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Space Forum / Amateur Astronomy / January 2006Predominate wavelength by spectral class?
Anyone know if there is a table or method to determine the _predominate wavelength_ emited by a star by spectral class. I would like to add a column to the following table showing the predominate wavelength of light emitted. For example, the Sun predominately emits yellow light. CI Spec Color -0.33 O5V Blue -0.27 O9I Blue -0.30 B0V Blue -0.01 A0I White -0.02 A0V White +0.17 F0I Yellowish +0.30 F0V Yellowish +0.58 G0V Yellow +0.81 K0V Orange +1.00 K0III Orange +1.41 M0V Red +1.56 M0III Red Cox(2000) and Kaler(1997) Color wavelengths Violet 380-440 nm Blue 440-485 nm Cyan 485-500 nm Green 500-565 nm Yellow 565-590 nm Orange 590-625 nm Red 625-740 nm - Canopus56 >Anyone know if there is a table or method to determine the _predominate >wavelength_ emited by a star by spectral class. I would like to add a >column to the following table showing the predominate wavelength of >light emitted. For example, the Sun predominately emits yellow light... How exactly do you define "predominant wavelength"? Does that mean the wavelength with the peak intensity? If so, you should just be able to calculate it based on the stellar temperature for each spectral class. A G0V star, for example, is around 6000K, so its peak output is at 483nm. If you are talking about color, however, the situation is considerably more complicated. 483nm is blue-green, but G0V stars appear as a warm white color. To come up with something approximating a color for these spectral classes you would need to convolve the blackbody curve with the spectral response of retinal cells, and then translate that into the eye's color space (and semi-arbitrarily assign color names). _________________________________________________ Chris L Peterson Cloudbait Observatory http://www.cloudbait.com <snip> Chris wrote: > How exactly do you define "predominant wavelength"? Does that mean the > wavelength with the peak intensity? Yes, I mean peak intensity. > If so, you should just be able to > calculate it based on the stellar temperature for each spectral class. A > G0V star, for example, is around 6000K, so its peak output is at 483nm. Any pointers or web page references on who to do this? Thanks, Canopus56 >Any pointers or web page references on who to do this? Do a search on "blackbody" and "planck's law" and you should turn up lots of info. The basic theory can be found at http://scienceworld.wolfram.com/physics/PlanckLaw.html . There are quite a few online calculators. Here are a couple: http://csep10.phys.utk.edu/guidry/java/planck/planck.html http://lectureonline.cl.msu.edu/~mmp/applist/blackbody/black.htm _________________________________________________ Chris L Peterson Cloudbait Observatory http://www.cloudbait.com <snip> Chris wrote - <snip> If you are talking about color, however, the situation is considerably > more complicated. 483nm is blue-green, but G0V stars appear as a warm > white color. To come up with something approximating a color for these > spectral classes you would need to convolve the blackbody curve with the > spectral response of retinal cells, and then translate that into the > eye's color space (and semi-arbitrarily assign color names). Thanks, Chris. I working in terms of resolution testing using the Rayleigh criteria. In Chapter 2 of Argyle's _Measuring and Observing Double Stars_, he has an excellent set of tables of stars with matched V-band magnitudes by aperature class (binos, 90cm, 8"-10" scopes, etc). In researching specific Arygle matched doubles, it became apparent that many of them are not O,B,A stars, but rather G and K stars. Now, I suppose that I could use my Johnson V filter and do resolution testing only in V band, but most amateurs do not have that kind of gear. It seemed to me that having the peak wavelength by spectral class, and experimenting with Argyle's matched doubles and Rayleigh's critera would make an enjoyable project during the upcoming warm season. - Canopus56 P.S. To lukers - Rayleigh's separation criteria is wavelength dependent, unlike the Dawes separation criteria, which implicitly is restricted to blue-white light at around 460nm. A_rad = (1.22 * lambda_meter) / D_meter where lambda is the wavelength of observed light, usually expressed in nanometers. 550nm light is green-yellowish and at the peak response of the human eye. Hi canopus56, > Anyone know if there is a table or method to determine the _predominate > wavelength_ emited by a star by spectral class. I would like to add a > column to the following table showing the predominate wavelength of > light emitted. For example, the Sun predominately emits yellow light. It depends what you mean by "predominate wavelength". In color measurement under photopic conditions (i.e. cone vision), one space used to quantify color includes a "dominant wavelength" parameter, together with an excitation purity and lightness. The dominant wavelength is presumed to be the wavelength of the monochrome stimulus which would be perceived by a human eye as the closest match to the given stimulus. It is not necessarily the same as the peak wavelength. These parameters are computed from the tristimulus values for the radiance, which are determined from its spectrum, and depend also on the field of view. The dominant wavelength calculation uses chromaticity, so it does not depend on the lightness, but does require a defined white point (tricky, for a star). The dominant wavelength can be determined graphically by plotting the white reference and the test point on a chromaticity chart. Under tight restrictions, a polynomial can also be used, but it is valid only in a region very close to conventional white points (i.e. illuminant C, D65, and so forth). Note that dominant wavelength has fallen out of favour in colorimetry, which nowadays uses CIE L*a*b* 1976 and modifications thereof. There is a brief but informative entry in Wikipedia on dominant wavelength. The international body setting standards on colorimetry is the CIE, and publication 15 defines color spaces. http://en.wikipedia.org/wiki/Dominant_wavelength http://www.cie.co.at/framepublications.html http://members.cox.net/astro7/color.html Best Regards, John. P.S. if you're interested in quantification of color, rather than mere spectral measurement, you might be sliding into a very deep hole... > CI Spec Color > -0.33 O5V Blue [quoted text clipped - 21 lines] > > - Canopus56 <snip> > P.S. if you're interested in quantification of color, rather than mere > spectral measurement, you might be sliding into a very deep hole... Thanks for the tips, John. Yes, it is _a deep hole_. I have looked into it before. Fortunately, I'm just interested in spectrometry on this one. - Canopus56 canopus56 Wrote: > Anyone know if there is a table or method to determine the _predominate > wavelength_ emited by a star by spectral class. I would like to add a > column to the following table showing the predominate wavelength of > light emitted. For example, the Sun predominately emits yellow light. > > - Canopus56 Spectral class + res of photo film - is there an active projects here? Could try prism before camera as http://home.freeuk.com/m.gavin/300d.ht Nyteca -- nytecam <snip> > Spectral class + res of photo film - is there an active projects here? > Could try prism before camera as http://home.freeuk.com/m.gavin/300d.htm Ultra-cool as always, Maurice. - C |
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