Redshift or just red?

Question

So Hubble figured out that all the stars where moving away from us because the light was shifted to red.

How did he know that it was red-shifted and not just red to start off with?

I appreciate that "red" is just a useful tag and understand the Dopler effect.

The thing I don't understand is how:

if you cannot detect the light/radiation/whatever as it leaves the star to compare it with how it arrives at earth how do you know a process/change has occured between the star and earth. How do you know it didn't arrive on earth exactly as it left the star?

Answer 1

All elements when heated give off a signature in the light spectrum. For example if you heated a piece of pure iron until it glowed then put a prism in front of the light, you would see the spectrum of light -- obvious stuff. However because it is the metal iron there would be slots-- frequencies that would be missing. These signatures are all very different from each other depending on which element(s) are giving off light. When something is red shifted the signature is still there but it's place in the spectrum is in the wrong place. It has shifted to the red end of the spectrum-- it is in the longer or red end of it.

There is two ways to think of this, the other is that when heated elements will give off light in only so many frequencies- the rest of the frequencies are missing.

Answer 2

Elements such as hydrogen give off a particular wave length of visible light. Hubble knew what that wave length was. When you see a star that is giving off a wave length that is shifted toward the red end of the spectrum then you know that means that the star is moving away. The wave length of these stars does not put them in the red category, that is, they are not 'red to begin with' they are simply shifted toward red. The only explanation for this is that they are moving away.

Answer 3

The concept of "red-shift" has little to do with the light from distant stars being red and more to do with the fact that their wavelengths are shifted towards the red end of the visible light section in the electromagnetic spectrum. That's all it really means.

Objects moving away from us emitting light have their wavelenghts actually stretched; longer wavelengths and lower frequencies are in the direction of the color red on the electromagnetic spectrum. Conversely, in the opposite direction (towards blue), objects emitting light moving towards us are set to be blue-shifted. Their light isn't necessarily blue, but the wavelengths of that light are simply "scrunched" rather than "stretched."

Not all the light emitted from all stars (because all distant objects are moving away from us) emit red light. Hubble simply looked at the two components of a light wave -- wavelength and frequency -- and found that the light was accelerating, that there was a continuous stretching of the wave, not a simple observation of a "red-like wave."

Answer 4

The term redshift has nothing to do with the color. All elements and molecules have a spectrum associated with them of wavelengths that are radiated or absorbed by them depending on what state they're in. When we look at the spectrum of light from distant objects, the entire spectrum from each element in the object can be seen to shift and they shift toward the longer wavelengths. The term redshift is just a convenient way to refer to the phenomenon.

Answer 5

He never saw the light. Red shift refers to the hydrogen line in a star's spectrum shifting toward the red.

Answer 6

Several ways.
First, he used as a measuring stick a specific kind of star called a "Cepheid variable." These types of variable stars take their name from the archetype in our own galaxy in the constellation Cepheus. Cepheid variables have a very specific brightening-dimming sequence, amount of variation, and timing that's directly related to both their mass and their absolute magnitude. Therefore, if you measure a Cepheid's variable period, you know it's mass and absolute magnitude (and it's "color").
Using both the 100" Hooker telescope at Mt. Wilson and the 200" Hale telescope at Palomar Mountain, Hubble found dozens of Cepheids in the relatively nearby Andromeda galaxy to start with. After measuring their periods, he was able to know their absolute magnitude, and comparing that to their visual magnitude gave him the distance to the Andromeda galaxy -- just over 2 milion light years. Next he found Cepheids in several other galaxies, and found that the further the measured distance (measured as above by absolute vs. visual magnitude), the further their colors were shifted towards the red end of the spectrum. Since Hubble, we've found Cepheids in thousands and thousands of galaxies, and the distance-red shift variation holds true for every single one of them.

The Andromeda Cepheids, by the way, are blue-shifted, not red-shifted. That's because Andromeda and our own galaxy are part of the "local group" of galaxies, and we are actually moving towards Andromeda at a rate faster than the expansion of space-time. Our galaxy will collide with Andromeda in a few billion years' time -- wish I could be around to see it :)

Answer 7

he didn't determine that from the subjective color of the stars. He determined it when he noticed that the spectroscopic signature lines of hydrogen from distant galaxys was shifted toward the red. (Each element has a distinct spectroscopic line pattern)