When Chinese and Arab astronomers watched the sky in the spring of the year 1054 A.D., they discovered a new star in the constellation of Taurus. According to their historical records, the "guest star" became brighter during several weeks and could be observed by July for 23 days even in the daytime. It remained visible to the naked eye for about two years.

Now we know that they observed the birth of the Crab Nebula by a gigantic supernova explosion. After millions of years of quiet evolution, a massive star had exhausted its supply of nuclear fuel, whose burning had provided the energy and pressure to stabilize the star against the pull of its own gravity. When the nuclear flame in its center died, the stellar core collapsed within fractions of a second to a neutron star, a compact object with more mass than the Sun but a diameter of only 20 kilometers. This neutron star is visible as the famous pulsar in the Crab Nebula, which sends periodic pulses of radiation as it spins around its axis 33 times per second.

Most of the star, however, was ejected in a violent explosion with an energy roughly equal to what the Sun has radiated in 5 billion years of its life. The hot stellar debris flashed up as the new star reported by the Chinese and Arab astronomers, and is nowadays visible as the filamentary gas cloud of the Crab Nebula measuring six light-years across and still expanding with a velocity of 1500 kilometers per second (Fig. 1). It contains not only the chemical elements which the star has built up in a sequence of nuclear burning stages — first fusing hydrogen to helium, then helium to carbon, and then carbon to neon, magnesium, and oxygen — but also material like radioactive nickel, which was freshly assembled during the explosion. The helium richness of the nebula and the low abundances of carbon and oxygen were interpreted as hints that the exploding star had a mass of only about 8 to 10 solar masses, just sufficient to end its life as a supernova.

But how did the star blow up? What was the reason why the star was disrupted? A group of researchers of the Max Planck Institute for Astrophysics is convinced that they have now found the answer of this long standing conundrum. Their refined computer simulations reveal that neutrinos are the driving force behind the explosion. These elementary particles are produced in huge numbers in the very hot and extremely dense interior of the newly formed neutron star, mainly by reactions of electrons and positrons with protons and neutrons, the constituents of atomic nuclei. Having made their way to the surface of the neutron star, most of these neutrinos stream off and carry away more than 99 percent of the energy liberated during the neutron star formation. Less than one percent of the neutrinos, however, is captured in the stellar gas surrounding the neutron star before being able to escape. The energy transfer by these neutrinos heats the gas and makes it boiling like the fluid in a pressure cooker (Fig. 2). The rising pressure finally accelerates the overlying stellar material and leads to the outburst of the supernova (see current research, february 2001).

Although this theory for the onset of the explosion is 25 years old, proving its viability with detailed computer models turned out to be extremely difficult (see current research, june 2003). Now at least for stars near the lower end of the mass range of supernova progenitors the models lend support to the theoretical idea. "With our refined description of how neutrinos are created and interact in the matter in the supernova core, we were able to confirm that neutrino heating indeed can drive healthy explosions of stars like the one whose relics form the Crab Nebula", says Francisco Kitaura of the team of astrophysicists who performed the computer simulations. The new models agree nicely with observations that the energy of the Crab explosion was only about one tenth of that of a typical supernova. Different from previous simulations they also predict only small amounts of ejected carbon, oxygen, and nickel (Fig. 3). Moreover, the strong enrichment of the chemical composition of the remnant with exotic elements is absent and thus a conflict of the older models with the observed abundances of rare elements in the Milky Way Galaxy. Since the disrupted star had a rather low mass and the explosion was sub-energetic with little production of radioactive material, other Crab-like supernovae must be expected to be fairly dim and therefore difficult to discover at great distances, although they could account for one third of all supernovae.

"Our computer models suggest that the Crab supernova was such a tremendously bright event only because it was just 6300 light-years from Earth," explains Wolfgang Hillebrandt, the leader of the research team. "Compared to other supernovae it actually was a fairly unspectacular case. Our computer models will tell us what we have to look out for in order to identify more such cases."

F.S. Kitaura, H.-Th. Janka, R. Buras, A. Marek, W. Hillebrandt

Reference:

F.S. Kitaura, H.-Th. Janka, and W. Hillebrandt, Explosions of O-Ne-Mg cores, the Crab supernova, and subluminous type II-P supernovae, Astronomy and Astrophysics, 450 (2006) 345-350

Further information:

Hans-Thomas Janka, thj mpa-garching.mpg.de