University of Michigan - Department of Astronomy


The Structure of the Milky Way Disk

Nothing has shown more fully the prodigious ignorance of human ideas and their littleness, than the discovery of Herschell, that what used to be called the Milky Way is a portion of perhaps an infinite multitude of worlds!

-- Horace Walpole



The Milky Way, like all spiral galaxies, has three basic parts: a central, dense bulge; a very thin roughly spherical halo; and the disk. The bulge is the densest region, though is difficult to view due to the dust in the center of the galaxy. Radio and infrared telescopes were required to fully map its size and shape. In contrast, the halo is difficult to view because it has so little in it. Its structure was recognized in the early twentieth century by Harlo Shapley, who mapped it out using globular clusters. The galactic disk has been recognized since the mid-eighteenth century, when Sir William Herschell (sometimes spelled Hershell) recognized the band of the Milky Way as being the disk of stars of the galaxy we live in. Unfortunately, he didn't know about the interstellar dust, so for over a century and a half, the galaxy was believed to be a slightly flattened disk of about 3 kpc across with the Sun near the center. 20th century astronomers have given us a clearer view of what the galaxy is truly like, from its size and structure to our location nearly 2/3 the way to the edge!

The disk of spiral galaxies is the most obvious (and usually most beautiful) part of the galaxy. The brilliant O and B stars tend to be confined to the spiral arms making it look like the disk is very lumpy. However most of the stars gas and dust follow an exponential law:

where i specifies the type of object you're looking at, R is the distance out from the center, ρi is the density of objects of type i at distance R, ρi,0 is the density of objects of type i in the central region, and H is some scale length. For all stars except the most massive O and B, H is equal to about 3.5 kpc. O and B stars and the gas and dust have different scale lengths that keep them more closely confined within the disk. Other galaxies may have different scale lengths.

In this lab you will create your own galactic disk and look at the distribution of most stars. Each group will have a different scale length so you can see the effect of the different scale lengths on the distribution of stars. You will need a group of 3 - 4 people and a clear table to do this activity.

Part 1: Setting Up the Model

  1. Get a 4-foot by 3-foot piece of paper from your GSI with circles marked on it. Tape the corners of the paper to the table.
  2. Measure the radius from the center to the outermost ring on the paper. Your galaxy should have a radius of 18 - 20 kpc. Choose a reasonably convenient scale for calculations (e.g. if the radius is 19 inches, a scale of 1 inch = 1 kpc and a radius of 19 kpc would be very convenient). Record your scale and radius here:

Scale: _____________________________ Radius: ___________________

  1. Get a table from your GSI. There should be a scale factor at the top of the page. Record the scale factor here:

  2. Since the disk is equivalant to a plane, the central density will be the surface density, or number of kernels per unit area. The number of kernals and the radius of the central circle are in the table given to you by your GSI. Use this information to calculate the central density of your model. Show your work here and record your answer here:

  3. Write the density law for your model, filling in all the constants. Check it with your GSI.

  4. The density and number of kernals for the inner 4 annuli ("rings") are given in the table. Caluclate the density for the outer annulus. Show your work and circle your answer.

  5. The number of kernals for the inner 4 annuli are given in the table. Calculate how many kernals you need for the outer annulus. Show your works and circle your answer

  6. Count out the correct number of kernals for each annulus and spread them on your model. Then take a look at the other models, including the vertical model your GSI has on the front desk, and answer the questions below.

Part 2: Looking at the models

  1. Observe all the models. How are they different? How are they similar?

  2. Describe how the models change as the scale length increases.

  3. The surface density for all the models decreased exponentially as you moved away from the center. However, in some of the models, the number of kernels actually increased at first before decreasing. Why did this happen?

  4. The dust in the galaxy is more closely confined to the center of the disk. Based on the observations of the models, is this a smaller or larger scale length? Explain your answer.

  5. How will the dust affect observations of the galaxy?

  6. Why are the O and B type stars not only concentraited near the center of the disk, but also to the spiral arms?

Part 3: Observing the Milky Way

Your GSI will give you a set of images. Sort them in order of increasing distance from the center of the galaxy. Describe the features you used to determine the distance and how they relate to the observations of the models.

updated: 6/27/05

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