<p>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.</p>

<p>A typical white dwarf is very hot, with $ T &gt; 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).
<a href="https://i.sstatic.net/GxgKp.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/GxgKp.png" alt="Plot for the White Dwarf (the y-axis is $ W m^{-2} sr^{-1} nm^{-1} $"></a></p>

<p>(The y-axis is $ W m^{-2} sr^{-1} nm^{-1} $.)</p>

<p><a href="https://i.sstatic.net/9AAgB.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/9AAgB.png" alt="Plot for Red Giant"></a></p>

<p>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.</p>

<p>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.</p>

<p>As @AtmosphericPrisonEscape already mentioned, dwarf and giant relate to the physical size as compared to the original star in the main sequence.</p>

CC BY-SA 3.0
2017-06-27T22:50:42.007Z
A typical white dwarf is more like $10^4$ K.
ProfRob
CC BY-SA 3.0
2017-06-28T04:18:13.683Z
@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?
NoFangs
CC BY-SA 3.0
2017-06-28T06:12:52.347Z
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.
ProfRob
CC BY-SA 3.0
2017-06-28T06:14:37.593Z
The "original" white dwarf, Sirius B, has a temperature of 10,000K.
ProfRob
CC BY-SA 3.0
2017-06-27T20:57:02.560Z
2017-06-27T20:57:02.560Z
NoFangs
<p>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. (<a href="http://www.etymonline.com/index.php?allowed_in_frame=0&amp;search=dwarf" rel="nofollow noreferrer">http://www.etymonline.com/index.php?allowed_in_frame=0&amp;search=dwarf</a>)</p>

<p>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.</p>

CC BY-SA 3.0
2017-06-27T22:29:13.903Z
2017-06-27T22:29:13.903Z
James K
<p>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?</p>

<p><strong>@EDIT</strong>
Is this somehow co-related with Stefan–Boltzmann law?</p>

CC BY-SA 3.0
2017-06-25T22:27:42.133Z
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.
AtmosphericPrisonEscape
CC BY-SA 3.0
2017-06-25T22:53:40.087Z
@AtmosphericPrisonEscape is this somehow connected to the Boltzmann law?
shurrok
CC BY-SA 3.0
2017-06-26T00:31:29.543Z
@Buszman it has more to do with color temperature than Stefan-Boltzman, but there is some relation.   https://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.
userLTK
CC BY-SA 3.0
2017-06-26T00:35:37.423Z
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.
userLTK
CC BY-SA 3.0
2017-06-28T05:26:31.553Z
Initially the star colors were designated by the wavelength peak of the light emission using photometry with different filters (U-V, B-V, etc). When it was realized that this correlated with the temperature of stars the colors (like the OBAFGKM designations) became "temperature labels" and never really were about what the star would look like to the naked eye. The "white" color is synonymous with the "A" spectral class. Unfortunately, stars are either dwarfs or giants by size. It's easier to live with old, bad terms than cross reference old literature. The name "white dwarf" wasn't taken yet.
Jack R. Woods
CC BY-SA 3.0
2017-06-28T05:37:29.827Z
White dwarfs actually range from spectral type B to K. See this diagram: https://en.wikipedia.org/wiki/Stellar_classification .. Therefore, a "white dwarf" could be yellow-white, yellow, or orange, too, but these names were taken (F,G or K). "Red giants" are commonly K stars (orange), but are definitely "giants". In other words, they are all just "labels" that give information, but are not to be taken literally.
Jack R. Woods
<star><white-dwarf><red-giant>
Origin of terms "white dwarfs" and "red giants"?
shurrok
# Origin of terms "white dwarfs" and "red giants"?

(Asked by: username_0 on 2017-06-25)

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?

**Tags:** star, white-dwarf, red-giant

> **Comments:**
> **username_1**: 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.  
> **username_0**: @username_1 is this somehow connected to the Boltzmann law?  
> **username_2**: @Buszman it has more to do with color temperature than Stefan-Boltzman, but there is some relation.   https://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.  
> **username_2**: 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.  
> **username_3**: Initially the star colors were designated by the wavelength peak of the light emission using photometry with different filters (U-V, B-V, etc). When it was realized that this correlated with the temperature of stars the colors (like the OBAFGKM designations) became "temperature labels" and never really were about what the star would look like to the naked eye. The "white" color is synonymous with the "A" spectral class. Unfortunately, stars are either dwarfs or giants by size. It's easier to live with old, bad terms than cross reference old literature. The name "white dwarf" wasn't taken yet.  
> **username_3**: White dwarfs actually range from spectral type B to K. See this diagram: https://en.wikipedia.org/wiki/Stellar_classification .. Therefore, a "white dwarf" could be yellow-white, yellow, or orange, too, but these names were taken (F,G or K). "Red giants" are commonly K stars (orange), but are definitely "giants". In other words, they are all just "labels" that give information, but are not to be taken literally.  

---

## 2 answers

### Answer by username_4 on 2017-06-27. Score: 1 | Accepted answer

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).
[![Plot for the White Dwarf (the y-axis is $ W m^{-2} sr^{-1} nm^{-1} $](https://i.sstatic.net/GxgKp.png)](https://i.sstatic.net/GxgKp.png)

(The y-axis is $ W m^{-2} sr^{-1} nm^{-1} $.)

[![Plot for Red Giant](https://i.sstatic.net/9AAgB.png)](https://i.sstatic.net/9AAgB.png)

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 @username_1 already mentioned, dwarf and giant relate to the physical size as compared to the original star in the main sequence.

> **Comments:**
> **username_5**: A typical white dwarf is more like $10^4$ K.  
> **username_4**: @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?  
> **username_5**: 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.  
> **username_5**: The "original" white dwarf, Sirius B, has a temperature of 10,000K.  

---

### Answer by username_6 on 2017-06-27. Score: 1

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.


---

