Spiral galaxy labeled7/24/2023 ![]() The black area shows Herschel’s old diagram, centered on the Sun, approximately to scale. ![]() (b) His diagram shows the location of globular clusters, with the position of the Sun also marked. Harlow Shapley and His Diagram of the Milky Way: (a) Shapley poses for a formal portrait. Shapley then made the bold assumption, verified by many other observations since then, that the point on which the system of globular clusters is centered is also the center of the entire Galaxy (Figure 3).įigure 3. When Shapley used the distances and directions of 93 globular clusters to map out their positions in space, he found that the clusters are distributed in a spherical volume, which has its center not at the Sun but at a distant point along the Milky Way in the direction of Sagittarius. Globular clusters can be found in regions that are free of interstellar dust and so can be seen at very large distances. (Recall that it is distance that makes the stars look dimmer than they would be “up close,” and that the brightness fades as the distance squared.) Knowing the distance to any star in a cluster then tells us the distance to the cluster itself. By comparing the known intrinsic luminosity of these stars to how bright they appeared, Shapley could calculate how far away they are. In 1917, he was studying RR Lyrae variable stars in globular clusters. The discovery of the Galaxy’s true size and our actual location came about largely through the efforts of Harlow Shapley. Until the early 1900s, astronomers generally accepted Herschel’s conclusion that the Sun is near the center of the Galaxy. Herschel therefore concluded that the stellar system to which the Sun belongs has the shape of a disk or wheel (he might have called it a Frisbee except Frisbees hadn’t been invented yet), and that the Sun must be near the hub of the wheel (Figure 2). They found that most of the stars they could see lay in a flattened structure encircling the sky, and that the numbers of stars were about the same in any direction around this structure. ![]() Using a large reflecting telescope that he had built, William and his sister Caroline counted stars in different directions of the sky. In 1785, William Herschel (Figure 1) made the first important discovery about the architecture of the Milky Way Galaxy. Similarly, it would be easier to map our Galaxy if we could only get a little way outside it, but instead we are trapped inside and way out in its suburbs-far from the galactic equivalent of Times Square. You could do a much better job from a helicopter flying over the city than you could if you were standing in Times Square. Suppose you were given the task of mapping New York City. However, the very fact that we are embedded within it presents a difficult challenge. The Milky Way Galaxy surrounds us, and you might think it is easy to study because it is so close. Identify the main components of the Galaxy.Describe the challenges of determining the Galaxy’s structure from our vantage point within it.Explain why William and Caroline Herschel concluded that the Milky Way has a flattened structure centered on the Sun and solar system.By the end of this section, you will be able to:
0 Comments
Leave a Reply. |