The night sky is filled with stars of different colors that astronomers rank in order of their luminosity, the brightest of which is Sirius in Canis Major, a blue-white star 8.611 light years distant with an apparent magnitude of -1.46. At the other end of the magnitude scale are the dimmest stars, which at +6.5 are on the very edge of our naked-eye stargazing capabilities.
There are 9,096 possible naked-eye stars
The brighter the star, the lower the apparent magnitude value assigned to it, with the most luminous given a negative number. In total there are 22 stars with magnitudes of between -1 and 1, making them the night sky’s brightest stars. In the meantime, there are 71 stars of 2nd magnitude, 190 stars of 3rd magnitude, 610 of 4th magnitude, 1,929 of 5th magnitude, and 5,946 of 6th magnitude. When we include another 3,150 stars at the limit of our visual acuity of magnitude +6.5, then this adds up to 9,096 naked-eye stars that it is possible to see in the night sky from both the northern and southern hemispheres combined.
4,548 stars seen from each hemisphere
Of course, we can only see half the celestial sphere at any one time from our particular location. Once the number of naked-eye stars of magnitudes between -1 to +6.5 is divided in half, we are then left with just 4,548 stars that can be seen by an observer living in either the northern or southern hemisphere. This value will vary depending on a range of factors, including the season, ideal viewing conditions, good eyesight, the Moon’s brightness, as well as the stargazer’s location on Earth.
446 stars can be seen from the suburbs
From suburban areas of a city, light pollution reduces naked-eye visibility down to a magnitude of about +4, making a total of 893 stars visible in the night sky, of which roughly half, or 446 can be seen from either of the Earth’s hemispheres. City dwellers themselves are unlikely to be able to see beyond magnitude +2, though, as more outdoor lighting will make the number of visible stars fall further to 71 worldwide or a mere 35 stars per hemisphere.
Sight test for the night sky
Pleiades
The Pleiades star cluster spans 13 light-years, contains around 3,000 stars, and has an apparent magnitude of +1.6, making it one of the brightest star clusters in the night sky. Also known as the Seven Sisters, it is easily found in the Taurus constellation to the top right of Orion, except most people will only be able to make out 6 stars under reasonably dark seeing conditions, while anymore will require a region with clear dark skies.
The individual magnitudes of the 5 brightest Pleiades stars (Alcyone, Atlas, Electra, Maia, and Merope) range from 2.9 to 4.2., so next time you’re out stargazing give this magnitude spotting test a go.
Pegasus
There’s a bounty of up to 70 stars you can find (or at least that’s the most ever seen) in the asterism known as the Great Square of Pegasus, depicted in the image at the top of this post. Back in the 19th century, German astronomer Johann Friedrich Julius Schmidt managed to count 102 stars with his acute eyesight alone while working at his observatory in Athens. For most people, spotting one star means you’re doing quite well at +4.4 magnitude, while 8 stars is magnitude +5.5, 13 stars is magnitude +6.0, and seeing the 23 stars as shown in the image would indicate a magnitude of +6.2. Spotting a total of 37 stars, however, means you are seeing at the near naked-eye limit of +6.5 magnitude.
Use of optical equipment
The magnitude scale was invented by Greek astronomer Hipparchus in 129 BC, and it wasn’t until Galileo Galilei pointed his newly constructed refracting telescope to the heavens in 1609 that it became possible to see stars dimmer than 6th magnitude. As Galileo wrote in his 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.”
Aperture
It’s important to realize that when seeing dim objects such as the stars, it is the amount of light you collect in your eyes that will determine the number of stars you will be able to see in the sky. The aperture of a telescope refers to the diameter of its main lens or mirror, and if your eye was a telescope, its aperture (retina) would measure up to 7 millimeters, or a 1/4 inch wide.
Optical equipment has improved significantly over the centuries, and these days even a pair of 50-mm binoculars will allow you to see up to 9th magnitude, or some 217,689 stars in the celestial heavens. Likewise, a 3 inch telescope (75mm) will take you up to around magnitude +11, or 1,196,690 stars; a 6 inch telescope (150mm) to magnitude +13 (15,431,076 stars); and a 12 inch telescope (300mm) to magnitude +15 (130,577,797 stars). Of course, even this is just a fraction of the 100 billion or so stars that are contained within our Milky Way Galaxy.
Magnification
Much greater enlargement power is available to us nowadays, even in hobby telescopes, which under very specific circumstances can manage 200-300 times magnification, but even they cannot see a perceptible “disk” as stars are simply too far away. Only the most powerful professional astronomical telescopes can see disks, and only for a few extremely large stars, such as Betelgeuse. Smaller scopes trying for higher magnification, on the other hand, will simply darken the image and cause it to blur.
Strong magnification is really only useful for planetary observation because planetary disks are comparatively small. Globular clusters, nebulae, and galaxies such as the Andromeda Galaxy are actually fairly large objects and so it’s more useful to use relatively low magnification so you can see them all at once. The Orion Nebula, for instance, appears double the size of the full Moon, and the Andromeda Galaxy appears six times as wide as the Moon in the night sky, that is if we were able to see its brightness with just the naked eye.