University of Michigan - Department of Astronomy

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The Distance Ladder and Google Sky

No conception whatever can be had of the magnitude of the visible universe until the distances of the stars are known.

--Edgar L. Larkin in Scientific American, 28 October 1905



This is an exercise in which you get the rare opportunity to apply your scientific knowledge in the analysis of pretty pictures. You’ll use Google Sky to look at images from the Hubble Space Telescope, Chandra X-ray Observatory, and Spitzer Space Telescope—many of these pictures are relatively famous, so some of them may look familiar! The goal is to not only get a sense for just what an array of stunning images these great satellite observatories have produced, but to figure out exactly what the images do and do not tell us about the objects in them. In particular, we want to relate the pictures to the theme of this course, the distance ladder.

Google Sky is a great starting point for many explorations in the world of astronomy. Hopefully this exercise gives you a sense of how it can be used as a launching pad for explorations of some of astronomy’s most famous images.

Additional Resources


Note that in all the images, you can recognize foreground stars (that is, stars that are between us and the object) by the diffraction “spikes.” These are artifacts produced by the way a star’s photons interact with the telescope optics, and only nearby (within the Milky Way) stars produce them.

Part 1: Using Google Sky to Gather Information

  1. Open Google Earth and change the view to the Sky map using the Map selection buttonbutton in the toolbar.
  2. Layers pane, best settingsTurn on the Hubble, Spitzer, and Chandra Showcase Lists in the Layers section, under Featured Observatories. You probably also want the constellation stick figures to help you identify where you are looking. See the image to the right for the best settings for the layers.
  3. The first column of table 1 is a list of objects with the name of the Showcase they are featured in in parentheses after the object name. Find each of these object by entering the full name below into the search field. It is best to search for the full name listed in the table to ensure you are getting the right object.
  4. For the HST images, clicking on “Read more” or “Learn more about this object” or “more” will take you to, official site of the Hubble Space Telescope, where you may find further interesting information. Likewise, clicking on these links for the Chandra and Spitzer images will take you to informational homepages for these observatories. For most of the photos, you will need to look up other websites to get all the information.
  5. Using the informational blurbs that Google provides for the photographs, and any other online resources you can find, put the following information into the table:
    1. The distance to the object in light-years.
    2. Where that distance places the object; for example: nearby within the Milky Way; far away within the Milky Way but on this side of the center; on the other side of the Milky Way’s center; orbiting the Milky Way; within the Local Group but not directly associated with the Milky Way; outside the Local Group but relatively nearby as galaxies go; very far away even for a galaxy?
    3. Of the techniques you have learned for determining distances, which techniques might have been used to determine the distance to the object?
    4. What are some important features/overall significance of the object; why did Google include the object in a photo showcase?

Part 2: Putting the Objects in Order

Now, using Table 2, put the objects in order from closest to furthest away. Again fill in the columns regarding where each distance places the object, and what techniques were applicable in finding the distance. This will give you a nice ordered sense of scale: you’ll have several distances, “sample objects” that correspond to those distances, a sense of how “far away” those objects are, and what point on the distance ladder that corresponds to.



Table 1: Image information
Object Distance to the Object (light-years) Where does that distance place the object?

What techniques might be used to determine the distance?


significance of the object

Open Star Cluster in SMC (Hubble)

Pinwheel Galaxy (Hubble)

Crab Nebula

Sagittarius Dwarf Galaxy (Hubble)

Cartwheel Galaxy (Chandra)

Sombrero Galaxy (Chandra)

M16/Eagle Nebula (Chandra)

M51/Whirlpool Galaxy (Spitzer)

Orion Nebula





Table 2: Objects in order from Closest to Farthest
Object Distance to Object (light years) Where is that? Technique(s) that could be used to find the distance











Concluding Questions:

Note that the HST images are all optical-only, while the Chandra and Spitzer images are composite images, including information from more than one wavelength range in the electromagnetic spectrum. On these composite images, sliding the opacity all the way to the left gives you an optical-only image (that is, an HST image), while sliding the opacity all the way to the right gives you an image that includes false-color X-ray information (Chandra), IR information (Spitzer), or both.

  1. Using the benefit of what you know about astronomy and distances, is there anything in the photos that you can use to infer, or at least make a guess about, the distance to these objects?

  2. What if you transported these images back in time and showed them to an astronomer 100 years ago? If you just gave the astronomer the images without any explanation, would she be able to infer anything about distances? What pieces of information could you give her that would help her judge distances?

  3. The first two questions probed to what extent knowing how to interpret the photos allows us to infer the distance. What about the other way around: how does knowing the distance (by measuring it using one of the distance-ladder techniques) allow us to interpret the photos? In each photo, what specific details would have been impossible to interpret had we not known at least an approximate distance to the object?

  4. The Cartwheel Galaxy image has data from infrared, visible, ultraviolet, and X-ray wavelengths. Where is the high-energy emission (UV and X-ray) concentrated? Where is the lower-energy emission (IR and visible) concentrated? What does this tell you about the structure of the object?

  5. How does the Sombrero Galaxy image change as you reduce the opacity from its highest setting to zero? What does this tell you about the distribution and temperature of gas in the galaxy?

  6. Change the opacity on the Eagle Nebula image as well. Note that it doesn’t change as much as the Sombrero Galaxy does. What does this tell you about the energy of the processes going on in the Eagle Nebula?

  7. Do the spiral arms of M51 become more or less apparent as you change the opacity on the image? What about the space between the spiral arms? What does this tell you about the distribution of matter in the galaxy?

  8. Why is the Orion Nebula photographed in infrared but not in X-ray?

  9. What information can you gain about the Pleiades by seeing the infrared image that you did not know when you had access only to a visible image in the Pleiades/spectral classification lab?

Last updated: 2/9/12 Original activity by CG

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