Supernova
Overview
Supernova is a violent, explosive death of a very massive star.
Nova is from the Latin word “novus” which means new, unusual or extraordinary. In 1572, the Danish astronomer Tycho Brahe used the term “nova stella” (new star) when he noticed a bright new star, as bright as Venus, suddenly appearing in the constellation Cassiopeia and then slowly fading away.
So, Nova refers to a newly appearing star with sudden increase in brightness for a few days and then slowly fading away. Astronomers initially thought that the event must be located far away from other stars. And, therefore, this phenomenon was named supernova. The plural of supernova is supernovae.
Now, let us explore what supernovae are, how they occur, and what are their types.
Evolution of Stars
First, let us make some groundwork on how stars evolve over time and then see how supernovae occur. Later, we will discuss types of supernovae. The figure below shows how stars evolve from birth to death depending on their mass – the mass of a star controls its destiny.
As you can see in the diagram above, Sun-like stars end their lives as white dwarfs while massive stars, after supernova explosion, end up as neutron stars or black holes depending on their mass.
Sun Like Star
becomes White Dwarf
For instance, our Sun, which is currently in the Main Sequence stage, is burning hydrogen in the core producing helium as ash. But when it runs out of hydrogen it will start burning helium in the core producing carbon and oxygen. Once all the helium is used up in the core, because its mass is below the Chandrasekhar limit, the Sun will become a red giant first and then turn into a white dwarf.
The death of stars like our Sun will be silent and peaceful. However, the end story of massive stars is completely different: most violent.
Massive Star
Goes Supernova (Type II)
Massive stars, with mass 8M☉ (M☉ is the mass of our Sun) or more, are able to take the nucleosynthesis process beyond carbon and oxygen. Such stars can continue synthesizing elements up to iron in the core.
Even these massive stars cannot continue the nucleosynthesis beyond iron because iron’s nucleus is, not only, very stable but also requires tremendous energy to fuse two iron nuclei. So, iron is the dead end for all these massive stars. In other words, transmutation of elements stops in the core when the core fully becomes a huge iron ball.
The Blow Out
As fusion stops in the iron core, there is no outward radiation pressure from the core to counter the crushing force of the outer layers. At this point, the gravitational force triumphs, and it literally crushes the iron core into its constituent atomic particles. The solid iron core disintegrates into electrons, protons and neutrons.
Upon further squeezing, the zipping electrons in the core reach the speed of light and start the neutronization process – electrons get pushed into protons to create neutrons. The entire core, as a result, turns into a neutron star, consisting of only neutrons, releasing a burst of neutrinos. Now, the core is a giant nucleus with no ‘empty space’ in it. At this stage, it is impossible to compress the core any further.
What happens next is more dramatic. As the matter in the core reaches nuclear density (a teaspoon of neutron star material weighs about a billion tons!!), the size of the core becomes very tiny. Therefore, the core implodes in a quarter of a second. As the core suddenly collapses, the outer envelope too falls onto the core’s surface at the speed of 40,000 km/sec. But the falling outer envelope encounters a hard wall, and it is stopped dead on the surface of the core. This creates such a massive shockwave that it results in a violent and most spectacular explosion.
This explosion is the Type II supernova.
During the supernova explosion, a star with mass of 25M☉ could eject 24M☉ of its mass as gas clouds and leaves the rest (1M☉) in form of the neutron star. The ejected gas clouds can span several light years across. For example, the Cassiopeia Supernova A remnant, shown in the picture 1 above, is 29 light years across and it is about 11,000 light years away from the Earth.
Types of Supernovae
Astronomers classify Supernovae into two broad classes – Type I and Type II.
Type I supernova is an explosion of a white dwarf whereas Type II supernova is a violent explosion (and death) of a massive star at the end of its life cycle.
The table below provides a quick view of the two major types of supernovae.
Type I
White Dwarf Explodes
Type I supernova occurs when a white dwarf explodes. This explosion does not accompany neutrinos emission. Type I supernovae are only observed in population II stars.
Population II consists of old stars which were present at the formation of galaxies without much heavy metals. Stars between 8M☉ and 10M☉ can explode as Type I supernovae, energy being supplied by combustion of carbon.
Type I Mechanism
Chandra Limit Violated
This involves a binary system consisting of a white dwarf and a Main Sequence companion star which is close by. The mass of the white dwarf is assumed to be just below the 1.44 M☉ (Chandra limit). The nearby star provides matter to the white dwarf whose mass, at one point, exceeds the Chandrasekhar limit. Because of the violation of Chandrasekhar limit, the white dwarf instantaneous gets transformed into iron, and results in sudden explosion.
Another model describes a binary system of two close white dwarfs composed of carbon and oxygen. Over time, as they lose energy, they spiral in toward each other and collide. Such a collision gives rise to the release of energy comparable to Type I supernova.
Type II
Massive Star Explodes
Type II supernovae are only observed in population I stars. Population I consist of young stars enriched, at their birth, with heavy metals made by previous generation of stars.
This type of supernova is an explosion of super massive star with mass greater than 10M☉. Type II supernovae are accompanied by the formation of a neutron stars.
Type II Mechanism
Neutronization
Recall the discussion on evolution of stars. Stars like our Sun end their lives as a white dwarf unable to continue the thermonuclear process in the core beyond carbon and oxygen. But super massive stars continue the nucleosynthesis progressively until it reaches iron. The transmutation of elements stops in the core when the core becomes a huge ball of iron. So, Iron core is the dead end for all massive stars.
As the fusion stops in the iron core, the relentless crushing force of the outer layers takes over and literally crushes the iron core. Now the core turns into a neutron star amid a huge burst of neutrinos and accompanied by a violent explosion. (Please refer to ‘The Blow Out’ section above for more details).
References:
- Black Holes, Cambridge University Press, by J-P Luminet
- https://chandra.harvard.edu/photo/2022/casa_streams/
- https://astronomy.swin.edu.au/cosmos/N/Neutron+Star
- Britannica, The Editors of Encyclopaedia. “supernova”. Encyclopedia Britannica, 21 Dec. 2022, https://www.britannica.com/science/supernova Accessed 9 February 2023.
- https://en.wikipedia.org/wiki/Supernova