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Old 12-27-2019, 06:34 PM
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Betelgeuse losing brightness


The gradual loss of Betelgeuse brightness although not a surprise has been reported as a possible sign that the star may explode.

If it does indeed destruct and we see the celestial supernova ......when did it actually happen?
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Old 12-27-2019, 06:40 PM
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Since that star is 642 light years away, I'd say it happened sometime around 1377CE. So we can rule out the "Bethlehem" star.
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Old 12-27-2019, 06:58 PM
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Everything we see in the sky happened some time in the past, whether 1.25 seconds for anything on the moon to billions of years for distant quasars. But it's not something to worry about. As far as we're concerned, they happen when we see them and not a millisecond earlier. There's occasionally times when it does make a difference, but generally you can just ignore the fact that they happened some time in the past. Astronomers do.
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Old 12-27-2019, 07:16 PM
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It's all relative. If it became apparently a supernova to us tomorrow we could say that it actually happened about 640 years ago.

But the photons that left the star then do not experience either time or distance. So from their point of view it just happened, and there is no time.
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Old 12-27-2019, 07:38 PM
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Another thread, with a somewhat more obscure title, on the topic.
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Old 12-27-2019, 08:35 PM
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Another thread, with a somewhat more obscure title, on the topic.
Obscure?!? Metaphorical, maybe, but obscure?
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Old 12-28-2019, 07:25 AM
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It's considered possible that Betelgeuse will explode in a supernova soon, but this "soon" is meant by astronomical standards; it can still be tens of thousands of years away and be considered "soon" by astronomers. So I wouldn't bet on us getting to see this supernova in our lifetimes.

But then, it's possible - as others have said - that it occurred already, within the last 640 years, and we simply haven't found out yet because the photons haven't reached us.
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Old 12-28-2019, 07:33 AM
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Since that star is 642 light years away, I'd say it happened sometime around 1377CE. So we can rule out the "Bethlehem" star.
I don't think you've thought this through.
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Old 12-28-2019, 09:32 AM
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The "Bethlehem star", if it happened at all, was something that was seen on Earth circa 0 CE. If it was a supernova, then it must have actually occurred significantly earlier than that.
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Old 12-28-2019, 10:11 AM
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It's considered possible that Betelgeuse will explode in a supernova soon, but this "soon" is meant by astronomical standards; it can still be tens of thousands of years away and be considered "soon" by astronomers. So I wouldn't bet on us getting to see this supernova in our lifetimes.

But then, it's possible - as others have said - that it occurred already, within the last 640 years, and we simply haven't found out yet because the photons haven't reached us.
When they say soon, they mean we will be able to see it soon. Astronomers typically don't say such and such happened in the distant past and we just saw it today, they say today such and such happened. Of course it happened in the past, that is generally understood.
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Old 12-28-2019, 10:34 AM
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And for what it's worth, the best estimate I saw (before these recent observations) was that we'd see Betelgeuse go "sometime in the next 1000 years". So yes, there's a pretty good chance that it's already happened and we're just waiting to get the news.
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Old 12-28-2019, 11:11 AM
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Is it really necessary or appropriate to say that an even we see on Betelgeuse today actually happened centuries ago? I thought Einstein's perspective ended the notion of absolute time.

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Henceforth, space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality.
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Old 12-28-2019, 01:41 PM
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Yes, it is necessary and appropriate. It's a common misconception that relativity means things happen when they're observed. You still take into account light travel time.
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Old 12-28-2019, 11:29 PM
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Anyone else upon reading the thread title think this was a misplaced and misspelled thread about the POTUS candidate from South Bend, IN?
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Old 12-28-2019, 11:35 PM
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No.
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Old 12-28-2019, 11:48 PM
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No.
But it would be beyond cool if it did!

The Crab Nebula, at its height, was visible during the day.
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Old 12-29-2019, 01:30 AM
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Yes, it is necessary and appropriate. It's a common misconception that relativity means things happen when they're observed. You still take into account light travel time.
I suspect that septimus was just pointing out that there is no absolute time.
That statements like "We think the supernova exploded today, but *really* it happened 20 million years ago" or whatever are laboring under a misconception, since "really" is also just picking a frame of reference.

