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?
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.
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.