The Magnitude Scale

How bright is that star? Astronomers use a logarithmic magnitude scale to assign brightnesses to stars. This notation is derived from that of the Greek astronomer Hipparchus, who assigned a brightness of "first magnitude" to the brightest stars, and "sixth magnitude" to the faintest stars visible with the unaided eye. The scale has been standardized and quantified, so that one star five magnitudes brighter than another gives 100 times as much light to Earth. This scale is useful for telescopic astronomy, as it can be applied to objects of any brightness.

Object                                  Magnitude

Sun                                       - 27
Full Moon                                 - 12
Venus at brightest                        - 4.9
Mars, Jupiter, and Mercury at brightest   - 2.8
Sirius (Brightest Nighttime Star)         - 1.5
Vega, Arcturus, Capella, Alpha Centauri   ~ 0.0
Saturn at brightest                         0.2
Betelgeuse, Antares, Aldebaran            ~ 1.0
Polaris                                     2.0
Andromeda Galaxy                            3.5*
Typical limiting magnitude**
  from Portland                             5.2
Uranus                                      5.6
Globular Cluster M13                        6.0
Standard limiting magnitude                                 
  (dark site)                               6.5
Faintest limiting magnitude I've seen     ~ 7.5
Neptune                                     8.2
Pluto                                      13.7
Faintest objects visible in photos taken
  with large optical telescopes          ~ 28

*Diffuse objects such as galaxies appear to be fainter than their published magnitudes, because their light is spread out over a large area rather than concentrated into a point. These objects are said to have low surface brightnesses.

**The limiting magnitude at any given time is represented by the faintest star which can be seen with the naked eye. Moonlight and light pollution, among other factors, will lower the limiting magnitude.

You can find your limiting magnitude by counting stars in specified areas of the sky. More information is at this site.