The Number of Stars

It was a cold winter night with a dark sky and the milky way was clearly visible over our heads. I was in Boy Scouts and my father was helping to orient me to the night sky. I was working toward the Astronomy Merit Badge. Dad had Peterson's Field Guide open to a star chart. He had used some of my mom's red nail polish to paint the bulb of a flashlight so we could look at the chart without ruining our night vision.

As a good parent, my father was trying to point out constellations and the names of certain stars. Like children all over the world have asked for millennia, I just wanted to know how many stars there were. It is not for nothing that the Lord God told his people that their descendants would be more numerous than the stars in the sky. There were a lot of stars out there to see.

Skip forward about 40 years, and I am standing on the back deck of a friend that lives above a quiet valley near San Antonio. (If you are reading this years from now, it was a time before the national Dark Skies movement began, when streetlights and security lights still lit up the night skies and made stars disappear.) As I look up, all I can see from his porch are stars that are second magnitude or brighter. There are not many of them. Here and there in the sky, I can see a star. I can see about half the sky from where I am - what with a house and some trees nearby - but when I scan the sky, all I see is about 5 stars. The kids that come out to the back yard to look through my telescope are fascinated by the view of Jupiter, but it probably never occurs to them to ask how many stars there are... They can easily count to 5 if they have any interest to do so.

Astronomers break down the brightness of stars into "magnitudes". This has nothing to do with the actual brightness of the star, as it would be seen from a standard distance. It has everything to do with the brightness of the star as seen from earth. The magnitudes range from quite bright to very, very dim. It might seem strange, but the brightest stars have the smallest magnitude numbers. Think of first class, second class, third class, to get the idea of what first magnitude, second magnitude, and third magnitude stars mean. (There are even a few stars that are brighter than first magnitude and they have negative magnitude numbers less than one.)

As the sun is setting and the sky starts to get darker, the brightest stars are visible first. Sometimes a planet like Venus, Mercury, or Jupiter is visible first, because they are oten brighter than the brightest of stars. But then the stars start appearing. The first magnitude stars are the first to be seen, then second magnitude, and so forth. If the sky is very dark (far from city lights, and the moon is not out) then most people can see stars down to about magnitude six.

But with bright city lights, or nearby streetlights, the magnitude of visible stars is reduced. At my friend's house, even on the darkest night with no moon, his sky is limited to second magnitude stars.

I became interested in finding out how many stars can be seen at different magnitudes. It turns out that (like almost every question I ask) an answer can be found on the internet. The source I used for answering this question is from: http://www.stargazing.net/David/constel/howmanystars.html.

It turns out that in my friend's sky, when I could see second magnitude stars, there were about 93 stars that that can be seen world wide that are second magnitude or brighter. (Some of these are only visible from the southern hemisphere.) A persistent Boy Scout could memorize that number of stars. That is the reason that for the small slice of the sky I could see from my friend's back porch, all that I could see was a few.

My house is a little further out in the country. From my house, on a moonlit night, I can just barely see third magnitude stars. But that raises the number of visible stars to almost three hundred - three times as many as my friend can see.

When the moon is below the horizon or near new moon, I can see fourth magnitude stars. With the additional darkness I can now potentially see about 900 stars. (Remember that a portion of this number are never visible from Texas - they are too far south.)

If I drive fourty minutes from my home to the Hill Country State Natural Area, incredible stars are visible. From my house, I can never see the band of the Milky Way. But from Hill Country the Milky Way is quite brightly obvious. When I look at the little dipper, I see 9 or more stars. I am in true sixth magnitude dark skies. In this darkness, almost 9000 stars are visible. It is incredible to see how constellations which were too dim to see in town are now like signposts in the sky. I can follow the outlines of the stars and find many more dim objects than I could in brighter skies. It also means that with a telescope, I can see objects that wash out into bright background light in my back yard. In the constellation of Virgo, for instance, there are about a dozen Messier object galaxies that are pretty easy to see in dark skies but which are completely invisible to me in my own back yard.

As I look back across the years to that astronomy session with my dad, I know I must have been looking at a very dark sky with a brightly glowing Milky Way. That is the reason that I was overawed by the number of stars. Thousands upon thousands of them crowded my eyes. I only hope that we find some way to preserve what dark skies we presently have and find ways to bring dark skies back to our cities. Children deserve to be awed by the sky. We all deserve the right to know how small we are and how enormous the heavens are.

Magnitude Cumulative Number of stars
-1.50 to -0.51 2
-0.50 to +0.49 8
+0.50 to +1.49 22
+1.50 to +2.49 93
+2.50 to +3.49 283
+3.50 to +4.49 893
+4.50 to +5.49 2,822
+5.50 to +6.49 8,768