What Factors Affect the Color of a Star from Earth?

[ranger]

Hey all,

What really contributes to the color of a star as we see it from earth? I've some articles which state that the doppler effect plays a role in this. As stars move away from us, the color appears towards to red end of the spectrum and moving closer to us; towards the blue end. I also gathered that the surface temperature of the stars affects this. Example our sun with a temperature of 6000 K, has a yellowish color.

Now I conclude that there are two things that affect the color, its relative motion towards or away from the Earth and the surface temperature of the star. But can't the two effects give misleading data? For example, if a star is moving away from us (doppler effect) how can we deduce the surface temp? The way I see it, the doppler effect will give a facade; not showing true color as it relates to surface temp.

--thanks

 

[Hurkyl]

You're right; the Doppler effect (as well as several other effects) need to be controlled if we want an accurate estimate of the surface temperature of the star!


It turns out that the light we receive from the star gives us enough information to account for the Doppler effect -- do you want to try and work it out yourself? The key is that stars don't emit just one frequency of light -- as you know, our own Sun (literally) emits every color of the rainbow, as well as infrared and ultraviolet rays. Can you imagine how we might be able to look at this spectrum and figure out how much it's been shifted?

 

[ranger]

Can you imagine how we might be able to look at this spectrum and figure out how much it's been shifted?

I did some research on this and this is what I could gather. Okay so we know that atoms and molecules can only absorb certain wavelengths (frequencies). Say molecule X can only absorb frequencies in the range of Y. When we analyze the [visible light] spectrum, we will see some dark lines in the spectrum corresponding to the wavelength (frequencies) which molecule X can absorb. These sets of lines are unique for each atomic element and always have the same spacing. Knowing this, we can deduce whether the object is red shifting or blue shifting (as the absorption lines move towards the blue end of the spectrum, the object is blue shifting; if the absorption lines are moving towards the red end, it is red shifting). And I guess we could actually tell how much its shifted by comparing it to the non shifted spectrum? I hope I got this down right.

 

[Hurkyl]

Yep; that's exactly my understanding of it!

Note that this method also accounts for gravitational redshifting.