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Old 12-29-2019, 01:54 AM
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You can't pick a frame of reference in which causality is violated. Since causality travels at c, there is a limit to what you can achieve. You can't pick a frame of reference that makes the time difference between here and Betelgeuse less than 642 years. You might be able to construct a non-inertial frame that makes it look longer to an observer in that frame.

If an observer at Betelgeuse was watching the Earth as Betelgeuse went nova, he would see the Earth as it was 642 years before information of the nova arrived at the Earth. Since 642 years is the minimum, it makes sense to go with that.
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Old 12-29-2019, 06:09 AM
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You can't pick a frame of reference that makes the time difference between here and Betelgeuse less than 642 years.
I didn't claim that there was. I am just suggesting (or rather, supporting what I think septimus was getting at), that there is no absolute time. It is misleading to talk about what time an event "really" happened because all we can ever do is pick a reference frame and say when the event took place in that frame.

I understand why it is convenient for astronomers to usually assume the reference frame of the distant object, watching the light travel X light years away, there's no issue with this.
It's only in cases like the OP where it is necessary to point out the distinction between doing this and saying when an event "really" happened.

It becomes more obvious when we examine accelerating reference frames because in that case the apparent order of events, or simultaneity of events, is not even preserved between different observers, and it becomes more obvious that there can be no universal clock.

Last edited by Mijin; 12-29-2019 at 06:10 AM.
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Old 12-29-2019, 07:24 AM
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You can't pick a frame of reference in which causality is violated. Since causality travels at c, there is a limit to what you can achieve. You can't pick a frame of reference that makes the time difference between here and Betelgeuse less than 642 years. [...]
You can't pick a frame of reference in which causality is violated, certainly. But that's a very different statement from "You can't pick a frame of reference that makes the time difference between here and Betelgeuse less than 642 years", which I think is incorrect, and that you can pick non accelerating or accelerating reference frames that do make the difference much less than 642 years. And, no, I don't mean failing to account for the light travel time to observe the events.

I think in any space time diagram there are world lines that, among other things, present limits to how far reference frame choice can be shifted. I think if you create such a diagram you could use it to test how far you could shift the time relationship between here and Betelgeuse.

https://en.wikipedia.org/wiki/Relati...f_simultaneity
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Old 12-29-2019, 08:14 AM
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Yes, you can pick reference frames where the time since the event is different. But in every reference frame, you would still say that it happened in the past, not now. And given that Earth and Betelgeuse are (nearly) at rest with respect to each other (at least, nearly at rest compared to the speed of light), there's an obvious choice of reference frame for this question, and in that choice of reference frame, the event happened 642 years ago.
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Old 12-29-2019, 08:24 AM
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The point is that the act of observation on the Earth is causally connected with the nova event. You cannot construct a frame of reference that sees the observation on the Earth of Betelgeuse's nova, and the nova itself appear at a closer distance/time.
You can construct a frame of reference that makes non causally connected events, one at Betelgeuse, one on the Earth appear in any order or time difference. But that doesn't change an observed distance between Betelgeuse and the Earth to anything other than 642 years. Because the events are not causally connected you have no way of using those events to derive a distance in your frame of reference (if only using those events.)
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Old 12-29-2019, 08:32 AM
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You can make the time difference any amount greater than 0. To an observer flying in the direction from Betelgeuse to Earth, the time difference (and hence also distance) would be less than 642 years, and to an observer flying in the opposite direction, the time difference (and hence also distance) would be greater than 642 years.

Though, just because you could use such a reference frame, there's no reason why you would.
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Old 12-29-2019, 09:31 AM
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If Betelgeuse does go kaboom, what would we see with the naked eye and for how long?
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Old 12-29-2019, 12:22 PM
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It would be considerably brighter than the full moon, easily visible during the day, and probably somewhat uncomfortable to look at directly. It would fade over timescales on the order of months.
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Old 12-29-2019, 12:52 PM
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Re frame of reference for the time measurement:

There is a natural frame of reference when talking about the age of the universe, that's the time measured on a clock carried by a co-moving observer, i.e. an observer at rest with respect to local space, moving only with the Hubble Flow, with the expansion of space itself. The frame in which you are at rest wrt to local space is the frame in which the universe is isotropic - it looks the same in all directions.

