Astronomers have noted that the mass of spiral galaxy is related to its rotation rate. Since they are so bright (sometimes exceeding the brilliance of an entire galaxy) they can be seen much farther than individual stars and distance can be obtained through the distance modulus by comparing the apparent magnitude to the known absolute magnitude. Type Ia supernovae tend to be caused by the same series of events and all tend to be the same absolute magnitude. The technique can be used for a few nearby galaxies, but is limited because outside of a few nearby galaxies, individual stars cannot be seen. Several other techniques for determining distances beyond our Milky Way Galaxy: The processes above are useful for determining distances within the Milky Way Galaxy. This again is compared to the apparent magnitude of the stars and the distance determined by the distance modulus relation. ![]() Main Sequence fitting is a process related to "spectroscopic parallax" that compares the Hertzsprung -Russell (HR) diagram of the stars in a star cluster, with a calibrated HR diagram to determine absolute magnitude.If we know the luminosity or absolute magnitude, we can compare that to the observed brightness to deduce mathematically the actual distance by the distance modulus. Thus, if we can identify a star as being a G2 from its spectrum, then we know its luminosity, because we know that it is essentially the same as the Sun's. More specifically, all G2 normal stars (such as our Sun) have approximately the same luminosity. All B-type normal stars are like all other B-type normal stars. That is, all G-type "normal" (non-Giant) stars are like all other G-type stars. That is, if we know the spectral type of a "normal" star, we know its luminosity (actual total energy output at all wavelengths). Spectroscopic parallax and related techniques based on spectral classification and the HR diagram (below).(Other stars with a similar relationship are the RR Lyra variables.) The specific equation is known as the "distance modulus." This is most useful for finding distances to stars clusters of which the Cepheid is a part. ![]() By then comparing the known brightness to the apparent brightness, the distance can be found by applying the inverse square law. By observing the star's period, we know its luminosity (which is directly related to another quantity called "absolute magnitude". Luminosity is a measure of how bright the star truly is at all wavelengths. It turns out that the period is related to the star's absolute magnitude or luminosity. They vary in brightness from brightest to dimmest to brightest in certain time periods. Cepheids are stars whose total brightness (luminosity) varies with specific periods.
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