Have you ever gone out on a clear night, looked up at a a star, and wondered if maybe someone on a planet around that star was looking back wondering the same thing?. If you have, you are probably not alone.
However, unless you are lucky, or are careful as to which star you pick, the chances are not good. Not due to any rareity of life in the universe, but for another reason. Even if every star similiar enough to our own supported a civilization, the chances are not good.
Here's why, in order for such an exchange of glances to work, not only do we need to be able to see their star, but they have to be able to see the Sun. The sun, as stars goes isn't all that bright. In fact, as seen froma planet with an atmosphere like ours by beings with eyes like our own, Sol would only be visible by naked eye from a maximum distance of 17 parsecs, not much beyond the local neighborhood.
Below is an image of the volume of space out to 17 parsecs of Earth, showing the naked eye visible stars from which it would be possible to see our sun.
There are 85 of them. When compared to the nearly 6000 naked eye visble stars in the Earth's sky, you can see that they represent a small percentage.
So what about the rest of those 6000 stars? They are all stars brighter than the Sun, most of them much brighter. the further the star, the brighter it has to be. Deneb, at its lowest estimated distance is some 500 parsecs distant. to be as bright as it is in our night sky it has to be tens of thousands of time more luminous than the Sun.
Now here's the rub. While the vast majority of stars we see at night are much more luminous than the Sun, this is not true for stars as a whole. Bright stars are rarer than dimmer stars. For example, of the 85 stars shown in the image, none of them are the of the brightest O or B classes of stars, and only 9 of of the A class.
What this means is that interspersed among those 6000 visible stars in the sky are an unseen hoarde of average stars, many of them much closer to us then the stars we do see.
And we don't even have to go out beyond the 17 parsec distance to see this,
Go back and look at the image again. The stars are color coded by class. A is white, F is pale yellow, G is yellow, K is orange and M is red.
Note that there are only two M class stars shown. The thing is, M class stars are the most numerous class of star there is.
So why only two?. Do we just live in a M class poor region? No. It is because M class stars are also the dimmest stars. There are many M class stars within 17 parsecs of the Earth, they are just too dim to see by naked eye. How many? Well, if I included all the stars within that region the image would have 658 stars! almost 8 times as many as shown.
Below is a chart comparing the number of stars in that volume by class for both those visible and not visible.
As you see, the M class stars totally dominate in the total star count and the trend is fewer stars as we move down to the brighter classes.
So the next time you look up at the sky, just think of that vast majority of the stars there that you are not seeing, and remember that what you are seeing are mostly the rarer stars which do not represent the galaxy as a whole.
