Red stars, white stars, blue stars
Anyone who has looked at the night-time sky for any length of time will have noticed that stars have different colours. Some stars are red, some are white, and some are blue. A nice example of this can be seen in the Orion constellation. Betelgeuse, the star in the top left hand corner, has a distinct red colour, Rigel, the star in the bottom right hand corner, has a distinct blue colour, and Saiph, the star in the bottom left hand corner is white.
The Orion constellation. Betelgeuse in the top left is a distinct red colour, Rigel in the bottom right is a distinct blue colour, and Saiph in the bottom left is white.
Stars have different temperatures
These differences in colour are due to stars having different surface temperatures. As the figure below shows, the blackbody curve peaks at different wavelengths depending on a star’s temperature, so a red star is cooler than a white star, which in turn is cooler than a blue star. This is because of Wien’s displacement law, which I discussed in this blog.
Stars with different surface temperatures will appear to have different colours. This is due to their blackbody spectra peaking at different wavelengths.
Fraunhofer’s spectrum of the Sun
In 1817 the German scientist Joseph von Fraunhofer published a spectrum of the Sun showing the continuum spectrum which had long been familiar, but superimposed on this were a series of dark lines. These dark lines were shown by Kirchhoff and Bunsen in 1859 to be due to absorption lines being produced by different elements. I discussed absorption spectra in this blog. We now know that the gases which produce the absorption lines are in the atmosphere of the Sun. The visible surface of the Sun is called the photosphere, and it is the photosphere of the Sun which produces its blackbody spectrum. The overlying, thinner gases in the Sun’s atmosphere produce the numerous absorption lines which Fraunhofer saw.
The spectrum of the Sun sketched by Fraunhofer in 1814/15, showing the dark lines he observed, superimposed on the Sun’s continuum spectrum.
The Harvard stellar classification scheme
In the 1880s the Harvard College Observatory, under the Directorship of Edward C. Pickering, set about gathering the spectra of thousands of stars. The initial catalogue was published as the Draper Catalogue of Stellar Spectra in 1890. Williamina Fleming classified the spectrum using the letters of the alphabet, with with stars with the strongest Hydrogen absorption lines having the designation A, then the next strongest B, then C etc.
The Harvard stellar classification scheme was originally in alphabetical order based on the strength of the hydrogen absorption lines of a star You can see in this figure how the strength of the Hydrogen absorption lines (e.g. the lines) are strongest for the A, B and F-type stars.
In 1901 Annie Jump Cannon revised the system. First of all she dropped most of the letters, leaving A,B,F,G,K,M and O. Secondly she subdivided each of these into 10 divisions, so for example A0, A1, A2…. A9. Thirdly, she re-ordered the letters based on the stars’ surface temperatures, not the strength of the Hydrogen line, with the hottest stars first. This is what has led to the O,B,A,F,G,K,M (Oh Be A Fine Girl/Guy Kiss Me) system we have today. The O-class stars are the hottest, the B-class stars the next hottest, all the way down to the M-class stars which are the coolest.
Annie Jump Cannon
The strength of the lines of different elements and their ions
The figure below shows the variation of the strength of the absorption lines of different elements and their ions as a function of temperature. Stars which have the strongest Hydrogen lines (A-class stars) have a surface temperature of around 10,000 K. The bright star Vega is an A0-class star, and is white in appearance. The Sun is a G2-type star, and its strongest lines are the Calcium II lines (singly ionised Calcium).
The strengths of the absorption lines of different elements (and their ions) as a function of temperature. The nomenclature e.g. Ca II means “singly ionised Calcium”, He I means “neutral Helium”, He II means “singly ionised Helium”, etc.
The reason the strength of the Hydrogen absorption lines peak at around 10,000 K whereas the singly ionised Calcium lines (Ca II) peak at around 6,000 K is something I will explain in a future blog.
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