What are Star Magnitudes?

Star Magnitudes
Image Credit: cnx.org

In astronomy, the magnitude of a star refers to its measure of brightness, and while absolute magnitude relates to the star’s intrinsic brightness, apparent magnitude, on the other hand, is a way of measuring how bright the object appears in the night sky to us human stargazers back on Earth. The system was introduced by Greek astronomer Hipparchus in his star catalog of 129 BC, and later incorporated by the influential astronomer Claudius Ptolemy around 140 AD.

Measures Order of Brightness

Basically, the stars were ranked in order of brightness starting with 1 and going all the way down to 6 for those stars which were on the very edge of human visual perception. Of course, the invention of the telescope allowed Galileo to see stars beyond 6th magnitude, and as he noted in his astronomical treatise called Sidereus Nuncius (1610):

“Indeed, with the glass you will detect below stars of the sixth magnitude such a crowd of others that escape natural sight that it is hardly believable.”

As the power and quality of telescopes has continued to improve over the years, so has their ability to image ever lower levels of magnitudes, and these days you can expect to see a 9th magnitude object using 50mm binoculars, while a 6″ telescope may yield an image of a 13th magnitude star. At the extreme end of the scale, the Hubble Space Telescope has been able to observe faint distant objects of just 31st magnitude.

Logarithmic Scale

While early astronomers had to use their own eyesight and judgment to rate a star’s brightness, advancements in science during the mid-19th century led to the discovery that 1st-magnitude star were 100 times brighter than 6th-magnitude stars. A rule was adopted to incorporate this more accurate system of measurement, and so a logarithmic scale was then used to determine the following brightness comparison ratio between a 1st and 6th magnitude star:

1: 100 Times
2: 39.8 Times
3: 15.8 Times
4: 6.3 Times
5: 2.51 Times
6: 1

And so on. Furthermore, those objects even brighter than magnitude 1 are assigned negative values, such as the night sky’s brightest star, Sirius, which has an apparent magnitude of -1.46. There are a total of 22 first magnitude stars ranging from Sirius in Canis Major (-1.44) to Regulus in Leo (+1.36).

22 First Magnitude Stars

1: Sirius (-1.46) in Canis Major
2: Canopus (-0.72 in Carina
3: Rigil Kent (-0.27) in Centaurus
4: Arcturus (-0.04) in Boötes
5: Vega (0.03) in Lyra
6: Capella (0.08) in Auriga
7: Rigel (0.12) in Orion
8: Procyon (0.34) in Canis Minor
9: Betelgeuse (0.42 var) in Orion
10: Achernar (0.50) in Eridanus
11: Hadar (0.60) in Centaurus
12: Altair (0.77) in Aquila
13: Acrux (0.77) in Crux
14: Aldebaran (0.85 var) in Taurus
15: Capella (0.96) in Auriga
16: Spica (1.04) in Vega
17: Antares (1.09 var) in Scorpius
18: Pollux (1.15) in Gemini
19: Fomalhaut (1.16) in Piscis Austrinus
20: Deneb (1.25) in Cygnus
21: Mimosa (1.30) in Crux
22: Regulus (1.35) in Leo

Examples of Magnitude

Here are some other examples of the different magnitudes of objects as seen with the naked eye, in order to give you a better comparison point:

Sun: -26
Full Moon: -13
Venus: -4
Jupiter: -2
Saturn: +1
Big Dipper: +2
Jupiter’s Moons: +5
Uranus: +6

The Great Square of Pegasus Star Test

Here’s a good way to test your visual ability to differentiate the different orders of magnitude of stars, as well as the clearness of the night sky in your area. Start by locating an asterism called the Great Square of Pegasus, which consists of four stars, all of which are of fairly equal 2nd magnitude luminosity.

If you can see 2 stars within the Great Square then you are at magnitude 4.6, while 8 stars is magnitude 5.5, and 13 stars is near the naked-eye limit of magnitude 6.0. Carrying on, 37 stars means you have exceptional eyesight and can see stars of magnitude 6.5, while the German astronomer Johann Friedrich Julius Schmidt with his impressive eyesight was able to count 102 stars while stargazing in Athens during the 19th century, which means he was able to see at magnitude 7.4. Why not take the test yourself, with the two sets of figures that follows referring to number of stars seen inside the Great Square, and their corresponding magnitude:

Stars in the Great Square of Pegasus1: 4.4
2: 4.6
3: 4.7
4: 4.8
5: 5.1
6: 5.3
7: 5.4
8: 5.5
10: 5.7
12: 5.9
13: 6.0
18: 6.1
23: 6.2
25: 6.3
30: 6.4
37: 6.5
47: 6.6
55: 6.7
59: 6.8
63: 6.9
70: 7.0

Want to learn more about star magnitudes, then click here to discover “10 Interesting Facts about the Magnitude of Stars.”

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