Most people think the night sky is studded with millions of stars. I try to make clear in this essay that our imaginations sometimes get the best of us; we can only see one or two thousand stars at any given time on a clear, dark night with the unaided eye. Follow through with this experiment to test your star-seeing ability.
How many stars can you see? Emphasis on the word you.
Here is a little experiment you can do to find out.
After even a quick look on a dark night it will be apparent that there are far more faint stars than there are bright ones.
Here is some background on how the brightness of stars is tallied, because the number of stars you can see surely depends on how faint a star you can see.
{Counting Stars. Copyright 2024 by Elisabeth Fritz-Mitchell.}
The brightness of stars is graded in magnitudes. The magnitude scale was established more than two thousand years ago, and in a slightly modified form is still used today.
Imagine that all the stars visible to the naked eye are sorted into six brightness bins, with the brightest stars in the fist bin and the faintest stars in the sixth.
Stars in adjacent bins differ in brightness by slightly more than a factor of 2.5. So, for example, a star in the third bin--a magnitude 3 star--is about 2.5 times brighter than a star in the fourth bin--a magnitude 4 star, and more than 2.5 times 2.5 = 6.25 times brighter than a magnitude 5 star.
The number of stars in each brightness bin is as follows:
Magnitude Approx. Num.
========================
1 15
2 35
3 130
4 350
5 1,100
6 3,500
------------------------
The Big Dipper and the Little Dipper provide good vision-testing ground.
This well-known constellation consists of seven main, bright stars: three in its handle and four in its bowl.
The middle of the Big Dipper's three handle stars--the star that gives the Dipper's handle a little hitch--is called Mizar. Mizar is a mag. 2 star. So if you can see Mizar, you can see about 50 stars.
The star that joins the handle and the bowl of the Big Dipper is named Megrez. Megrez is a magnitude 3 star, so if you can see it, you can potentially see 180 stars.
Now glance back at Mizar and look closely. Mizar has a faint, very nearby companion, named Alcor. Alcor is pretty faint, but most people can see it. Alcor is a magnitude 4 star, so if you can spy it, you can see as many as 430 stars in all.
To see still fainter stars, shift your gaze to the Little Dipper.
The two stars that form the far end of the Big Dipper's bowl point the way. These two stars--called the pointer stars--point to the North Star. The North Star is the end-most star is the Little Dipper's handle.
The faintest Little Dipper star, which is sometimes called Alasco, is the bowl star just below the star that joins the bowl to the handle. Alasco is a magnitude 5 star, so if you can see it, you can see about 1,500 stars in total.
Now on a clear, dark night, the faintest stars most anyone can see with their eyes alone are magnitude 6 stars. There are literally thousands of them, so we'll have to leave our star counting experiment like this: If you can see stars that appear to be about two times fainter than Alasco, then all total, you can see about 4,000 stars.
Of course, you must remember, that at any given time you can see only half of the entire sky--the half that is above the horizon.
So 2,000 is a fair estimate for the number of stars you can see at any given time on a clear, dark night.
Is this more or less than you expected?