For the next six months or so, the most prominent and visible asterism (star pattern) in the northern-hemisphere sky will be Orion. I find Orion much easier to spot than even the Big Dipper: it travels roughly along the ecliptic through most of the fall, all of winter, and most of spring. In fact, I find the Pleiades Cluster easier to spot than the Big Dipper, even in the city. Maybe that is because the stars in Orion’s belt point to it. Most people who look at Orion don’t realize they’re looking at not only a Giant Molecular Cloud that contains carbon monoxide, but also the source of some of the most powerful gamma-ray flares ever recorded.
This source is the Crab Nebula, the aftermath of a supernova explosion first noticed here on Earth by Chinese astronomers in 1054. This object, also known as M1 (or Messier 1, after Charles Messier, an 18th Century French astronomer who set out to catalogue all the objects in the night sky that were not comets), is Number One on NASA’s list of top ten gamma ray sources in the universe.
The energies associated with the gamma photons from the Crab Nebula are more than astronomical; they’re cosmological. The space.com article I linked to says they are a “thousand trillion” times greater than the energies of visible light. Visible photons have energies roughly in the 1 electronvolt (eV) range. Gamma photons have energies starting at tens of thousands of eV; there is no upper limit on gamma energy. For example, the gamma photons associated with the electron-positron annihilations that are the basis of PET scans have energies of 511,000 eV. So for the gammas from the Crab Nebula to have energies in the order of a thousand trillion (1015) eV… that is very high energy.
The Crab Neblula is just one of nearly 2,000 extraterrestrial gamma sources detected by NASA’s Fermi Gamma-ray Space Telescope, an orbiting device that was launched in 2008.
The sheer number of these sources suggests that outer space is a rather radiant environment. But the earth is too, and I’m not just talking about the radiations arising from the roughly 46 trillion metric tons of uranium estimated to be in the planetary crust. Fermi has detected numerous terrestrial sources of gamma photons that cannot be associated with uranium and the other elements in its decay series. In 2010, the telescope, orbiting Earth just above Egypt, picked up positrons from a thunderstorm in Zambia, which was over the horizon. These antimatter particles cascaded upward through the atmosphere into space, passed Fermi, bounced off a magnetic “mirror point” which reversed their direction, then passed the telescope again, where some of them collided with electrons and annihilated, producing gamma photons which were picked up by Fermi’s gamma detectors. This chain of events was all deduced from the energies of the gammas: 511 keV, the telltale energy of electron-positron annihilations. (You can read NASA’s absolutely fascinating account of this event here.)
This provided conclusive evidence that thunderstorms, which occur in the troposphere where all weather happens and where most of us humans live, produce gamma ray flashes. Our built-in photon detectors, a.k.a. eyes, cannot see these flashes; gamma photon energy is at too small a wavelength. We can only see light in the visible spectrum, which is at wavelengths in the range of 10-6 meters (one millionth of a meter); such wavelengths correspond to energies in the range of ~1 eV. We cannot see these Terrestrial Gamma-ray Flashes (TGFs), because their energy, 511 keV, corresponds to wavelengths on the order of 10-12, or one trillionth of a meter; to be precise, 2.428 trillionths of a meter. Because we can’t see TGFs, they are known as Dark Lightning. If you have experienced thunderstorms, then you have probably been the recipient of a lot of gamma photons in your lifetime; you just didn’t know it.
With all the gamma photons whizzing through space and our planet’s atmosphere, you might wonder how any life ever developed on Earth. Gamma photons have so much energy that they can knock electrons out of the atoms they collide with. The U.S. EPA warns that gamma radiation is “the primary hazard to the general population during most radiological emergencies.” How does this square with the fact, noted by the Health Physics Society, that of the roughly 5,000 decays of potassium-40 per second occurring in the body of a 70-kilogram human, ten percent—500 decays per second—involve the emission of gamma rays? These photons come with an energy of 1.46 million eV.
Clearly, the “radiological emergencies” the EPA refers to must involve gamma-emitting activities of much greater than 500 becquerels. The HPS says that all sources of radioactivity in our bodies, including potassium-40, give the average man (70 kilograms) an effective dose of 0.3 millisieverts. That is a full mSv less than the average background effective dose in Vancouver, Canada, and less than one-thirteenth that in Winnipeg.
At the cockamamie end of the spectrum, the Sierra Club of Canada, in a typically error- and innuendo-laden litany of reasons to be very, very afraid of anything remotely associated with nuclear energy, says this:
The level of gamma radiation inside an operating irradiation facility is anywhere from ten to hundreds of times the level that would kill a human in a single short exposure.
—Sierra Club of Canada, 2002
Woo-woo… scary. Until you realize you could say that about a house furnace or car engine, or even about the engine on one of the airplanes that Sierra Club officials take to international climate conferences.
Nevertheless, cockamamie anti-nuclear nonsense aside, gamma radiation, like many other substances and phenomena, can present a hazard if it is of sufficient intensity. That of course also explains why we humans, and many other life forms on this planet, have managed to survive the incessant gamma bombardment from terrestrial and extraterrestrial sources. Though the bombardment has indeed been incessant, and has come at us from all angles, it just has not been sufficient to prevent us from surviving and thriving on the planet.