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I Frequency of nearby Supernova Explosions

  1. May 16, 2018 #1
    I have heard an interesting claim that looks fishy to me.
    The claim is that during the last 10,000 years there hasn't been a single Supernova eruption closer than about 5,000 light years. And that somehow this is an anomaly. That the background rate of nearby supernovas is much higher. And so we are living in some sort of goldilocks time with respect to this threat.
    I suspect this is not correct (thats its anomalous ) but am looking for guidance. Any help much welcome.
     
  2. jcsd
  3. May 16, 2018 #2

    mfb

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    The estimated supernova rate in our galaxy is about 2 per century. In 10,000 years we expect 200, and if we naively divide the galaxy in 200 regions of equal area you expect the nearest supernova ~5000 light years away, give or take a factor of 2-4 to account for all sorts of things I simplified away. Nothing closer than 5000 light years in 10,000 years is not exceptionally rare.

    Another question: Are we sure about that? Would every supernova closer than 5000 light years have left clear traces over 10,000 years?
    Where did you hear that? "I have heard" is not a good reference.
     
  4. May 16, 2018 #3
    Thanks for your reply. The reference is below. It is also not a good reference. Its from a religious organisation that tries to use physics to promote their agenda. That was one reason I thought it was fishy to start with.
    I know, one should not take them seriously but it spurred my interest to find out what the real science was. But if you have any more comments on the link below Id be interested to hear them.
    http://reasons.org/explore/blogs/to...e-are-living-in-the-ultimate-supernova-moment
     
  5. May 16, 2018 #4

    phyzguy

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    I'm not quite sure why you would view a supernova within 5000 light years as a "threat". For example, when Betelgeuse (640 light-years away) blows, it will be a spectacular sight, probably brighter than the full moon, but it will not be dangerous to life on Earth.
     
  6. May 16, 2018 #5

    mfb

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    That is a very poor argument, especially as the list has 10 entries AD, but none BC. The probability that an event got recorded in such a way that we are aware of the records increased over time.

    The claims about all the bad effects of somewhat nearby supernovae are very questionable, highly exaggerated, and all unsourced.

    The last paragraph is obviously nonsense.
    Even if (a) we would live in a time with surprisingly low supernova activity and (b) all the claims about supernovae would be true, it wouldn't mean anything. If we wouldn't live in such a time then there wouldn't be the internet to write articles like this. That's like claiming it would be a miracle that Earth is in the habitable zone.
     
  7. May 16, 2018 #6
    Which I think they do indeed claim. One question ,what is the threat radius for a nearby supernova. How close does it have to be to be a worry.
     
  8. May 16, 2018 #7

    phyzguy

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    I don't think there are any massive stars close enough to worry about. A star has to be at least 8 solar masses to go supernova. I think the nearest star that qualifies is Betelgeuse, and it is too far away to be a threat. The brightest supernova are about absolute magnitude -20.

    Edit:

    [ IGNORE THIS, I made an error, as snorkack pointed out. The sun is of magnitude -13. So if one of these went off at 25 parsecs (about 80 light years), it would be as bright as the sun for a few weeks. This would obviously cause major weather disturbances, but even then I don't think it would be a huge catastrophe.]
     
    Last edited: May 16, 2018
  9. May 16, 2018 #8

    mfb

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    Twice the heating, with a much higher UV component? Sounds very uncomfortable for all surface life.
    It depends on the supernova, but if it is more than 1000 light years away we are probably fine. Note that we have much better tools today than people in the stone age, and we understand the threat much better.
     
  10. May 16, 2018 #9
    No.
    It is Moon which is magnitude -13.
    Sun is -27.

    But Clarke Burst would have been as bright as Sun from a distance of 8000 lightyears.
    Which is the distance to Eta Carinae.

    It would be as bright as Sun for just half a minute, though.
     
  11. May 16, 2018 #10

    Vanadium 50

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    You're right. It's a crap source, and you should have been more forthcoming about that rather than hide it with an "I heard that".

    The galaxy has an area of around 8 billion square light years, and the area of a 5000 ly circle is about 1% of that. So by statistics alone one would expect 1% of the supernovae to be within this radius. With a galactic supernova rate of 1-2 per century, you expect 0.5-1 events inside this radius within 5000 years.

    So it's not just a crap source, it's a crap argument from a crap source.
     
  12. May 16, 2018 #11

    phyzguy

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    Thanks for pointing out my error.
     
  13. May 16, 2018 #12

    russ_watters

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    Yeah, and do we really want to quibble about arbitrary limits like this anyway? I can think of two off the top of my head that come close or may even fit:

    Crab Nebula: 6500LY, 900 years
    Veil Nebula: 1500 LY, 5-8,000 yrs. Fits?
     
    Last edited: May 16, 2018
  14. May 16, 2018 #13

    russ_watters

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    The paragraph before Table 2 doesn't make a lot of sense and contains a couple of things that are definitely wrong. They should be taking the table at face value; those supernovae happened whether humans saw them or not (and they probably did).
     
  15. May 17, 2018 at 9:55 AM #14

    Ken G

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    I don't think the visible light from supernovae are anything significant because even a bright one would need to be at most a light year away to rival the intensity we already get from the Sun. By the estimates above, we would only expect a few SNe within a few light years of Earth in the history of the planet, and they probably wouldn't be the brightest ones. I've heard about gamma-ray bursts as being potentially harmful to life, because they are beamed emissions that could have consequences much farther away if you happen to be in the beam, but they would be even more rare. It doesn't sound like we've dodged any bullets there, and as mentioned, even if we had, it would merely be another example of the anthropic principle. I often find that probabilistic arguments about the miraculousness of life tend to overlook just how big the universe actually is (a size we don't even know and likely never will).
     
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