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.
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.)
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.
If Betelgeuse does go kaboom, what would we see with the naked eye and for how long?
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.
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.
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.
Tycho’s New Star, however, appears to have only been visible by night.
So, will depend on how big a ka-boom it produces.
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.
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.
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.
If Betelgeuse does go, are we in any danger of a toxic shower of gamma rays?
Wouldn’t he be seeing the earth as it was 1,284 years before the nova info got to earth?
Probably no danger. Significant gamma rays would probably only be along the polar axes.
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.
Maybe you should have tried that before sticking your foot in your mouth.
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
For instance, would you concider Astronomy Magazine to be somewhat more knowledgeable than random social media?
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.
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.
And could we bring it back if we say its name three times?
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.