"We are stardust"

Carl Sagan made the phrase "We are stardust" popular in his television series Cosmos, a personal journey in 1980, it was an impact in its time, in my youth, it is a poetic and metaphorical phrase, but it is also a very useful phrase to explain how most of the chemical elements that make up our bodies such as carbon, oxygen, nitrogen and iron arose, since these were not born from the Big Bang. Approximately between 100 to 300 seconds after the beginning of the expansion of the universe, after the Big Bang, the matter of the universe was composed of 75% hydrogen and 25% helium, there were traces of other light elements, but in negligible quantities, everything else, the oxygen we breathe, the carbon in our cells, the calcium in our bones, the elements that form us and form the planets arose from the fusion of atoms within stars. Initially, a star fuses hydrogen in its core, which it then transforms into helium. When the hydrogen runs out, it begins to fuse helium, forming increasingly heavier elements. There comes a point at which the star cannot fuse any more. Stars like the sun are not going to explode, neither in a nova, nor in a supernova, nor in anything. What will happen is that they will swell and become a giant, a red giant. 5,000 million years, in this process it will swallow the Earth, it is one of the debates that exists, it is assumed that it does, but the red giants also expel their outer layers, thus losing mass, in fact, it is estimated that the Sun will lose between 40 and 50% of its current mass and that means that it will also lose its attractiveness, so the planets may move away, perhaps the Earth will be saved, although who will be there at that time? and it will not be saved from other things.

But in this process of expelling its outer layers, which will be made up mainly of hydrogen and helium, there will also be carbon, nitrogen and oxygen, which will serve to create new planets and possible forms of life in the future, but if all the stars were made like the sun, we would not exist and nor would elements heavier than iron exist, because that would mean that in the universe, with an age of about 13.8 billion years, there would only be heavy elements from just over 3000 years ago. 4000 million years. The Earth is 4.5 billion years old. Where then did all the elements that build most of what we see come from? Well, of massive stars, because luckily the universe has a very curious rule and that is that the larger a star is, the faster it consumes its fuel and the less time it lives, the most massive stars live only a few tens of millions of years. The largest ones, even fewer end up exploding in supernovas, hypernovas and kilonovas. A star of one solar mass lives about 10,000 million years, a star of 10 solar masses lives only 20 or 30 million years, with 25 solar masses between 3 and 5 million years, with 100 solar masses or more, which do exist, those giants exist, they only live one or 3 million years, if you think about the poor planets of those stars, don't worry, they barely have time to form planets and, therefore, Of course, they have even less time for life to emerge in them. Be concerned about the planets that orbit stars like the sun, but that are a few light years from one of those exploding giants, because if there was life on one of them it would be the end of them, or at least a mass extinction. There are orange dwarf stars that are slightly smaller than yellow dwarfs like the sun. They are very interesting for searching for life because they are stable, they live three or four times longer than the sun and their habitable area is smaller than that of a star like the sun, but it is also of a curious size, which allows the planets to orbit an orange dwarf star and not be gravitationally tied, that is, these planets can have normal days and nights, not be tied with one hemisphere always looking at the star and the other always at night, which would generate a rather chaotic and brutal climate.

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