Doppler shift or spectroscopic displacement


What sounds so complicated is in reality a very simple principle, comprehensible by every layman. As you learn in school every chemical element emits characteristic spectral lines if burnt or somehow else excited. The phenomene is called emission spectrum. These characteristic lines can in the simplest case get resolved by a simple prism (if you haven't got one at hand as probably most of the readers of these pages, you got nevertheless a spectral analyzer nearly everyday in your hands. In every household today there are many: CD's or DVD's. Look from the side on the shimmering surface and you will see in plain sunlight rainbow colors. It's the spectrum of the sun. Hold it towards a neon lamp and you will see the spectrum of the neon lamp, which is a modified mercury spectrum. Modified by the inner white coating of the lamp which is in reality a mercury discharge lamp with an UV-spectrum. Admitted, this spectrum analyzer is not the best in the world.).

The solar spectrum was very early detected and described by Joseph v. Fraunhofer(1787-1826) in 1814. He gave the first exact description of the absorption spectra in the light emitted by the sun. Now since stars are nothing else but distant suns you can measure these lines in stars too. And other than the intensity of the light which gets fainter the farer the star is and can easily get degraded by athmospheric disturbancies, these lines don't get modified by any distance - as long as there is visible light. This means on the other hand that any shifting in these lines is an exact testemony that the star is moving away from you or is moving towards you, depending on the direction the lines are shifted or displaced. This is due to the same phenomena as the sound of a train approaching you sounds higher when it comes nearer and sounds lower when it leaves you (this effect is extensively used and everybody knows it in music electronics, it's name is after the researcher who found it "doppler effect".). At good last a spectroscopic displacement is nothing else than a spectroscopic shift.

The only problem in these measurements in the 19th and early 20th century were in the expenditure of photographic material you had to use before there were electronic means to measure these shifts. Since you can't know beforehand when the biggest elongations of the orbit of the star you want to measure is, you got to take with photographic means a lot of photos in discrete time intervalls. But the maesurement itself was possible right after Fraunhofer's description in 1814. You only had to place a prism at the end of the optical path of your telescope! (But since photography was not invented before 1830 you would have had to note the lines by hand in 1814.). So after Kirchhoff and Bunsen found the connection between the chemical elements and the spectra these new ideas found soon their usage in astronomy. Secchi(1818-1878) was the first to investigate the spectrum of stars, H.C.Vogel(1814-1892) was the first to use doppler effect maesurements. Here you can read that H.C.Vogel made 1890 exactly those measurements that today scientific pages ("During the past several years the astronomical techniques used for observations have become more and more sophisticated leading to precise indirect methods of detecting planetary bodies orbiting stars other than our Sun.") and semi-scientific pages assert they were only possible in the 1990th: "Only in the mid 1990's were instruments developed that were sensitive enough to record the telltale signs that indicate the presence of a planet orbiting a star." Exactly 100 years wrong! Compare these statements to the links below: More than 20 exoplanets were found with cheapest amateur backyard telescopes! Should this be the reason why this site is so well hidden??

But even the today used electronic spectroscopes are that simple devices that every layman can understand the principle: you only need a rotating prism and a photodiode (or a prism/rotating mirror and photodiode)! Parts which cost less than a dollar! And available in the fifteeths(germanium transistors were incredibly sensitive photodiodes/phototransistors e.g. OC71) and sixteeths (silicium photodiodes) of last century. Photocells and selen cells were available even earlier, in the thirteeth of last century, they allowed actors to speak in sound film. (Actually spectroscopes are build today a little bit different, even simpler, but the principle stays absolutely the same. CCD rows or CMOS rows or high resolution camera chips simplify the design drastically...)

But now again this is an example how certain scientists obviously worked. This shift of the spectral lines was very early detected, around 1910(actually already in 19ths century, see above). But since these scientists knew nothing about the wobbling of the stars (and perhaps because everyone would have said they're crazy) they obviously suppressed halve of their measurements: 'Because what may not be, cannot be'. They decided there is only a red shift*: the starting point of the whole Bigbang-theory ! The blue shift part of their measurements they did suppress. Actually there were from time to time in later years always news from different researchers that they measured instead of the red-shift a blue-shift, but that was misinterpreted as 'different shifts depending on the direction' or as mismeasurements or....

And finally you see that the explanation that astro-physics and astronomers give why the exoplanet systems were detected so late, now only 15 years ago, that the measurement techniques were not that evolved earlier is not correct. Astronomers simply started to search for exoplanets after they learned  from my description that our sun and thereby all stars wobble under the gravitational forces of the planets.

To state it clearly: since nearly 200 years these maesurements could have been accomplished!

At least most of the so far found big planets orbitting in tight orbits their central star. The lately (2009) found small planet of the size of the earth needs higher resolutions and with every new chip generation of CCD's or CMOS smaller planets will get found. And some wobble surely so faint that detection will get really hard.

But some can never get detected by this method: those with orbits at exactly 90 degrees. Their spectrum does not get shiftet, because they are always at the same distance to us. But happily these are just those cases that show up best by normal telescope viewing. So these techniques are complimentary for finding wobbling stars: where one of them is 'blind' the other views best and vice versa.

Finally, there are quite naturally other ways of detecting extrasolar systems and planets. Besides normal telescope viewing another one is the 'transit method' which detects planets by the small dip in luminosity of the star when the planet passes in front of the star.


* Red shift is a transmogrify of phycisists and astronomers. More exactly expressed it is a shift towards the red end (in frequency terms the lower end) of the spectrum in contrast to a shift towards the blue end (in frequency terms the higher end) of the visible spectrum. So if you speak of a red-shift you want to express that the stars move away from you. See above the train example.


Copyright © 2009 R.Cooper-Bitsch