The Milky Way Galaxy contains at least one hundred billion stars. We can write one hundred billion as 100,000,000,000. The way we normally write numbers is called

. As numbers get really large, it's hard to count all those zeros. So we write 100,000,000,000 like this: 1.0x10^11. This way of writing is called

. In this example the

is 11. So we multiply 10 by itself 11 times: 10*10*10*10*10*10*10*10*10*10*10 = 100,000,000,000. That's why there's 11 zeros. The number 10 to any exponent is a

. There are ten trillion or 1.0x10^13 galaxies in the universe. To estimate how many stars are in the universe we can multiply 1.0x10^11 by 1.0x10^13. Using the

, we just add the exponents. So 11 + 13 = 24. The universe has about 1.0x10^24 stars. That's 1,000,000,000,000,000,000,000,000 stars! The reverse operation also works. We can divide 1.0x10^24 by 1.0x10^11 to get 1.0x10^13. When you divide powers, just subtract the exponents: 24 minus 11 equals 13. This rule is the
. We also use scientific notation for tiny numbers. For example, an electron has a mass of 9.11x10^-31 kilograms. The makes it 9.11 divided by 10^31.