It’s in a nebula with the boring name of NGC-3372, and the un-boring name of Homunculus, maybe 7,500 light years from Earth. It’s called Eta Carinae. But you might call it the Death Star.
Luckily for us, it’s not pointed our way. We think.
It’s currently a million times more powerful than the Sun, but the output has fluctuated wildly in historic times, like a guttering candle. A major flare that started in 1843 lasted 20 years and made Eta Carinae the next brightest star in the sky after Sirius.
Eta Carinae is 872 times the distance of Sirius. That flare, alone, was as bright as a supernova.
Victorian-age astronomers looked and marvelled. But they didn’t really have the gear that we do. But get this – we can make up for that now. Some of that light was reflected off dust clouds about 85 light years beyond Eta Carinae, from Earth’s vantage point. That ‘echo’ bounced back, whipped past Eta Carinae – and is reaching us now. Let me say that again. We’re picking up light 170 years after we first saw it – by its reflection! Scientifically, that’s sub-zero cool.
Today the gas blown off in that event appears as a nebula surrounding a blazing blue-white star roiling in pre-hypernova mode.
There are a couple of theories to explain how a hypernova works. Both likely true in different circumstance, but the one that counts is the ‘collapsar’ model. High-mass stars fuse hydrogen around their core at a phenomenal rate. After a while the star starts fusing helium into lithium, and so on up the periodic table until it reaches iron. That’s stable and can’t fuse. So it cools. And then – if the core has more mass Chandashekar’s Limit, extreme things happen, faster than any human could perceive.
First, the core of the star collapses at light-speed into a black hole. A reflected blast wave erupts at light-speed. The advancing wave intersects with the stellar matter to produce ultra-energetic radiation at gamma frequencies, focussed by the star’s magnetic field and squirted in a beam down its polar axes. Meanwhile, the main blast, following at a more sedate fraction of light-speed, rips the star apart with a glare that can out-shine whatever galaxy the star is inhabiting.
What’s gamma radiation? It’s at the sharp end of the electromagnetic spectrum. Frequencies start at 10,000,000,000,000,000,000 hertz and go up, which gives a gamma burst a wavelength smaller than the diameter of an atom. Gamma ray bursts are among the most energetic phenomena observed. We can detect them in galaxies billions of light years away.
If Eta Carinae blew and its GRB hit us, it’s been calculated that would deliver energy equivalent to one kiloton of TNT in energy for every square kilometre of the hemisphere facing the radiation. Ouch.
Probably it’s not pointed our way, but if Earth was hit by a burst approaching that intensity, we’d know. A lot depends on energy and duration. It is unlikely a burst would reach ground level; air is a shield against gamma and x-rays. But part could penetrate at ultraviolet frequencies and burn anybody under it.
The larger problem is gamma radiation hitting the upper atmosphere – smashing the ozone layer, at least. Potentially, a GRB could create nitrogen dioxide smog – plunging Earth into darkness. While this would occur only on the side struck by the beam, high-altitude winds would spread the effects. And we’d get rain. Nitric acid rain. Here’s the paper analysing the biological effects.
The thing is, while we think Eta Carinae isn’t pointed at us, the Wolf-Rayet star WR-104 might be.
Do I lie away at night worrying? No. Gamma ray bursts are rare. Really big ones happen in the order of once every 100,000 years in the galaxy. Their effects occur in a cone spreading from the poles of the exploding star. The chance of one hitting Earth is remote.
Still, there is a theory that a GRB might have caused the Ordovician-Silurian extinction event about 450 million years ago. Another theory suggests a burst hit us in the eighth century – it didn’t have enough energy to smash everything, but may have caused climate change
Realistically, for us, there are more likely things that will knock off humanity.
Meanwhile, we can look up at the skies and marvel at the wonders of extreme physics we’re discovering…out there.
Copyright © Matthew Wright 2013