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.
Another excuse (as if we needed any) to enjoy the spectacle of a thunderstorm: getting a few extra gamma rays for one’s health.
More interesting reading:
Lightning-produced radiation a potential health concern for air travelers December 7, 2009
“If an aircraft happened to be in or near the high-field region when either a lightning discharge or a TGF event is occurring, then the radiation dose received by passengers and crew members inside the aircraft could potentially approach 10 rem in less than one millisecond,” the paper says.
Ten rem is considered the maximum safe radiation exposure over a person’s lifetime. It is equal to 400 chest X-rays, three CAT scans or 7,500 hours of flight time in normal conditions. All airplane passengers are exposed to slightly elevated radiation levels due to cosmic rays.
A new study suggests that GRBs could cause mass extinctions, and could be responsible for “the Ordovician extinction, which occurred around 440 million years ago… .”
The study authors say such an extinction could be caused by a GRB 5,000 light years away. Well, photons from the Crab Nebula, which is 5,600 light years away from earth, reached earth in 1054. From my study of history, I cannot recall a mass extinction occurring around 1054. Maybe I’m misinterpreting the study, but I wonder if the Crab Nebula provides evidence against the study’s hypothesis.
I think that ordinary supernovae and GRBs are not the same thing — the latter are much rarer and far more powerful.
Apparently the explanation for the big difference is simply that the polar jets from a supernova must be pointing directly at the earth for it to appear as a GRB.
If GRBs are associated with axial jets from such collapses rather than being omnidirectional, an explosion in the Crab would have likely been aimed elsewhere so not been a threat to us.
Check out this NASA video: http://youtu.be/JgK4Ds_Sj6Q
And the associated article posted yesterday:
Given that all radioactive elements decay (and become less hazardous) by decaying into less “aggressive” elements or isotopes at a given rate that we call their half-lives, it’s therefore a given that moving back in time takes us into times when natural radiation was far higher than at present. Move back a few billion years to a time when early life was beginning, and you get a hint of the fact that our “ancestors” must have developed robust defenses against irreparable cell damage. That defense mechanism – there might have been others – still exists in our bodies’ hugely efficient ability to repair radiation and oxidation-caused damage to our DNA. We now know that DNA strands break and repair some 10,000 times per
day per cell, and MIT researchers observed that 100 mSv/y radiation dose rates increased this number by only 12 per day. The huge majority of breaks are caused by ionized oxygen molecules from metabolism within the cell. See http://energyrealityproject.com/nuclear-power-climate-change-warrior-for-the-21st-century/
Wish FUD-busting articles like this could get out the mass media!