University of Michigan - Department of Astronomy

Version: plan

Brightness and Surface Brightness


Part 1: Surface Brightness

Your instructor will set up the planetarium with Orion up and the lighting set to about the same level as the light pollution at the Angell Hall Student Observatory. You will receive a red flashlight to help you see this worksheet.

  1. When the planetarium is ready, compare what you see in the planetarium to the magnitude charts to determine which chart matches best with what you see. Record the chart number in Table 2.
  2. Your GSI will adjust the sky brightness to simulate the other locations in Table 2, including Peach Mountain in Dexter, the location of the UM radio telescope and the 24” McMath Telescope.  For dark sites, it is helpful to pay attention to the area around Orion's belt. Your GSI will bring up the lights so you can pick which magnitude chart best matches Peach Mountain on a good night.
  3. Your GSI will point out the Orion Nebula, which is a glowing cloud of gas with new-born stars. Label it on one of your sky charts.
  4. Use the sky charts and Table 1 in the Introduction to determine the sky surface brightness in mag/arcsec2 for these observations, and enter the values in Table 2.
Table 2
Setting Chart Number/NELM Sky Surface Brightness (mag/arcsec2) Limiting Magnitude (mag) Orion Nebula visible?
Angell Hall        
Downtown Detroit        
Peach Mountain      


Part 2: Limiting Magnitude

  1. Your GSI will point out a star and tell you its name. Compare it to the stars in Orion and determine its magnitude from the sky charts. The number of the sky chart shows the magnitude of the faintest stars shown. Record the star's name and magnitude here:

  2. Use your sky chart to determine the limiting magnitude above which stars are invisible for the observations in Table 2, and enter the values in the Table.

Concluding Questions:

  1. Estimate the surface brightness of the Orion nebula. Explain how you obtained it.

  2. Most of the Andromeda galaxy has a surface brightness of 22.3 mag/arcsec2. Could you see this from Angell Hall? From Peach Mountain? Explain your answers.

  3. Explain why the surface brightness of the Milky Way is roughly the same as the surface brightness of the Andromeda galaxy (22.3 mag/arcsec2), even though the latter is 1000 times farther away than most of the Milky Way.

  4. A laser emits light only in a concentrated beam in a single direction. Explain why the observed brightness of a laser does not follow the inverse square law for light. This is why a laser should never be pointed at your eye, no matter how far away it is.

  5. The star Betelgeuse is magnitude 0.2. How much more light do we receive from it than from the star whose magnitude you estimated in Question 5? Show your work.

Last updated: 8/22/12 by SAM & MSO

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