The luminosity of an object in space is the amount of energy that it radiates each second in all directions. Luminosity is also referred to as the absolute magnitude or absolute brightness of an object. It is the real brightness of a celestial object.

The apparent magnitude or apparent brightness of an object is a measure of how bright an object appears to be to an observer. It is the amount of energy from an object in space which reaches a square centimeter of a detector each second. Apparent magnitude is also referred to as flux. It is a measure of how bright a celestial object appears to us. The apparent magnitude of an object depends upon its real brightness and on its distance from us.

A visible light view of a star cluster. Notice how the stars appear to have different brightnesses or apparent magnitudes.
Hillary Mathis & N. A. Sharp, KPNO, AURA, NOAO, NSF
If you look up at the night sky on a clear night, you will notice that the stars appear to have different levels of brightness - some are bright and some are dim. The apparent brightness of an object is measured in magnitudes. This system was developed over 2000 years ago by the Greek astronomer Hipparchus to rank how bright different stars appeared to the eye. In his magnitude system, the brightest stars were called first magnitude stars, and the dimmest were sixth magnitude stars. So, in this system, brighter objects have lower magnitudes than dimmer objects. Much later, when astronomers were better able to measure the brightness of stars and other celestial objects, they kept the traditional magnitude scale of Hipparchus and added magnitudes that go beyond the range of 1 to 6. Objects which appear to be much brighter than the stars, such as the Sun, Moon and Venus are given negative magnitudes or -26.7, -12.6 and -4.4 respectively. With modern telescopes we can measure objects which appear as faint as +28 magnitudes. Each number on the magnitude scale is about 2.5 times apart in brightness from the next number. For example, first magnitude stars (stars with a magnitude of 1) are about 2.5 times brighter than second magnitude stars (stars with a magnitude of 2).

When we look at an object in space we see its apparent brightness. If we know the object's distance from us, it is easy to calculate its absolute brightness or luminosity.

absolute magnitude = apparent magnitude - 5 × log(distance in parsecs) + 5.

From this formula, you can see that if a celestial object is 10 parsecs away from us, then its apparent magnitude is equal to its absolute magnitude. Both the apparent and absolute brightness of objects in space will be different at different wavelengths, for example the infrared magnitude will not be the same as a visible light magnitude, however, the above formula still applies. Below is a table showing the visible light absolute and apparent magnitudes of some well known stars.

Star Apparent Mag. Distance(pc) Absolute Mag.
Sun -26.74 000000.48 4.83
Sirius -1.44 2.64 1.45
Arcturus -0.05 11.25 -0.31
Vega 0.03 7.76 0.58
Antares 1.00 130.0 -4.7
Barnard's Star 9.54 1.82 13.24

By using large-format detector arrays and innovative choices in orbit and cryogenic architecture, The Spitzer Space Telescope will be able to detect objects which are many magnitudes fainter than have ever been detected by previous infrared missions.