If you have ever had a medical checkup, you have probably heard about the Body Mass Index (BMI) that it supposed to have an optimal value around 25. Probably you were told to take your weight in pounds, divide it by the square of your height in inches and multiply the result by 703 to find your BMI.
Well, astronomers think there are large number of stars and guess (estimate) that there are around 1 septillion stars in the Universe1. If you look at the night sky, you can see around 3,000 stars with the naked eye. A galaxy is a group of hundreds of billions of stars that are relatively close to each other.
At night, the sky appears to be a mostly dark surface or region scattered with stars. During the day, the Sun can be seen in the sky unless obscured by clouds. In the night sky (and to some extent during the day) the moon, planets and stars are visible in the sky. Some of the natural phenomena seen in the sky are clouds, rainbows, and aurorae.