The Mysterious Heart of the Crab

The debris from a vast cosmic explosion, the Crab Nebula in Taurus is a well-known spectacle. But what is it and how was it formed?

Stardoom The nebula really does look like a vast cosmic explosion in this Hubble Space Telescope image (Image Credit: NASA, ESA, J. Hester, A. Loll (ASU))

On 4 July 1054 AD Yang Wei-T’e (?-?), astronomer to the Chinese emperor, carefully recorded a ‘guest star’ in the constellation of Taurus. Yang was puzzled by this newcomer to the heavens as it was unmarked in his charts and over the centuries none of his predecessors had mentioned it in their meticulous records.  The new star was brighter than Venus, bright enough to be seen in daylight for 23 days before it faded away. It was also observed by Muslim astronomers and in the Americas the Anazasi people of Arizona who depicted it in a rock painting.  Despite its brilliance in the sky at the time it went unrecorded in Dark Ages Europe.

Yang’s work was unknown outside of China until the 1940s so when John Bevis (c1695-1771) in England noted a misty patch in the same location in 1731 as Yang’s guest star he had no idea of its significance. Shortly afterwards Charles Messier (1720-1817) catalogued it among other comet-like objects as M1.

Almost 800 years after Yang, in 1844 Lord Rosse (1800-67) brought his telescope (not the famous ‘Leviathan of Parsonstown’ which was not completed until the following year), to bear on the faint nebula in Taurus. Rosse was unaware of Yang’s observations, but would have known this nebula had changed its appearance since it was first studied through telescopes in the Eighteenth Century. His sketches based on his observations made over several years showed the nebula to contain wisps and filaments vaguely reminiscent of a crab’s pincers. As a result this object is now known as the Crab Nebula. The Crab nebula lies about 6500 light years (2000 parsecs) from Earth but is a faint (magnitude 8.4), unimpressive object best observed on a moonless night.

Since Rosse’s time the Crab has expanded further, and based on this known rate of expansion (about 1500 km per second) its time of origin can be calculated with sufficient accuracy to determine that the Crab Nebula and the Chinese guest star are the same object.

Over the decades, astronomers have continued to observe the Crab and by the 1950s many were puzzled by some of its features. Rosse’s filaments were glowing red in a wavelength associated with hot hydrogen (not unexpected) while much of the rest of the nebula was emitting a pale blue. This blue light originates from high energy electrons spiralling at near light speed through an intense magnetic field, an effect known as synchrotron radiation. This was a surprise as the power source accelerating the electrons to such a fantastic speed was unknown.  What is more, the nebula’s rate of expansion should be slowing as it encountered resistance from the surrounding interstellar matter, yet the rate appeared constant implying that something is maintaining the expansion. Clearly, the Crab contained a powerful energy source (estimated to be 75 000 times as powerful as the Sun). As a result the nebula is a bright object at radio, X-ray and gamma wavelengths. In 1968 this mysterious power source was confirmed to be a pulsar, a rapidly spinning neutron star. To explain what a neutron star is, we need to look at how giant stars die.

An aging giant star ends not with a whimper but a bang, exploding as a supernova, perhaps the most powerful events in the Universe. In their final red supergiant stages, such stars are ticking timebombs. Deep inside the star is frantically trying to stay alive. It no longer has the plentiful hydrogen of its youth to fuse into helium; instead it is generating energy by ‘burning’ ever heavier elements. The star races up the Periodic Table until it is fusing silicon and sulphur into iron in its core. That is its doom. Fusing iron into heaver elements does not release energy, rather it absorbs energy. Like a house with its ground floor demolished, the star collapses and the outer layer fall inwards. As it implodes, the material from the star’s outer layers, still relatively rich in lighter elements ‘ignites’ in an awesome nuclear explosion. The star tears itself to pieces, blasting its material across space, enriching the interstellar medium with heavy elements including everything that makes up a human being. But what about the former star’s core?  The initial implosion has left it compressed to an incredibly dense state; a really large star would have ended up as a black hole, but the star which died to create the Crab Nebula was not quite that massive, probably it was not unlike Antares (about ten times the mass of the Sun). Instead it became a neutron star.

Neutron stars are formed in supernovae as the star’s core implodes. Neutron stars are about the size of a city yet are about twice as massive as our Sun. They are essentially balls of neutrons crammed tightly together. ‘Normal’ matter, the stuff which makes up you and me is made of atoms with nuclei of protons and neutrons. The atoms are finished off by shells of orbiting electrons. The electrons are comparatively distant from the nucleus, as a result any matter we are familiar here on Earth is mainly empty space.  This not so for neutron star material (sometimes called neutronium) where the atoms are so crushed together that the electrons have merged with the protons in the nuclei, forming more neutrons. Matter cannot be more compressed than this weird material. Compared to neutronium a block of lead is a mere wisp, an insubstantial ghost. A spoonful of this stuff would weigh about a billion tonnes or so.

The neutron star retains the magnetic field it has as a living star, but it is compressed into a smaller volume of space, leading to incredible magnetic fluxes. The Crab neutron star’s magnetic field is about a trillion times stronger than Earth’s! By a still unexplained mechanism, a neutron star generates awesome beams of energy from distinct regions near its surface (almost certainly its magnetic poles). As it spins, these beams may sweep across the Earth, revealing the pulsar’s presence to us, just as its rotating lamp reveals a distant lighthouse. This property has led to the term ‘pulsar’.

Ghost of a dead star. w This image from the Chandra X-ray observatory shows the Crab Nebula’s central pulsar surrounded by tilted rings of high-energy particles. Perpendicular to the rings, jet-like structures produced by high-energy particles blast away from the pulsar. The diameter of the inner ring in the image is about a tenth of a light year across. (Image Credit: NASA/CXC/SAO)

All that is left of the 1054 supernova is such a pulsar, a mere 30km (19 miles) in diameter. Pulsars are believed to slow down as they age, but the Crab pulsar spins rapidly, making it among the youngest known pulsars. At present, the pulsar is rotating thirty times per second but is slowing at a measurable rate, and it is the energy lost as it slows which illuminates the surrounding nebula. In the vicinity of the pulsar the nebula is a violent and dynamic environment, which changes over days. Hubble Space Telescope observations of the nebula around the pulsar have revealed evidence of waves of gas rippling away from the pulsar, possibly energised by the pulsar’s beam. Glowing a moody blue, swirling with flowing gases and crackling with energy, the Crab Nebula’s interior would be a perfect dramatic location for a showdown between science fiction dreadnoughts. In reality though, it would be an utterly lethal location to visit thanks to extreme radiation levels permeating the nebula.

This image of the Crab Nebula combines visible light (green, from the Hubble Space Telescope), radio waves (red, from the National Radio Astronomy Observatory) and x-ray (blue, from the Chandra Observatory). (Image credit: NASA/CXC/ASU/J. Hester et al, NASA/HST/ASU and NRAO/AUI/NSF)

The Crab still has surprises for astronomers; in April 2011 the Crab was seen by NASA’s Fermi satellite to emit a six-day long flare of high energy gamma rays. Further research with the VERITAS array in Arizona revealed gamma rays of higher still energies, approaching those of the particles accelerated by CERN’s Large Hadron Collider. These results are inexplicable by theories as they stand today.

An utter mystery to astronomers of old, today the Crab Nebula is still not wholly understood but is regarded as the classic example of a supernova remnant. Yang and all those other observers saw a mighty star die that day, If they had known the truth, I wonder what myths and legends would have been born that day..Even today there is still much about it that is unknown. On the millennium anniversary of Yang’s discovery in 2054 it will still be a source of fascination to stargazers around the world.