This time instead of looking at which lines are present, as we do to get the temperature or spectral class, we look at the width of the lines. One way to classify by luminosity is to look at the star's spectrum. Doing this would allow us to convey a lot of information about a star by its spectral and luminosity classification, including placing it fairly well on the H-R diagram. Spectral classification can place a star on the horizontal axis H-R diagram, but what the vertical axis? We need to classify stars according to their luminosity. This class is actually determined by the lines in the star's spectrum, which are heavily dependent on temperature. The spectral class of a star is closely related to its temperature. Stars may be split up even more than just into Giants, Dwarfs, and Main Sequence stars. Tapscott 1978, Revised MK Spectral Atlas for Stars Earlier than the Sun (Yerkes Observatory, University of Chicago and Kitt Peak National Observatory) This image is a negative. In fact, many of the brighter stars on the diagram are not among the closest to Earth. It is important to note that the location of a star on the H-R diagram does NOT relate to its position in space. These categories are main sequence stars, red giants, and white dwarfs. Stars fall into three general categories. Figure 12.19 shows these constant radius lines on the H-R diagram. These constant radius lines go from the upper left to the lower right (negative slope) of the diagram. Looking back at the Stefan-Boltzmann Law we may notice that by holding R constant and plotting the luminosity as the temperature varies, we generate a set of diagonal lines. They can have radii as small as the Earth, having temperatures around 10,000 K. These small, hot stars are called White Dwarfs and lie below the main sequence. This must mean that it has a very large radius, in fact about 600 times larger than the solar radius! Similarly, stars that are very hot and yet still dim must have small surface areas. Its temperature is a cool 3548 K (the Sun is about 5770 K), while its luminosity is about 50,000 times brighter than the Sun. Antares is a good example of a red giant. These enormous stars are called Red Giants and lie above the main sequence line. Also, stars that have the same luminosity as dimmer main sequence stars, but are to the left of them (hotter) on the H-R diagram, have smaller surface areas (smaller radii).īright, cool stars are therefore necessarily very large. From this we see that stars above the main sequence on the H-R diagram (higher luminosity), with the same temperature as cooler main sequence stars, have greater surface areas (larger radii). Where L is the luminosity, R is the stellar radius, and T is the temperature (alpha is a constant). This puts it around the middle of the diagonal line. Our own star, the Sun, is nearly in the middle of both the temperature and luminosity scales relative to other stars. This places the cooler, dimmer stars towards the lower right and the hotter, more luminous stars at the upper left. The Luminosity scale on the left axis is dimmest on the bottom and gets brighter towards the top. ![]() This is contrary to the normal convention, where values increase going left to right on an axis. The temperature scale along the bottom axis goes from coolest on the right to hottest on the left. The majority of these stars when plotted on an H-R diagram go down from left to right in a diagonal line. Imagine plotting a random sampling of stars from our galaxy. The resulting diagram was named after the two discovering astronomers, Ejnar Hertzsprung of Denmark and Henry Norris Russell of America. This index comes from a difference in the amount of blue light a star emits and the amount of visual (more green) light emitted and is related to the temperature. Probably more like the absolute magnitude (Mv), which is determined from how bright the star looks from Earth and its distance away, versus the B-V color index. The actual properties originally plotted were properties which can be determined observationally. ![]() They found that when stars are plotted using the properties of temperature and luminosity as in the figure to the right, the majority form a smooth curve. A significant tool to aid in the understanding of stellar evolution, the H-R diagram was discovered independently by two astronomers in 1912 using observational comparisons. That was not the case however, for the Hertzsprung-Russell diagram. Many scientific discoveries are made first theoretically and then proven to be correct, or nearly so, in the laboratory. Origin of the Hertzsprung-Russell Diagram
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