The Crab Nebula

The Crab Nebula

The Crab Nebula is one of the most interesting objects in the sky. The nebula, discovered by John Bevis in 1731, is a supernova remnant in the constellation of Taurus and is listed as M1 in the famous catalog of Charles Messier.

The supernova SN 1054 generated the Crab Nebula

This nebula is about 6 light years wide and is expanding at a speed of 1,500 km/s. Thanks to observations that have taken place over the years, it has been possible to extrapolate the presumed date of the explosion of the supernova, estimated in the eleventh century.

In fact, the Chinese recorded in their astronomical annals the appearance of a star so bright that it could be visible even during the day, even exceeding the brightness of Venus, on July 4, 1054. Today, this supernova is listed as SN 1054, and remained visible for 23 days and 653 consecutive nights. The supernova almost certainly was type II.

What does this mean? Type II supernovae arise when massive stars, at least nine times the mass of the Sun, meet the final stages of their life. Once the main fuel, hydrogen, is exhausted, these stars begin to use helium, forming increasingly heavy elements up to the iron-56.

The Crab Nebula

At this point, the energy curve reverses, and melting the iron-56 would require too much energy, so the star does not go any further with the formation of heavier elements but begins to accumulate the iron-56 produced within the nucleus. When the latter reaches the critical mass of 1.44 solar masses, known as the mass of Chandrasekhar, the final explosion takes place. The outer layers are expelled violently, dispersing the chemical elements produced in the last phase of the life of the stars and producing new ones, through a neutron capture mechanism. Indeed, neutrons, having no electric charge, can penetrate more easily into nuclei of atoms than protons.

What remains after the explosion of a supernova?

After the explosion of a type II supernova, a pulsar remains in place of the star. Pulsars are neutron stars that rotate rapidly on themselves, emitting radiation into very narrow cones. The classic metaphor is that of the lighthouse. These objects are extremely particular: think of the mass of the Sun confined within a sphere of 20 km in diameter. Density is very high, and neutrons are so close to each other that significant quantum effects also come into play, but they are outside the scope of this article.

Also inside the Crab Nebula there is a pulsar, listed as PSR B0531 + 21. It was discovered in 1968 thanks to the Arecibo Radiotelescope. The pulsar about 30 times per second emitting in the X-ray band 100 times the radiation that emits in the visible. Where does all this energy come from? It comes from the rotational kinetic energy of the pulsar, and this implies that, by emitting radiation, the pulsar over the years slows its rotational motion very slowly.

The Crab pulsar is very important for radio astronomy: having a constant X-band flux density and being very luminous, it is perfect for calibrating the X-band sensors. Furthermore, it is one of the very few pulsars that can be observed even in the spectrum of visible.

Physical characteristics of the Crab Nebula

The Crab Nebula is formed mainly of ionized helium and hydrogen, along with small percentages of carbon, neon, oxygen, nitrogen, sulfur and iron. The temperature of these gases is between 11,000 and 18,000 K. The nebula would have an estimated mass of about 4.6 ± 1.8 solar masses, which together with the mass of the central pulsar would give a total of between 6 and 9 solar masses. According to a research proposed in 1953 by Iosif Sklovskij, the blue region would be generated by synchrotron radiation, ie by electrons forced to move at relativistic speeds along curved trajectories due to the magnetic field produced by the pulsar. Three years later, the theory was confirmed.

The Crab Nebula

The Crab Nebula

Image of the Crab pulsar obtained by combining the data from the Hubble and Chandra space telescopes.

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