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Name: Kumar
Status: student
Grade: 9-12
Country: India
Date: Winter 2012-2013

How do we know how many stars are in the galaxy? How do we know how many galaxies are in the universe? How many visible stars are there?

Dear Kumar,

Good question. We do not. Based on many studies of the Milky Way, I think the highest estimate came from Bart Bok around 1980, who estimated around four hundred billion suns in our galaxy. The true number is probably between 200 billion and 400 billion. It is worked out from studies of the spiral arms, the different types of stars, and other factors. The number of galaxies in the universe is another matter. I do not believe that we know the answer. The idea that there may be more galaxies than there are grains of sand on all the beaches of Earth makes sense to me.

Sincerely David H. Levy


Such numbers are only estimates. We can keep records and photos of all stars that have been seen. We cannot know that every visible star has been recorded. We cannot know how many invisible stars exist. All scientific data has uncertainty to it. That is part of how data works.

Nothing in science is known exactly. Also, scientific theories and laws are all uncertain. We cannot prove that they must be true. We can only show that they work, that they have not yet disagreed with measurements. Many laws are known to be false. Because they do work well in many cases, we still use them. In science and engineering, usefulness is much easier to identify than truthfulness.

Dr. Ken Mellendorf Physics Instructor Illinois Central College


We really do not know absolutely, for sure, how many stars and galaxies are in the universe. Besides, stars are increasing and decreasing in numbers as you are reading this message and the number that are visible depends on which hemisphere you are in and how clear the atmosphere is when you are looking.

The numbers you see in scientific articles and journals are just estimates. Some of the estimates are honest attempts to get the right number, others are just inventions of numbers we have never heard of before like a googol. Look ?googol? up at

Sincere regards, Mike Stewart

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