1: Naked Eye Vision Scale From Magnitude 1 to 6
If your familiar with the stars of the night sky, then you will know that the brighter the star, the smaller the magnitude assigned to it. The brightest stars that we can see with the naked eye, for instance, range in visual magnitude from -1 to +1, while the fainter ones go down as low as magnitude 6. In fact, these 6 magnitude stars are on the extreme edge of our visual perception, and are so faint that unless you have both good eyesight and clear viewing conditions, then you may not be able to see these at all.
2: Devised by Hipparchus in 2nd century AD
It was Greek astronomer Hipparchus who first introduced the magnitude scale for measuring a star’s order of brightness from 1 to 6 in his star catalog of 129 BC. Claudius Ptolemy (100-170 AD) added further credence to the system by utilizing it in his scientific treatises, but by the early 1600s the invention of the telescope meant that dimmer stars previously unknown of lower magnitudes could then be observed using optical equipment. These days, a pair of binoculars may yield a view of an object up to 9th magnitude, while you might expect to see a 13th magnitude star using a 6″ telescope.
3: 1st Magnitude Star 100x Brigher than 6th Magnitude
In the mid-19th century, astronomers no longer had to rely on just their eye sight to differentiate between stellar brightness. With advancement in optical equipment and photometric measurements, a more accurate logarithmic scale was created in which a 1st magnitude star is measured to be 100 times brighter than a 6th magnitude star. Likewise, a 2nd magnitude star is 39.8 times brighter, a 3rd magnitude stars 15.8 times brighter, a 4th magnitude star 6.3 times brighter, a 5th magnitude stars by 2.51 times, and so on.
4: There Are 22 First-Magnitude Stars
According to the Tycho Catalogue detailing the sky’s 2.5 million brightest stars, there are the following number of individual stars in each of the listed magnitude ranges. Also note that a magnitude -1 star’s brightness ranges from -1.50 to -0.51; a magnitude 0 from 0.50 to +0.49; a magnitude 1 from +0.50 to +1.49, and a magnitude 2 star’s brightness ranges from +1.50 to +2.49; all the way up to a magnitude 20 star’s brightness of between +19.50 and +20.49:
5: Absolute Magnitude Measures Intrinsic Brightness
Visual magnitude allows astronomers to measure a star’s brightness using the naked eye, but if we wanted to gauge its intrinsic brightness we would use absolute magnitude. This allows us to measure different objects’ true energy output, irrespective of their actual distance from Earth, by imaginatively placing them all 32.6 light years away, thus creating a kind of hypothetical apparent magnitude.
6: Magnitude of the Sun, Moon and Planets
The brightest object that we can see in the sky is the Sun, which has a visual magnitude of -26.74, and an absolute magnitude of +4.83, while a Full Moon shines with an apparent magnitude of -13. The brightest planet we can see is Venus (-4.9 to -3.8), followed by Mars (-3.0 to +1.6), Jupiter (-2.94 to -1.6) Mercury (-2.6 to +5.7), Saturn: (-0.24 to +1.47), Uranus (+5.32 to +5.9), and Neptune (+7.78 to +8.02). In the meantime, Jupiter’s four Galilean moons have a magnitude which varies from between +4.6 and +5.6, although you are unlikely to be able to see them with the naked eye due to their proximity to Jupiter’s glare.
7: Magnitude of the Stars
There are 22 first-magnitude stars, 71 second-magnitude stars, 190 third-magnitude stars, 610 fourth-magnitude stars, 1,929 fifth-magnitude stars, and 5,946 sixth-magnitude stars, according to the Tycho Catalogue. As we know, the stars get progressively more difficult to see as their magnitude rises, so if you would like to test your visual acuity, locate the constellation of Ursa Minor, which contains the north star Polaris, and see how many of the stars you can see that are identified in the image at the top of this article.
Here’s a slightly more challenging game you might like to try out. First try locating the asterism called the Great Square of Pegasus, which is easily located by drawing an imaginary line through the “pointer stars” in Ursa Major through Polaris in Ursa Minor and then onto the constellation of Pegasus. The four stars which delineate the Great Square are of magnitude 2, while if you can see another star inside the square then you are seeing at magnitude 4.4. Spotting a total of five inner stars would bring you up to magnitude 5, and eight stars up to magnitude 5.5, while thirteen stars means you have exceptional eyesight and can see to magnitude 6.0. More precise figures on The Great Square of Pegasus Star Test can be found here.
8: Magnitude of the Galaxies
All the night sky objects we can see without optics are contained within our own Milky Way, with the exception of four galaxies, including The Andromeda Galaxy (M31), which at magnitude +3.44 is easy to see on a dark night, and The Triangulum Galaxy (M33), just west of Andromeda, which at +5.72 is barely visible at all without having to resort to using binoculars or a telescope. From the southern hemisphere, both the Large Magellanic Cloud of magnitude +0.9, and the Small Magellanic Cloud of magnitude +2.7 are both easily seen.
9: Magnitude of the Nebulae
There are a handful of nebulae that can be seen with the naked eye, the most famous of which in the northern hemisphere is the Orion Nebula (M42). At first glance the second object in Orion‘s sword appears to be a star, but it is in fact a 24 light years wide nebula situated 1,344 light years distant with an apparent magnitude of +4.0. In the southern hemisphere, The Coal Sack nebula is readily seen as a dark area that stands out against the bright backdrop of the southern Milky Way; with another nebulae also visible without optics including the Northern Coalsack in the northern Milky Way, and The Pipe Nebula in the constellation of Ophiuchus.
10: Magnitude of Double Stars and Star Clusters
In addition to individual stars, there are a number of multiple stars which are fun to see unaided, including the double star Mizar and Alcor in Ursa Major, as well as the multiple star Epsilon Lyrae in the constellation of Lyra. In terms of open star clusters, the Pleiades (M45) in Taurus presents a beautiful target with its young, blue stars shining with a magnitude of +1.6; while another example is provided by the Hyades, also in Taurus, which is the nearest open cluster to our solar system at 151.1 light years distant, and has a magnitude of 0.5. M41 in Canis Major, containing around 100 stars, is situated four degrees south of Sirius, and has an apparent magnitude of +4.5, making it visible to the naked eye.
In terms of globular clusters, Omega Centauri in Centaurus shines at magnitude +3.9, and has been known about since antiquity, although back then it was considered a star; while the Hercules Globular Cluster (M13) contains around 300,000 stars but is harder to see, even on a clear night, as it has an apparent magnitude of just +5.8.