Edwin Hubble's Redshift and the Hubble Space Telescope

[Holocene]

I have read that Edwin Hubble first noted the "reddening" of galaxies, and that this redshift was evidence for the idea that the universe is expanding.

Now, why do none of the galaxies seen in pictures from the Hubble Space Telescope appear to be red?

Does redshift simply mean that the light from the galaxies is reaching us at longer wavelengths, or does it literally mean that galaxies ought to look red when seen through a telescope?

 

[berkeman]

I'm no expert, but the shift to longer wavelengths of the emission and absorption spectra is small. Certainly not enough to be noticed by the naked eye -- you need a sensitive spectrometer. You match up the spectra of far-away stars with the spectra of nearby stars, and see that the spectral lines of the distant star are shifted slightly in the direction of longer wavelengths. Like in this picture:

http://en.wikipedia.org/wiki/Red_shift

 

[SpaceTiger]

I have read that Edwin Hubble first noted the "reddening" of galaxies, and that this redshift was evidence for the idea that the universe is expanding.

Now, why do none of the galaxies seen in pictures from the Hubble Space Telescope appear to be red?

Depends on which galaxies you're referring to. The ones very nearby will only have very small redshifts that won't be noticable to the naked eye, as berkeman already said. However, if you look at the Hubble Deep Field or Ultradeep Field, the galaxies should look quite a bit redder than, say, M31, even without a spectrograph. This is because they are much more distant. Crudely speaking, the more distant the galaxy, the more its light is redshifted.

 

[Garth]

Holocene, if you look at very distant galaxies, such as those in the Hubble Ultra Deep Field, then you will find they are red.

The most distant ones are those circled in green in that remarkable photograph.

Garth

 

[berkeman]

Holocene, if you look at very distant galaxies, such as those in the Hubble Ultra Deep Field, then you will find they are red.

The most distant ones are those circled in red in that remarkable photograph.

Garth

Wow! That's an incredible image. I just read on APOD this morning that a supernova out at 5 billion years has a red shift of 0.28, so yeah, I guess it really is visible!

 

[Garth]

Wow! That's an incredible image. I just read on APOD this morning that a supernova out at 5 billion years has a red shift of 0.28, so yeah, I guess it really is visible!

In the HUDF we are talking about z > 6!

http://hubblesite.org/newscenter/archive/releases/2004/07/text/

"Hubble takes us to within a stone's throw of the big bang itself," says Massimo Stiavelli of the Space Telescope Science Institute in Baltimore, Md., and the HUDF project lead. The combination of ACS and NICMOS images will be used to search for galaxies that existed between 400 and 800 million years (corresponding to a redshift range of 7 to 12) after the big bang. A key question for HUDF astronomers is whether the universe appears to be the same at this very early time as it did when the cosmos was between 1 and 2 billion years old.

Garth

 

[jlorda]

I believe greater red shift also means greater speed for those distant objects.

 

[Chris Hillman]

Well, "distance" and "speed" become tricky concepts at cosmological scales, but roughly speaking, that is probably a good way to think about it.

 

[pixel01]

If those distant galaxies illuminate only a narrow band of light then you will see it red or more exactly redder. But they emit long range of wavelength so some light become redder, but other in the UV range will become visible light. The only things you can see (only through a photospectrometer) are stripes that move to the Red region .

 

[George Jones]

If those distant galaxies illuminate only a narrow band of light then you will see it red or more exactly redder. But they emit long range of wavelength so some light become redder, but other in the UV range will become visible light. The only things you can see (only through a photospectrometer) are stripes that move to the Red region.

Stars (and thus galaxies) actually do emit a a band light given by a blackbody curve, so we can see a change in colour, as our eyes are tuned by evolution to the band of a typical star, the Sun. Not all stars have peak intensity at the same colour, but the overall effect is visible.

 

[pixel01]

Stars (and thus galaxies) actually do emit a a band light given by a blackbody curve, so we can see a change in colour, as our eyes are tuned by evolution to the band of a typical star, the Sun. Not all stars have peak intensity at the same colour, but the overall effect is visible.

No we can not see the change in color which shifts to the red. The visible band of light is very narrow compared to the whole range of 'light' the stars emit. I do not talk about lights from plantary nebulae or something like that which have narrowbands of light reaching us.

Possibly some stars may have peak intensity, but also the peak can be in UV region and for the redshift, we can see it very bright or more bright then.

 

[russ_watters]

The peak is big enough and the band narrow enough that it is visible for large redshift objects (as seen in the previous links to the HUDF). Here's the emission band of some stars: http://csep10.phys.utk.edu/astr162/lect/sun/spectrum.html