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Astronomy with an online telescope
Astronomy with an online telescope

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1.3 Estimating magnitudes by comparing stars

Although they are harder to see there are far more faint stars in the night sky than bright ones, as you may have noticed when out under the stars with dark-adapted eyes.

While there are only a handful of stars of magnitude +1 or brighter (including Sirius) there are many thousands of fainter stars. Including stars up to magnitude +6, there are approximately 5000 stars visible to the naked eye across the whole of the night sky. Since these are spread across the whole sky, perhaps half of these will be visible at any given time and location.

Activity 3: Estimating magnitudes

Timing: Allow approximately 30 to 45 minutes plus travel time if necessary

Although magnitudes can be measured precisely using sensitive detectors and instruments it is relatively easy to estimate magnitudes visually by comparing stars. If you are able to get out to a dark observing site, you can try this for yourself. If not, you can use the image of Orion in Figure 1 to do a similar thing following the same steps to estimate the magnitudes of stars in the image before checking the exact values reported by Stellarium.

  1. First, use Stellarium to select a prominent constellation that will be visible at the time you intend to do this activity.
  2. Click (or tap) on each of the main stars in the constellation in turn and make a note of the magnitude value reported by Stellarium. These will be your reference stars.
  3. Now head out to your observing spot, remembering to stay safe, and to use a red torch to look at your notes! Make sure that you can see your chosen constellation and identify the reference stars whose magnitudes you have noted.
  4. Now pick another star nearby whose magnitude you wish to estimate. Compare the brightness of this star with your reference stars – is it brighter, or fainter? Ideally, you will want to compare against two references – one brighter and one fainter, although this is not always possible. [If you are not able to go outside you can do this and the subsequent steps in the same way using Stellarium and the Orion image from Fig 1 – pick a target star in the image and compare with your reference stars, then check the value against your estimate.]
  5. If you have a comparison star either side you can refine your estimate by deciding whether your target star is midway in brightness between the two, or whether it is closer in brightness to that of the brighter or the fainter star. For instance, if you had comparison stars at +3 and +4 magnitudes and you felt that the target star was two-thirds of the way from the fainter star to the brighter, this would give an estimated magnitude for your target star of about +3.3.
  6. Check your estimated magnitude in Stellarium by clicking on your target star. Don’t worry if you haven’t got it exactly right - this technique takes some practice but is a very useful skill to have for observing with the naked eye, through binoculars or with a telescope. If you have time and can try a few more you should find that your estimates improve with practice.

Until the development of photography and digital imaging magnitudes were always estimated by eye in this way. Using modern technology the brightness of stars can be measured very accurately from digital images or with sensitive instruments, and magnitudes determined very precisely.