Composite image of the Crab Nebula using different wavelengths of light. [Credit: X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA-JPL-Caltech]

The first entry in Charles Messier's catalog of nebulous objects is M1, the Crab Nebula in the constellation Taurus. In 18th-century telescopes it was no more than a fuzzy patch. Yet imaged by the Hubble Space Telescope the Crab Nebula is fascinating and intricate. What is it? Why is it called the Crab Nebula? And what is the amazing secret hidden in its center?

The nebula
The nebula is around three times the size of our Solar System, and it's expanding at the rate of 1500 km per second (900 miles/sec). It's a long way off – 6500 light years – but it's a bright object in a wide range of light wavelengths. Here you can see four examples.

The Crab Nebula can't be seen without a telescope, and it wasn't discovered until the eighteenth century when telescopes were in widespread use. English astronomer John Bevis found it in 1731, and it was independently discovered by French comet-hunter Charles Messier in 1758. In the following century, improved telescopes meant that some structure was becoming visible. When Irish astronomer William Parsons (Lord Rosse) observed it in 1844, he thought there was something crablike about it. Yet four years later with a much bigger telescope he didn't see any resemblance to a crab – nor has any observer since. But the name stuck.

In the twentieth century the Crab Nebula was recognized as a supernova remnant.

The supernova: SN 1054
When a massive star runs out of fuel, its core collapses dramatically, setting off a stupendous explosion. For a time, it's brighter than an entire galaxy – this is a supernova. The supernova remnant is an expanding gas cloud created in the explosion from the outer layers of the original star. In the Hubble image there are filaments around a blue central region. The filaments are what's left of the material from the star's outer layers.

Almost a thousand years ago Chinese astronomers recorded a “guest star” in the area of sky where we see M1 now. It was visible in the night sky for about two years from the original sighting. But even more spectacularly, if we had a time machine that would let us visit the summer of 1054, we could see the supernova during the day. The Chinese records are the most comprehensive, but we also have references to this strange star from Japan and from the Middle East. In addition, there is artwork done by native Americans in the southwest which may refer to it.

M1 is in the right part of the sky to be the remnant of SN 1054. An understanding of supernovae plus the observed expansion rate of the nebula let astronomers work backward to the explosion. This provides a convincing time frame. The Crab Nebula was the first celestial object to be linked to a historical celestial event.

Yet there must be more to the story than the nebula. If there was a supernova, where is the collapsed core of the exploded star?

The pulsar
The core of the star became a neutron star. The collapse was so complete that even the atoms it was made of collapsed. Orbiting electrons joined with protons in the nucleus to form neutrons. The neutron star has a mass greater than that of the Sun, yet it's only about the size of a city. A teaspoon of its matter would weigh several million tons.

The neutron star in the Crab Nebula is a pulsar. A pulsar is a neutron star which has a powerful magnetic field, spins rapidly, and emits a beam of radio waves. Like a lighthouse, each time the radiation beam points our way we can detect a pulse.

The very first pulsar was discovered by Jocelyn Bell and Antony Hewish at Cambridge University in 1967. Before that, neutron stars were merely an exotic theoretical construct. The discoverers of the first pulsar had no idea what they had found. They tried unsuccessfully to link the strong regular pulses to various kinds of interference. Before the pulsing object was identified as a collapsed star, the discoverers even considered the possibility of a distant alien communication beacon, jokingly calling it LGM-1, with LGM standing for “little green men”.

In 1968 astronomers found a pulsar that was somewhere in the general direction of M1. This "lighthouse" was pulsing an impressive 30 times a second. It was intriguing, but it's difficult to identify a radio source optically. Nonetheless the Crab Pulsar was found optically in the center of the nebula. It was the first pulsar to be identified through a visible light pulse. Even now, although there are around two thousand known pulsars, only a small fraction have an optical light pulse.

Supernovae and their remnants
What do the following have in common? Gold jewelry; silver for water purification and in surgical masks; copper wiring for your electrical needs; semiconductors for your computer, tablet or smartphone; iodine for your thyroid gland to work properly. All of these products – and many more – use heavy elements that are made in supernova explosions.

The expanding supernova remnant ejects the heavy elements into space, where they may then be incorporated into new stars and planets. And not only does the supernova remnant supply heavy elements, but its associated shock wave is also a trigger for new star formation. This is recycling on a grand scale.