Spectral Classifications Help
On a clear night, especially when the Moon is not above the horizon and there are not many lights to produce skyglow, it’s easy to see that stars have different colors, as well as different levels of brilliance. This is so because of differences in the amounts of energy stars emit at various wavelengths.
The wavelengths of visible light are extremely short and are commonly measured in units of nanometers (nm), where 1 nm = 10 –9 m, or in Ångström units (Å), where 1 Å = 10 –10 m = 0.1 nm. These units are microscopic in size. The visible spectrum extends from about 750 nm, representing red light, down to 390 nm, representing violet light. From longest to shortest wavelengths, colors proceed through the spectrum as red, orange, yellow, green, blue, indigo, and violet. The first letters of these colors come out as the odd name Roy G. Biv. Some people find this helpful in remembering the order in which the colors of the visible spectrum proceed.
All stars emit energy over a wide range of electromagnetic (EM) wavelengths, from low-frequency radio through microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays. The EM spectrum is sometimes portrayed in logarithmic form by wavelength (Fig. 13-2). The visible spectrum is a small portion of this.
Astronomers use an instrument called a spectroscope to scrutinize the spectra of the Sun, the planets, and distant stars. A spectroscope works according to the same principle by which a prism splits light into a rainbow. The spectroscope, however, is much more sophisticated. It can resolve the rainbow down into tiny slices. It also can examine wavelengths that are not visible to the unaided eye, particularly the near infrared or near IR (energy at wavelengths slightly longer than 750 nm) and the near ultraviolet or near UV (energy whose wavelengths are a little shorter than 390 nm).
When the spectra of distant objects are examined with a spectroscope, dark lines appear at certain places. Each chemical element is known to produce a certain pattern of such lines. In this way, astronomers can tell what distant objects are made of. This was first done with the Sun; the dark lines were discovered by accident. They were first studied seriously by a German astronomer named Joseph von Fraunhofer around the year 1800. Today, dark lines in stellar spectra are sometimes called Fraunhofer lines . Because they are caused by the absorption of energy at specific wavelengths, they are also known as absorption lines .
The first serious attempts to study the spectra of stars revealed dark absorption lines, just like the ones observed in the spectrum of the Sun. However, not all stars have the same pattern of lines. In the late 1800s, an astronomer at Harvard University named Annie Cannon compiled a record of the spectra of nearly half a million stars. This became known as the Henry Draper Catalogue .
There are seven main categories of stars, classified according to the type of spectrum they have. The main categories have been given the names O, B, A, F, G, K, and M. Each of these seven classes is divided into subcategories from 0 through 9. Thus a type A9 star is followed by a type F0 star, which is followed by F1, F2, F3, and so on. In all, there are 70 different spectral types of stars. What do these letters and numbers actually mean?
It turns out that the spectrum of a star tells us the surface temperature. Type O stars are the hottest and appear bluish to the eye. Type M stars have the lowest surface temperatures, and they appear orange or ruddy. Within a particular alphabetic subdivision, the number 0 represents the highest temperature, and the number 9 represents the lowest. The hottest possible star would be symbolized O0 (the letter O followed by the numeral 0); the coolest would be M9. On this scale, our Sun is a type G2 star. This means that it is medium cool. Of course, hot and cool are relative terms; even an M9 star is scorching hot by Earthly standards.
The surface temperature of a star is related to its absolute visual magnitude. This relationship was found in the early 1900s by a Danish astronomer named Ejnar Hertzsprung and an American named Henry Russell. These two scientists, working independently, graphed the absolute magnitudes of some nearby stars as a function of spectral classification. They found that, in general, as the surface temperature increases, so does the absolute brightness. This is not surprising, but it does not tell the whole story. Temperature is not the only variable in star classification. Size matters too.
Our Sun is a rather small star. The largest stars are called giants . The smallest are called dwarfs . Some relatively cool stars are bright because they are huge: the red giants . Some hot stars are dim because they are tiny: the white dwarfs .
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