Daughter, age 7: “Daddy, will the Earth always go around the Sun forever?”
Louis CK: “Well no, at some point the Sun is going to explode.”
Daughter: (starts crying)
Louis CK: “Oh honey, this is not going to happen until you and everyone you know has been dead for a very long time.”
Daughter: (continues crying)
As you all know, the closest supernova to us in the last quarter-century has recently gone off, currently shining in the faint, but relatively close Pinwheel Galaxy. (And starting tonight, early in the night, those of you with telescopes should go look for it!)
But when I talked about it, there was some confusion as to what it means that this is a supernova, as opposed to a regular nova. Let’s start at the very beginning, with the seven different main classes of stars.
When stars are first born, they’re made almost exclusively (more than 98%) out of hydrogen and helium. The lowest mass ones are red, cool, and very long-lived, while the highest mass ones are blue, hot, and very short-lived. Our star — a G-type star — is somewhere in the middle in terms of mass, color, temperature and for the purpose of this discussion, lifetime.
When our Sun runs out of fuel, it will do the same thing that most stars will do: blow off its outer layers, while the inner core contracts to form a white dwarf star.
But what is this white dwarf star like? Luckily, we have a photo from the Hubble Space Telescope to show us! The A-type star, Sirius, the brightest star in our night sky, has a white dwarf companion star that orbits it, clearly visible next to Sirius itself.
White dwarfs can be the same color, temperature, and nearly the same mass (up to about 70%) of the bright stars that gave rise to them. However, they’re both about 10,000 times less bright and a factor of a million lower in volume than the stars they came from.
Made out of the condensed atoms packed closely together under the tremendous force of gravity, but without any nuclear fusion at the core, white dwarf stars, although they can easily be as massive as our Sun, are only physically about the size of Earth, making them around 300,000 times as dense as our planet.
Now, in our Solar System, there is but one star. But, much as is the case for Sirius, a substantial portion of stars exist in binary or even trinary star systems. When a white dwarf star lives in a binary star system with a relatively close companion star, it can begin to gravitationally siphon off some of the mass of the larger, less dense companion!
Unlike the white dwarf star itself, which is made out of heavier elements such as carbon, nitrogen, and oxygen (as well as sometimes neon, silicon, and sulphur, among others), the stolen mass is almost always contains copious amounts of hydrogen, the nuclear fuel that normal stars burn for their energy!
And even though it usually takes thousands of years for enough hydrogen to build up on the surface of a binary white dwarf for this burst of fusion to occur, the sheer number of stars we have access to ensures that our galaxy never has a shortage of white dwarf star bursting with hydrogen fusion. This flare-up is what’s known as a nova.
But after a nova, the white dwarf star is fine, and will go back to building up hydrogen on its surface in preparation for another burst, until it recurs.
Some of these white dwarfs, however, were born very massive, with many even exceeding the mass of the Sun. But as you pile more and more mass onto your white dwarf, something counterintuitive happens.
The more massive your white dwarf is, the smaller in size it becomes! The very atoms that make up the star are the only thing holding it up against gravitational collapse, and as you add more and more mass to your white dwarf, the atoms find themselves being compressed into an ever-smaller volume. There are two ways theorized to build up the mass of the white dwarf: one gradual and one sudden.
At some critical point, the nuclei and electrons become so compressed that atomic structure itself begins to buckle. When a white dwarf star gains enough mass so that its total mass exceeds about 138% the mass of our Sun, something spectacular happens!
The atoms at the core of this white dwarf fail to support the star under the tremendous stress of gravity! The white dwarf star is completely destroyed, as the interior collapses down to form a black hole, where nothing can escape! The process of collapse, even though it lasts just a few seconds, results in a tremendous release of energy into the outer layers.
And the outer layers heat up tremendously, expand incredibly rapidly and get strewn across space for light years! This is what a supernova is; in particular, a type I supernova, the same type as the one you can see in the Pinwheel Galaxy, and the same type that occurred in the famed 1572 supernova.
When the fuel in the core runs out, the innermost layers lose the pressure that has held them up against the tremendous pull of gravity. Without that fusion, the cores of these stars collapse, producing a type II supernova. Unlike the one presently in our sky, these supernova often collapse their cores into a single gigantic atomic nucleus, known as a neutron star! Although they are — again — of approximately the same mass as our Sun (although many are perhaps two or three times as massive), they are only a few kilometers in size! Some of these neutron stars rapidly rotate and emit massive amounts of radiation; these rapidly rotating neutron stars known as pulsars, such as the one left over from the 1054 explosion that created the Crab Nebula.
Unlike a regular, plain old nova, a supernova completely destroys the star that emits it, and at peak brightness, a supernova can be so luminous that it may outshine the entire galaxy it occurs in for a time!
When you see a supernova, you are seeing the culmination of a star’s life, after it has exhausted the entirety of its core’s nuclear fuel. Some of them (the ones that are type I) take billions of years to form, and do it after growing from a white dwarf, while others (the ones of type II) can do it in just a few million.
So what makes a supernova so super? Try living for ten billion years and then exploding with such incredible power that you outshine your entire galaxy. That’s what makes them super! Now go out there and see it for yourself!
Update 9/17/11: There are a number of details that I have given that are out-of-date in the article. (See here.) Normally, I would strike them out and replace them with the correct ones, but there is too much this time. A full update on supernovae is coming next week!