Xanthir wrote:Many stars, definitely. The Sun is a third-generation star - do you count only the stars that died immediately before the sun, or the ones that died to make *them*, too?
starslayer wrote:Ooh, tough question. The trouble with answering this is that different elements come from different places. For example, Type II (core collapse) supernovae spew alpha elements (take C and keep adding alpha particles) everywhere, but virutally no iron. Type Ia's, since they leave no central remnant, make tons of iron peak elements. S-process elements come from AGB stars and supernovae, and we don't know where the hell the r-process elements come from.
Still, we can try to come up with an order of magnitude estimate. Since supernovae are the main seeds of the process, we should be able to ignore the other nucleosynthesis sites for the purposes of the calculation. Therefore, we need to calculate how many stars had to explode to bring the stellar neighborhood up to solar metallicity. To do that we need to know the star formation history of the galaxy, which is difficult at best. The present star formation rate (SFR) and supernova rate (SNR) are 1 solar mass/yr and about 2 per century at present. Both of these were higher in the past. The Milky Way was probably never a starburst galaxy, but we can't be sure. As a complete guess, we'll say the SFR was a constant 3 solar masses/yr between the galaxy's formation and the Sun's (so, ~7-8 billion years). This feels right to me, though this average rate was probably higher. Still, we can get a nice lower bound on how SNe it took to enrich the galaxy. SFR and SNR are approximately linearly correlated, so we would expect ~5 SNe per century over the lifetime of the Milky Way. Run for the required time, and we get about 350-400 million SNe in the Milky Way before the Sun showed up.
However, the galaxy is not well-mixed, so as you go further in towards the galactic center, the metallicity gets higher, as those parts of the galaxy are denser and older. Since metals from the bulge don't make it out to the suburbs where we are, we need to revise our estimate downward. I've no idea by how much, but a factor of 5-10 seems reasonable enough, though this is still a complete SWAG. This would mean you need somewhere in the neighborhood of 50 million SNe to give you the conditions to replicate the Earth we know and love.
mfb wrote:=> I would expect atoms from nearly every supernova in the milky way in our solar system. And probably some from other galaxies, too.
Diadem wrote:Let's assume that a supernova spreads its heavy elements uniformly over the region of space its shock wave has reached (it travels at about 10% of the speed of light). This is of course a huge assumption, but if it's not true, that just means some regions gain more from one supernova and less from another, so I hope it'll still average out approximately right.
Remember, I quoted a (completely made up) average SFR over the lifetime of the galaxy. It has for the most part been slowly, constantly decreasing over the lifetime of the galaxy, and was indeed much higher when the Milky Way was young. And SNR goes hand-in-hand linearly with SFR, assuming the initial mass function1 (IMF) is essentially universal. All signs so far point to the idea that it is.Diadem wrote:That is considerably more than I expected. And judging from your figures it sounds like a pretty low estimate. Only a factor 2.5 increase in supernova rate seems rather low. I always thought star formation was much, much higher in young galaxies? Also, why would heavy elements from the galactic centre not reach us? Their velocity is a significant fraction of the speed of light, well above the escape velocity. And there's plenty of time. Traversing a 100K lightyears at relativistic speed does not take long, on the time scales involved.
Nah, Diadem's got it. They're symmetric enough that it all averages out, especially when you have a lot of them going off in a relatively short time (~10 Myr or so). Most of the SNe in the galaxy, like 70-75%, occur in OB associations,lorb wrote:I think this assumption is too much of a stretch, especially since supernovae are known to be asymmetric, and there may be plenty of objects/forces around which do have an impact where the matter goes.
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