Supernovae: Catastrophic Explosions of Giant Stars

This is what's left when a massive star explodes as a supernova. The Hubble Space Telescope captured this image of the Crab Nebula, a supernova remnant more than 6,000 light-years away from Earth. NASA

Supernovae are the most dynamic and energetic events that can happen to stars. When these catastrophic explosions happen, they release enough light to outshine the galaxy where the star existed. That's a lot of energy being released in the form of visible light and other radiation! It tells you that the deaths of massive stars are incredibly energetic events.

There are two known types of supernovae.

Each type has its own particular characteristics and dynamics. Let's take a look at what supernovae are and how they come about in the galaxy. 

Type I Supernovae

To understand a supernova, you need to know a few things about stars. They spend most of their lives going through a period of activity called the main sequence. It begins when nuclear fusion ignites in the stellar core. It ends when the star has exhausted the hydrogen needed to sustain that fusion and begins fusing heavier elements.

Once a star leaves the main sequence, its mass determines what happens next. For type I supernovae, which occur in binary star systems, stars that are about 1.4 times the mass of our Sun go through several phases. They move from fusing hydrogen to fusing helium, and has left the main sequence. 

At this point the core of the star is not at a high enough temperature to fuse carbon, and enters a super red-giant phase.

The outer envelope of the star slowly dissipates into the surrounding medium and leaves a white dwarf (the remnant carbon/oxygen core of the original star) at the center of a planetary nebula.

The white dwarf can accrete material from its companion star (which can be any type of star). Basically, the white dwarf has a strong gravitational pull that attracts material from its companion.

 The material collects into a disk around the white dwarf (known as an accretion disk). As the material builds up, it falls onto the star. Eventually, as the mass of the white dwarf increases to about 1.38 times the mass of our Sun, it will erupt in a violent explosion known as a Type I supernova.

There are some variations of this type of supernova, such as the merger of two white dwarfs (instead of the accretion of material from a main sequence star). It is also thought that type I supernovae create the infamous gamma-ray bursts (GRBs). These events are the most powerful and luminous events in the universe. However, GRBs are likely the merger of two neutron stars (more on those below) instead of two white dwarfs.

Type II Supernovae

Unlike Type I supernovae, Type II supernovae happen when a isolated and very massive star reaches the end of its life. Whereas stars like our Sun won't have enough energy in their cores to sustain fusion past carbon, larger stars (more than 8 times the mass of our Sun) will eventually fuse elements all the way up to iron in the core. Iron fusion takes more energy than the star has available. Once a star begins to try and fuse iron, the end is very, very near.

Once the fusion ceases in the core, the core will contract due to the immense gravity and the outer part of the star "falls" onto the core and rebounds to create a massive explosion. Depending on the mass of the core, it will either become a neutron star or black hole.

If the mass of the core is between 1.4 and 3.0 times the mass of the Sun, the core will become a neutron star. The core contracts and undergoes a process known as neutronization, where the protons in the core collide with very high energy electrons and create neutrons. As this happens the core stiffens and sends shock waves through the material that is falling onto the core. The outer material of the star is then driven out into the surrounding medium creating the supernova. All of this happens very quickly.

Should the mass of the core exceed 3.0 times the mass of the Sun, then the core will not be able to support its own immense gravity and will collapse into a black hole.

This process will also create shock waves that will drive material into the surrounding medium, creating the same kind of supernova as the neutron star core.

In either case, whether a neutron star or black hole is created, the core is left behind as a remnant of the explosion. The rest of the star is blown out to space, seeding nearby space (and nebulae) with heavy elements needed for the formation of other stars and planets. 

Edited and updated by Carolyn Collins Petersen.