Stellar Magnitude

In astronomy, magnitude refers to the logarithmic measure of the brightness of an object, measured in a specific wavelength or passband, usually in optical or near-infrared wavelengths.

Origin and why bright objects are negative magnitude, dim objects positive

It traces to the Greek astronomer Hipparchus (or the Alexandrian astronomer Ptolemy—references vary). He classed stellar objects on how bright they appeared — the brightest were "magnitude 1", the next brightest were "magnitude 2", on down to "magnitude 6", the faintest he could see. Thus the scale is roughly 2000 years old.

Absolute scale based on Vega

The star Vega is defined to have a magnitude of zero, or at least near. Modern instruments as bolometers and radiometers give Vega a brightness of about 0.03. The brightest star, Sirius, has a magnitude of −1.46. or -1.5.

Problems

The human eye is easily fooled, and Hipparchus's scale has had problems. For example, the human eye is more sensitive to yellow/ Red light than to blue, and photographic film more to blue than to yellow/red, giving different values of visual magnitude and photographic magnitude. When we use precise instruments to actually measure light from stars, we find a rough multiplicative factor of roughly 2.5 between magnitudes (e.g. a magnitude 2 star is roughly 2.5 brighter than a magnitude 3 star). The actual value is closer to 2.512 (the 5th root of 100); the scale is logarithmic, not linear. The use of the 5th root of 100 is difficult in computations as it is an irrational number. Furthermore, many people find it counterintuitive that a high magnitude star is dimmer than a low magnitude star.

The modern world

Astronomers can now measure differences as small as one-hundredth of a magnitude. Stars between magnitudes 1.5 and 2.5 are called second-magnitude; there are 20 stars brighter than 1.5, which are first-magnitude stars.

Apparent and absolute magnitude

Two specific types of magnitudes distinguished by astronomers are:

• Apparent magnitude, the apparent brightness of an object. For example, Alpha Centauri has higher apparent magnitude (i.e. lower value) than Betelgeuse, because it is much closer to the Earth.
• Absolute magnitude, which measures the luminosity of an object (or reflected light for non-luminous objects like asteroids); it is the object's apparent magnitude as seen from certain location. For stars it is 10 parsecs (32.6 light years). Betelgeuse has much higher absolute magnitude than Alpha Centauri, because it is much more luminous.

Usually only apparent magnitude is mentioned, because it can be measured directly; absolute magnitude can be derived from apparent magnitude and distance using the distance modulus.