|MadSci Network: Astronomy
Robert: That is a really good question. It is actually extremely complicated and lots of astronomers and even mathematicians spend their entire careers trying to answer it. I will tell you what we do know, sort of. The number of atoms in the Universe is a relatively finite number. That is to say, it does not really change by any appreciable amount. So, if there are 100 atoms in the Universe, there will always be 100 atoms in the Universe. There is a small problem though. According to Albert Einstein, matter and energy are the same, and under certain circumstances they can be made to interchange (this happens in nuclear reactions that take place in nuclear bombs and at the center of the Sun). So, it is probably more accurate to say that the total energy of the Universe is constant, it doesn't change. Counting all of these atoms is the difficult part. In astronomy, we use a lot of math to make guesses about how much matter (atoms) we should have by looking at how stars and galaxies respond to each other (gravity). Then we look at how much stuff (atoms) we can actually count and we always come up short. This is called the dark matter. A lot of work is being done to find this dark matter since we know it is there but we cannot see it. For example some of this might be locked up in black holes, or in black holes in the middle of galaxies to name a few possible places. I apologize for such a long answer, but it was a very difficult question you asked. Just to give you some perspective on how many atoms it might be, the number of atoms alone in the graphite in your pencil is about 25000000000000000000000 atoms... In just the pencil lead! WOW! But, we should probably give you a number to use, just in case you are interested. This won't include dark matter, brown dwarf stars, dwarf galaxies and such, but we will count the atoms in a star and multiply this by the number of stars in the Universe, since that is mostly what we can see when we look out. A typical star weighs about 2x10^33 Grams, which is about 1x10^57 atoms of hydrogen per star... That is a 1 followed by 57 zeros. a typical galaxy has about 400 billion stars so that means each galaxy has 1x10^57 X 400,000,000,000 = 5x10^68 hydrogen atoms in a galaxy There are possibly 80 billion galaxies in the Universe, so that means that there are about: 5x10^68 X 80,000,000,000 = 4x10^79 hydrogen atoms in the Universe. But this is definately a lower limit calculation, and ignores many possible atom sources. That number Bob, is a 4 followed by 79 zeros. I hope This helps, for some more astronomy questions, check out: http://astrosun.tn.cornell.edu/students/lazio/sci.astro.html
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