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residue of stars [Q&A]


Vibha (Poornaprajna PU College) and Tilak (SDM College) ask, there is a lot of stuff in the Universe, especially Hydrogen, where did they all come from? How did they form?

Hydrogen is easy (well, in a manner of speaking). It is just an electron paired with a proton. A lot of these were produced during the early stages of the Universe, and as it expanded and cooled, they combined to form H atoms and eventually H2 molecules.

In the first 20 minutes of the Universe, the temperatures and pressures were large enough to fuse protons and neutrons into nuclei of He, Li, and Be. This is called Big Bang Nucleosynthesis (@wikipedia). Elements heavier than Be could not be synthesized because there wasn’t enough time to do so before the Universe cooled below the threshold required for fusion.

Long after the formation of the Universe, stars formed, and the temperatures and pressures at their cores became large enough to sustain fusion. At this stage, elements more massive than Beryllium, and in fact all the way up to Iron, can be produced. This is called Stellar Nucleosynthesis (@wikipedia). The more massive the star, the farther it gets on the atomic number scale.

Now, stars can produce Iron, but they are locked up in their cores. How to get them out? Mid-sized stars (up to a few tens of solar masses) dredge up the heavier elements and put them into the interstellar medium via an increasingly strong stellar wind. But to make and release even heavier elements is also possible if you start with an extremely massive star. As it turns out, producing elements more massive than Iron requires energy (i.e., the reactions are endothermic [with some exceptions like in slow neutron capture processes]). So, when a star starts producing Iron, it is pretty much at the end of its tether. It cannot maintain its pressure balance, and the core collapses, releasing a tremendous amount of gravitational energy. This does two things. First, it allows the temperature to go higher and form even heavier elements (all the way up to Plutonium and beyond), and second, it blows off the outer envelopes of the star in a tremendous explosion that we call Supernova. This explosion takes all the extra elements that have been synthesized and puts them into interstellar space. This is called Supernova Nucleosynthesis (@wikipedia).

Over many generations of star formation and supernova explosions, the metallic content (astronomers call everything beyond He a metal) of the Universe keeps increasing. Currently, H is still the most dominant element in the Universe by far, followed in a distant second place by He, followed by the rest in essentially trace amounts. In the Sun, the abundance of He is ≈8% of that of H, O is at ≈0.08%, C is at ≈0.03%, N and Ne are at ≈0.01%, Fe is at ≈0.005%, and so on. On the Earth, we have lost most of the H and He and the abundances are skewed strongly towards the more massive elements.

The human body is made up of 10% Hydrogen by weight, locked up in water. The rest, 90%, was all made in stars. We are, literally, the residue of stars.

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