"Star brightnesses are measured in magnitudes, a system invented by the Ancient Greeks 2000 years ago and refined by modern astronomers. The Greek divided the stars into six classes, sixth-magnitude stars being the faintest visible to the naked eye, and first-magnitude stars being the brightest.

When in the last century astronomers developed instruments to measure star brightness accurately, the magnitude scale was redefined so that a difference of five magnitudes exactly equals a brightness difference of 100 times. Hence a star of magnitude 1.0 is 100 times brighter than one of magnitude 6.0. Once the magnitude scale had been defined in this way it was necessary to modify it to include the brightest stars, which are more than 100 times brighter the faintest ones visible. So the magnitude scale now continues up through magnitude 0 and into negative magnitudes for the very brightest objects. Vega, a brilliant star of summer, has a magnitude of almost exactly 0, while Sirius, the brightest star in the sky, is magnitude -1.46.

The magnitude scale is used for all celestial objects, not just stars. The planets Venus and Jupiter (magnitudes respectively -4.7 and -2.9 at best) are even brighter than Sirius, and sometimes so is Mars (-2.8 at best). In fact, the magnitude scale can be extended indefinitely in both directions, up to the very brightest objects such as the Moon and Sun (magnitudes -12.7, when full, and -26.8 respectively) and down to the faintest objects visible through large telescopes, which are around magnitude 24 (the magnitude is assumed to be positive if no sign is given). With modern instruments, astronomers can measure brightnesses to a precision of a hundredth or even a thousandth of a magnitude.

The faintest magnitude viable is known as the limiting magnitude. While the nominal naked-eye limit is sixth magnitude, this is true only under clear, dark skies away from haze and streetlights. The actual limiting magnitude on a given occasion depends on the sky conditions and also on the observer's eyesight. In the polluted air of towns, the limiting magnitude may be no better than fourth magnitude. Under ideal conditions observers with exceptional eyesight have reported seeing stars of seventh magnitude. Much fainter stars can be seen through binoculars and telescopes.

Objects near the horizon look much dimmer because their light has passed through a greater thickness of the Earth's atmosphere. This effect, called extinction, amounts to about one magnitude at 10^o from the horizon, and even more for objects closer to the horizon. Therefore it is always best to observe objects when they are high in the sky."

From the American Nature Guides, Astronomy by Ian Ridpath pg. 13-15.

Magnitude	Example	Comments
-12	full moon
-5	Venus	Brightest starlike object. Can cast shadows.
-1.5	Sirius	Brightest star
0	Vega	Only eight stars are this bright or brighter.
1	Antares	Twenty stars are this bright
2	Polaris	Medium bright: 60 stars
3		Dull medium: 150 stars
6		Borderline visible: 6,000 **
9		Binocular limit: 50,000
12		Barely visible through small telescope; 60,000 times fainter than Vega.
20		Visual limit of Hale telescope 100 million times fainter than Vega.
25		Faintest Hale telescope stars using electronic amplification.
29		Detection limit of Hubble Space Telescope; 250 billion times fainter than Vega.

*Table from Secrets of the Night Sky. The Most Amazing Things in the Universe You Can See with the Naked Eye by Bob Berman. page 11-12.
** 6th magnitude stars are visible in a very dark location.

How do you pronounce all of those hard star and constellation names? Go to the Astronomical Pronunciation Guide and find out.
Now that you know how to pronounce the star and constellation names, how do you pronounce the names of the planets, their satellites and prominent surface features. Go to the Astronomical Pronunciation Guide - II and you will know.

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