Star Magnitudes: Smaller Numbers, Brighter Stars

Smaller numbers mean more received light

Apparent magnitude describes an object's brightness as seen from our viewpoint in a specified wavelength band. A magnitude 1 star is brighter than a magnitude 4 star. Negative numbers describe brighter objects still; they are not mistakes or negative amounts of light.

The scale is logarithmic. A difference of five magnitudes corresponds to a brightness ratio of exactly 100. One magnitude is a factor of about 2.512. Subtracting the numbers gives a step on the scale, not the number of times one star is brighter than another.

Reference: NASA Chandra: Magnitudes

Compare two imaginary stars

Suppose star A is magnitude 2 and star B is magnitude 5 in the same band. A is the brighter star because its number is smaller. Their three-magnitude difference corresponds to 100 raised to the power 3/5, or about 15.85. A supplies roughly 16 times as much measured light.

For mental estimates, use the landmarks below. Decide which object is brighter before assigning the ratio; this avoids accidentally turning the result upside down. A measurement in a blue filter should not be compared uncritically with one in an infrared filter.

Reference: Las Cumbres Observatory: Magnitude and Color

Apparent brightness is not stellar power

A nearby modest star can look brighter than a much more luminous distant star. Absolute magnitude compares stars at a standard reference distance of 10 parsecs. It helps separate intrinsic brightness from the effect of distance; intervening dust and the measurement band still need consideration.

An object's total magnitude also does not guarantee easy visibility. A galaxy spreads its light across an area, while a star appears nearly point-like. Background skyglow can conceal an extended object even when its catalog number sounds encouraging. Read the target's size and observing guidance as well as its magnitude.

Reference: Las Cumbres Observatory: Absolute Magnitude

Try a small comparison by eye

Pick three charted stars close together and compare their brightness ordering before reading their labels. Keep them at similar elevations so that looking through different amounts of atmosphere does not dominate the exercise. Your eyes need not reproduce a precise ratio for the comparison to be useful.

Write down the Moon conditions, haze, and nearby lights. The faintest star you detect is a result for that session, not a permanent rating of your eyesight or a universal limit for everyone.