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Did you ever wonder what stars are made of? You might not be surprised to know that stars are made of the same stuff as the rest of the Universe: 73% hydrogen, 25% helium, and the last 2% is all the other elements. That’s it. Except for a few differences here and there, stars are made of pretty much the same stuff.
After the Big Bang, 13.7 billion years ago, the entire Universe was a hot dense sphere. The conditions inside this young Universe were so hot that it was equivalent to being inside the core of a star. In other words, the entire Universe was like a star. And for the brief time that the Universe was in this state, nuclear fusion reactions converted hydrogen into helium to the ratios we see today.
The Universe kept expanding and cooling down, and eventually the hydrogen and helium cooled down to the point that it could actually start collecting together with its mutual gravity. This is how the first stars were born. And just like the stars we have today, they were made up of roughly 73% hydrogen and 25% helium. These first stars were enormous and probably detonated as supernovae within a million years of forming. In their life, and in their death, these first stars created some of the heavier elements that we have here on Earth, like oxygen, carbon, gold and uranium.
Stars have been forming since the Universe began. In fact, astronomers calculate that 5 new stars form in the Milky Way every year. Some have more of the heavier elements left over from previous stars; these are metal-rich stars. Others have less of these elements; the metal-poor stars. But even so, the ratio of elements is still roughly the same. Our own Sun is an example of a metal rich star, with a higher than average amount of heavier elements inside it. And yet, the Sun’s ratios are very similar: 71% hydrogen, 27.1% helium, and then the rest as heavier elements, like oxygen, carbon, nitrogen, etc. Of course, the Sun has been converting hydrogen into helium in its core for 4.5 billion years.
Stars everywhere are made of the same stuff: 3/4 hydrogen and 1/4 helium. It’s the stuff left over from the formation of the Universe, and one of the most elegant pieces of evidence to help explain how we’re here today.