Supernovas: Learn about the explosive end of the stars’ lives

*This text was written by a columnist from TechWorld; learn more at the end.

Have you ever wondered how do stars die? Or even why stars die? Next, we’ll understand what supernovas are and why they represent the end of life for high-mass stars.

Supernova 1E 0102.2-7219 observed by the Hubble Telescope, located in the small Magellanic Cloud (dwarf satellite galaxy of the Milky Way)Fonte:  NASA/ESA Hubble Space Telescope

Stars, throughout their existence, live a constant battle: internal pressure against gravity. Basically, right in the center of the stars is a “fuel”. This fuel is the fusion of hydrogen atoms, a process that creates helium atoms. This fusion releases some energy. It turns out that with tons and tons of hydrogen atoms, there is enough energy and pressure to be released to support the weight of the star itself and keep it in balance. That is, without that fuel, there would be nothing to support the star against its weight. And the result: imminent collapse. And why does it need to be at the core? Because for hydrogen fusion to happen extreme conditions of heat and density are necessary, found only in the central region of stars.

Illustration of the battle between pressure and gravity.Illustration of the battle between pressure and gravity.Fonte:  Astronomy Notes

But, as we already know, nothing lasts forever. And even the fuel at the center of the stars doesn’t last forever. Sometime this Hydrogen in the star’s core runs out. Of course, even consuming all the available Hydrogen takes many years. For the Sun, for example, about 9 billion years. But when that fuel runs out, the stars begin an evolution towards the end of their lives.

This stellar evolution will depend on the star’s mass. High-mass stars, above 10 solar masses, for example, are starting to use alternative fuels. In other words, other atoms like Helium, Carbon, and Oxygen start to fuse. It turns out that all these other atoms are not as effective and are short-lived. Forcing the star to always change fuel and look for the next option. In the process, synthesizing new elements.

Schematic structure of the interior of a high-mass star at the end of its life.  It is possible to notice shells of different chemical elements that were synthesized during the star's life.Schematic structure of the interior of a high-mass star at the end of its life. It is possible to notice shells of different chemical elements that were synthesized during the star’s life.Source: Meteoritic

This goes on until it becomes literally unsustainable. Massive stars can synthesize even the element Iron. Always fusing lighter elements into heavier elements and releasing energy in the process. It turns out that once you reach Iron, it is no longer possible to continue with the process. This is because the fusion of two iron atoms does not release energy, as has been happening so far. On the contrary, it consumes energy. Thus, the star no longer has a source of energy to fight gravity itself. And the inevitable end takes place: the victory of gravity. The collapse of the star into itself.

This collapse, for massive stars, becomes an explosion—a supernova. The entire envelope of the star falls towards its core, now composed of Inert Iron. When there is a collision between the stellar envelope and the Iron core, this core starts to collapse as well, releasing a huge amount of neutrinos. The shock wave between the nucleus and the envelope plus the energy released in the form of neutrinos creates the supernova.

These neutrinos push the star’s material outward, explosively and violently. This is what we see as supernova. In fact, 99% of the energy released in supernovae is actually in the form of neutrinos.

Supernovas are explosive and catastrophic ends of high-mass stars. A supernova alone could be momentarily brighter than its entire host galaxy! At the time of explosion, you have construction of new elements, especially heavier ones!

Supernova 1994D (bright dot lower left corner) in galaxy NGC 4526Supernova 1994D (bright dot lower left corner) in galaxy NGC 4526Source: Wikipedia

Another important role of the supernova is to disrupt the interstellar medium. The shock wave spreads among the molecular clouds, which can trigger their collapse. The collapse of molecular clouds represents the birth of new stars, or even groups of stars!

After the supernova, you have two final possibilities, depending on the mass of the starting star: a neutron star or a black hole. When the iron core collapses, the matter is ultra-compacted, becoming a degenerate neutron core. Degenerate matter would be this state of compaction. If the collapse stops, we have the formation of a neutron star. But if the initial star is too heavy, this degenerate neutron state won’t be enough to stop the core’s collapse, and then that collapse continues until the black hole forms!

Stellar evolution scheme for stars of different masses.  First line represents evolution of high-mass stars, going through the supernova explosion and ending up as a black hole or a neutron starStellar evolution scheme for stars of different masses. First line represents evolution of high-mass stars, going through the supernova explosion and ending up as a black hole or a neutron starSource: ESA

Camila de Sá Freitas, columnist of TechWorld, holds a bachelor’s and master’s degree in astronomy. She is currently a PhD student at the European Southern Observatory (Germany). Self-styled Galaxy Examiner, he investigates evolutionary scenarios for galaxies and possible changes in star-making. It is present on social networks as @astronomacamila.

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