Finding the Center of the Milky Way Galaxy Using Globular Star Clusters
Abstract
The Milky Way is the edgewise view of our home galaxy, a disk made up of billions of stars. The Sun resides on one of the spiral arms of the disk, 30,000 light-years from the thick hub of the galaxy. The actual center, with a black hole 3-4 million times the Sun's mass, is hidden by dust clouds in space. In this astronomy science fair project, you will use astronomical data to locate the center of this galaxy.Summary
Jacob Arnold and Jean Brodie, University of California. Santa Cruz, Department of Astronomy
Edited by David Whyte, PhD, Science Buddies
Edited by Sandra Slutz, PhD, Science Buddies
Objective
The objective of this astronomy science fair project is to use Internet-based software tools and databases to locate the center of the galaxy, based on the distribution of globular clusters.
Introduction
Our solar system is located nearly 25,000 light-years from the center of our Milky Way galaxy. We now know that we live in a spiral galaxy, consisting of billions of stars, and that our galaxy is just one of hundreds of billions of galaxies in the universe. However, the location of our Sun in the Milky Way, the size of our galaxy, the number of stars in it, and its structure were all unknown just 100 years ago. During the early 20th century, astronomers were trying to answer these questions using a variety of techniques. You will use one such method to determine the location of the center of our galaxy.
The most direct approach, adopted by Jacobus Kapteyn in order to determine the structure of the Milky Way, inferred distances for a number of stars in various directions to create a 3-dimensional view of our galaxy. Kapteyn found that our Sun lies at the very center of a nearly spherical distribution of stars, and he incorrectly concluded that we lie at the center of the galaxy. What Kapteyn was unaware of was that our galaxy is filled with starlight-absorbing dust, or interstellar dust. This means that stars far away from our Sun appear dimmer or are not even visible from Earth. This effect means we preferentially see the stars nearest to our Sun and cannot easily observe the other side of the galaxy. Therefore, this is not a good technique to use in determining the structure of the Milky Way.
Instead, you will adopt a method, used by Harlow Shapley, that correctly infers the direction of the center of our galaxy. Throughout most of the galaxy, stars are separated by a few light-years. However, globular star clusters contain anywhere from 10,000 to 1 million stars, densely packed into a region only a few tens to a few hundred light-years wide. Figure 1 shows a nearby galaxy surrounded by globular clusters. Because globular clusters contain so many stars, they are much brighter than individual stars and can be seen in the Milky Way, even at very far distances. Unlike stars, which tend to rotate around the Milky Way Galaxy in a flattened disk, globular clusters are distributed in a roughly spherical distribution around the center of the Galaxy. Thus, if we look toward the center of the Galaxy, we should see more globular clusters than if we look in the opposite direction.
Figure 1. The famous Sombrero galaxy (M104) is a nearby bright spiral galaxy. The prominent dust lane and halo of stars and globular clusters (globular clusters are the bright white spots) give this galaxy its name. (Wikipedia, 2009.)
In this science fair project, using a compiled list of the Milky Way's globular clusters (approximately 150), you will count the number of clusters found in each constellation. Constellations, like the Big Dipper or Orion, serve as a way to orient ourselves and define directions in our galaxy. You will determine which top three constellations contain the most globular clusters, and therefore, in which direction most Milky Way globular clusters exist. Using Google Earth in sky mode, you will determine a best-guess location for the center of the galaxy and compare this to the correct location.
Terms and Concepts
- Solar system
- Light-year
- Milky Way galaxy
- Spiral galaxy
- Jacobus Kapteyn
- Interstellar dust
- Harlow Shapley
- Globular star cluster
- Spherical distribution
- Constellation
- Google Earth
Questions
- What is a globular star cluster?
- Why are clusters better than individual stars for creating a 3-dimensional view of our galaxy?
- How are globular clusters distributed around galaxies?
- How big is the Milky Way?
- What is a constellation?
Bibliography
The original data for this project can be found at:
- Fromert, H. and Kronberg, C. (2011, June 24). Milky Way Globular Clusters. Retrieved October 29, 2015.
You'll need to download Google Earth to do this science project:
- Google. (2009). Google Earth. Retrieved January 12, 2009.
