University of Michigan - Department of Astronomy

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Finding the Center of the Milky Way From Globular Clusters

gazing far up the lanes of Sagittarius
richest stream of our sky—
a cup to the center of the galaxy!.

- Gary Snyder "Burning Island"

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Introduction

In the early 20th century, the shape and extent of the galaxy were not yet known. 19th century astronomers arrived at the size and shape by studying the band of light we usually refer to as the Milky Way. However there were several things they didn't know about the galaxy, so they arrived at a model that was roughly disk shaped, about 20,000 ly in diameter and 6000 ly thick with the Sun near the center.

Most of the stars, and especially the short-lived stars, do in fact reside in the aptly named galactic disk. Numerous techniques were developed in the 20th century for accurately measuring the properties of the disk, such as size, shape, mass, content, and rotational speed. We now know a great deal about the disk, including that it is only part of the structure of the galaxy. (For more information on the structure of the disk, see the Structure of the Milky Way Disk activity or see The Milky Way Disk for more on the stellar populations and the disk.)

M13Figure 1: M13 globular cluster in Hercules

In the 1920s, a young astronomer named Harlo Shapley began mapping the positions of the globular clusters. Globular clusters are a special type of star cluster. They have tens of thousands to a few million stars, and are spherical in shape. The stars are generally older, mid to low mass stars, and were probably among the earliest stars to form in our galaxy. Shapley’s maps led to the discovery of the galactic halo, and the Sun’s true position in the galaxy. The spherical halo completely surrounds the disk, but it is nearly empty. Only 2% of the galaxies stars are in the halo, and almost all of them are in globular clusters.

Modal of the Milky WayFigure 1: Modern model of the galaxy

There were two important pieces to determining the center of the galaxy. First was the distribution of the globular clusters on the sky. Herschel noted in the early 19th century that the clusters weren't evenly distributed and many maps had been made by the time Shapley began his work. The second piece was to determine the distance to the clusters, and map their true locations from Earth.

Shapely used RR Lyrae variables to determine the distances to nearby globular clusters, but telescopes of the time weren't good enough to resolve individual stars in the more distant clusters. However, since globulars are all so similar, he assumed they should all be about the same size and brightness, and used their angular size and apparent magnitude to estimate their distances from Earth (see the Cepheid Variables activity for more on using variable stars to determine distance, and Galactic Star Clusters for more on determining the distance to globular star clusters and reddening from dust)

Unfortunately, Shapely did not know about interstellar dust, which dims and reddens starlight. This "reddening" makes the clusters seem farther away than they really are, which led him to estimate the size of the galaxy as around 300,000 x 30,000 ly. Additionally, at the time there were only 78 known globulars. There are now more than 150 known to reside in the Milky Way.

We now know the disk is about 100,000 ly across and only 1000 ly thick, and it contains most of the gas, dust, and stars have fallen into the disk. But this is only part of our galaxy.

 

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Updated: 10/31/12 by SAM

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