Star Colors Explained – One Minute Astronomer
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In the late 19th century, Harvard astronomers developed a system to classify stars not according to color, but by the strength by which hydrogen gas absorbed light at particular wavelengths. The star classes were labeled A to N in order of decreasing hydrogen absorption strength.
This is the Harvard spectral classification, which is still used today. So what does this have to do with star color? New e-Book gives you the best articles from the first three years of One-Minute Astronomer.
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Click here to learn more. They discovered the type-O stars are hotter than type-B stars, and type-B stars are hotter than type-A stars, and so on. But hot stars are blue, and medium-hot stars are white, and cool stars are red. They emit visible light of all colors to some degree.
You can tell that many of the stars are similar in color; however some stand out as being much redder than the others.
These red stars have the coolest temperatures among the stars in the cluster. Another good example is this color image of Albireo taken by students at the University of California, Berkeley. Want to learn more? Recall from Lesson 3 that the spectrum of a star is not a true blackbody spectrum because of the presence of absorption lines.
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The absorption lines visible in the spectra of different stars are different, and we can classify stars into different groups based on the appearance of their spectral lines. In the early s, an astronomer named Annie Jump Cannon took photographic spectra of hundreds of thousands of stars and began to classify them based on their spectral lines.
Originally, she started out using the letters of the alphabet to designate different classes of stars A, B, C…. However, some classes were eventually merged with others, and not all letters were used.
The original classification scheme used the strength of the lines of hydrogen to order the spectral types. That is, spectral type A had the strongest lines, B slightly weaker than A, C slightly weaker than B, and so on. For more information on her life and work, visit the homepage for Annie Jump Cannon at Wellesley College.
Recall from Lesson 3 that the electrons in a gas are the cause of absorption lines—all the photons with the correct amount of energy to cause an electron to jump from one energy level to a higher energy level get absorbed as they pass through the gas.
Star Colors Explained
The absorption lines from hydrogen observed in the visible part of the spectrum are called the Balmer series, and they arise when the electron in a hydrogen atom jumps from level 2 to level 3, level 2 to level 4, level 2 to level 5, and so on. The strength of the Balmer lines that is, how much absorption they cause depends on the temperature of the cloud.
If the cloud is too hot, the electrons in hydrogen have absorbed so much energy that they can break free from the atom. So, very hot stars will have weak Balmer series hydrogen lines because most of their hydrogen has been ionized.
Recall also that it takes energy to raise an electron from a lower level to a higher level. So, if the cloud of gas is too cool, the electrons will all be in the lowest energy level the ground state, level 1. Since the Balmer series lines require electrons to already be in level 2, if there are no hydrogen electrons in level 2 in the gas, there will not be any Balmer series hydrogen lines created by that gas. So, very cool stars will have weak Balmer series hydrogen lines, too. Thus, the stars with the strongest hydrogen lines must be in the middle of the temperature sequence, since their atmospheres are hot enough that hydrogen will have its electrons in level 2, but not so hot that hydrogen becomes ionized.
This theory for the absorption by hydrogen was not understood until after much of the work on stellar classification had been completed. So, after the origin of the strengths of the lines was understood to have some dependence on temperature, the spectral classes for stars were reordered with the hottest stars at the beginning of the sequence and the coolest stars at the end of the sequence.