These resources contain good information about astronomy in general and the Milky Way in particular:
- Dolan, C. (n.d.). The Constellations and their Stars. Retrieved July 1, 2010 from the University of Wisconsin Department of Astronomy.
- Flanders, T. (n.d.). Constellation Names and Abbreviations. Retrieved July 1, 2010.
- Smith, H.E. (n.d.). The Structure of the Milky Way. Retrieved January 24, 2009, from the University of California, San Diego, Center for Astrophysics & Space Sciences.
- Cudworth, K.M. (1999, March 7). Short Essays: Galactic Structure, Globular Clusters. Retrieved January 24, 2009.
Materials and Equipment
- Personal computer with Internet access and Google Earth installed; see the Experimental Procedure below for more details
- Lab notebook
Experimental Procedure
- Do your background research so that you are knowledgeable about the terms, concepts, and questions above.
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Go to this link to see a list of all the globular clusters in the Milky Way: http://messier.seds.org/xtra/supp/mw_gc.html
- Columns 1-3 are names, or other identifiers, commonly used when to referring to each globular cluster.
- Column 4 ("Con") contains the name of the constellation where the globular cluster is found.
- The remaining columns are not necessary for this project, but you may find them interesting. Some simple text searches online or an introductory astronomy text book can help you figure out their significance. The abbreviations refer to:
- RA, Dec (2000): right ascension and declination for epoch 2000.0
- R_Sun, R_gc: distance from our Sun and the Galactic Center in thousands of light years (kly)
- m_v: apparent visual magnitude
- dim: apparent dimension in arc minutes
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Count how many globular clusters are in each constellation.
- NGC104 is the first globular cluster in the list. It is seen in the constellation Tucana.
- Make a data table in your lab notebook, add the constellation Tucana, and put NGC104 next to it.
- Repeat the process for each globular cluster. Add a new line in your data table for each constellation, but if a cluster is in a constellation that you already have in your list, put the cluster's name on that line instead of on a new one.
- Count the number of globular clusters you found in each constellation and record the numbers in another column in your data table. Note: Every entry in the data table is a globular cluster—some begin with "M," but most are names, numbers or codes. With a little patience you can easily sort all of the entries by constellation.
- Identify the three constellations with the most globular clusters seen in them.
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Now go to http://earth.google.com and click "Download Google Earth."
- Click "Agree and Download."
- Once the file has been downloaded, install the program.
- Open the Google Earth program.
-
Set up Google Earth in Sky Mode.
- At the top, click "View," then "Explore," then select "Sky."
- On the left-hand side of the window, you should see "Layers."
- Uncheck every item, except "Imagery" and "Backyard Astronomy."
- Click the arrow next to "Backyard Astronomy."
- Uncheck every item except "Constellations."
- Try to become familiar with the Google Earth navigation controls by panning around and zooming in and out, using the controls located in the top right corner of the screen.
- Notice the bright band that stretches across the sky. This is the disk of our Milky Way galaxy!
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Find the three constellations that contain the most globular clusters, which you identified in step 4.
- On the left-hand side is a search bar; type in the name of the first constellation.
- Repeat for the other two constellations.
- Zoom out and pan the sky until you can see all three constellations at once.
- Are the three constellations near each other? Most of the Milky Way's globular clusters should be in the direction of the center of the galaxy. Where do you think the center of the galaxy is?
- In the search bar, type "Galactic Center" to find the true center of the galaxy. How close was your guess?
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Try going back and using three other constellations with fewer globular clusters to predict the center of the galaxy. Is this second prediction more or less accurate (meaning closer or farther away from the true center of the galaxy) than the first one? Based on your results, do you think the distribution of stars really does increase as you approach the center of the galaxy?
- For your project display board, you can create a table or graph showing how the accuracy of your prediction changed as you used sets of constellations with fewer and fewer globular clusters to create those predictions.
Ask an Expert
Variations
- Find the distribution of globular clusters in the Milky Way by plotting their locations using Google Earth.
Careers
If you like this project, you might enjoy exploring these related careers:
Related Links
- Science Fair Project Guide
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