colour and no star radiates in just one colour! This would be much easier to explain on a graph but basically the black body curve is too wide to allow any star radiate only green light.

Bart.
My Home Page - www.bartbusschots.ie
19 years 5 months ago #483

Please Log in or Create an account to join the conversation.

  • Posts: 8851
  • Thank you received: 237

Replied by dave_lillis on topic Re: Why are there no green stars?

To say it simply,,,

There are plenty of stars out there pumping out plenty of green light, it just happens that these stars also pump out alot of blue and red light so the star appears white.. .
Dave L. on facebook , See my images in flickr
Chairman. Shannonside Astronomy Club (Limerick)

Carrying around my 20" obsession is going to kill me,
but what a way to go. :)
+ 12"LX200, MK67, Meade2045, 4"refractor
19 years 5 months ago #496

Please Log in or Create an account to join the conversation.

  • Posts: 3663
  • Thank you received: 2

Replied by voyager on topic Re: Why are there no green stars?

To say it simply,,,

There are plenty of stars out there pumping out plenty of green light, it just happens that these stars also pump out alot of blue and red light so the star appears white.. .


Exactly!

(now why didn't I think o saying it like that!)
My Home Page - www.bartbusschots.ie
19 years 5 months ago #502

Please Log in or Create an account to join the conversation.

Time to create page: 0.066 seconds
Powered by Kunena Forum