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Estimates of the supernova rate in the Milky Way put it at a few per century, but very few of these are visible from Earth (at least with the naked eye) because of intervening gas and dust clouds). On the other hand the Andromeda galaxy (M31) is sufficiently face on that we should see at least most supernovae there, and is several times more massive than the Milky Way, so one would naively expect to see at least one every few decades. Nevertheless, we have seen only one supernova in M31 in the last 150 years (SN1885a).

Are these accepted (or even seriously proposed) explanations for this?

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The low supernova rate in M31 can be directly attributed to the fact that the galaxy's star formation rate is much lower than the Milky Way's.

Andromeda is currently in a relatively quiet phase in terms of star formation, currently experiencing rates of $\sim0.40M_{\odot}\;\mathrm{yr}^{-1}$. The Milky Way, on the other hand, has a star formation rate several times higher; as such, it contains more massive, short-lived stars which are supernova progenitors. This leads to a correspondingly higher supernova rate than Andromeda. Estimates of the supernova rate in M31 vary, but a value of 0.5 to 1.0 per century is reasonable. This in turn matches well with observations.

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  • $\begingroup$ Thanks. Perhaps I should ask this as a separate question, but is there a clear cause for the lower star formation rate? Closely related, is this a long-term state of affairs, or just a "chance of the moment"? $\endgroup$ – Steve Linton May 14 '20 at 16:36
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    $\begingroup$ @SteveLinton It's certainly a relatively recent change. The likely culprit is gas depletion, possibly spurred by star formation bursts within the last couple billion years and a high star formation efficiency, leading to a gas fraction that's lower than the Milky Way's. $\endgroup$ – HDE 226868 May 14 '20 at 16:49

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