How many stars can you count?
Have you ever been under a completely dark night sky, on a night with no Moon light? There’s nothing quite like that carpet of stars that stretches from horizon to horizon with the cloudy Milky Way running through the middle.
But just how many stars can you see at any one time on a night like this? 20,000? 100,000? More?
While it might seem there’s almost too many stars to count, the number you can see is much lower than you might think.
A full sky survey reveals there’s just 9010 stars of Magnitude 6.5 and greater across the entire sky. As we can only half the sky at any one time, this means the maximum number of stars you can see above you is about 5,000. (This is because visible stars are not distributed evenly across the sky.)
And this number is assuming you’re under a perfectly dark sky with extremely good eyesight. Move closer to street or building lights and the number you’ll see drops dramatically.
There’s a gazillion more stars in the sky that are , but our eyes can’t see them because they’re too faint. A telescope will help you see these fainter stars and other astro objects. (A good rule of thumb in astronomy is that there’s lot more faint things than bright things)
What’s a Magnitude?
The way astronomers measure a star’s brightness when observed from Earth is a unit called an Apparent Magnitude, often just shortened Magnitude.
The lower the number, the brighter the star. Humans can see roughly down to just over magnitude 6 under ideal conditions.
BINTEL Astro Nerd Facts:
Each magnitude is about 2.512 as bright as the next one. A Magnitude 1 star 2.512 times brighter than a Magnitude 2 star.
You can also have negative magnitudes. It was discovered that the brightest star in the sky, Sirius, wasn’t actually Magnitude 0. It’s brighter than that – it turned out to be -1.46. Magnitudes are also used to measure other objects in the sky. The brightest planet seen from Earth, Venus, can shine at up to -4.6.
What Telescopes do part I – concentrate light.
The first job of a telescope is to collect and concentrate light.
The light that falls onto the front lens or main mirror of a telescope is concentrated and presented to your eyeball (or camera) at the eyepiece end of a telescope.
The front lens of even a small telescope is much bigger than your eye’s own lens. This means the light gathering power of a telescope is larger than your eyes. Things that are too dark to be seen by your eye alone might be visible through a telescope.
(Image via Sky&Telescope)
The bigger the front lens or main mirror of a telescope, the more light it will collect.
Think of it like leaving a bucket out in the rain. The larger the bucket, the more water you’ll collect. It’s the same way with light. The most important measurement for a telescope is the diameter of the front lens or main mirror. This is called the telescope’s aperture. It’s measured in mm or sometimes, inches. The two little telescopes you carry around in your head – your eyeballs – have an aperture of about 7mm.
A 70mm telescope will collect around 100 times more light than your eyes.
Larger apertures mean an even better ability to collect light to increase brightness and details on the objects you wish to view or photograph. As aperture increases, so does the cost and size of the telescope.
For example, Celestron has 70mm and 80mm versions of their popular StarSense Explorer telescopes.
(Celestron StarSense Explorer LT80AZ Telescope)
You might think, “there’s only a 10mm difference between them. Why is the 80mm is $100 more expensive?” A telescope collects light that falls over the entire area of its front lens or mirror.
A 70mm telescope has a front lens with an area of roughly 3,848 square mm. The 80mm has an area of 5,026 square mm – or about 30% larger. It will collect 30% more light than the smaller version.
In other words, while a 70mm telescope will collect 100 times as much light as your eyes, an 80mm will collect 130 times as much light.
Even small jumps in aperture will see big increases in a telescope’s ability to see things too faint for our eyes alone. The more light we can see, the fainter the stars we can see.
What it all means.
Here’s handy table that shows you the light gather power of various sizes of telescopes as well what magnitude you can see to – along with roughly how many stars will be visible to you:
| Optical System Diameter | Light Gathering | Magnitude | Approx. Number of stars visible |
| Just your eyes (7mm) | 1x | 6.5 | 5,000 |
| 70mm | 100x | 10.7 | 342,000 |
| 80mm | 130x | 12.2 | 900,000 |
| 130mm | 345x | 13.3 | 7.5 million |
| 6” (150mm) | 460x | 13.6 | 19 million |
| 8” (203mm) | 840x | 14.2 | 26 million |
| 10” (250mm) | 1275x | 14.7 | 31 million |
| 12” (300mm) | 1835x | 15.1 | 65 million |
Conclusion:
There’s a vast number of faint stars and other fascinating astronomical objects in the night sky. Most are too faint to be seen with just your eyes alone. A telescope will let you see these and the larger the telescope, the more you’ll see.