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I've seen quite a few news about astronomy but I'm not deep into knowing how it works or how people observe it. It got me thinking: if I want to watch a supernova for example, how long will I have to look for it to watch the whole "event"? Or events like NASA "seeing something coming out of a black hole", were they "looking" at the blackhole when something came out really quickly or did they observe it for months? Or maybe the birth of a blackhole?

Now, I do know black holes that quite a few trillion of years to vanish, I'm just curious how much time does it take for astronomers to observe such events.

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    $\begingroup$ Not quite the same thing but this figure in Rau et al 2012 gives some idea of the decay time of some astronomical transients. Basic rule of thumb is that events can't happen faster than the light-crossing time of the varying region. So if a black hole accretion disc 1 light day across changes in brightness, then that makes the time for the variation 1 day. Smaller varying sources could vary quicker. $\endgroup$
    – astrosnapper
    Jun 14, 2019 at 15:24
  • $\begingroup$ How long does it take for a burned out star to collapse? That is, when the actual collapse starts? $\endgroup$
    – d-b
    Jun 15, 2019 at 0:00
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    $\begingroup$ The answer to this likely depends heavily on how you define when the "event" starts and ends. $\endgroup$
    – probably_someone
    Jun 16, 2019 at 22:01

6 Answers 6

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A supernova (and many other interesting events) starts abruptly -- a flash of neutrinos lasting just a fraction of a second, but then different aspects of the event play out over minutes, days, hours or years. The astronomy community has for some years been developing increasingly sophisticates ways to deal with such events early on. Instruments with a very wide field of view (gravitational observatories, neutrino observatories, gamma ray detectors on satellites) detect the events initially and bring first networks of robotic telescopes and then humans and still larger telescopes into the loop to try and get as much information as possible about the violent and fast developing early stages. See for example http://growth.caltech.edu/

Other things are slower. The "something coming out of a black hole" (not really that, just something coming from very close to a black hole) event takes a long time, this was just the first occasion when they'd observed that black hole with the right instrument.

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As described in other answers, most astronomical events vary over rather long time scales. Even a supernova, which starts suddenly, has a subsequent light curve that peaks, then decays usually over many months. However, there are some shorter events.

In the recent LIGO gravitational wave observations, the final stage inspiral and merger of binary black holes or neutron stars generates a detectable GW signal for less than a second.

Other shorter term astronomical observations occur within the Solar system. Of course, the Sun and planets move across the sky over the year, and the Moon changes its phase continuously, repeating about every month. Occasional Solar and Lunar eclipses occur over a time scale of hours, with totality lasting only a few minutes for a Solar eclipse. Similiar time frames apply to other Solar system transits. Occultations, where a solar system object passes in front of a star, are nearly instantaneous for a visual observer. Meteor trails in the atmosphere are common, and are rarely visible for more than a few seconds.

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  • $\begingroup$ But is that really the supernova that lasts that long? The boom is over very quickly, the light after is what's being emitted by the material blown off. We normally consider that light as part of the supernova but I don't feel that's really correct. $\endgroup$
    – Loren Pechtel
    Jun 16, 2019 at 2:59
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    $\begingroup$ Note also that while the final in-spiral and merge of black holes lasts less than a second, the "everything else" (once the two objects are gravitationally bound to each other) leading up to that moment takes millions of years. @LorenPechtel Also consider that the light can move away from us, bounce off other material in the system, and reflect back towards us again, so its not just "afterglow" radiation. $\endgroup$
    – Draco18s no longer trusts SE
    Jun 16, 2019 at 4:37
  • $\begingroup$ @Draco18s With your black holes there's millions or billions of years of spiraling in, a small fraction of a second of boom and then the shockwave radiating away. Three effects, not one. $\endgroup$
    – Loren Pechtel
    Jun 16, 2019 at 13:03
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    $\begingroup$ @LorenPechtel, I agree, binary systems spiral in for a very long time before losing enough energy by emitting GW to merge. That does not change the fact I pointed out that LIGO can only detect the final short pulse of the strongest GW leading to the merger. The existance of the merger event, and substantial detail about masses, spins, etc. of the merging objects are extracted via modeling from this short signal. Other observations of gamma ray burst, or later X-ray emission from a shockwave interacting with surrounding matter provide additional information and confirmation. $\endgroup$
    – amateurAstro
    Jun 16, 2019 at 14:49
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    $\begingroup$ @LorenPechtel The part of my comment before your name was more directed at Amateur. And additionally did include that information ("the final in-spiral and merge"), but I was specifically pointing out that the lead up lasted a very long time and that he hadn't mentioned that at all. The fact that the final spiral and the shockwave are separate events was not relevant. $\endgroup$
    – Draco18s no longer trusts SE
    Jun 16, 2019 at 16:49
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In our galaxy, supernovae visible from Earth are very rare events, so if you wait for astronomers to raise the alarm when they see the next one you might have to wait hundreds of years. The prospects of seeing one in faraway galaxies are much better, but even so they are not everyday events. The chances of seeing one from the very beginning when the star collapses and explodes are almost non-existent because when they occur, they have to reach considerable brightness to become noticeable from Earth. However, searching for supernovae is something a well-equipped amateur can do, but it requires a lot of patience. As for the energetic particle beams that some black holes send out, they don't arise from within the black hole itself but from the matter in the accretion disc which is accelerated by the poles of the black hole's magnetic field. As this process continues for many years, you don't need to react so rapidly as a supernova discoverer, but you would need some very high tech equipment to observe and photograph it (the particle beams eject large amounts of matter which form large swirls, sometimes light years in length, from both poles of the black hole, and need a radio telescope to image them).

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The answer is that some phenomenas are incredibly fast. Others take billions or trillions of years (or more). But, most observed events don't just occur one time. So we can watch for longer events many times, and observe "snapshots" of many different times during the process, and interpolate to get then process as a whole.

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Your question strikes me as being more about humanity than astronomy. We define what is interesting to us: there is no physical law as to what "interesting" is.

The "interesting" part of a supernova might be a couple of months (when it's at its brightest), however, if we could predict when a star was within ten years of becoming a supernova, that would increase the window of "interesting".

The sun has been observed by humans for as long as humans have existed. We still find it interesting. We now know much more about it than when humans first appeared on Earth.

In short, the length of an interesting event is entirely up to us, and each type of interesting event will have its own typical length. It could be milliseconds to millennia.

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The universe is rather robust in that if physics allows a particular situation, there will usually be some place or time in the universe where that situation will occur. Thus any timescales that are allowed by physics, the universe will have astrophysical phenomena that occur on those timescales.

That's a long way of providing the answer "as fast as can be and as slow as can be, and everything in between."

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