Most stars on the main sequence are relatively average: not too big and not too small. But every so often, a star begins its life as an absolute monster: a supergiant.
These supergiants do join the main sequence, but due to the sheer amount of gravitational force and pressure, they burn through the hydrogen in their cores in a fraction of the time that smaller stars do. They quickly leave the main sequence and expand in size to become thousands of times larger than the sun. Like smaller stars, these supergiants begin helium fusion in their cores and begin hydrogen fusion in shells around the core. But unlike smaller stars, which stop their fusion at this point, supergiants form several layers throughout the star of differently fusing gases, giving it an onion-type look.
Supergiant stellar evolution
Fusion in the core will eventually reach iron. At this point, nuclear fusion no longer produces energy, and so it stops. Without outward radiation pressure, the intense gravity causes the star to collapse. Protons and electrons get forced together in the core, turning it into a rigid sphere of neutrons. As the outer layers reach the limits of the neutron core, they rebound off and get propelled outward with huge amounts of energy. These shockwaves tear the star apart in a supernova.
Supergiant stellar
Credit: NASA, ESA, and G. Bacon (STScI) 
All that remains is the rigid sphere of neutrons, known as a neutron star. However, some supergiants are so massive that even the neutron star continues to collapse, creating an object so dense that it creates a singularity, also known as a black hole. Supergiants are rare in our universe, but their existence is crucial. Supernovas are so energetic that they create all of the elements heavier than iron, and many star systems, including our own, are made from the remnants of these explosions. Without them, life itself wouldn’t exist.
Written By: Scott Yarbrough
Additional Resource: Mouser

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