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Spectral Classifications Help (page 2)

By — McGraw-Hill Professional
Updated on Sep 17, 2011

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 and Nebulae How Bright? How
Distant? The Hertzsprung-russell (h-r) Diagram

Figure 13-3. Most stars fall along a characteristic curve called the main sequence in the Hertzsprung-Russell diagram. Our Sun (large dot) is a main-sequence star.

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.

Practice problems of this concept can be found at: Stars and Nebulae Practice Problems

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