[...] and Deadly Radiation Last year, there was some excitement that our Solar System was "looking down the barrel of a gun". The gun in question was an unstable and violent star called WR 104 and the barrel was the [...]

[...] was growing for a Wolf-Rayet star named WR 104 that appeared to be aiming right at Earth (see Looking Down the Barrel of A Gamma Ray Burst). A Wolf-Rayet star is a highly unstable star coming to the end of its life, possibly culminating [...]

Possible … but then how do we know that on every such incident life was not wiped off (partially or completely)

Essel – the current consensus is that the opening angle of the beam is a few degrees. So, not an extremely tight beam, but still pretty small. So the ones we see we are really looking down the barrel, but they are so bright that most of the ones we see are hundreds of millions to billions of light years away.

The Earth may get blasted once every billion years or so.

The Tunguska event was definitely an asteroid, I don't think there's any disagreement about that. A GRB would certainly not flatten a forest – mostly it ionizes the atmosphere, destroying ozone and leading to really bad sunburns!

But! The GRB itself is a blast of gamma rays, not visible light. However, the gamma rays would interact with the atmosphere to down-scatter and probably produce some visible light, kind of like how sunlight scatters and makes the sky blue. There is an "afterglow" that includes optical light, but it's much less powerful that the gamma part.

So, it would be much dimmer than daylight, but still could be pretty bright, especially compared to other night-time objects.

In the 4.5 Billion lifetime we must have been down such barrel several times.

Could Tunguska be such an incident?

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do you have an estimate how bright that explosion would be.

Will it turn night into day?

The two stars are not actually going to collide. Instead it is the implication of this face-on orbiting system that is the potential threat. The colliding winds from the two stars create 'focused streams of gas that move outwards, and as the system rotates it looks like a spiral.

1. The lack of 'tilt' in the spiral indicates the orbits are face-on here.
2. A face-on orbiting system likely means their poles point approximately straight towards us Likely, but not necessary.
3 Gammaray bursts are suspected to come from some types of supernova, being shot out from the magnetic poles – magnetic poles often align somewhat with the rotational poles – thotugh some stars seem to have really large magnetic tilts.
4. Both stars are WR.stars, expected to go supernova – one after the other. And WR-stars dont have long before they end their life, they are expected to go boom quite soon (in astronomical timings)

So this is a possible threat, but there are still alot of uncertainties in this scenario.

Blazars also put out jets at almost relativistic speeds (within a small percentage of the speed of light).

If such a jet from a blazar were to be heading our way, we wouldn't know about it until very very shortly before it struck our solar system.

At 8000 ly there would definitely be a measurable effect and maybe some impact on the biosphere, but probably not at mass extinction levels. Still, wear your sunscreen!!

You can read the details of our work at the sites below and others. There was also an episode about GRBs on the program "Mega Disasters" which aired on the History Channel.
http://www.nasa.gov/vision/universe/starsgalaxies/gammaray_extinction.html
http://arxiv.org/abs/astro-ph/0505472

No. Whatever has happened to this star in the 8000 years since the light we see left it, is in our future, and cannot be observed until then. Look up 'light cones' in the Wiki articles on relativity.

Unfortunately, any lensing would apply to the gamma ray burst as well. If the axis appears to be pointed at us when the system goes supernova, a cone of super-intense gamma rays will be ejected in our direction, following the same gravitational contours as the light we've been viewing.

As for "how do we know that it didn't go supernova 7999 years ago…" well, that's a fair question. We don't–because it takes 8000 years for that light to reach Earth.

Thing is, this is just one case that is easy to see, and obviously dicey. For all we know, something much closer could have gone bang about six thousand years ago, before the Chinese were keeping dutiful astronomical records, and we could even now be facing a shower of deadly gamma rays speeding towards us. There is no way to tell until they get here, and by then, it's too late.

But the odds are much higher that most life on Earth will be wiped out by a collision with a high-speed Near Earth Object long before then. So don't lose any sleep over the gamma rays. Earth has many other dangers to face from space… provided we manage to avoid killing ourselves even faster with pollution and overpopulation, which is by no means certain.

The Universe tends to be an "all bets are off" kind of place, and because it's so scary, and because our technology is woefully insufficient to protects us from most of the dangers out there, it's just easier to say "well, if it happens, it happens… so what's for lunch?"