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Comment by jmartrican | Hacker News Reader

So that means that for life to form, we probably need a star to die so that the heavier atoms used in complicated life forming chemical reactions (correct me if i am wrong here as what I'm about to say depends on it), hence it could be the case that if the universe is 13.5 billion years old, then we humans are appearing in the universe at the earliest possible time.

13.5 billion years seems like the time required to create a star, have the star die and blow up, have all that material settle and create a new star, then the planets are formed, than enough time on one of those planets needs to pass for life to form, then complicated life.

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Not necessarily. First generation stars were, theoretically, enormous both due to low metallicity of the collapsed medium and a higher average concentration of said medium. These stars lifespans were extremely short, shorter that blue giants we see today. So novas due to the death of these stars happened fairly early in the lifespan of the universe (talking about few million years after the big bang).

Therefore, life could have developed in a few tens to few hundreds of millions of years after the big bang. That's still true even if we assume that heavier elements are created mainly when neutron stars collide and not by super/hypernovas as we theorized before LIGO/Virgo observatories.

Consequently, we likely are not a "progenitor" civilization in the universe if we only consider planets formation. We might not see anyone out there either because there's a great filter for intelligent life to emerge (so the bottleneck is in our past) or because few/no civilizations get to have an impact on their host stars (the filter is in our future) that would allow us to see them.

Basic life (single-celled?) requiring the elements above lead might have a chance at that time, but complex life like us wouldn't do so well if there were still supernovas going off left and right. There's a theory with decent evidence that at least one of the mass extinctions was caused by a supernova: https://www.space.com/supernova-caused-earth-mass-extinction...

That being said, I wasn't aware of how LIGO changed the understanding of how heavier elements are usually formed, guessing it changed the expected neutron star prevalence? Do you have any additional reading on that?

You are right about supernova hampering life evolution, but it's unclear how long the fireworks lasted. In my comment I argued that it is possible to have the conditions of life emerge much earlier than 13.5 billion years. Not that it necessarily happened.

Regarding the second point have a look at https://www.ligo.org/science/Publication-GW170817Kilonova/in... . That isn't my field of specialization, so I am not sure about recent publications. At the time though this was a big deal as kilonovas seem to be the primary source of heavy nuclei in the universe. That particular event crested between 1/100th to 1/1000th solar masses worth of heavy ( heavier than iron) nuclei. This is a greater rate than supernovas estimations.

> how LIGO changed the understanding of how heavier elements are usually formed, guessing it changed the expected neutron star prevalence?

It's not about the prevalence, but about the light curves observed during the event AT 2017gfo. They indicate significant heavy metal ejection but, what's interesting, also production.

> mergers of neutron stars contribute to rapid neutron capture (r-process) nucleosynthesis

These two articles cite the relevant papers:

https://en.wikipedia.org/wiki/GW170817#Scientific_importance

https://en.wikipedia.org/wiki/Nucleosynthesis#Neutron_star_c...

I'm just a layman but I believe by the time our sun has formed, we've gone through multiple star cycles. The early stars were very pure - made basically purely of hydrogen (maybe some helium?). They were huge, burned very bright and died comparatively quickly. Each time stars died, more heavy elements (and heavier elements than before) were produced. Over time the heavy element content (called metallicity) has increased in all stars. I believe there are also theories of white dwarf mergers undergoing runaway fusion and a lot of heavy elements being generated during the explosion.

You raise an interesting question though: what is the earlier point of time where the heavy elements were abundant enough for life (as we know it) to form? Just because we started existing at +13.5 billion years, it doesn't mean carbon based life couldn't have formed much earlier.

Very much a laymen also, however funnily enough I was listening to a bbc program called in our time, a couple of nights ago, where a similar topic was discussed one comment was that life is carbon based and for carbon to exist a star has to die, so yes therefore we are in the early stages. Will try to fin the episode….

I have zero ability to answer your question but I would love to know about about this. If life (like we know it) requires the explosion of aged stars, what is the earliest it would take. What is the minimum time needed to form, grow and explode a single star? Has there been time for this to occur 10s, 100s of times since the Big Bang? (obviously they can happen in parallel, but I'm thinking about how many in series).

There's a large range of stellar lifetimes, from a few million years up to trillions of years.

> 13.5 billion years seems like the time required to create a star, have the star die and blow up, have all that material settle and create a new star, then the planets are formed, than enough time on one of those planets needs to pass for life to form, then complicated life.

Maybe for a main sequence star, but there other processes that involve nucleosynthesis.

That assumes that life as we know it is the only possible form of life.

Not necessarily, but look up the phosphorus problem.

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