So I think it's correct to say that we'd get 642 years if we measured the age of the universe (as described above) at the time light reaches earth, and subtracted the age of the universe when the light was emitted from Betelgeuse.
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Old 12-29-2019, 01:03 PM
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If Betelgeuse does go kaboom, what would we see with the naked eye and for how long?
According to Chinese accounts, the "guest star" of 1054 was first visible on the morning of July 4, 1054. It remained visible during the day for 23 days, therafter being seen only at night. It faded over the next 20 months, being last seen on April 6, 1056.

https://en.wikipedia.org/wiki/SN_1054#Chinese_astronomy

Tycho's New Star, however, appears to have only been visible by night.

So, will depend on how big a ka-boom it produces.
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Old 12-29-2019, 01:10 PM
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And that was for a supernova ten times further away (and hence, one hundredth as bright, if all else were equal). And I'm pretty sure that Betelgeuse is more massive than the progenitor of the Crab Nebula, so it'd be even brighter than that, yet.

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Quoth Riemann:

There is a natural frame of reference when talking about the age of the universe, that's the time measured on a clock carried by a co-moving observer, i.e. an observer at rest with respect to local space, moving only with the Hubble Flow, with the expansion of space itself. The frame in which you are at rest wrt to local space is the frame in which the universe is isotropic - it looks the same in all directions.
The local co-moving reference frame, Betelgeuse, and Earth are all moving very slowly with respect to each other, so you'll get very close to the same result using any of those frames (i.e., close enough that measurement error is more significant than the difference between frames). Though there's very little relevance to the local co-moving reference frame, when we're talking about objects within the same Galaxy.
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Old 12-29-2019, 01:19 PM
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Addendum: I just looked it up, and Betelgeuse is probably larger than the progenitor of the Crab, but there are considerable error bars on both, making it difficult to determine for sure. But if the Crab's progenitor were the same inherent brightness as Betelgeuse is now, then it would have been a dim star, but visible to the naked eye, and I don't think there are any records of that star being known before the supernova. There may not, however, have been any stellar catalog comprehensive enough to include 5th-magnitude stars at the time, so the absence of such a record might not be remarkable.
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Old 12-29-2019, 08:11 PM
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If Betelgeuse does go, are we in any danger of a toxic shower of gamma rays?
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Old 12-29-2019, 08:46 PM
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If an observer at Betelgeuse was watching the Earth as Betelgeuse went nova, he would see the Earth as it was 642 years before information of the nova arrived at the Earth. Since 642 years is the minimum, it makes sense to go with that.
Wouldn't he be seeing the earth as it was 1,284 years before the nova info got to earth?
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Old 12-29-2019, 08:52 PM
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Probably no danger. Significant gamma rays would probably only be along the polar axes.
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Old 12-29-2019, 09:06 PM
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Betelgeuse is not "gradually losing brightness" -- it's a variable star with a very complex history of variation. It's been waxing and waning for centuries. The Encyclopedia Britannica doesn't seem to realize this, but the American Association of Variable Star Observers has about a century's worth of data on its brightness variations.

John Herschel (son of the discoverer of the planet Uranus, and an extremely noted astronomer and scientist in his own right) discovered the variability of Betelgeuse not just once, but twice. He observed its variability, then forgot about it and observed it again.

I'm not up on the latest information about Betelgeuse, but the fact that its intensity is changing ought not to be cause for concern -- it's been doing it for hundreds of years (documented). In a book I haven't yet published, I make the case that, like Algol, Mira, and delta Cephei, its variability was known since the days of Classical Greece, and was incorporated into myths.
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Old 12-29-2019, 09:26 PM
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I'm not up on the latest information about Betelgeuse
Maybe you should have tried that before sticking your foot in your mouth.
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Old 12-29-2019, 09:26 PM
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Phil Plait had a column about all this a few days ago. In addition to noting (as has already been pointed out in this thread) that Betelgeuse is a well-known variable star, and this sort of behavior is hardly unprecedented for old Alpha Orionis; and also that even if Betelgeuse does/did explode Next Tuesday/650 years before Next Tuesday, we wouldn't be in any danger; he also says that
Quote:
...it'll probably be about 100,000 years before it explodes. That's not a guarantee, but it's where the science points right now. Even if that's off by a lot, the odds of Betelgeuse going supernova in the next century are extremely low.
So, darn.
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Old 12-29-2019, 09:31 PM
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Maybe you should have tried that before sticking your foot in your mouth.
For instance, would you concider Astronomy Magazine to be somewhat more knowledgeable than random social media?
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Old 12-29-2019, 09:45 PM
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They are building a Dyson sphere. It was scheduled to take 10,000 years, and they were planning to live on the outside. But they have seen what humans are up to, accelerated the project, and are now planning to live on the inside.
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Old 12-29-2019, 09:52 PM
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Wouldn't we have a couple of hours of warning from neutrinos admitted before the radiation?

