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From this subsection in Wikipedia's aricle on extinction event:

A nearby gamma-ray burst (less than 6000 light years away) would be powerful enough to destroy the Earth's ozone layer, leaving organisms vulnerable to ultraviolet radiation from the sun.[92] Gamma ray bursts are fairly rare, occurring only a few times in a given galaxy per million years.[93] It has been suggested that a supernova or gamma ray burst caused the End-Ordovician extinction.[94]

Does this mean if this were to occur it would take 6000 years for the gamma ray burst to destroy our ozone (potentially)?

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    $\begingroup$ It means it would take 6000 years for the light to reach us from the time that it is emitted from the star. As a side note, a gamma ray burst as the cause of mass extinction is almost impossible to test as a hypothesis so make of it what you will. $\endgroup$
    – bon
    Jun 15, 2017 at 21:15
  • $\begingroup$ @bon And may this hypothesis never be tested. $\endgroup$
    – kingledion
    Jun 19, 2017 at 11:37

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As bon noted in the comments, that it the time it would take the gamma rays to reach Earth. Gamme rays travel at the same speed as all other forms of electromagnetic radiation, including visible light, so we will not have any warning. This kind of makes the "6000" number meaningless. We will know that the star exploded and the gamme rays will reach earth at the same time. The destruction of ozone will then probably be instantaneous (on a geological time scale).

From your quote:

It has been suggested that a supernova or gamma ray burst caused the End-Ordovician extinction.

It has been suggested because there was no other plausible terrestrial cause for the mass extinction, so people started to look at extra-terrestrial causes. But a paper published earlier this year documents a mercury anomaly associated with the extinction event. This gives strong evidence for a volcanic cause for the extinction (similar to the P-T or K-Pg events), and there is no need to make up a gamma-ray burst.

David S. Jones, Anna M. Martini, David A. Fike, and Kunio Kaiho: A volcanic trigger for the Late Ordovician mass extinction? Mercury data from south China and Laurentia. Geology, July 2017, v. 45, p. 631-634, 2017, doi:10.1130/G38940.1

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  • $\begingroup$ We could easily have warning, by observing the behavior of the star and predicting that it's going to give off a GRB. Of course getting people to pay attention to the prediction is another matter entirely... $\endgroup$
    – jamesqf
    Jun 16, 2017 at 6:09
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    $\begingroup$ @jamesqf do we know how a GRB proceeds? Or is this kind of like earthquake prediction: "we know this star's gonna pop, but not sure if tomorrow or in 1000 years" $\endgroup$
    – Gimelist
    Jun 16, 2017 at 10:56
  • $\begingroup$ I meant that not as saying we know how to predict a GRB (or an earthquake) now, but that we possibly could do so in the future. For a GRB caused by the merger of two neutron stars, for instance, observing a pair, would give a pretty good handle on when they'll merge. $\endgroup$
    – jamesqf
    Jun 17, 2017 at 18:41

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