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How to Measure Magnitude for Objects in Space

For thousands of years people have looked to the sky with admiration and fascination. Some have also looked with wonder and curiosity. To understand the variety of objects visible in the night sky, the first step was to have some measure of their differences. The most obvious is brightness --- some objects in the night sky are brighter than others. To quantify this, astronomers created and refined a brightness scale called visual magnitude, which can be measured for anything in the heavens.

Things You'll Need

Telescope
Visible light filter
Optical power meter
Meter mask

Instructions

- 1
  Select a reference star with a known visual magnitude. You could measure without reference to an existing object, but that requires many measurements in a variety of conditions, and it doesn't offer a real advantage.
- 2
  Fit the visible light filter on the eyepiece of the telescope. Visual magnitude is defined in terms of a set of wavelengths that correspond to the sensitivity of the human eye. Astronomical visible light filters are manufactured to that standard.
- 3
  
  As long as the same detector is used for both measurements, the measure of optical power should be comparable.
  
  Focus the light from the reference star through the eyepiece onto the optical power meter. If necessary, place a mask on the power meter, so only the light from the reference star makes it into the meter. Record the reading as I_reference.
- 4
  
  A fancy telescope is not necessary to calculate visual magnitude, but it does make it possible to measure fainter stars.
  
  Focus the telescope on the star of interest and measure as in the previous step. Record the reading as I_measured.
- 5
  Calculate the difference in magnitude. Astronomers have agreed on a magnitude scale where a difference of 5 in magnitude corresponds to a difference of 100 in intensity. The means the magnitude difference is given by
  
  m_measured - m_reference = -2.5 * log ( I_measured / I_reference). Insert the readings recorded in Steps 3 and 4 into this equation and get the magnitude difference.
- 6
  Add the magnitude difference calculated in the previous step to the magnitude of the reference star. That is the magnitude of the measured star.

Astronomy