On the Cover

The stunning image on the cover, reminiscent of Sauron’s eye, is M1, the Crab Nebula. M1 is located in Taurus, about one degree to the right of Zeta Tauri, the star at the tip of the bull’s left horn. The crab nebula is the remnant of a super-nova that blew its guts all over the sky on the rather auspicious date of July 4th, 1054 AD. At a distance of 6500 light years, that explosion appeared to Chinese observers as a new star, roughly as bright as Jupiter. Indeed, it was visible during the day! Over the next six months it slowly faded from naked-eye view.

The cover image is a composite of visible and X-ray light. The visible image was taken by the Hubble telescope and forms the outer envelope. The inner object that looks like a blue archery target was taken by the Chandra x-ray telescope.

The visible image depicts a rapidly expanding cloud of dust and gas laced with heavy elements left over from the supernova explosion. That cloud is now 11 light-years in diameter, weighs in at 4.5 solar masses, and is expanding at the furious rate of 1500 kilometers per second. The kinetic energy of that old explosion is impressive to say the least.

At the very center of the target is a bright blue dot. That’s where the pulsar is. It was the formation of the pulsar that caused the star to blow up in the first place. Nearly a solar mass of material in the core of the doomed star imploded into a sphere of neutrons about 30 kilometers in diameter. The kinetic energy of that implosion, coupled with the incredible barrage of neutrinos created when all those neutrons formed, ripped the star open, and blew it to kingdom come.

The pulsar is spinning about 30 times per second; and it flashes as it spins. We can see it blinking in our telescopes. Those pulses of light are the reason we call it a pulsar, which is short for Pulsating Star.