David Calder Hardy's Cosmology
Red Shift
And I guess that if we were actually located in one of those rapidly receding galaxies we would still think we were in the middle of the universe and that everything was receding from us relative to its perceived distance from us. Incidentally, I would like to know what happens to the colors red and blue if something out there is hurtling toward us.
Which, of course, raises the question, what proportion of galaxies are receding compared with those approaching? All we really hear about is the expansion factor. DCH
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How is the red shift of a galaxy measured? I thought the spectrum of a galaxy is continuous, but apparently there are some prominent lines. Which are they, and where are they originating from?
Andreas Glinz
Redshift is measured, as you imply, by measuring the displacement of lines in spectra from their rest wavelength. Galactic redshifts arise because of the apparent recession of the galaxies, caused by the expansion of the Universe.
Most astronomical objects have both a continuous spectrum and a line spectrum, galaxies included. The Sun, for example, has a continuous spectrum that is roughly that of a 5,800 K blackbody, and it has a complicated line spectrum that arises from the many different elements in its atmosphere. Galaxies are comprised of stars, so it is reasonable to assume that galactic spectra would look like a blend of many stellar spectra. Galaxies also have large quantities of gas in the space between their component stars (this gas is called the interstellar medium), that can also give rise to lines.
As hydrogen is the principal constituent of the Universe, some of the easiest lines to detect and measure in galactic spectra are those of hydrogen. In particular, the strong Lyman alpha emission line, whose rest wavelength is 1215 Angstroms, can often be used to measure the galaxy's redshift. The lines in the Balmer series can be used as well. Since hydrogen is located in every part of the galaxy, these lines arise in material located throughout the galaxy.
Redshift is denoted by the letter Z, and you can calculate it using the formula Z = dL / L,where dL is the total amount by which the line is shifted, and L is the rest wavelength. So, a galaxy whose Lyman alpha line was shifted to 3645 Angstroms would have dL=3645-1215=2430 Angstroms, L=1215 Angstroms, and Z=2. Z of 1 to 2 is a moderate redshift, indicative of a speed of about 80% of the speed of light.. When Z gets up to 4 or so, we are looking at galaxies that appear to be receding extremely rapidly, and that are very, very far away.
Jeffrey Hall / 1997apr25