Essential astronomical terms

magnitude ... a number on a scale of apparent brightness. The brightest star (Sirius) has magnitude -1.4. The faintest 'naked eye' star has magnitude 6.

Note: a decrease of one magnitude unit represents an increase in apparent brightness by a factor of about 2.5.

The magnitude scale is a modern interpretation of the ancient Greek scale that listed bright stars as 1 and fainter stars as 2 .... 5 and 6 for the faintest.

The faintest stars that can be resolved in ground-based telescope images are of magnitude 22. That limit has been extended to 30 by the Hubble Space Telescope.

absolute magnitude ... the apparent magnitude of a star if it were placed at a distance of 10 parsecs.

parsec (pc)... the parsec is the distance at which a star has a parallax angle of one second of arc. One parsec is 3.26 light years.

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