Betelgeuse?

[FraserClarke]

Yes -- but now you've started all the astronomers talking about supernovae

(and there aren't any Finnish astronomers at Palomar...)

[Ben Ritchie]

Ok, don't people get that it was an April Fool?!?

Yes, just ignored that bit

[brianb]

don't people get that it was an April Fool?!?

The "imminent" bit - to me that was so obvious that it wasn't worth commenting on.

But there's a serious vein to the ongoing discussion.

[brianb]

you see elements more massive than iron in RSGs and asymptotic giant branch stars, so they don't just form during late-stage core burning and core-collapse SNe

Of course ... r & s process nucleosynthesis goes on in the cores of all stars, even less massive main sequence stars like the Sun which don't have hot / dense enough cores to burn helium. The rates are low but not completely negligible. In fact everything is a statistical equilibrium process, but it's at the "silicon burning" stage that equilibrium processes become dominant.

Except in very massive stars (over 60 to 80 solar masses) where pair production instability can generate a core collapse to a black hole without the formation of a "heavy element" core. It can even happen during the main sequence phase if the star doesn't manage to lighten itself rapidly enough by radiative stellar wind.

Estimates of Betelgeuse's mass are of the order of 20 solar masses, plenty massive enough for an iron disintegration catastrophe but nowhere near enough for a pair production catastrophe.

[Ben Ritchie]

Except in very massive stars (over 60 to 80 solar masses) where pair production instability can generate a core collapse to a black hole without the formation of a "heavy element" core.

I think "may" rather than "can" is a better way of putting it - some caution is needed with the pair-instability process, because it's not really known if massive stars do explode that way. SN 2006gy is probably the most persuasive example (it is, at the very least, a very unusual supernova) but there are a number of issues there too as the metallicity of the host galaxy appears rather high for a classical pair-instability process. Conventionally it's a process thought to occur for very massive stars of low metallicity, therefore likely in the first ("Pop-III") stars but less so in the current era.

In general mass loss is the critical issue in determining the end-point of massive stars - there are a number of neutron stars that had progenitors more massive than ~50-60 solar masses (e.g. SGR 1806-20 or CXO J164710.2-455216) , implying that at least some stars can lose ~95% of their mass before or during a supernova. Also the binary fraction amongst massive stars appears to be very high, and massive binaries tend to be very efficient at shedding mass, but quite what goes on is still a very active research topic.

[brianb]

some caution is needed with the pair-instability process, because it's not really known if massive stars do explode that way.

Massive enough stars are not exactly common, they have difficulty forming from "recycled material" because of optical density issues. Even though the lifetime is short, there are so few of these stars that we don't expect to see them "popping" all over the place.

It is quite certain that, if a star which is massive enough doesn't manage to blow off its mass excess in time, a pair production induced core collapse will ensue ... you're quite right to point out that the observational characteristics of such an event are rather uncertain, but it is bound to be spectacular.

You're also quite right to point out that mass loss is critical - I've heard it said that, for stars which reach helium ignition, you can ignore anything which formed with a mass exceeding 30 solar masses, because the rapid mass loss during the main sequence phase due to radiation pressure of the envelope will reduce the mass of all heavier stars that reach the end of the main sequence stage to ~ 30 solar masses.

All this is of little interest in the case of Betelgeuse, which has quite clearly evolved out of the main sequence and is therefore quite certainly not massive enough to undergo a pair production induced core collapse, though it is also massive enough that a iron disintegration induced core collapse is (in the long term) almost certainly inevitable.

[brantuk]

Hmmm.... I noticed it was getting redder too. If it does go SN - won't we have to wait a few years before we see it? Or if we see it - does that mean it happened a few years ago? Or - if looking through our scopes is a "look back in time" will people with bigger scopes see it before I do?

Just a thought - but all this light year stuff has me dumbfounded sometimes.

[brianb]

It is possible (unlikely IMO) that Betelgeuse has already gone SN with the light "flash" still be on its way towards us. After all it is several hundred light years away.

Funny thing is, even though the neutrinos travel (fractionally) slower than light, they still beat the light pulse ... the neutrino pulse from 1987A beat the light pulse to Earth by a few hours despite travelling for a couple of hundred throusand light years .... the key is that the neutrino pulse comes directly from the core collapse, the light comes from beta decay of nickel-56 and cobalt-56 created in the event, which takes a few days to get blasted through the remains of the outer layers of the star so that it can be seen.

When Betelgeuse does go bang, if they're still around, the neutrino observatories will detect it first ... and it will be an unmissable event at the level of neutrino detectors, many orders of magnitude above the normal flux.

[brianb]

"The final stage of nucleosynthesis - the forging of iron from silicon - takes only a single day",

Ummm. If that was true then the pre-collapse core would be mostly nickel not iron - the "ash" from silicon burning has to have the same mix of protons & neutrons as the Silicon 28 fuel (14 of each) which makes it nickel-56. Ni56 is unstable and decays through Co56 to Fe56 the process having a half life of some weeks.

Yet the models show the SNII progenitor core as iron.

Together with the luminosity / radiation pressure argument, that pretty well confirms that the iron core generation is a "slow" process - like Rome, it takes substantially more than a day to build. (But still very fast compared with the main sequence evolution of a massive star!)

[George]

So are we unanimous in the event happening over the SGL5 weekend is unlikely to happen? or do I take a camera just in case :(

[Demonperformer]

So are we unanimous in the event happening over the SGL5 weekend is unlikely to happen? or do I take a camera just in case :(

Yes to both

[Demonperformer]

I am perfectly willing to bow to Brianb's obviously superior knowledge on this subject. I was only passing on what was printed in AN, in answer to a question I had actually submitted. This was asking if spectroscopic measurements of the iron content of a star be used to assess its likelihood of going supernova soon. This seemed to me to be pretty much on topic to this thread.

As I said in my first post, I was not seeking to be controversial, and if the answer I was given by AN has resulted in me passing on misleading information in my posts here, I apologise.