[note: this is the original post. There is an important update at the bottom; sadly the event was not ‘real’, but I leave the text here for some time for those who want to share the excitement of the moment a few hours ago]

I just need to draw your attention to a breaking science news story. A Gamma-Ray Burst has been detected in the Andromeda Galaxy, the closest galaxy to the one we live in. That really is news.

It could not happen any closer, unless it was in our own galaxy. Just to give you the parameters, one of the closest ones observed to date was 1000x further away than the one yesterday. Since it literally happened to the next-door neighbour in inter-galactic terms, it’s fair to ask what would have happened if this had occurred in our galaxy? Well, it depends, but our galaxy has a diameter of approx 200,000 light-years, and if it happened in our ‘neighbourhood’ (some 10,000 light years or less), every place on Earth would feel like it was somewhere near an exploding nuclear warhead, i.e. there would be very real trouble down here. It serves as a reminder that even things that only ever happen on galactic time-scales still do happen once in a while. On the balance of probabilities, there is really no point being scared of it happening (after all, we aren’t typically scared every time we cross the road or get into an airplane), but the consequences — in any sense other than a purely scientific one — would be unthinkable.

I will post more soon, but for now, there are two explanations, one more exotic than the other. The more likely is that two neutron-stars collided. Those are objects as heavy as the sun but smaller than Manhattan. They are literally as dense as atomic nuclei, but obviously much larger. It’s an atom-smasher’s dream to observe something like that. The process would likely give birth to a new black hole in Andromeda (and we ‘know’ of only 26 or so of those). The other possible explanation is that an existing black hole swallowed a large amount of mass. Data over the next 24 hours will likely favor one of the two explanations. It depends on whether the burst lasted longer or shorter than 2 seconds. But because 2 seconds is no enough to train your telescope on something new happening, no-one currently knows how long it really lasted. Everyone expects more data soon. Stay tuned.

update (2 hours after original text above was posted): This now looks like a false alarm, as one of the data team scientist explains on http://www.star.le.ac.uk/~pae9/twitter/GRBM31.html