Quick explanation
A supernova is the violent explosion of a star ending its life. For a brief period, a supernova may outshine the remaining billions of stars in its galaxy.
For a particular type of supernovae ("Type Ia"), we know exactly how bright they shine. This information can be used to calculate their exact distances. In that way we're able to map the Universe and its expansion.
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Death of a massive star
Stars shine by burning — or rather fusing — hydrogen to helium. That is, the lightest element to the second-lightest. The energy produced keeps the star from collapsing under its own gravity.
But as the star is running out of fuel, the star can no longer resist gravity and it starts contracting. The contraction makes the pressure and temperature inside the star rise, enabling the star to start fusing heavier elements.
For a long time, it can shine by burning helium to carbon, but when there's not enough helium the star contracts further, burning heavier and heavier elements.
Eventually, the star forms layers of increasingly heavier elements. The exact composition depends in part on the mass of the star, but is outermost hydrogen (H), helium (He) and carbon (C). If the star is a "massive star" — i.e. if it's more than eight Solar masses — it will be able to merge heavier elements, forming layers typically consisting of neon (Ne), oxygen (O), magnesium (Mg), silicon (Si) and ultimately iron (Fe).
Since you can't get any energy out of fusing elements heavier than iron, nothing can prevent the catastrophe: The star now has less than a second left of its life…
Credit: anisotropela/data from Stan Woosley.
Gravitational collapse
Whereas small stars end up as compact white dwarfs about the size of the Earth, the pressure in the center of the massive star becomes so high that electrons are, so to speak, squeezed into the protons, converting them into neutrons. This happens if the mass of the white dwarf is more than 1.4 Solar masses (the so-called "Chandrasekhar-limit"). The pressure is then relieved so that the core can continue its collapse.
Now things go fast! The nucleus contracts at a quarter of the speed of light — in less than a second it reaches a radius of some 10 km, but with several Solar masses of matter.
This implosion creates a neutron star; the densest and hottest object in the Universe since the Big Bang. In the center of a neutron star, the density is up to a billion tons per cubic centimeter — denser than a atomic nucleus — and the temperature is 100 billion degrees.
If the mass of the neutron star is above 2–3 Solar masses, not even the neutron pressure can support the star, so the collapse continues until a split second later the star becomes a black hole.
The explosion
If, however, the core succeeds in resisting the ultimate collapse, a shock wave is formed when the density exceeds the density of a atomic nucleus; here the nuclear forces that usually bind the atomic nuclei become repulsive rather than attractive.
The rest of the original star's mass bounces on the neutron star's compact surface and is ejected in a fantastic explosion — a supernova! The energy of the shock wave in this explosion is about 1044 Joules — as much energy as our Sun will produce throughout its 10 billion year long life — but is absorbed in the outer layers which are also very dense.
The absorbed energy creates more heavy elements by fusion, and the radioactive decay of the heavy atomic nuclei — especially nickel decaying to cobalt before turning into iron — causes the supernova to brighten tremendously; for several days, the supernova may shine brighter than all the other billions of stars in the galaxy combined.
As if this explosion weren't enough, the formation of the neutron star releases a lot of tiny elemental particles called neutrinos. The energy of these neutrinos is 100 times greater than the energy of the explosion itself, and because neutrinos barely interact with anything at all, they are not absorbed, but fly straight through the outer layers.
The outer layers themselves are blown away in a radioactive cloud at 10,000 km/s or more. This cloud, visible long after the explosion is over, forms a supernova remnant.
Type Ia supernovae
The kind of supernovae I describe above — that is, stars that implode because they can no longer produce enough energy to resist gravity — are called "core-collapse" supernovae. Collapsing in this way requires an original star of at least eight Solar masses.
But in fact it's possible for a smaller star to explode.
Most stars aren't "lonely" like our Sun, but are members of a binary stellar system where two (or more) stars orbit each other. If both are "small stars" (i.e. below \(8\,M_\odot\)), they first evolve to red giants, and later to white dwarfs.
The most massive member burns out first and becomes a white dwarf. If the stars are not too far from each other, the white dwarf may, by its gravity, pull and accrete gas from the other star (usually when it swells to a red giant) and slowly increase its mass. Alternatively — and in fact more recent research indicate that this is more common — the two stars may both evolve to white dwarfs and subsequently spiral in toward each other until they merge.
As the white dwarf's mass increases, so does its pressure and temperature. During its life, a "normal" star can regulate its pressure and temperature; if they increase, the star produces more energy, which makes the star swell a little so that they fall again. But a white dwarf is in a special quantum mechanical, so-called degenerate, state where this is not possible.
Should the white dwarf exceed the mass limit of 1.4 Solar masses, it would implode to a neutron star. However, shortly prior to this (at about 99% of the mass limit), temperature and, in particular, pressure increases enough to ignite fusion of first carbon and then oxygen..
This increases the temperature, and because it does not swell like ordinary stars and "relieve its pressure", in a few seconds the whole star can fuse to heavier elements in a thermonuclear chain reaction . The energy created in this process is roughly the same as in the core-collapse supernovae (without the neutrinos though, since no neutron star is created), and is enough to completely disrupt the star in a giant explosion.
