Astronomy Essentials

What is stellar magnitude?

The stellar magnitude scale. Black line from -30 (on the left side) to 30 (on the right side). Different stars point at this black line.
Stellar magnitude scale. The higher the number, the fainter the object. Negative numbers indicate the brightest objects, like Venus, the moon or the sun.

Stellar magnitude: An ancient system

A star’s luminosity refers to its intrinsic brightness. But the word magnitude in astronomy usually refers to how bright a star (or other space object) looks. That’s what we’re talking about in this post: how bright stars look and how astronomers categorize the different brightnesses of the stars.

The magnitude scale used by astronomers dates back a long way. The early astronomers Hipparchus (c.190 – c.120 BCE) and Ptolemy (c.100 – c.170 CE) used this scale. Both men compiled star catalogs that listed stars by their apparent brightnesses, or magnitudes.

This system remains intact to this day, though with some modifications.

And maybe – to us today – that’s why the magnitude scale doesn’t seem as simple as it might be. According to this ancient scale, the brightest stars in our sky are 1st magnitude. And the very dimmest stars visible to the eye alone are 6th magnitude.

So a 2nd-magnitude star is modestly bright. But it’s fainter than a 1st-magnitude star.

And a 5th-magnitude star is still pretty faint. But it’s brighter than a 6th-magnitude star.

So the magnitude scale in astronomy is a lot like a score in golf, in that the lower number means a greater brightness on the magnitude scale (and a better score in golf).

Hundreds of black points represent different stars. There are green lines representing many constellations.
People often find the magnitude system confusing because the brightest stars have negative magnitudes. For instance, the star Sirius, the brightest star of the nighttime sky, has an apparent magnitude of -1.44.

1st-magnitude stars, like Spica

In the spirit of Hipparchus and Ptolemy, the modern-day charts of the constellation Canis Major above and the constellation Virgo below scale stars by magnitude.

Take the star Spica, the sole bright star in Virgo. It serves as a prime example of a 1st-magnitude star.

In other words, although Spica’s magnitude is slightly variable, its magnitude almost exactly equals 1.

Hundreds of black points represent different stars. There are green lines representing many constellations.
Notice the magnitude scale at the lower left corner of this star chart and the one above. Sky charts via the International Astronomical Union (IAU). Visit IAU for sky charts of all 88 constellations.

Stars and planets brighter than Spica

Loosely speaking, the 21 stars that are brighter than magnitude 1.50 are called 1st-magnitude stars.

However, the 0-magnitude star Vega (magnitude = 0.00) is actually one magnitude brighter than Spica (m = 1.00).

And the star Sirius with a negative magnitude (m = -1.44) is nearly 2 1/2 magnitudes brighter than Spica.

Big white circle represents magnitude 1 on the top. A little white circle represents magnitude 6 on the bottom.
The apparent magnitude scale, from GCSE Astronomy.

One magnitude = 2.512 times brighter

Modern astronomy has added precision to the magnitude scale. A difference of 5 magnitudes corresponds to a brightness factor of a hundredfold. So a 1st-magnitude star is 100 times brighter than a 6th-magnitude star.

Or, conversely, a 6th-magnitude star is 100 times dimmer than a 1st-magnitude star.

So a difference of 1 magnitude corresponds to a brightness factor of about 2.512 times.

Red line to the left represents the brighter celestial objects (from 0 to -9). A purple line to the right represents the fainter celestial objects (from 0 to 9).
A higher positive number means a fainter celestial object; whereas a higher negative number means a brighter celestial object. For instance, Venus at its brightest has a magnitude of -4.6 and the faintest star visible to the unaided eye has a magnitude of +6.0.

Extending the magnitude scale

The sun and moon, plus the planets Venus and Jupiter, are much, much brighter than 1st magnitude. And modern telescopes let us see stars that are millions of times fainter than 6th magnitude.

Nowadays, the magnitude system includes not just stars but also the sun, moon, planets, asteroids and comets within the solar system, and star clusters and galaxies that reside outside the solar system. Astronomers even list the magnitudes of man-made satellites circling our planet.

Because a difference of 5 magnitudes corresponds to a brightness factor of 100 times, then a difference of 10 magnitudes corresponds to a brightness factor of 10,000 times (100 x 100 = 10,000). In addition, a difference of 15 magnitudes corresponds to a brightness factor of 1,000,000 times, and a difference of 20 magnitudes corresponds to a brightness factor of 100,000,000 times. Hipparchus and Ptolemy would be amazed!

Bottom line: Stellar magnitude is a measurement of brightness for stars and other objects in space.

Posted 
March 9, 2022
 in 
Astronomy Essentials

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