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ATLANTIC SKIES: Magnitude explained: How astronomers determine the brightness of stars

Mid-October is the time of year for the Orionids Meteor Shower. Over a dozen meteors were caught in successively added exposures over three hours in this October 2006 photo shared with NASA by Tunc Tezel taken near Bursa, Turkey. Multiple brilliant meteor streaks can all be connected to a single point in the sky just above the belt of Orion, called the radiant. The Orionids meteors started as sand-sized bits expelled from Comet Halley during one of its trips to the inner solar system. - TUNC TEZEL, courtesy of NASA
Courtesy of NASA - SaltWire Network

Astronomers use two different types of stellar magnitudes to measure the brightness of a star or other celestial body: apparent magnitude and absolute magnitude.

Although it is generally termed "stellar magnitude," the same principles of magnitude apply when measuring the brightness of other celestial objects, such as the planets, comets, galaxies, the moon and the sun. Apparent magnitude (sometimes referred to as "visual magnitude") relates only to how bright a particular star appears visually to an observer on Earth.

Absolute magnitude, meanwhile, is the intrinsic brightness a celestial object would exhibit if it were viewed from a distance of 32.6 light-years (approximately 310 trillion kilometres). Most amateur astronomers are more interested in apparent magnitude than in absolute magnitude.

As the magnitude scale currently stands, brighter objects are assigned negative numbers (the higher the negative number, the brighter the object), and fainter objects are assigned positive numbers (the higher the positive number, the fainter the object).

It was the Greek astronomer Hipparchus who, in the second century B.C., created the magnitude scale. He labelled the brightest stars he could see in the night sky as first-magnitude (+1.0) stars. Stars dimmer than +1.0 magnitude were labelled second-magnitude (+2.0), stars dimmer than +2.0 magnitude were labelled third-magnitude (+3.0) stars, and so forth, down to sixth-magnitude (+6.0) stars, the dimmest stars that he could see. This is primarily the stellar magnitude scale that modern-day astronomers still use, though with a slight, but significant, change.

In 1850, English astronomer Norman Robert Pogson established a new system of ascribing magnitudes to celestial objects. He determined that a difference of five magnitudes corresponded to a brightness factor of one hundredfold. Simply put, a first-magnitude (+1.0) star is 100 times brighter than a sixth-magnitude (+6.0) star. Since the fifth root of 100 is approximately 2.512, a difference of one magnitude corresponds to a brightness or dimness factor of about 2.512, meaning that a +6.0 magnitude star is 2.512 times dimmer than a +5.0 magnitude star.

Pogson was eventually forced to add negative magnitude values in order to accommodate the vast range of celestial object magnitudes. As the magnitude scale currently stands, brighter objects are assigned negative numbers (the higher the negative number, the brighter the object), and fainter objects are assigned positive numbers (the higher the positive number, the fainter the object). For example, the faintest star that can be seen (under a clear sky, away from city lights) with the naked eye by the average human is usually around +6.0 magnitude, while the brightest star Sirius in Canis Major has a magnitude of -1.5, the full moon approximately -11, and the sun a whopping -26.7 magnitude.

While this manner of ranking magnitudes appears "backward" to most people, it does become easier the more one uses it.

Can you see it?

Here is a small test for you. On a clear spring or summer night, go outside and see if you can spot, with just your naked eye (remember to give yourself about 15-20 mins for your eyes to dark-adapt), the following stars in the northern sky (star/constellation and magnitude):

- Mizar/Ursa Major - +2.27

- Talitha/Ursa Major - +3.14

- Alcor/Ursa Major - +4.01

- Chalawan/Ursa Major - +5.05

- Polaris/Ursa Minor - +2.02

- Pherkad/Ursa Minor - +3.05

- Yildun/Ursa Minor - +4.36.

You may need to consult a star atlas or go online to find some of these stars. Ursa Major and Minor are visible all night long, every night of the year. Let me know how you make out.

This week's sky

Mercury, heading towards superior solar conjunction (passes behind the sun as viewed from Earth) on April 18, is currently too close to the sun to be seen. Venus is likewise too close to the sun to be observed.

Mars (magnitude +1.4, can be found in Taurus - the Bull) becomes visible in the evening sky about 43 degrees above the western horizon by 8:40 p.m., before dropping to the horizon and setting by 1:15 a.m. Look for the waxing, crescent moon near Mars on the evening of April 16. The Red Planet will form a triangle with two other celestial red objects in the mid-evening, western sky: Betelgeuse, in the constellation Orion - the Hunter, to its lower left, and Aldebaran, in the constellation Taurus - the Bull, to its lower right.

Saturn (magnitude +0.7, seen in the constellation Capricornus - the Sea Goat) rises in the southeast around 4:10 a.m., reaching 14 degrees above the horizon before fading with the approaching dawn by about 5:50 a.m.

Jupiter (mag. -2.1, in the Capricornus constellation) rises in the east-southeast shortly after Saturn, around 4:40 a.m., reaching a height of 13 degrees above the horizon before fading from view by about 6:10 a.m.

Until next week, clear skies.


  • April 14 - Moon at apogee (furthest from Earth)
  • April 16 - crescent Moon near Mars, in the western sky, mid-evening

Glenn K. Roberts lives in Stratford, P.E.I., and has been an avid amateur astronomer since he was a small child. He welcomes comments from readers at [email protected].


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1 being least likely, and 10 being most likely

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