Water is a pretty simple molecule. Two hydrogen atoms attached to one oxygen atom; about as basic as you get. Hydrogen — that is, protons — was created in the moments after the Big Bang, but oxygen is more complex. It needs to be forged in the thermonuclear fires in the cores of massive stars, which explode and seed the atoms into space.

Water couldn’t exist before oxygen did, so it must have come after. But when? And more importantly, how?

New research investigated this in an interesting way, simulating the lives and deaths of stars in the early Universe, and the aftermath of their demise. What they found is pretty cool [link to journal paper].

Conditions in the early Universe were different than they are today. For one thing, once the Big Bang settled down, the only elements around were hydrogen (about 75% of all normal matter), helium (about 25%), a bit of lithium, and even less beryllium. It was hot but cooled as the cosmos expanded. Eventually it cooled enough that the clouds of gas could condense and form stars. It’s not exactly clear when that happened, though theoretically the first stars formed about 100 million years after the Bang.

These stars were essentially pure hydrogen and helium. Because of this some of them could grow to enormous size, with some hypotheses putting them at over 200 times the mass of the Sun, far more massive than you can get today (why? Answer in a sec). They behaved in a similar way to stars today, fusing hydrogen into helium, then, when the hydrogen ran out, helium into carbon, and so on up the periodic table. Stars this massive probably become unstable when they start fusing neon into oxygen, undergoing what’s called a pair instability (I wrote about this in BAN #619, but also on The Old Blog™ here and here).

When that happens, the superstar explodes. It’s a colossal explosion, even by today’s standards of supernovae. New elements are created in the intense heat and pressure of the explosion, which are then expelled outward at very high speed. This stuff collides and merges with pre-existing gas between the stars. That gas then forms the next generation of stars, which then already have the heavier elements in them when they’re born (these elements can absorb and prevent energy from leaving the star, which heats them up — that’s why stars today can’t get as massive as they could in the earlier Universe; they get too hot and either can’t coalesce or they become unstable and shed the excess mass).

Clearly, that’s where the first oxygen in the cosmos came from.

In the new research, the astronomers simulated conditions in the early Universe and then traced the lives of two different stars. One had 13 times the Sun’s mass, and the other 200 as described above. Both go supernova, though the 13-solar-mass one explodes the way most stars do these days, with its core collapsing and the outer layers blasted away. The lower mass star blows up after about 12.2 million years, and the higher mass one in only 2.6 million; it goes through its fuel far faster even though there’s more fuel to fuse.

Both blast out a lot of oxygen into space; 0.05 and a staggering 55 times the mass of the Sun, respectively. What happens next is the key.

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