Professional and amateur astronomers all talk about the objects in the sky in an order of magnitude.
The brightness magnitude, that is.
All the stars and objects seen in the Universe are designated with a brightness number that is its “magnitude.”
That brightness magnitude is important to professional and amateur astronomers alike. But for astronomy educators, writing and talking about magnitude to the general public can be confusing.
Stellar magnitude is a completely arbitrary system to give our brains a reckoning point for how much brighter or fainter objects are compared to one another.
One of the fathers of astronomy, Hipparchus of 150 BC Greece, made accurate star charts and divided the various star brightnesses into six groups—from 1st to 6th magnitude.
Two hundred fifty years later, the records of Hipparchus’ sky observations were refined by the Greek scientist and philosopher Claudius Ptolemy (pronounced TOLL me). He, like many of the Mediterranean intellectuals, lived in Egypt because of the legendary library and school at Alexandria.
Over the centuries, the magnitude scale was refined farther, with stars being reassigned brightness values that include fractions. Four stars have “zero” magnitude, and the brightest, Sirius is minus -1.45 mag. The planets Venus (-3 to -4 mag.) and Jupiter (-2 to -3 mag.) are always brighter than Sirius, and sometimes Mars (+2 mag. to -2 mag.). The distance from Earth changes the brightness of the planets.
Confused? But it really isn’t that complicated. Magnitude numbers are a logarithm scale that separates each number by 2.512 times in brightness. So, a 2nd magnitude star is 2.5 times fainter than a 1st magnitude, but it is 2.5 times brighter than a 3rd magnitude star. And that 2nd magnitude star is 6.5 times brighter than a 4th mag. star, and 15 times brighter than a 5th magnitude star, and so on.
The lower the number, the brighter the object. The higher the number the fainter. The minus number is assigned to the brightest objects, like planets, the Sun and Moon.
The Moon is –12 mag. at full phase, around –5 mag. at First Quarter. The Sun is –27 magnitude all the time.
There are whole bunches of stars that vary in brightness over period of hours, days, even years. The magnitudes of these “variable stars” have been plotted accurately for decades, providing clues to the astrophysics involved.
The faintest stars seen in the light polluted skies of suburbs are around 4th magnitude. And in the dark skies of a lake, mountain or countryside, the human eye can see as faint as 6th magnitude. That’s a 2.5 x 2.5 x 2.5 magnitude difference, meaning you can see stars 15 times fainter at a dark lake compared to your backyard. And that means many more stars to see.
Binoculars see stars in the 8-10 magnitude range. A telescope probes deeper into magnitudes, like 12-14. But objects fainter than around 15th magnitude are better left for the large, professional telescopes. They probe the Universe in the 18-20-magnitude range. The Hubble Space Telescope can see objects as faint as 32nd magnitude.
A 1st magnitude star is 250 times brighter than a 6th, which is a comfortable human threshold for eyesight—though some people can see stars as faint as 7th magnitude.
A star’s brightness has little to do with its power source. Generally, the brightest stars are the closest, and some of the most powerful stars are so far away that they are visible only in large But there are exceptions, like Deneb in Cygnus a whopping 3,200 Light Years away yet at 1st magnitude it is the 14th brightest star in the Northern Hemisphere.
Star charts depict the different stellar brightness by making the bright ones bigger. A scale of star point sizes on the map denotes the magnitudes.
When you see a star chart it is made up of star “dots” of various sizes, and that helps understand how easy or hard the celestial object is to see visually as well as photographically. Monthly astronomy magazines and star charts in libraries can help the new stargazer easily understand this.
When an object is described by its magnitude, remember that the faintest star you are going to see is around 6th magnitude, that 4th magnitude is about the faintest you’ll see from a neighborhood backyard, and 1st magnitude is a bright, eye-catching star.
Of course, the darker the viewing site, the fainter the magnitude of stars can be seen, and thus, more stars are visible in the sky. That’s why at dark sites like Mt. Rogers, South Holston Lake and the High Country of North Carolina, we see maybe 2,000 stars with our naked eye. But in suburban Boone’s Creek, Tennessee, you might be lucky to see a hundred stars.
Astronomers call artificial light from civilization “light pollution.” It is a serious problem to professional and amateur stargazers as new shopping centers, residential housing, and streets add more security lighting and rob the night of darkness.
The International Dark Sky Association campaigns for the proper security lighting that shines down, not upward in the sky. Even the neighbor’s back porch light can limit the things a backyard telescope can see in the night sky. So, serious stargazers are always looking for dark sky sites. And they are willing to drive an hour or so just to set up their telescopes at an optimum view of the heavens.
This matter of magnitude gives the constellations their character, boundaries and asterisms. That is the fun of learning the night sky. The bright stars of the familiar constellations all have a name and story behind them—first told in antiquity.
In fact, many of the star names and constellation lore have Arabic names and roots that go back to the Fertile Crescent of 3,000 B.C. This is the home of the first and little known civilizations of Sumaria, Acadia and Babylonia. That location is today’s Iraq and Iran.
The heavens are a matter of magnitude that really isn’t that hard to understand—one that bright people will easily figure out!