What type of cosmic event would cause immediate species extinction with no hope of survival?

What type of cosmic event would cause the immediate death of the entire human species with absoutely zero chance of protection or defense against said event? For example, a GRB or collision with a big asteroid might kill a good % of the population but would not not cause "immediate" (within 12 hours or less) and 100% extermination.|||I don't think there's anything that would do it within 12 hours except for a large meteor impact. It would have to be bigger than the one that killed the dinosaurs because some of them survived afterwards and died slowly. One that would be big enough to basically kill everyone on Earth with it's shockwave.





Some people have said a GRB, or Gamma Ray Burst, could do it, but this seems unlikely. One could kill us all, that's for sure, but not in the 12 hour deadline that you've given us. They last for extremely short periods of time, like 10 seconds or so, and only directly hit half of the planet. A sufficiently close gamma ray burst would result in serious damage to the atmosphere, perhaps instantly wiping out half the ozone layer, and causing nitrogen-oxygen recombination, thereby generating acidic nitrogen oxides. These effects could diffuse across to the other side of the Earth and result in long-term climate and atmospheric changes and a mass extinction. GRB's would not cause such an immediate extinction.





If the sun exploded, we might only last a few days before it got too cold to live on Earth, but not really as soon as 12 hours. Also, there's really no reason for the Sun to suddenly explode. A strong Coronal Mass Ejection (CME) could destroy our electronics, and possibly infect people close enough to the poles with radiation poisoning, but would never come close to completely killing everyone on the planet.





And a supernova would have to be extremely close (relatively) to kill everyone too. If its close but not too close, it would have the same basic results as a GRB.





So in conclusion, other than a collision between Earth and some other relatively large body, it seems unlikely that anything would kill everyone on the planet in half a day.|||Our sun going super-nova would do it.|||In order of probability;





Asteroid impact





Coronal mass ejection





Gamma ray burst





Any will ruin the day for most of us.|||Oh, one more, a supernova next door.|||To completely and, within 12 hours, annihilate life on Earth, it would need to be struck by an object 3/5 the mass of Earth in a direct blow.





There would be no defense or protection against such an object.|||if the earth collided with the sun|||If a black hole was close-enough to Earth, Absolutely NOTHING would survive.