This kind of supernovae are called Type Ia (pronounced "type one a"). Because we know quite accurately its mass when it exploded (\(1.4\,M_\odot\)), we also know how bright it is. And that's super useful, because then we can infer its exact distance.
Click to animate.
Credit: Walt Feimer, NASA/Goddard Space Flight Center.
Inverse square law
The farther away a luminous object is — e.g. a star — the fainter it looks, because its light is speard out over a larger area. If you put the star at twice the distance, it gets four times fainter; put it at three times the distance and it becomes nine times fainter, etc.
That is, the brightness decreases with distance squared (i.e. distance\(\times\)distance). This relation is called the inverse square law. If you know how bright an object is, you can calculate its distance by measuring how much light we receive. On the other hand, if you don't know how bright it is, intrinsically, you don't know whether it's faint because it doesn't emit much light, or because it's far away.
Calculating distances in the Universe is very difficult, exactly because we generally don't know this. But with Type Ia supernovae we do (though it's a bit less straightforward to infer than described here), and since they shine so bright, we can even observe them to very large distances; more than 1/3 of the way to the edge of the observable Universe.
Credit: anisotropela.
The expansion of the Universe accelerates
Particularly in one respect, this property of Type Ia supernovae has been ground-breaking in astronomy, more specifically in cosmology. The reason is that they have enabled us to determine the distances of galaxies far, far away, where we at the same time could measure their velocity with respect to us through their cosmological redshift.
And why is that interesting?
Well, we know that the Universe expands. We've known that since Edwin Hubble and others in the 1920's measured distances and velocities to galaxies in our "neighborhood". With Type Ia supernovae we can investigate the relation thousands of times farther away than Hubble did. Because it has taken the light some times to reach us, that in turn means that we can see how fast the Universe has expanded through most of its history, billions of years back in time.
And here, astronomers discovered something terribl… something very interesting in the late 90's:
In the beginning of its life the Universe expanded incredibly fast, but the gravity from everything in the Universe (primarily your mom) gradually decelerated the expansion. Whether the gravity would one day decelerate enough to make the Universe contract, or whether it had received a large enough kick in the beginning to expand forever, depends on how much stuff there is in the Universe. Both ordinary matter (such as stars, planets, and bicycles), dark matter, and in fact also radiation, helps slowing down the expansion.
What the observations showed us was that the Universe indeed did slow down in the beginning. But relatively recently (cosmologically speaking — it's roughly 3½ billion years ago), something peculiar happened: the Universe started speeding up. And not only did it speed up, its expansion accelerates, so it goes faster and faster.
We don't know with certainty the reason for this, but the most "mainstream" explanation is that it's a property of empty space itself. Various possible mechanisms go by the name dark energy. The more the Universe expands, the more empty space is created, and hence the more dark energy, which makes the Universe expand even faster.
In other words, the Universe seems to keep on expanding forever, so everything gets farther and farther away from each other (it might reassure you that this is only on "cosmological scales"; galaxies, stars, and planets are held together by gravity, and you are held together by electromagnetic forces.
Credit: anisotropela.
Historical supernovae
The first supernova we know was observed was in the year 185 AD by Chinese astronomers, although some think that the Greek astronomer Hipparchus observed one in 134 BC.
In 1054, a supernova exploded in the constellation Taurus and was recorded by both Arab, Chinese, and Japanese astronomers, and probably also by the Pueblo Native Americans, though apparently not by a single European. It was visible in daylight for 23 days, and on the night sky for almost two years (this was before telescopes). Today, the supernova remnant is known as the "Crab Nebula". It's really beautiful, and easily spotted by amateur astronomers with a small telescope.
The Danish astronomer Tycho Brahe is famous for having observed a supernova in the constellation Cassiopeia in 1572. He called it stella nova ("new star") and showed that the celestial sky is not — as most people thought at the time — eternal and immutable. This helped changing the European notion of the cosmos.
After Tycho Brahe, for many years European astronomers called supernovae and other transient phenomena on the sky "novae". Nowadays, though, nova means something different. The Chinese called them guest stars, and the Icelandics call them something as cool as sprengistjörnur, meaning "exploder stars".
All these supernovae were located in our own galaxy, the Milky Way. Tycho Brahe's assistant, Johannes Kepler, observed one 32 years later, in 1604, and in fact that's the last one that was seen explode in the Milky Way, even though we expect one to go off every 100–200 years, on average.
But in 1987, a supernova exploded in one of our closest neighbor galaxies, "The Large Magellanic Cloud". It was the first one in modern times that could be observed in detail. For instance, at the same time a few neutrinos from the formation of the neutron star were detected.
In 2005 I observed a supernova myself. It exploded while I were at the Danish 1.5 m telescope at La Silla, Chile, looking for gamma-ray bursts. You can see it here (I mean, it's just a white dot, but it's pretty cool nonetheless, and its host galaxy is quite nice).
Credit: NASA/ESA.