Can someone explain my why "white dwarfs" and "red giants" were so named? And maybe that's a silly question but why not in the opposite way?
@EDIT Is this somehow co-related with Stefan–Boltzmann law?
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$\begingroup$ It's their colour, how they appear or would appear to the human eye. 'Dwarf' and 'Giant' relates to physical size relative to our own sun. $\endgroup$– AtmosphericPrisonEscapeJun 25, 2017 at 22:27
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$\begingroup$ @AtmosphericPrisonEscape is this somehow connected to the Boltzmann law? $\endgroup$– shurrokJun 25, 2017 at 22:53
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$\begingroup$ @Buszman it has more to do with color temperature than Stefan-Boltzman, but there is some relation. en.wikipedia.org/wiki/Color_temperature It's worth noting that nobody had ever seen a white dwarf (er, I think), they were theoretical when named and they probably would appear much more blue than white due to high surface temperature. But somebody with more knowledge than me should answer that. I'm just putting it out there. $\endgroup$– userLTKJun 26, 2017 at 0:31
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$\begingroup$ The name "Red giant" however, is entirely obvious and 100% observation. Wow, some of those red stars are HUGE. Red Giant, Blue Giant - named off observation. I think this is actually a good question. There's probably some interesting history on the early observations and naming/classification of stars. $\endgroup$– userLTKJun 26, 2017 at 0:35
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$\begingroup$ It has to do with the Hertzsprung–Russell diagram (en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_diagram). In the main sequence, white stars are generally very large and red stars are generally very small. Small white stars (white dwarfs) and large red stars (red giants) are the largest categories of non-main-sequence stars. Now we use "dwarfs" and "giants" for other types of stars, but this is (I believe) the history behind those two terms. $\endgroup$– user21Jun 26, 2017 at 11:53
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2 Answers
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The terminology is related to Planck's Radiation Law. We have (from Wikipedia): $$ {\displaystyle B_{\lambda }(\lambda ,T)={\frac {2hc^{2}}{\lambda ^{5}}}{\frac {1}{e^{\frac {hc}{\lambda k_{\mathrm {B} }T}}-1}}} $$ where $ k_B $ is the Boltzmann constant, h the Planck constant, and c the speed of light, $ \lambda $ is the wavelength, and T is the temperature.
A typical white dwarf is very hot, with $ T > 10^5 K $ even. Here let's assume it is about 50,000 K. A typical red giant is of lower temperature than the sun. Let's take it to be about 4,500 K (it will usually be a bit lower). Then we can plot the Spectral Irradiance of both the sources and compare them (I had an older Matlab code).
(The y-axis is $ W m^{-2} sr^{-1} nm^{-1} $.)
For the white dwarf we see that the peak occurs at a significantly lower wavelength than visible light (at about 0.58 nm). Most white dwarfs are hotter so the peak would shift further to the left. Thus, we expect the white dwarf to appear white with a bluish tinge as blue is still marginally higher than the other wavelengths.
For the red giant, the peak occurs at about 640 nm, which is near the red end of the spectrum. Red giants colder than this would have peaks shifted to the right. So they appear orange-red.
As @AtmosphericPrisonEscape already mentioned, dwarf and giant relate to the physical size as compared to the original star in the main sequence.
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$\begingroup$ A typical white dwarf is more like $10^4$ K. $\endgroup$– ProfRobJun 27, 2017 at 22:50
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$\begingroup$ @RobJeffries I was about to use B type star temperatures, but White Dwarfs' Wikipedia page gave a range of 150,000 K to 4,000 K. Also I think the older ones will be colder and more abundant. Doesn't this bias the average temperatures to be lower? $\endgroup$– NoFangsJun 28, 2017 at 4:18
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1$\begingroup$ Only an extremely young white dwarf has temperatures of tens of thousands. The cooling times are short. The bias is in fact that we tend to see hotter young white dwarfs, but to say that a typical white dwarf is 1E5K, when they spend less than a million years at such temperatures, is incorrect. $\endgroup$– ProfRobJun 28, 2017 at 6:12
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$\begingroup$ The "original" white dwarf, Sirius B, has a temperature of 10,000K. $\endgroup$– ProfRobJun 28, 2017 at 6:14
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The use of giant and dwarf in reference to stars of the highest and lowest luminosity is attested by 1914, said to have been suggested by Danish astronomer Ejnar Hertzsprung, (1873-1967); hence red dwarf (attested by 1922) etc. (http://www.etymonline.com/index.php?allowed_in_frame=0&search=dwarf)
The terms "giant" and "dwarf" refer to the position on the Herzprung-Russel diagram.It is apparent that there are some stars that are much brighter (and, we deduce, larger) then the temperature of their surface would suggest (these are the giant stars) There are others that are much dimmer dispite being very hot - white dwarfs. More recently, all non-giant stars have become classified as dwarfs.
