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Stars with more than 8-16 solar masses usually go supernova. These are OB stars, of which 0.2% currently exist in our galaxy. The average lifespan of said star ranges from 10 to 40 million years. So 200 million OB stars ÷ 40 million years = 5 per year. That's how many supernovae we should be seeing annually in our Galaxy alone. So why do we see only an average of one supernova every 50 years?

Clearly, I am not considering something else important here. What am I missing?

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    $\begingroup$ Where does your "0.2% currently exist in our galaxy" estimate come from? $\endgroup$ May 21 at 18:06
  • $\begingroup$ Welcome to Stack Exchange! "Why is X true?" questions need to include a short explanation of what it is that makes you believe X is true. Can you edit your question and add to it a link or cite a source where your values (like PeterErwin mentions above) are coming from? Thanks! $\endgroup$
    – uhoh
    May 22 at 1:54
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There are far fewer than 0.2% of stars that are as massive as $8M_\odot$. It is possible you've confused it with the percentage of stars born with $M>8M_\odot$.

The thing is, those massive stars die very quickly, but most stars, those with masses $<1M_\odot$, are still alive. So the fraction of massive stars in existence now is much lower. i.e. Almost all the massive stars ever born have already died, but almost all the low mass stars ever born are still alive.

What matters for the present supernova rate is the current birth-rate of massive stars.

The current star formation rate in the Milky Way is something like 1 solar mass per year. The way the birth distribution of stellar masses works out, most stars born are of order $0.3 M_\odot$, but about 0.2% are $>8 M_\odot$, which means, if you work through the numbers, that you need round about $150 M_\odot$ of star birth to produce one massive star - which means you get a type II (core collapse) supernova every 150 years. These numbers are uncertain by factors of two.

Another factor to bear in mind is that we don't "see" a supernova every 50 years in the Milky Way. In recorded history, it is more like one every 300 years. However, the majority of supernovae would have been totally hidden from an observer on Earth by dust in the Galactic plane (where star formation occurs). Thus the true rate in the Milky Way might be closer to 1 in 50 years.

Finally, note that some of these supernovae are caused by exploding white dwarfs, not the core collapse of massive stars. Thus the rough number of one every 150 years is probably consistent with the historical observational record.

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    $\begingroup$ Your footnote's also a major factor, so you should probably expand on it. $\endgroup$
    – Mark
    May 25 at 22:06
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    $\begingroup$ @Mark done. However, this is a minor issue compared with the 2 order of magnitude discrepancy in the question and is counterbalanced by the fact that a fraction of observed supernovae are of type Ia. $\endgroup$
    – ProfRob
    May 25 at 22:24

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