Quote:
Originally Posted by New Science
A redshift is measured by comparing the observed spectrum of an object to the Suns spectrum.
Since the Sun radiates a continuum, the colors blend.
So, IMHO, I believe they use the hydrogen absorption lines to make these measurements or possibly the 'sodium' lines that conform to exact wavelengths.
These can be measured by computer to give exact redshifts.
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This is essentially correct – as the emission spectrum of a star is a fairly smooth continuum, the discrete absorption lines of other elements are better to use in measuring its red/blue shift. Modern
astronomical spectroscopy identifies thousands of absorption lines, while the earliest published scientific papers, ca. 1817, mention 10 lines resulting from 6 molecules, O2, H, Na, Ca, Fe, and CH.
The spectral shift of stars were first systematically measured in the mid 19th century, long before the availability of computers. The “old fashioned” technique involved comparing the emission lines of known sources, such as
sodium vapor lamp, to those of a star being viewed. By the mid 19th century, this was done mostly using photographic plates. In its early days, it was done by hand, marking a piece of target paper with a fine pen. In both cases, shift is measured using high precision mechanical measuring devices – rulers.
Quote:
When an object shows a shift of 'one' wavelength, regarding any line that exhibits that shift, it is a shift of 'one'.
Any fractional shifts are given in percentages IMHO.
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I interned in an observatory (
), but never heard the term “fractional redshift”, and can’t see any significance or utility to such a term for ordinary measurments.
(An internet search reveals the term "fractional redshift" use in papers such as
this one, in which it refers to a very small effect where photons lose energy via absorption and reemission by free electrons in a plasma in a magnetic field, such as the star’s corona)
The usual measure of
spectral shift - redshift or blueshift - is called “z”.
If
, it is called redshift. If
, it is called blueshift. If
, no spectral shift has been measured.
, that is, z must be greater than -1, and has no upper limit.
It’s also common to describe redshift by the velocity required to produce the observed redshift. For example, we observe an average redshift of the stars in our neighboring
Andromeda galaxy of about
, but usually state it as -300000 m/s
So a redshift of
indicates a doubling of the wavelength of the observed photon vs. its emitted wavelength, which is equivalent to a half-ing of its frequency. There’s nothing particularly special about integer redshifts, other than that they are unusually high. Typical
s for objects like rapidly orbiting binary stars, planets, or stars orbiting galactic centers are
. The greatest stellar redshift observed to date is
, or tentatively,
.
The cosmological redshift of the first light predicted by the Big Bang model to have been emitted is much higher:
, suggesting that the peak frequency of that light was 1090 times the current observed peak frequency of 160 GHz for the
CMBR. This redshift is not explained as being due to the source of the CMBR receding from the observer at a high velocity, and is not measured by comparing absorption lines in its spectrum. It is explained as being due to the expansion of space, a much more complicated (and less directly observable) phenomenon.
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The core of the redshift concept is the shift itself, hence any movement toward the red would be a shift... a redshift.
The history and conventional terminology of astronomical spectroscopy
