Why stars explode, creating the universe as we know it

 

 There are quite a few ways for a star to die, but in general people tend to think of stars as going out with a bang.
The term “supernova” refers to the incredibly energetic explosions caused when certain stars reach certain points in their life-cycles. Supernovae can often briefly outshine whole galaxies of billions of stars, and wreak utter destruction on anything unlucky enough to be within a hundred or so light years of the event. But supernovae aren’t just incredible natural events — they’re also the most important type of event for the development of complex matter and, by extension, life.

This image composite shows the search for the supernova, nicknamed Refsdal, using the NASA/ESA Hubble Space Telescope. The image to the left shows a part of the the deep field observation of the galaxy cluster MACS J1149.5+2223 from the Frontier Fields programme. The circle indicates the predicted position of the newest appearance of the supernova. To the lower right the Einstein cross event from late 2014 is visible. The image on the top right shows observations by Hubble from October 2015, taken at the beginning of observation programme to detect the newest appearance of the supernova. The image on the lower right shows the discovery of the Refsdal Supernova on 11 December 2015, as predicted by several different models.

Artists conception of the first stars. Image Credit: Wikipedia

First, why supernova occur. Essentially, when enough gas all collects in one place, it starts to have enough mass to exert a meaningful amount of gravitational energy, focused most powerfully on the center of the growing sphere-like cloud. When this pressure builds past a certain point, hydrogen atoms at the center of the sphere begin to undergo fusion, which ignites the ball of gas into a star — great! But at all times, as the star continues to live and burn, and likely acquires new matter as it goes, there is an interplay between the outward pressure of the thermal reaction, and the inward pressure of the star’s own gravity.
As the star burns down over billions of years, that outward pressure becomes weaker, while the magnitude of gravitational force stays largely the same. So, as a smallish or medium-sized star cools down, its gravitational potential comes to dominate, but since it’s a fairly small star, that potential is too weak to do more than just continue holding the star together. This safely cooled star is called a white dwarf. The mass threshold below which a star will not create enough gravitational force to cause a supernova is called the Chandrasekhar Limit, which lies at about 1.4 times the mass of the Sun. If you’re smaller than that, you can expect a relatively quiet stellar exit.

Supernova are so bright they shine out even against the backdrop of galaxies.

However, we need not give up hope that a white dwarf could still end its life with some fireworks. White dwarfs are still stars, after all, and in principle they can be reignited. This can happen in one of two ways. Either it can acquire enough mass to create an absurd amount of pressure at the core, and fuse carbon (as opposed to hydrogen and helium), causing a runaway fusion reaction that causes the star to explode.
On the other hand, if the white dwarf’s core is mostly made of neon, as some are, then it will undergo core collapse not unlike that which ignited the star in the first place. This super-collapse also results in a stellar explosion, but this time it leaves a neutron star behind. This almost always occurs in binary systems like this twin-star system, in which one star slowly approaches the Chandrasekhar Limit by sucking up matter from its partner. Since astronomers currently have no way of seeing what’s in the growing star’s core, they don’t know which of the two paths it will will follow once it passes that limit.

This image of the Tycho supernova remnant contains evidence of a double-star collision.

So, that’s what happens when a white dwarf passes the Chandrasekhar Limit, but white dwarfs are already considered largely dead stars. Stars bigger than 1.4 suns while still alive (and they can get much, much bigger than that) have different life-cycles. A red giant star will slowly burn down, and thus gravity will come to dominate as before — but this time, that gravity is strong enough that if its isn’t offset by fusion, it can create core collapse and trigger a supernova. Stars above the 1.4 solar masses but below about three solar masses tend to collapse down to form neutron stars, much like the core-collapse of a white dwarf, seen above.
Stars heavier than about three of our sun also collapse, but they actually keep on going and can form a black hole. This is the most famous upshot of a star’s death, yet it actually only occurs in a small minority of stars. Black holes are fairly numerous in the universe (there’s thought to be a supermassive black hole at the center of every major galaxy, for instance,) but they are still far less common than other types of stellar remnant.

Artist’s impression of a binary star system. (Credit: NASA)

There are other, less common ways for a supernova to start. For instance, while most white dwarfs that acquire new mass will do so slowly, inching toward the Chandrasekhar Limit before explode as they pass it, some other stars will acquire a ton of mass all at once (like from a direct stellar collision) and rocket way, way past that limit before they’ve really had a chance to start collapsing. These sorts can vary widely in terms of their radiation output, and scientists are interested in their chaotic, poorly understood mechanisms and implications.
Supernovae of various types actually have some fairly useful real-world applications, at least for astronomers. In particular, Type Ia supernovae (the white-dwarf-undergoes-carbon-fusion type from above) seem to send out uniform signals, time after time. This has led to their being dubbed astronomy’s “standard candles,” since their uniformity can make them useful as optical measuring sticks. However, recent research seems to indicate that while they are useful, they might be slightly less reliable than previously believed. At the least, there is likely more variation is how Type Ia supernovae proceed than perviously believed.
Yet, I said that supernovae are the most important events to complex matter, not just that they’re big and cool and useful. Well, you’ll notice that in the above explanation, we talked about the fusion ignition of carbon. Carbon is the heaviest metal (neon is heavier, but not a metal) created by stars in their normal state. That is it say: if you want heavier elements like sodium, lead, gold, or uranium, you’re going to need more power than a puny old red giant star can provide. And what has more energy than a star? A dying star.
Virtually everything you interact with was, at one point, thrown out by a star in its final moments. The Earth is a rocky collection of debris thrown out by supernovae, as are comets, asteroids, and everything else composed of heavy matter. And we, being made of the matter that collected into the Earth, are made of supernova shrapnel as well. This is why Carl Sagan said that we are star stuff — because, in a very real way, we are.