I would wonder though whether neutrino bursts can be identified in real time.
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Old 12-29-2019, 09:57 PM
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If Betelgeuse does go kaboom, what would we see with the naked eye and for how long?
And could we bring it back if we say its name three times?
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Old 12-29-2019, 11:40 PM
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If the core is collapsing enough to produce a neutrino pulse, there's probably going to be some indication of that in light, too, and we're watching Betelgeuse very closely (as opposed to the progenitor of 1987a, which was a totally nondescript speck on some archived photographs full of a bunch of other nondescript specks).

But yes, the neutrinos would certainly be definitive, and there are procedures in place for quickly spreading news of a burst like that.
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Old 12-29-2019, 11:45 PM
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Actually, speaking of that: We can't say whether Betelgeuse's recent activity is a sign of imminent supernova, because we've never watched a star before it went supernova, and so we don't have any real idea, beyond unreliable models, what a star that's about to explode actually looks like. Maybe what we're seeing from Betelgeuse means it'll be a few weeks, or maybe it means it'll be a milennium, and we just don't know.

It's the same issue with unprecedented geological observations at Yellowstone. We've never watched a supervolcano go off, so we don't know what to look for.
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Old 12-30-2019, 12:20 AM
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With respect to the latter, iím hoping we remain without a real-life example for a long time to come. Let the scientists continue with their models.
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Old 12-30-2019, 12:40 AM
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It's the same issue with unprecedented geological observations at Yellowstone. We've never watched a supervolcano go off, so we don't know what to look for.
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... iím hoping we remain without a real-life example for a long time to come. Let the scientists continue with their models.
But we need data!
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Old 12-30-2019, 07:57 AM
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Maybe you should have tried that before sticking your foot in your mouth.
No foot in the mouth. As I say, Betelgeuse has done this before. People just aren't aware of its frequent changes in brightness.
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Old 12-30-2019, 08:06 AM
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No foot in the mouth. As I say, Betelgeuse has done this before. People just aren't aware of its frequent changes in brightness.
And you still haven't read the linked article? I congratulate you on the flexibility of your joints.

Do you imagine that the cited professional astronomers are unaware of the star's history?

Last edited by Riemann; 12-30-2019 at 08:09 AM.
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Old 12-30-2019, 08:07 AM
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The gradual loss of Betelgeuse brightness although not a surprise has been reported as a possible sign that the star may explode.

If it does indeed destruct and we see the celestial supernova ......when did it actually happen?
If it goes super nova, it will have happened 642 years ago because it would have taken the light from the explosion 642 years to reach us.
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Old 12-30-2019, 08:12 AM
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If it goes super nova, it will have happened 642 years ago because it would have taken the light from the explosion 642 years to reach us.
Let us know when the light from the first twenty posts in this thread reaches you, then we can work out how far away you are.

Last edited by Riemann; 12-30-2019 at 08:13 AM.
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Old 12-30-2019, 08:58 AM
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Where's the kaboom? I was expecting an Earth-shattering kaboom!
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Old 12-30-2019, 09:31 AM
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It would be considerably brighter than the full moon, easily visible during the day, and probably somewhat uncomfortable to look at directly. It would fade over timescales on the order of months.
And would it occur virtually instantaneously, i.e. from normal (albeit faded) Betelgeuse to cosmic kablooie in seconds, or would it gradually get brighter over a long period of time?
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Old 12-30-2019, 10:00 AM
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And would it occur virtually instantaneously, i.e. from normal (albeit faded) Betelgeuse to cosmic kablooie in seconds, or would it gradually get brighter over a long period of time?
Type II supernova light curves

It will not get brighter over a long period of time. Note the rapid rise to peak luminosity over a week or so as the nova expands, followed by a gradual decline.
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