Visualizing the Number Of Stars in Our Galaxy

When astronomers use numbers in the billions and larger, it’s hard to find a frame of reference to compare it to. For example, our galaxy, the Milky Way, is described as having about 250 billion stars*. What does that mean?

Consider a single grain of table salt. What if we used that to represent our sun? What would 250 billion of them look like? A handful? A cupful? A bathtub full? Let’s work it out:

If you place three grains of salt side by side, the combined length is about 1 mm. That means a grain of salt—which are more or less regular cubes—is 0.3 mm (or 0.03 cm) on each side. From that we can figure out that the volume of a single grain of salt is (0.03 cm)³ which equals 0.000027 cm³ … to make the number easier to handle, we’ll convert this into scientific notation: 2.7 × 10^-5 cm³.

With the volume of our stellar grain of salt all calculated, all we need to do now is multiply it by 250 billion (or 2.5 × 10¹¹) to get an idea of how many stars that is. So, (2.7 × 10^-5 cm³) × (2.5 × 10¹¹) equals 6.75 × 10⁶ cm³. Converting from cubic centimeters to cubic meters (i.e. divide by a million) gives us 6.75 m³.

For the U.S. readers, I’m now going to shift over to feet measures so you can visualize better. The value 6.75 m³ is about equal to 238 cu.ft. That means, if you go into a small room, 8′ x 12′, and fill it with table salt about up to your hips—assuming you’re about 180 cm (5′ 10″)—you’ll have around 250 billion grains of salt.

Now that you are buried up to your hips in salt, take a moment and look at a single grain. That’s us. All the rest that is going to be a vacuuming nightmare? In the whole vast universe those are our nearest and dearest relatives. Every grain represents another star system in our galaxy, though sizes vary. On this scale, the distance between our little grain and the grain that represents our closest neighbor is about 9 km (over 5.5 miles)**. Wow. That’s a whole lot of nothing between two little grains of salt…and we have 250 billion of them. The mind boggles.

* Actually, I’ve seen estimates between 150 billion and 400 billion. Most common is between 200 billion and 300 billion, so I chose the middle ground.

** This comes out to about 2 km per light year.