8/16/2023 0 Comments Hr diagramThis is highly significant as it suggests that there may be some relationship between the luminosity and temperature of a star. If we now plot a Hertzsprung-Russell diagram for a few thousand nearest or brightest stars we see the following:Īs we can see, stars do not appear randomly on the plot but appear to be grouped in four main regions. By comparing the spectra of an unknown star with spectra of selected standard reference stars a wealth of information, including its colour or effective temperature can be determined. The main spectral classes for stars range from O (the hottest) through B, A, F, G, K and M (coolest). Stars of similar size, temperature, composition and other properties have similar spectra and are classified into the same spectral class. By splitting the light from a star through a spectrograph its spectrum can be recorded and analysed. The third possible scale for the horizontal axis is a star's spectral class. Stars with a negative colour index are bluish whilst cooler orange or red stars have a positive colour index. In practice astronomers actually measure a quantity called colour index that is simply the difference in the magnitude of a star when measured through two different coloured filters. One quirk here is that the temperature is plotted in reverse order, with high temperature (around 30,000 - 40,000 K) on the left and the cooler temperature (around 2,500 K) on the right. The effective temperature of a star is plotted on the horizontal axis of an H-R diagram. Also take care if using magnitude to work upwards to negative values. Note how the temperature scale is reversed on the horizontal axis. Possible axes for a Hertzsprung-Russell Diagram. Our Sun has an absolute magnitude of + 4.8. By definition a star of magnitude 1 is 100 × brighter than one of magnitude 6. Just to confuse things, the lower or more negative the magnitude, the brighter the star. Absolute magnitude is simply a measure of how bright a star would appear if 10 parsecs distant and thus allows stars to be simply compared. Astronomers also use the historical concept of magnitude as a measure of a star's luminosity. This can be expressed as a ratio of the star's luminosity to that of the Sun L */ L sun. In an H-R diagram the luminosity or energy output of a star is plotted on the vertical axis. ![]() It is named after the Danish and American astronomers who independently developed versions of the diagram in the early Twentieth Century. These can be plotted to form what is one of the most useful plots for stellar astronomy, the Hertzsprung-Russell (or H-R) diagram. So far we have discussed the luminosity and colour or effective temperature of stars. One important technique in science is to try and sort or classify things into groups and seek out trends or patterns. There are a few hundred billion stars in our galaxy, the Milky Way and billions of galaxies in the Universe. Classifying Stars - the Hertzsprung-Russell Diagram
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