Put the Stars in Order of Distance
One of the methods astronomers use in determining stellar distances is
the relationship between the apparent magnitude of a star and its absolute
magnitude. The apparent magnitude scale was established in antiquity by
the Greek astronomer Hipparchus, who placed the stars visible to the naked
eye into six classes with first magnitude being the brightest and sixth
magnitude the faintest.
Now this system has been scientifically quantised but using a similar
scale. Consequently some bright objects have entered the negative end of
the scale most notably, Sirius, the brightest star in the sky. It has an
apparent magnitude of -1.45. The absolute magnitude of a star is defined
as the apparent magnitude the star would have if it were at a standard distance
of 10 Parsecs. If both the apparent and absolute magnitudes of a star are
known then the distance to the star can be calculated using the following
formula:
The Little Dipper is a good indicator of apparent visual magnitudes because
it is easy to find and the stars contained in it are of a variety of magnitudes.
It is especially good for determining the limiting magnitude, ie the faintest
stars you can see, because of the different magnitudes.
Using these images of Orion:
- Compare the stars of Orion with those of the Little Dipper and guess
at the magnitudes of Orion's stars.
- Use the table below, which gives the absolute magnitudes of those stars,
with the formula given above to calculate the distance to the stars. If
you don't have a calculator handy the form at the bottom of this page will
calculate distances using magnitudes for you.
| Star |
Absolute Magnitude |
| Rigel |
-8.1 |
| Betelgeuse |
-7.2 |
| Bellatrix |
-3.9 |
| Mintaka |
-5.8 |
| Alnilam |
-7.0 |
| Alnitak |
-6.2 |
| Nair al Saif |
-5.6 |
| Saiph |
-7.0 |
- Check your results with those
given here.
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