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I have a lay person's question - the wiki states:

James Webb Space Telescope - Wikipedia, the free encyclopedia

The JWST will not be a complete successor, because it will not be sensitive to all of the light wavelengths that Hubble can see. The main scientific goal is to observe the most distant objects in the universe, those beyond the reach of either ground based instruments or the Hubble.

So, how is it possible that the JWST, which apparently is less sensitive to all light wavelengths than Hubble, should be able to observe objects that where beyond what Hubble could perceive?

by Fran on Sun Jan 24th, 2010 at 04:02:24 AM EST
[ Parent ]
Good layman question!

The JWST will be less sensitive to more bluish wavelengths than the HST, however, it will be much more sensitive to reddish and infrared wavelengths. Now, the light of the most distant objects is significantly redshifted. So, while all galaxies are the brightest in or near the band of visible light, the light of the furthest-away galaxies was released so long ago and redshifted so much until it gets into our telescopes that these galaxies can be best detected in the near infrared.

As for redshift: this is usually explained for laymen in terms of the Doppler effect (e.g. the lower pitch of the noise of receding trains), but for cosmological redshift, that is less helpful. That the Universe expands should be understood as space itself expanding -- thus, a wave of light travelling across it is stretched, too. That means that the wavelength of the wave of light is increased. Red light has longer wavelength than blue light, hence it's called redshift.

*Lunatic*, n.
One whose delusions are out of fashion.

by DoDo on Sun Jan 24th, 2010 at 06:15:01 AM EST
[ Parent ]
Thanks DoDo, makes sense to me. :-)
by Fran on Sun Jan 24th, 2010 at 11:55:02 AM EST
[ Parent ]
Illustrating what I said above, from Open University, here is the Sun's spectrum:

...and a typical spiral galaxy spectrum:

A redshift of z=9 (which is in the middle of where the JWST wants to look) means that wavelengths expand 10x, that is by an order of magnitude, or one tick mark to the right on the horizontal axis of the above diagram.

*Lunatic*, n.
One whose delusions are out of fashion.

by DoDo on Sun Jan 24th, 2010 at 06:24:39 AM EST
[ Parent ]
BTW, if anyone is curious what's the possible maximum in redshift: 1089, that's the redshift of the Cosmic Microwave Background, which was a yellowish light when released (the thermal radiation of non-transparent plasma turned transparent gas), but the Universe expanded 1090x since then. However, it took some time from there for the first stars to light up, so the most distant galaxies observable even in theory will have redshifts in the dozens at most.

*Lunatic*, n.
One whose delusions are out of fashion.
by DoDo on Sun Jan 24th, 2010 at 06:32:43 AM EST
[ Parent ]

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