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Comment by mrfusion | Hacker News Reader

I've always wondered how elements produced in past stars end up in our solar system. I wouldn't think the whole universe is getting remixed all the time. It seems like areas stay pretty isolated.

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At the universal scale, we can observer several galaxies in various stages of colliding with each other (1). In fact our galaxy and Andromeda are on course to collide with each other in about 4 billion years (2).

Massive stars end their lifecycle in a supernova explosion, which blasts much of their mass off into space. the remaining stellar core either forms a neutron star or a black hole. The mass that is blated off contains many heavy elements formed in the star by nuclear fusion, and contributes to nebula formation. These nebulas coalesce to form new stars and their associated planetary systems. We can actualy see this happening in various places throughout our galaxy, with nebulas in various stages of coalescing and with multiple stars forming within them. Absorbtion spectra tell us about the materials these nebulae are composed of.

There is some speculation that heavy elements such as iron are also produced and scattered about in nutron star collisions.

(1) http://en.wikipedia.org/wiki/Galaxy_collision (2) http://en.wikipedia.org/wiki/Andromeda_galaxy#Future_collisi...

Interesting. So where did our sun come from? Are you saying it started from a multilight year wide cloud?

Why is it 4+ light years from any other stars? Did our sun pull everything out of that radius when it was forming leaving a whole bunch of empty interstellar space?

Bear in mind the galaxy is continuously rotating, our sun is orbiting the galactic nucleus, as are the other stars around it. Right now the nearest star is 4 lightyears away, but that's not always the case and it's quite likely other stars have passed ours by much closer than that during it's lifetime. In fact our sun has orbited the galactic core many times; it does so roughly every 250 million years.

The sun would have accumulated material from the nebula it formed in as it drifted though it. The stars near us now are not the ones our sun was near when it formed though (except by extemely unlikely co-incidence), as each star is on a slightly different course round the galaxy, like water droplets in a very slowly rotating cyclone.

If another star were to pass by very close, it could be disastrous for us as it could severely disrupt the planetary orbits in our system, but that's fairly unlikely and there's no prospect of that happening for many millions of years at least. The space between stars is vast, even by comparrison to the size of stars themselves and their solar systems.

EDIT - on that last point, 4 ly is ~250,000 AU (the distance from the earth to the sun). If we asume out solar system is 100 AU across, that's still only 1/2500 the distance to the next star.

Assuming I remember correctly, I should clarify that specifically it's the supernova itself that produces the heavy elements. The fusion that occurs during the bulk of the star's life doesn't tend to get much beyond oxygen.

This is actually my PhD research! Yes the current thought is that neutron star mergers are the source of heavy elements (iron and greater) over supernova. Looking at the elements produced by nucleosynthesis from the outflow yields very positive results when compared with abundance curves in our solar system.

Also because we know how much material is released in a mrger and how much material there is in our galaxy, we can work out how frequent neutron star mergers are and these results agree with other independent estimates of frequency of merger.

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