# The distance from the Sun to the nearest star is about #4 x 10^16# m. The Milky Way galaxy is roughly a disk of diameter #~10^21# m and thickness #~10^19# m. How do you find the order of magnitude of the number of stars in the Milky Way?

##### 1 Answer

Approximating the Milky Way as a disk and using the density in the solar neighborhood, there are about 100 billion stars in the Milky Way.

#### Explanation:

Since we are making an order of magnitude estimate, we will make a series of simplifying assumptions to get an answer that is roughly right.

Let's model the Milky Way galaxy as a disk.

The volume of a disk is:

Plugging in our numbers (and assuming that

Is the approximate volume of the Milky Way.

Now, all we need to do is find how many stars per cubic meter (

Let's look at the neighborhood around the Sun. We know that in a sphere with a radius of

Using the volume of a sphere

Going back to the density equation:

Plugging in the density of the solar neighborhood and the volume of the Milky Way:

Is this reasonable? Other estimates say that there are are 100-400 billion stars in the Milky Way. This is exactly what we found.