Is the Andromeda Galaxy Green Shifted and How is Red Shift Measured in Practice?

[Yolander]

I've always read that the proof of the expanding universe is the red shift in all observable stars/galaxies showing velocity away from Earth observation. I've also read that the Andromeda galaxy is on a collision course with our galaxy. Would'nt that mean the stars in Andromeda would be green shifted to an observer in our galaxy?

 

[Borg]

It is referred to as a Blue Shift. However, the shift would be minor.

 

[Yolander]

It is referred to as a Blue Shift. However, the shift would be minor.

Forgot about Blue Shift. I work as a paint color matcher and green is always considered the opposite of red. But it's interesting to note that red shift is not universal!

 

[Borg]

Forgot about Blue Shift. I work as a paint color matcher and green is always considered the opposite of red. But it's interesting to note that red shift is not universal!

The Andromeda Galaxy's blue shift due to its eventual collision with the Milky Way is one of the few. It is pretty minor compared to really distant objects when talking about red shift.

Redshift z = −0.001001
(minus sign
indicates blueshift)[1]

 

[mathman]

The use of the terms "red" and "blue" refer to the direction of the shift (red - lower frequency, blue - higher frequency). It has nothing to do with the colors as colors.

 

[Janus]

Forgot about Blue Shift. I work as a paint color matcher and green is always considered the opposite of red. But it's interesting to note that red shift is not universal!

The galaxies in our local group are close enough to be held together by their mutual gravitational attraction and are not effected by the expansion of the universe. To put it another way, galaxies form clusters that hold together and it is these clusters that are moving apart from each other.

 

[Yolander]

The use of the terms "red" and "blue" refer to the direction of the shift (red - lower frequency, blue - higher frequency). It has nothing to do with the colors as colors.

Yup I totally had a brain cramp knowing that compression of light wavelength moves toward ultra violet on the spectrum, sorry.

 

[berkeman]

Thread closed temporarily for Moderation...

EDIT -- Thread re-opened.

 

[ebos]

I've always wondered why light that moves towards us isn't called 'violet-shifted' (or indigo or green etc.). Is there a reason they stop at blue?

 

[rbelli1]

Is there a reason they stop at blue?

http://sciencequestionswithsurprisinganswers.org/2013/06/27/have-astronomers-ever-observed-a-violet-shift-like-they-have-blue-shifts-and-red-shifts/

It seems that limitations of the human sensory system gave us blue instead of purple. Black body radiation never approaches purple due to the way we perceive color.

https://en.wikipedia.org/wiki/Black-body_radiation#/media/File:PlanckianLocus.png

BoB

TIL: Why purple shift is not a thing.

 

[rootone]

I've always wondered why light that moves towards us isn't called 'violet-shifted' (or indigo or green etc.). Is there a reason they stop at blue?

I think it's just a convention that was arrived at because of the historical notion of 'primary colors', as used by artists.
That in turn arises from the fact that biologically normal humans have 3 kind of color receptor in the retina.
One of these is most sensitive to blue light, another to red light, and the third sensitive in the middle yellow area.
Hence we came to regard those 3 colors as 'pure' or 'primary', while all other colors were regarded as mixtures of the 3 primaries.

Now we know that the EM spectrum extends way beyond the visible range, it wouldn't be illogical to refer to 'X-ray frequency shifted' or 'radio frequency shifted', but a convention related to our everyday perception is easier to comprehend.

 

[sophiecentaur]

I've always wondered why light that moves towards us isn't called 'violet-shifted' (or indigo or green etc.). Is there a reason they stop at blue?

it wouldn't be illogical to refer to 'X-ray frequency shifted' or 'radio frequency shifted',

The original astronomers were only using optical telescopes and the limit of the distances they could observe was such that the detectable shifts were very much in the optical region (with some spillage over into IR, I guess) So the terms are based on history. But that goes for many of the phrases used in Science. I can be a waste of effort to worry too much about such things.

 

[rbelli1]

it wouldn't be illogical to refer to 'X-ray frequency shifted' or 'radio frequency shifted',

Except to a *human* using their *eyes* perceiving a red or blue shifted broad spectrum object they will see a change from red to blue. No matter how far it is shifted towards red or radio or DC it will simply fade to red then to black. No matter how far it is shifted to blue or violet or gamma rays it will fade to blue then to black.

For a narrow enough spectrum object the blue end may shift into purple. However normally visible celestial objects generally emit in a black-body-ish fashion.

Now what happens when a narrow spectrum high energy object gets red shifted into the purple? What do we call that?

BoB

 

[sophiecentaur]

Now what happens when a narrow spectrum high energy object gets red shifted into the purple? What do we call that?

This is the phenomenon on which red shift is actually measured in practice. Looking at the peak of the spectrum of a hot object is a hopelessly inaccurate way of assessing speed of recession. What is normally used is detailed spectral analysis of the light received and the line spectra of individual elements in the gases surrounding stars is used. Normally, I believe, they use the absorption spectrum of those gases -very narrow absorption lines due to Hydrogen etc atoms, through which the star's light is shining can be identified. The relative positions of the characteristic spectral lines can be seen (the fingerprint of the element) but all the lines will be shifted. The amount of that shift will give an accurate measure of the amount of red shift