Spectral Classifications Help (page 2)
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 .
The Hertzsprung-russell (h-r) Diagram
When the spectral type of a star is graphed along with its absolute magnitude, the star is represented by a single point on a coordinate plane. Figure 13-3 shows what Hertzsprung and Russell found. Such graphs are used by astronomers to this day and are called, appropriately enough, Hertzsprung-Russell (H-R) diagrams . On the horizontal axis, the highest temperature is toward the left, and the coolest is toward the right. On the vertical axis, the brightest absolute visual magnitude is toward the top, and the dimmest is toward the bottom. Our Sun is shown by the large dot.
Stars denoted in the upper right part of the H-R diagram are red giants. Blue giants are in the upper left corner, at the top of the main sequence . The smallest and coolest stars, the red and orange dwarfs, are at the lower right end of the main sequence. White dwarfs appear at the bottom and are not on the main sequence. There are also supergiants that do not fall onto the main sequence; these are shown at the top middle.
So what exactly is this main sequence? As Hertzsprung and Russell plotted their diagrams, they noticed an interesting correlation. Most stars fall along a curve running diagonally from the upper left to the lower right. This became known as the main sequence because it contains the majority of stars.
When astronomers began investigating the relationship between the location of a star in the galaxy and its position on the H-R diagram, some fascinating discoveries were made. Hot, massive stars seem to be concentrated mostly in the flat, disk-shaped part of the Milky Way, in the spiral arms but not in the central region. The spiral arms happen to be where most of the interstellar gas and dust is found. It was theorized that stars formed from this material, and we should therefore find more young stars in the spiral arms of the galaxy than near the center. There is not much interstellar material in the central part of the galaxy, and the stars there are much older. This led to the notion that the galaxy has evolved from the center outwards. If this theory is correct, the galaxy looked much different a few billion years ago than it does now.
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