"Touch the Invisible Sky" Chapter 5: Crab Nebula

A supernova is the dramatic end of a supergiant star's life. The Crab Nebula is the remnant of a powerful supernova which was visible from Earth in the year 1054. This supernova was so bright that it could be seen in the daytime sky for 23 days, and it was documented by astronomers throughout the Far East. The Crab Nebula is found in the constellation of Taurus and got its name because its outer shape roughly resembles a Crab's pincer.

Figure 14: The Crab - visible image. this image from Hubble reveals a chaotic nebula that is the remains of the outer layers of a once majestic star. the jagged outer edge of the nebula is still expanding outwards at 1500 kilometers per second (900 miles per second), almost a thousand years after the explosion occurred!

Figure 15: The Crab - infrared image. The Crab has a similar shape in infrared light. this is because the near-infrared emission is coming from the same places as the visible light. Visible and infrared emission is from the wispy clouds of gas in the nebula. The remains of the star's core are indicated by a dot in the center of the nebula.

Figure 16: The Crab - composite X-ray and visible image. Not only did something survive the supernova explosion, but whatever is left still powers the expansion of the Crab. Trace the jagged outline of the visible Crab. Now move towards the center of the image to find the X-ray emission, a tilted disk of high-energy particles surrounding a tiny point source called a pulsar. Emerging from the pulsar are two jets of sub-atomic particles (such as electrons) shooting away at the two and eight o'clock directions. The pulsar is a rapidly spinning neutron star no larger than an average city, but spinning on its axis 30 times a second. As the neutron star spins, it flashes or pulses its jets towards the Earth. this pulsar is the leftover core of the original supergiant star that exploded.

Figure 17: The Crab - radio image. Pulsars generate the most powerful magnetic fields in the universe. These magnetic fields capture sub-atomic particles in the nebula and whip them up to incredible speeds. As the particles such as electrons accelerate, they emit radio waves. The whole Crab Nebula is glowing in radio light, and the shape of the radio cloud matches closely the visible and infrared outline of the nebula.

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