Even though the name is a little confusing, you probably already know that a light year is the distance that light travels in a full year. At speeds of almost 300,000 kilometers per second that gets you pretty far from home.
So take that distance and turn it into a cube, each side one light year in length. Imagine that giant volume of space, a little challenging for some of us to get our heads around. How much “stuff” would be in there, and not just “stuff”… how much nothing is in there? There is an answer, but it all depends on where put your giant cube.
Measure it at the core of the galaxy, and there are stars buzzing around all over the place. Perhaps in the heart of a globular cluster? In a star forming nebula? Or maybe out in the suburbs of the Milky Way? There’s also great voids that exist between galaxies, where there’s almost nothing.
There’s no getting around the math in this one. First, let’s figure out an average density for the Milky Way and then go from there. Its about 100,000 light-years across and 1000 light-years thick. According to my buddy Phil Plait, the total volume of the Milky Way is about 8 trillion cubic light-years. And the total mass of the Milky Way is 6 x 10 to the power of 42 kilograms.
Divide those together and you get 8 x 10 to the power of 29 kilograms per light year. That’s an 8 followed by 29 zeros. Is that a lot? It sounds like a lot.
Actually, that’s about 40% of the mass of the Sun. In other words, on average, across the Milky Way, there’s about 40% the mass of the Sun in every cubic light year. But in an average cubic meter, there’s only about 950 attograms. Almost a femtogram; a quadrillionth of a gram of matter. Which is pretty close to nothing. Seriously, air has more than a kilogram of mass per cubic meter.
In the densest regions of the Milky Way, like inside globular clusters, you can get densities of stars with 100, or even 1000 times greater than our region of the galaxy. Stars can get as close together as the radius of the Solar System.
But out in the vast interstellar gulfs between stars, the density drops significantly. There are only a few hundred individual atoms per cubic meter in interstellar space. And in the intergalactic voids; the gulfs between galaxies, there are just a handful of atoms per meter.
How much stuff is there in a light year? It all depends on where you look, but if you spread all the matter around by shaking the Universe up like a snow globe, the answer is very close to nothing.