Great question and if we this becomes a tangent, would it be possible for a moderator to create and move this to another thread?
The real story, from my research, stems from the fact that the Sun is a yellow star as a way to qualify it based on its spectrum. Modern textbooks and papers seem to qualify this more and more, as opposed to suggest a yellow color is the color of the Sun seen visually. [Since color is only seen visually, it's still odd.
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So, in the mid 1800s, IIRC, when it was determined that a narrow beam of light would produce a spectrum, and that if done carefully, it would reveal strange narrow dark bands with all those colors, then things started to get interesting. Then it was discovered from lab experiments that things like Na (sodium) and other elements had the same dark lines at the same places in those colors, it became obvious that the composition of stars could be determined. Father Secchi, College Romano, was the Jesuit astronomer who pioneered the effort to categorize stars by their spectrum. He visually observed and drew (in color) likely hundreds of these spectrums. Fortunately for him, new photography techniques allowed him to image these spectrums, though in black and white. He cataloged about 4000 stars.
Perhaps a chronological outline will minimize my blather:
1817 - Joseph Fraunhofer studied stellar spectra and found them dissimilar to the Sun, though the Moon’s light was similar (of course). He also demonstrated that the lines were not likely telluric, meaning due to atmospheric effects.
1855 – Father Angelo Secchi (College Romano) began his work with spectroscopy.
1860 – Kirchoff showed metal gasses were familiar with the sun’s atmosphere. This spurred renewed interest in spectroscopy in Italy, England and in America.
1860 – Kirchoff showed metal gasses were familiar with the sun’s atmosphere. This spurred renewed interest in spectroscopy in Italy, England and in America.
1861 – Rutherford (Columbia Obs.) pub. In 1862 his spectral classification for stars.
Reddish or golden stars’ lines resemble the sun’s. These include Capella, Beta Geminorum, alpha Orionis, Aldebaran, gamma Leonis, Arcturus, Procyon
The white stars, sp. unlike the Sun. Sirius
White stars that show no lines – alpha Virginis and Rigel
1862 – Givoanni Donati (Florence) published in 1862 a classification based on star colors.
White stars – Sirius, Vega, Procyon, Fomalhaut and Rigel
Yellow stars - Altair and Capella
Orange stars - Arcturus and Pollux
Red stars – Aldebaran, Betelgeuse, and Antares
1862 – Father Secchi resumes his spectral work, publishing his improved classifications from 1863 to 1868, thanks to better instruments. He studied over 4000 stars.
1867 – Fr. Secchi published his three types:
Type I – white stars. Vega
Type II – Yellow stars. Arcturus
Type III – Orange and red stars. Betelgeuse
[My finger slipped and this was prematurely posted, so ....]