The Crab and the Jellyfish

by Nicholas Mee on July 22, 2017

Fritz Zwicky

Fritz Zwicky was a Swiss astronomer who worked for most of his career at Caltech (California Institute of Technology) and the Wilson and Palomar Observatories. Zwicky was a creative and original thinker who ignored fashionable trends and pursued his own ideas.  He referred to himself as a lone wolf. He certainly had a rather misanthropic streak that left him with little sympathy for less talented colleagues, who he described as:

sycophants and plain thieves who have no love for any of the lone wolves who are not fawners and apple polishers,’ they ‘doctor their observational data to hide their shortcomings and to make the majority of the astronomers accept and believe in some of their most prejudicial and erroneous presentations and interpretations of facts’ and this is why they publish ‘useless trash in the bulging astronomical journals’.

His ultimate contempt was reserved for those he referred to as ‘spherical bastards’, because they were ‘bastards whichever way you looked at them’. It is perhaps no surprise that Zwicky had few academic friends. Justified or not, his tirades did not help his valuable insights to gain acceptance and his work was slow to receive the credit it deserved. Some of his imaginative ideas were mistaken, but more often he was on the right track decades ahead of the pack. He discussed gravitational lensing, dark matter and neutron stars in the 1930s. One of his most important breakthroughs relates to exploding stars.

Guest Stars

Tycho’s chart showing the new star of 1572 labelled I. Stars FEDBG form the familiar ‘W’ of Cassiopeia.

A star that appears, as if from nowhere, is known as a nova, meaning new star. Gradually the nova fades and disappears again. Such outbursts have been observed by astronomers for thousands of years. We now know that they are produced by gigantic nuclear explosions on white dwarf stars. (There is more about white dwarfs in my post: Diamonds in the Sky.) Some white dwarfs regularly produce brilliant outbursts such as RS Ophiuchi which flared up in 1898, 1933, 1958, 1967, 1985 and 2006.

The historical records also include very rare accounts of extremely bright stars that have suddenly appeared, such as a new star witnessed by Tycho Brahe in 1572 and one studied by Johannes Kepler in 1604. Several of these are recorded in the Chinese annals where they are referred to as guest stars. The brightest such guest star appeared on 1st May 1006 in the southerly constellation of Lupus (The Wolf). It was recorded all around the world; accounts survive from Japan, China, Iraq and even the monastery of St Gall in Switzerland. For several months it was so bright that it was visible during the daytime, and the Egyptian astronomer Ali ibn Ridwan recorded that it was comparable in brightness to a quarter Moon. Guest stars such as these are much brighter than ordinary novae. But by 1930 none had been seen for well over 300 years.

The Vastness of the Universe

Detail of the Hubble Ultra Deep Field. Every object in the image is a separate galaxy. Credit: HST/NASA.

The true scale of the universe only became apparent in the 1920s. Galaxies that were previously thought of as relatively nearby gas clouds were found to be vast collections of stars millions of light years distant. Our Milky Way was relegated to just one of hundreds of billions of galaxies in the universe.

Occasionally novae had been seen in other galaxies, such as one that occurred in the Andromeda galaxy in 1885 (then known as the Andromeda nebula). Zwicky and Walter Baade realised in 1931 that if these novae were really in galaxies millions of light years distant, they must belong to the rare class of superbright stellar outbursts like Tycho’s star. They labelled such spectacular events supernovae and argued that, as they were so much brighter than novae, they must have a different and even more violent origin. They soon found examples in other distant galaxies and instigated systematic searches for more.

Supernova SN1994D on the outskirts of galaxy NGC4526 about 50 light years away. The supernova is as bright as the core of the galaxy, which could contain 100 billion stars. Credit: Hubble Space Telescope, NASA.

Cosmic Fireworks

Zwicky and Baade began their studies of supernovae just before the birth of nuclear physics when our understanding of stars was in its infancy. We now know that stars generate energy through nuclear fusion reactions. For most of a star’s life these reactions steadily convert hydrogen into helium in the star’s core and the energy this releases supports the star against its tendency to collapse under gravity.

Very massive stars burn their nuclear fuel at a prodigious rate and when the hydrogen in their core runs out, the core contracts and the temperature rises until a new round of nuclear reactions is triggered. Helium is converted into carbon and oxygen, and in the most massive stars this is followed by fusion processes that create the nuclei of heavier atoms such as neon, magnesium, silicon, sulphur and iron. But eventually no further new nuclear reactions are possible and in stars of several solar masses the final collapse begins, this liberates huge amounts of energy, and the star blasts itself apart in a supernova explosion.

When the smoke clears the core of the star may have been transformed into an object around 30 kilometres across — the size of a major city — but with the density of an atomic nucleus. These weird objects are known as neutron stars. In even bigger stars the collapse of the core doesn’t stop. The result is a black hole.

The Jellyfish Nebula, known more formally as IC 443, is the remnant of a supernova that would have been seen several thousand years ago. (c) Alessandro Falesiedi

The most famous supernova remnant is the Crab nebula, which is the highly energetic remains of a supernova that was seen in 1054. The stunning image above shows a much older supernova remnant known as the Jellyfish nebula. The Crab and Jellyfish nebulae are both known to contain a rapidly spinning neutron star, which is all that is left of the original star that exploded.

Remnant of the supernova of 1006. This false colour image combines data from a wide range of the electromagnetic spectrum, X-ray data in blue, optical data in yellow and radio data in red. Credit: NASA, ESA, Zolt Levay (STScI)

There is another important class of supernovae that results from a different type of event, a runaway thermonuclear explosion of a white dwarf triggered by the detonation of large amounts of material accumulated from the outer layers of a companion star. These enormous blasts are thought to completely obliterate the white dwarf leaving nothing but a rapidly expanding cloud of material stuffed with the heavy elements cooked up in the explosion. They include the brilliant supernova of 1006, the remains of which were tracked down in 1965. All that is left is an extremely hot gas cloud that is expanding at around 10 million kilometres an hour. It is now about 60 light years across and lies at a distance of 7000 light years from us.

Your Gold Ring

It is an amazing fact that all the atoms in your body, other than the hydrogen atoms, must have been created within a star. It is also rather incredible that all the atoms in the gold ring on your finger were created in a supernova explosion.

Further Information

For more information about the synthesis of the elements in the stars and supernova explosions see my book Higgs Force: Cosmic Symmetry Shattered.

There is more about the remarkable lives and work of Tycho and Kepler in my book Gravity: Cracking the Cosmic Code.


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