Translated by DeepSeek V4 Pro. Translations can be inaccurate, please refer to the original post for important stuff.
Su JianlinFebruary 28, 2010#501
Question: Why are green stars so rare?
For example, Zubeneschamali (Beta Librae) in the constellation Libra
has a magnitude of 2.61. Based on its temperature, it should appear
blue-white, yet it is often perceived as green.
Regarding the green stars we see, I think it is simply that the
proportion of the green spectrum in these stars is slightly higher. Just
like the Sun, the Sun also has a green spectrum, but its proportion is
relatively small, so we do not see its green color.
Why do most stars have so little green in
their spectra?
The color of a star is merely the sensation of its visible spectrum
integrated by the human eye. The spectral energy distribution of a star
is primarily determined by its surface temperature (its distribution is
roughly consistent with blackbody radiation at the same temperature). It
seems that within the stellar temperature range of 3000\text{K} to 50000\text{K}, no stellar spectral
distribution gives the human eye a green sensation. If you want to see a
green star, buy a green filter; then all stars will turn green.
It is said that in some special cases, when the sun sets, the
atmospheric scattering can be just right for a few seconds, causing it
to appear green (the green flash).
Does the spectrum jump directly from yellow
to blue? Is green a point of discontinuity? Green is everywhere on
Earth, isn’t it?
Why do you say it "jumps" just because the apparent color of stars is
not green? The color sequence established by extracting narrow bands
from a continuous spectrum is entirely different from the color change
sequence given to the human eye by the full spectrum at different
temperatures. Although the latter also roughly gives you an order from
red to purple, the color caused by a slight excess in a certain band is
very different from the pure color sensation given by a narrow band. The
stars most likely to appear green are those whose surface temperature
happens to place the maximum of the continuous spectrum in the green
band (likely A or F-type stars). Go and see if those stars are
green.
.
Second:
Color Temperature
Stellar radiation is caused by heat. As long as it is standard
blackbody radiation, green stars will not appear. Green is a pure color,
while blackbody radiation inevitably results in mixed light perceived by
the human eye—that is, mixed colors. In the image above, the entire
triangular area is the color gamut, also known as the color space, which
belongs to the range perceptible by the human eye. The curved path is
called the Planckian locus, and the scale represents the temperature of
the blackbody radiation source (absolute temperature K). It is easy to
see that green is not on the Planckian locus; blackbody radiation does
not emit pure green light.
According to the definition of color temperature, a "green" star
should actually look white, and purple is even more impossible. As
analyzed before, we should not confuse monochromatic light with
continuous radiation. In terms of pigments, Red + Green + Blue = Black,
but in terms of light, Red light + Green light + Blue light = White
light. Stellar radiation is continuous, not a monochromatic light source
at all. Furthermore, as mentioned earlier, strong green emission lines
are rare. Among the most abundant elements in stars (H, He, C, O, etc.),
there are essentially no strong green emission lines.
Planckian Locus
I am uploading a large image of the color temperature definition from
Wikipedia, hoping everyone will not get confused on this issue again.
Note that the brightest is cyan-white; this is what is meant by the
idiom "the peak of perfection" (literally "the fire turns pure
blue").
.
Third:
The colored stars we see more often, such as red, orange, and blue
stars, are due to their temperatures determining the position of the
peak of the emission spectrum relative to the visible light band. For
spectral types K and M, the peak of their Planck curve is beyond red
light; in the visible light band, their emission energy distribution
decreases monotonically as the wavelength shortens, so it is easier for
the naked eye to perceive their "color." High-temperature stars, such as
O and B-type stars, are in the opposite position on their Planck curves
within the visible light band; their blue-end emission is much higher
than the red-end, which is also easily perceived by the naked eye.
So, why do F and A-type stars with surface temperatures of 8000\text{--}10000\text{K} look white to the
naked eye instead of green?
Reason 1: The peak of the Planck curve is not steep
enough, and the wavelength range of visible light is very narrow.
Visible light spans only 400\text{nm},
and a Planck curve with an emission peak around 520\text{nm} is almost flat in this interval,
with only a very small decrease at the red and violet ends, making it
undetectable by the naked eye.
Reason 2: The wavelength range that can trigger a
green sensation is too small. In the visible light band, only a narrow
range of 495\text{--}540\text{nm} is
perceived by our eyes as green light.
Reason 3: In the green light (wavelength) region,
strong emission lines of specific elements are relatively few. Since we
cannot rely on temperature, we must rely on specific elements emitting
particularly strong radiation in the green region. However, according to
the data I have on hand, only one spectral line of sulfur (545.39\text{nm}) and one spectral line of
titanium (498.173\text{nm}) fall near
the green light region, and the formation conditions for these two lines
still need to be investigated. Fewer emitters or stricter conditions
result in very few stars emitting an unusually large amount of green
light.
.
Fourth: A Moment of Joy:
I guess if you asked that star at the time, "How much love can be
reckless?" the star’s face would have turned green with fright.
