Overview:Astronomers measure the distance to stars within a few hundred light years (1 ly ~ 6 trillion miles!) of the Sun by noting how much each star appears to move against the back drop of far more distant stars as the Earth orbits the Sun: exactly the age-old technique used by geographers to survey terrain that was not readily accessible. With this information in hand, we can work out from how bright stars appear, how luminous they really are. Furthermore, we find a close relation between their color and their luminosity. If we assume the same relation holds for vastly more distant stars, we can measure their color, work out what their luminosity must be, and, from how bright they actually appear, work out how far away they are. Thus, we obtain the distance to loose clusters of stars in the disk of our Galaxy called Open, or Galactic clusters. Some stars in these clusters are pulsating stars called Cepheids, and, because we know their distance, we know how luminous they are. We find that their luminosity is closely related to their period of pulsation. Thus, when we see such stars in the spiral arms of other galaxies, we can measure their period, infer their luminosity, and, from how bright they appear, work out their, and, therefore, their host galaxy's, distance.Exercises:
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Part II: Extragalactic Astronomy
Overview: Once the distance to nearby galaxies is known through the use of Cepheids (see Part I, above), we can explore the properties of galaxies and use both the Cepheids and our new found knowledge of galaxies to get the distance to galaxies in clusters that are sufficiently far away such that the expansion of the Universe becomes evident: The Hubble Law. That is a profound discovery, but also a very practical one, because once we know that the more distant a cluster of galaxies, the more rapidly it recedes from us, we can measure the speed of recession for any distant galaxy, and from the Hubble Law work out its distance. Thus, we can survey the Universe! Comparing the structures that we observe with those whose origin we can simulate on a computer gives us insight into the nature of dark matter and the fate of the Universe.Exercises: |