Doppler effect and color of stars

[nseth1]

I was reading somewhere that the because of the red shift it can be concluded that the galaxies are moving away from us. But still when we talk about stars, red color refers to cooler stars rather than stars moving away from us.
Why?

 

[Garth]

Welcome to these Forums nseth1!

There are two important mechanisms that emit light.

One is the random motion of atoms in a hot body, and therefore the motion of electrons in the atom’s outer shells. This produces Black Body radiation in the form of a very specific continuous Planck spectrum. It is this form of radiation that depends on the body's temperature. The hotter a body is the 'bluer' is the radiation, each body radiates at all wavelengths but the peak of maximum intensity is at a specific wavelength that is inversely proportional to the temperature. You at about 300⁰K are radiating mainly in the infrared, the Sun at about 6,000⁰K is radiating mainly in the visible wave band with a peak at the wavelength of yellow light.

The second form of radiation is emitted/absorbed by an electron in one shell around the nucleus of an atom jumping to another shell of lower/higher potential energy. It is forced by quantum effects to jump from one specific ‘orbit’ to another specific ‘orbit’ and therefore emit a specific quantum of energy. Therefore each element may emit, or absorb, at a number of specific wavelengths of radiation. These show up on a spectrograph as a series of bright (emission) or dark (absorption) lines against the rainbow background of a body's black body spectrum. It is the spectrum of absorption, or emission, lines that is red shifted by recession in the Doppler effect.

I hope this helps,

Garth

 

[matt.o]

don't forget the continuum emission is also redshifted, not just the line emission!

 

[Garth]

Thank you - of course!
Garth

 

[DaveC426913]

"I was reading somewhere that the because of the red shift it can be concluded that the galaxies are moving away from us. But still when we talk about stars, red color refers to cooler stars rather than stars moving away from us.
Why?"

When we look at the stars in our own galaxy, they are - on average - not moving with respect to us. We'll see all colour of stars, from red through orange yellow to blue. When we take all the light from them together, they will average white.

When we look at stars in other galaxies, we know that, taken together, they should average white. We still see the whole spectrum of colours, red, yellow, blue, but we notice that every star's colour is very slightly reddened. This is due to the Doppler Effect, and tells us that the stars are receding from us.

 

[nseth1]

But for a lot of these stars we cannot be sure if they are stationary because of the distances involved (TRUE or FALSE?).
If we are not sure, then we can also conclude that their blue color is because they are moving towards us. What makes it so easy to distinguish between the two effects?
You can guess that I am a novice at Physics and trying to figure out some concepts :-).
Is that why the Gauls in the Asterix comics were scared of the falling sky since it is blue in color? ;-)

 

[matt.o]

stars in galaxies far enough away to have measurable cosmological redshift are not resolved. hence we don't measure the redshift of a single star in another galaxy, but take a measure of the whole galaxy spectrum.

the stars in our galaxy are definitely not stationary, in fact we can measure their red and blue shifts, but these are due to the motions of the stars(eg due to the galaxies gravitational field), not due to a cosmological redshift.

the colours of the nearby stars are not affected by redshift too much. for a star to appear red when it is actually blue, the star would have to be traveling away from us at a velocity such that it's entire spectrum was shifted by 200 angstroms. ie. at around $$133.3 \times 10^{6}$$m/s. That is a redshift of about 0.4, which is a long way away and not due to motions in our galaxy.

 

[Garth]

What makes it so easy to distinguish between the two effects?

The temperature of a star is given both by its colour and its spectral class, these two measurements of temperature ought to agree, if they don't then some correction is necessary.

Absorption by the interstellar medium will cause it to redden and has to be corrected for.

The Doppler shift can be read straight off the star's spectrum as the wavelength of, say the sodium doublet from the star, can be compared with that of sodium in the laboratory at zero red shift. Once the red shift is known, which gives the star's radial velocity, then the necessary correction to the observed colour is also known.

The measurement of colour, by the colour index and the measurement of red shift are two independent observations and there is normally no confusion between them.

I hope this helps,

Garth

 

[Chronos]

Kudos to matt and Garth. Individual stars in other galaxies are too distant to resolve. There is no cosmological redshift in stars within our own galaxy, hence any red or blue shift detected is strictly due to proper motion.

 

[Janus]

But for a lot of these stars we cannot be sure if they are stationary because of the distances involved (TRUE or FALSE?).
If we are not sure, then we can also conclude that their blue color is because they are moving towards us. What makes it so easy to distinguish between the two effects?
You can guess that I am a novice at Physics and trying to figure out some concepts :-).
Is that why the Gauls in the Asterix comics were scared of the falling sky since it is blue in color? ;-)

The color of the Star has nothing to do with determining its blue or red shift.
The actual color is related to its temp. Red shift is determined by the shift of the spectral lines, which is not affected by the stars color.

When we look at a Stars spectrum (break its light up into its component colors) we note that there are patterns of bright lines and black gaps, these are spectral lines and are produced by the element from which the star is composed. each element has a distinctive pattern od spectral lines like a fingerprint. The brightness of these lines and the actual element patterns that show up, define the spectral class of the star and its color.

A red star will have a certain pattern of spectral lines and a blue star another. With red or blue shifting due to Doppler effect what happens is that these patterns are shifted in position either to the red or blue end of the specturm. Thus a red star might show dimmer spectral lines for a certain element than a blue star, but all other things being equal, those lines appear at the same place in the spectrum for both stars. If we see those same lines for either star appear closer to the red part of the spectrum than they should be, that shift is due to Doppler shift.

Hence it is easy to tell the difference between color due to spectral class and temp and the red-shift caused by Doppler effect.

 

[Garth]

Chronos of course, those extra-galactic stars that we can resolve are in nearby galxies that do not have much red shift, if at all. However when considering the evolution of distant, i.e. early, galaxies the integrated light of billions of stars is red shifted relative to such an integration of the light from a nearby galaxy, and has to be corrected for.

BTW Well done the Huygens team!

Garth