How Much Stuff is in a Light Year?

Article Updated: 8 Mar , 2017

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.

This image from the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile, shows the bright star cluster NGC 6520 and its neighbour, the strangely shaped dark cloud Barnard 86. This cosmic pair is set against millions of glowing stars from the brightest part of the Milky Way — a region so dense with stars that barely any dark sky is seen across the picture.

Millions of glowing stars from the brightest part of the Milky Way — a region so dense with stars that barely any dark sky is seen across the picture. Credit: ESO

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.

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4 Responses

  1. Arnold Rimmer says:

    So how much stuff is in a cubic light year between the galaxies?

    • Arnold Rimmer says:

      A handful pr. cubic meter in intergalactic voids/A few hundred pr. cubic meter in interstellar space = 1/40 (?) * 0.4 M sun = 1% mass of the sun = 10 Jupiters. Is that about right?

  2. pafnooty says:

    Kind of tells you a bit about the significance of gravity! 🙂

  3. Laced says:

    I hadn’t heard of the term retcon before and had to look it up. A quick google search led me to what you are referring to, although I didn’t see the explanation I had thought to be correct prior to today. I always took it to mean the distance required to lose someone chasing you, like “Dude that’s nothin I managed the 7-11 Run last week and lost the cops in less than 2 miles” meaning upon fleeing the scene of the crime, the cops lost my tail just two miles from my starting point.”

    Reading now what Lucas has said, I guess I’m wrong, but I think for the sake of Official Canon, a consensus retcon SHOULD be decided upon. As much as I like to marvel at how such a dunce as Lucas created the best trilogy of all time, simply having the canon be that George-is-a-moron isn’t worth it, because then you have the official canon breaking the 4th wall.

    Plus, the explanation given in The Expanded Universe about the Kessel Run being a dangerous pathway through a cluster of black holes–meaning the faster the ship (remember in canon we can pass light speed without relativistic ramifications) can go (greater momentum) the closer it can skirt to each black hole’s event horizon, and the closer to the minimum distance (a straight line, again canon says relativity can fuck itself) the ship can go. I think that’s WAY better than my losing-the-cops retcon and infinitely better than the George-is-a-dumbass entry being made permanent. Plus how cool would an 8 or 9 member octinary/noninary star system that had every member turn into black holes, thus allowing the system to continue whatever orbits each star had allowing the black holes to stay separate? And isn’t it better to have Han be THAT much of a badass to navigate through that mother, cutting corners past the various infinite gravity wells? Of course 8 parsecs is way too large for gravitationally bound objects, but maybe the Kessel system changed its distance from the black hole maw periodically, and different points in the period allow for different entry paths into the maw, necessitating the larger total distance. I know, special pleading, but this is science fiction not fact right? I say screw Lucas he’s a dumbass, we can’t let him cause Han Solo to be a dumbass too.

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