What is the Crab Nebula?
In Messier's time, astronomers didn't know exactly what M1 was. All they could really tell what that it looks like a blurry star. The Crab Nebula is a big cloud of hydrogen gas that measures, as viewed from Earth, 4 arcminutes in diameter. To compare, the full Moon is 30 arcminutes across. 7,500 years ago the Crab Nebula was a red supergiant, thousands of times brighter than our sun. When the iron core of that star reached 1.4 solar masses, the force of gravity forced it to suddenly collapse into a 12-mile wide neutron star. The outer layers of gas began falling inward, rushing towards the center of gravity. As the gas fell it picked up speed, reaching about a fourth the speed of light. When that gas smashed into the neutron star, it could not compress anymore. Other than a black hole, nothing can be more dense than a neutron star. The falling gas was reflected backwards, making a huge explosion: a supernova. The star's outer layers were blown off, leaving a planetary nebula with a neutron star floating silently at its center.
In 1054 the light from this supernova reached the Earth. Ancient astronomers in China and the Middle East watched as a star suddenly became as bright as the full moon. For 25 days it was actually bright enough to be able to see during the day. When modern astronomers found accounts of this strange star, they pointed their telescopes where these ancient scrolls said to look and found the Crab Nebula.
The neutron star left in the nebula's center emits light across all wavelengths of light. Because of the shape of the star's magnetic field, it only emits this light from its magnetic poles. When a neutron star's magnetic poles and rotational poles do not align, it creates a type of lighthouse effect. From the Earth's perspective, we see a flash of light 2 times for every revolution. This type of pulsating star is called a pulsar. The flashes of light from pulsars are extremely regular, and used to be the most acurate way of telling time before more complicated methods were invented using radioactive decay. The Crab Pulsar is an interesting case in that it is one of the rare pulsars that emits visible light and is also one of the brightest sources of gamma rays in the sky. It has a diameter of 25 kilometers and rotates 30 times every second.