Mass Estimate for the Milky Way

The article, “New Mass Estimate for the Milky Way,” explains how the question of the size of our galaxy has been a challenging topic for decades. The reason it is so difficult to estimate is because it is hard to measure something you are inside of, and it’s difficult to measure something you can’t see. Astronomers make do with what they do see, such as stars in the Milky Way’s halo. Astronomer Gwendolyn Eadie chose to use globular clusters, spherical collections of old stars that reside in the stellar halo, outside the disk. The farthest of these clusters floats 400,000 light years from the center of our galaxy. Using globular clusters does not give a perfect estimate of our galaxy because the galaxy’s dark matter halo extends much farther. Making do with what they know, Eadie and her team estimated that the Milky Way contains between 600 and 750 billion suns worth of mass within 600,000 light years. This is a lot lower than previous estimates which set the mass of the Milky Way at about a trillion suns.

Our objective was to be able to describe the structure and size of the Milky Way galaxy. This article relates because it discusses the technique of using globular clusters to estimate the size of the galaxy. The Milky Way galaxy consists of a thin disk about 100,000 light years in diameter with a central bulge and a spherical region called the halo that surrounds the entire disk. Most of the galaxy’s bright stars reside in its disk. According to the textbook, the disk contains most of the gas and dust of the interstellar medium, while the halo contains only a small amount of hot gas and virtually no cold gas. The most prominent stars in the halo are found in about 200 globular clusters of stars. The power point slides in class stated that the orbit of clusters can be used to estimate the mass of the galaxy, just like what the astronomers in this article are doing. In order to view the galaxy from within it we can measure the distance to globular clusters or measure distribution of hydrogen gas in the disk.

This was an article that I put in my Astro Journal earlier in the semester. I remember being really intrigued by it because I could not imagine how we could know the size and structure of the Milky Way galaxy when we are inside of it. It seems as though every estimate that has been made has been very different from the others. I liked this objective because it answered a lot of the question that I had before about our galaxy.


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