What is a supernova? Are all stars destined to go through it?

Credit: interestingengineering.com

A supernova is one of the most spectacular fireworks displays in the universe. The last gasp of a sufficiently large star in its death throes, the star’s explosive finale often becomes the brightest point of light in an entire galaxy in less than a second and can remain so for several weeks afterward before fading away into a spectacular nebula spread across dozens if not hundreds of light-years.

What causes such a violent end to the life of a star, though, and is a supernova the fate of every star? Can a supernova produce a black hole? And what about our own Sun? Is it, too, destined for a spectacular final bow several billion years from now? Fortunately, supernovas are some of the most actively studied celestial events, so there is a lot that we know about them, even as they keep some secrets to themselves, for now.

What is a supernova?

A supernova is a key stage in the “dying” process of certain stars. There are two broad classes of supernova, each the product of a different process and conditions. The Type II supernova is the kind that most people think about when they think about supernova; the classic explosion of a dying star brought on by something known as core collapse. There are several stages that most stars pass through over the course of their lives, and as far as supernovas are concerned, we’re talking principally about main sequence stars like our Sun, only larger.

Stars below the level of the main sequence, like red dwarfs, are the proverbial tortoise to the main sequence star’s hare and can steadily burn their limited hydrogen fuel judiciously for trillions of years while larger stars like our Sun burn themselves out after several billion years or less. It is at this late stage of a main sequence star’s life that a supernova becomes possible. While not every star in the main sequence will go supernova, those that do will first pass through a “giant” phase after they exhaust their hydrogen reserves and instead start fusing helium and residual heavier elements like oxygen, silicon, and carbon rather than hydrogen (limited fusing of these heavier elements takes place in young stars as well, but doesn’t account for their primary fusion reactions).

These nuclear reactions will sustain a star for another few billion years after it transitions to its giant phase, but eventually, the fusion process climbs up the periodic table to the point that it starts fusing lesser elements into iron or even heavier elements like gold. Once you start fusing atoms into iron, the process actually absorbs energy instead of producing it, so once a star starts to grow a heavy iron core, its end approaches at an accelerating pace as heavy metal fusion sucks up whatever energy is being generated elsewhere.

This becomes a huge problem for the star because it’s not like it’s lost any mass over the preceding billions of years. Its heavy outer layers of gas and plasma, previously held aloft by the energy produced by fusion reactions beneath, suddenly cross a tipping point and their mass is too great for the star to hold up. These layers rapidly collapse in toward the star’s core, kicking off the supernova process.

The other class of supernova, a Type I supernova, is less well-understood and is actually subdivided further into Type Ia, Ib, and Ic supernovas. Astronomers are fairly comfortable with the mechanics of the Type Ia supernova, which is thought to occur in binary star systems with at least one white dwarf star, so we’ll use that as a representative example. As a white dwarf co-orbits its companion star, it steadily sucks material off of it into an accretion disk around itself. There is an upper bound of mass, known as the Chandrasekhar limit, under which a white dwarf can accrete material without becoming unstable (about 1.44 solar masses). In the case of a Type Ia supernova, a white dwarf accretes material from its companion that pushes it over this mass limit and it can no longer support its own mass.

Unlike a nova—where a white dwarf accretes a more modest amount of material below this limit and produces a relatively mild explosion of energy from the additional mass but otherwise remains intact—in a Type Ia supernova, the white dwarf destabilizes in an explosive fashion and destroys itself in the process. Type Ib and Ic supernovas are very similar to Type II supernovas in that they are a function of core collapse after the stars have exhausted their fuel to sustain fusion reactions in their core. The only real difference, in this case, is fairly academic; Type Ib and Type Ic supernovas occur during a core-collapse event in stars that have shed their outermost layer of hydrogen, or their outermost hydrogen layer, and a substantial part of the helium layer below that, for Type Ib and Type Ic, respectively. 

Which stars go supernova? 

The mass of a star plays a huge role in whether a star will go supernova at the end of its lifecycle. Generally speaking, stars heavier than eight solar masses are candidates for supernovas, though even at these enormous masses, particular circumstances can sway things in one way or the other.

In 2008, a star with 25 solar masses exhausted its fuel and should have progressed to its final supernova event, but instead shuffled off the galactic stage with barely a recordable blip of activity, most likely collapsing directly into a black hole. Computer simulations had previously shown researchers that the stars they thought should have blown up didn’t, and still there isn’t a clear explanation why some stars in the eight to 30 solar mass range explode in supernova and others seem to fizzle out. Further investigation points to a range of eight to 17 solar masses as the most likely to collapse into a supernova, with stars between 17 and 30 solar masses often taking a less flashy end-of-life path, but not always.

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