Rob Knop on Supernovae

Rob Knop, having decided to leave, has discovered that the SB Overlords will remove the archives of his blog sometime during the next couple months. Rather than let them vanish entirely into the memory hole like a factual comment on a creationist blog post, I thought I’d mirror a few here at Sunclipse. (In no way will Dr. Knop be responsible for answering comments on these mirrors, or anything like that; I’d just like to preserve the information.) First up is an essay from 10 October 2007, “Supernovae: the source of cosmic rays.” I have something of an interest in big stars going boom — check the SNEWS link over in the sidebar and say hello to my old colleagues — so I don’t want to see good explanations of this stuff slip away into the aether.


Astronomers have long assumed that supernovae are the source of at least most of the cosmic rays that hit Earth.

Woah, slow down… cosmic rays? Right, you hear the term all the time, but do you really know what they are? They are charged particles that rain down on Earth from space. Really! Kinda cool, huh? There are charged particles— mostly protons, or hydrogen nuclei, but with some heavier ions mixed in— smacking into our atmosphere all the time. Some of them have extremely high energies, higher energies than those to which we can accelerate particles in our best particle physics accelerators. Of course, the very highest energy cosmic rays are the rarest.

Thanks to a recent study by the Chandra Space Telescope, we have direct confirmation of the model that cosmic rays are produced in supernovae.

In fact, the Solar System is awash in charged particles, many of which come streaming off of the Sun. A “cosmic ray” proper, however, has a higher energy than most of the particles coming off of the Sun, and comes from outside the Solar System. The production mechanism for these cosmic rays is called the “Fermi mechanism,” and involves compressing magnetic fields in supernova remnants.

The path of a moving charged particle will be bent in the presence of a magnetic field. Indeed, magnetic fields can “capture” moving charged particles (both from the Solar wind and cosmic rays), causing them to spiral about it. We have bands of charged particles, the so-called “Van Allen Belts”, around the Earth; these are particles trapped in the Earth’s magnetic field. As the particles spiral along the fields, they crash into the atmosphere near the North and South Poles, where the magnetic field lines dip down into the earth. The result of the collisions of these particles with the atmosphere is what can be seen on earth as aurorae.

Supernova remnants have two things. First, they have strong magnetic fields; we’ve known this for a long time. Second, they have expanding gasses. In general, if you have a gas whose particles are partly charged, magnetic fields will move along with the gas. As the high-velocity gas in a supernova expands into the interstellar gas around it, you get shocks where the expanding gas collides with the ambient gas. You will also have magnetic field lines getting compressed, as the magnetic fields in the expanding gas plow in to the ambient magnetic fields out there.

Charged particles moving about the field lines of expanding gas will bounce back and forth between the expanding magnetic fields and the ambient magnetic fields. As the two field lines come closer together, the magnetic fields pick up energy. It’s similar to bouncing a tennis ball between two trucks coming towards each other. Each time the tennis ball collides with one track and bounces back towards the other, it picks up a bit of the kinetic energy of the oncoming truck, getting faster and faster and faster.

Normally, the charged particle would stay trapped in these strengthening magnetic fields (the compressing magnetic field lines) forever. However, there is enough junk there that eventually the charged particle— potentially moving quite fast now— will bounce off of something and get scattered out of the supernova remnant. At that point, it goes flying through the Galaxy as a cosmic ray.

The new observations show hot spots in X-rays appearing and disappearing in the shocks at the edge of a supernova remnant, which results from the sporadic production and release of the charged particles, some fraction of which will run into planets and be observed by the residents there as cosmic rays.

(Hat tip: Roger Amdahl of the Second Life “Astro News” group.)

5 thoughts on “Rob Knop on Supernovae”

  1. It’s sad to see Rob go, I usually loved his writing (other than his posts on religion, I could rarely find fault with them). The horror stories Rob and others have been telling about academic politics are part of what have made me weary about pursuing an academic research career (that, and the prospect of working insane hours for dirt pay for 8+ years before you’ll have any chance of landing a tenurable position). I’ll probably be doing something else. I’ll make sure to leave a note of appreciation at Rob’s blog for his writing.

  2. This brings a question to mind: does seem remarkably light on the physical sciences? They’ve got a powerhouse biology contingent, led by PZ Myers, but math and physics seem to get less weight. They’ve got Orzel, Sigurðsson and Chu-Carroll. . . anybody else? I mean, compare how much physics gets written about at SB with the throughput of Jacques Distler’s aggregator!

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