What is Star Magnitude?

stars
All objects in the universe have a brightness that is defined by a number called its "magnitude". Each of these stars has a different magnitude. European Southern Observatory

Magnitude is a term that describes the brightness of an object. It can be applied to all celestial objects, from stars and planets to nebulae and galaxies. We most often see it used to state different brightnesses of stars. It is formally defined as the degree of brightness of a celestial body designated on a numerical scale.

The one most astronomers use is called "visual magnitude", which refers to light that we can see with our eyes — in the visible part of the spectrum of light.

That's usually around 5500 Angstroms (550 nanometers). You often hear observers refer to a "6th-magnitude star" or a 1st-magnitude object. Tgise simply define the objects' level of brightness to the naked eye. Astronomers also use magnitude to refer to objects much dimmer than we can see, objects normally visible only through telescopes. 

An Historical Look at Magnitudes

The attempts to determine magnitudes goes back far in human history. Around 120 BCE, a Greek astronomer named Hipparchus created the first known catalog of stars. While his listings do not survive today, it is thought that he included around 850 stars. His work was later edited and increased to 1,022 stars by Ptolemy in the second century B.C.E. and included in the famous work, the Almagest. The Greek astronomer Hipparchus listed the stars that could be seen in each constellation, described their positions, and rated their brightness on a scale of 1 to 6, the brightest being 1 and the dimmest at 6.

This method of describing the brightness of a star survives today. Of course, Hipparchus had no telescope, and so could only see stars as dim as 6th magnitude. Today we can see stars with ground-based telescopes down to about 22nd magnitude. Hubble Space Telescope sees stars as dim as magnitude 31.

Refining Magnitudes

As more accurate instruments came into use by astronomers, dimmer objects came into view. Astronomers wanted to come up with a more rigorous way of defining magnitudes. Eventually, they came up with a scale that puts each magnitude about 2.5 times brighter than the next greater magnitude. This means that magnitude 1 stars are around 100 times brighter than magnitude 6. Also the more accurate measurements allowed the astronomers to assign stars decimal values, like 2.75, rather than rounding off to magnitude 2 or 3.

Today, there are stars known to be brighter than magnitude 1. For instance Vega (alpha Lyrae) has a visual magnitude of 0. Any other star brighter than Vega will have a negative value for its magnitude. For example, Sirius is magnitude -1.46 and is the brightest star in our night-time sky. The Sun is magnitude -26.74.

Usually, when an astronomers talk about magnitude, they mean "apparent magnitude." This is usually written with a lower case m, as in 3.24m. This is the brightness of an object as seen from an observer on Earth, but adjusted to how bright it would look if there were no atmosphere to affect our view.

Other Measures of Magnitude

However, a star's brightness is not just a matter of how brightly it shines, but also how far away it is.

For example, step outside at night and turn your porch light on, then walk to the end of the block. You'll see a difference in brightness, but the bulb hasn't changed. So, astronomers came up with another way to measure brightness and called this absolute magnitude. This is defined as how bright a star would appear if it were exactly 10 parsecs (about 33 light-years) away from Earth. For example, the Sun has an apparent magnitude of -26.7 (because it's very, very close) and an absolute magnitude of +4.8.

Absolute magnitudes are often written with a capital M, as in 2.75M. They measure a star's "intrinsic" brightness—that is, the luminosity of a star without the effect of distance  or other factors (such as clouds of gas and dust absorbing or scattering a star's light). 

Edited and updated by Carolyn Collins Petersen.