Crown jewel

A white dwarf is the remnant of an average-to-small star at the end of its life.  When a star like our own Sun exhausts its hydrogen fuel, it goes through a brief period of fusing helium into carbon and oxygen, but that too eventually runs out.  This creates an imbalance between the two opposing forces ruling a star's life -- the outward thermal pressure from the heat released by fusion, and the inward compression from gravity.  When fusion ceases, the thermal pressure drops, and the star collapses until the electron degeneracy pressure becomes high enough to stop the expansion.  The Pauli Exclusion Principle states that two electrons can't occupy the same quantum state, and the force generated in order to prevent this happening is sufficient to counterbalance the gravitational pressure.  (At higher masses, even that's not enough to stop the collapse; the electrons are forced to fuse with protons, generating a neutron star, or at higher masses still, a black hole.)

For a star like our Sun, in a single-star system, that's pretty much that.  The outer layers of the star's atmosphere get blown away to form a ghostly shell called a planetary nebula, and the white dwarf -- actually the star's core -- remains to slowly cool down and dim over the next billion-odd years.  But in multiple-star systems, something far more interesting happens.

White dwarfs, although nowhere near as dense as neutron stars, still have a strong gravitational field.  If the white dwarf is part of a close binary system, the gravitational pull of the white dwarf is sufficient to siphon off gas from the upper atmosphere of its companion star.  The material from the companion is heated and compressed as it falls toward the white-hot surface of the white dwarf, and once enough of it builds up, it suddenly becomes hot enough to fuse, generating a huge burst of energy in a runaway thermonuclear reaction.

The result is called a nova -- a "new star," even though it's not new at all, it has merely flared up enough to see from a long way away.  (The other name for this phenomenon is a cataclysmic binary, which I like better not only because it's more accurate but because it sounds badass.)  Once the new fuel gets exhausted, it dims again, but the process merely starts over.  The siphoning restarts, and depending on the rate of accretion, there'll eventually be another flare-up.

Artist's concept of a nova flare-up [Image courtesy of NASA Conceptual Image Lab/Goddard Flight Center]

The topic comes up because there is a recurrent nova that is due to erupt soon, and when it does, a "new star" will be visible in the Northern Hemisphere.  It's in the rather dim, crescent-shaped constellation of Corona Borealis, between Boötes and Hercules, which can be seen in the evening in late spring to midsummer.  The star T Coronae Borealis is ordinarily magnitude +10, and thus far too dim to see with the naked eye; most people can't see anything unaided dimmer than magnitude +6, and that's if you've got great eyes and it's a completely clear, dark night.  But in 1946 this particular star started to dim even more, then suddenly flared up to magnitude +2 -- about as bright as Polaris -- before gradually dimming over the next days to weeks back down to its previous near-invisibility.

And the astrophysicists are seeing signs that it's about to repeat its behavior from 78 years ago.  The best guesses are that it'll flare some time before September, which is perfect timing for seeing it if you live in the Northern Hemisphere.  If you're a star-watcher, keep an eye on the usually unremarkable constellation of Corona Borealis -- at some point soon, there will be a new jewel in the crown, albeit a transient one.

You have to wonder, though, if at some point the white dwarf in the T Coronae Borealis binary system will pick up enough extra mass from its companion to cross the Chandrasekhar Limit.  This value -- about 1.4 solar masses -- was determined by the brilliant Indian physicist Subrahmanyan Chandrasekhar as the maximum mass a white dwarf can have before the electron degeneracy pressure is insufficient to halt the collapse.  At that point, it falls inward so fast the entire star blows itself to smithereens in a type-1a supernova, one of the most spectacular events in the universe.  If T Coronae Borealis did this -- not that it's likely any time soon -- it would be far brighter than the full Moon, and easily visible in broad daylight, probably for weeks to months.

Now that I would like to see.

****************************************

A cataclysmic pirouette

Hamlet famously states to his friend, "There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy," and every time we look into the night sky, we're reminded how true that is.

In the last hundred years astronomers have discovered deadly gamma-ray bursters and black holes, neutron stars for which a teaspoon of their material would weigh as much as a mountain, planets made of stormy swirls of ammonia, methane, and hydrogen, ones made of super-hot molten metal, water-worlds completely covered with deep oceans.  We've seen newborn stars and stars in their violent death throes, looked out in space and back in time to the very beginning, when the universe itself was in its infancy.

Even with all these wonders, new and bizarre phenomena are still being discovered every time our technology improves.  Take, for example, the "cataclysmic variable" that was the subject of a paper in Nature this week, a pair of stars locked in such a tight dance that they whirl around their common center of gravity in only fifty-one minutes.

Given the euphonious name ZTF J1813+4251, this pair of stars is comprised of a white dwarf -- the burnt-out core of a low-mass star like the Sun -- and an even more lightweight star not much bigger than the planet Jupiter.  The white dwarf has been swallowing (the astronomical term is "accreting") the hydrogen fuel from its partner, and they're drawing closer together, meaning that the process will speed up.  Eventually all that will be left of the partner star will be its core, and astronomers predict that at that point, they will have an orbital period of eighteen minutes.  But once the accretion process ends, drag in the pair's movement will rob energy from the system, the wild stellar pirouette will slow down, and they will gradually start to move apart again.

It's fortunate that the partner star is as light as it is; if it had more mass, it would be headed toward one of the most violent fates a star can have -- a type 1a supernova.  White dwarfs are the remnants of stars that have exhausted all their fuel, and they shrink until the inward pull of gravity is counterbalanced by the mutual repulsion of the negatively-charged electrons that surround the atoms they're made of.  There's a limit, though, to how much this repulsive force can withstand; it's called the Chandrasekhar limit, after its discoverer Subrahmanyan Chandrasekhar, and is equal to 1.44 solar masses.  For a lone white dwarf -- as our Sun will one day be -- this is not a problem, as there won't be anything substantial adding to its mass after it reaches that point.

The situation is different when a low-mass star is in a binary system with a giant star.  When the low-mass star burns out and becomes a white dwarf, it begins to rob its partner of matter -- just as ZTF J1813+4251 is doing.  But in this case, there is a lot more mass there to rob.  Eventually, the white dwarf steals enough matter from its companion to go past the Chandrasekhar limit, and at that point, the mutual repulsion of the electrons in the stars atoms lose their contest with the inward pull of gravity.  The white dwarf's core collapses completely, making the temperature skyrocket so high that its helium ash can fuse into carbon and other heavier elements, suddenly releasing catastrophic amounts of energy.  The result is...

... boom.

In the process, the matter from the exploded dwarf star is scattered around the cosmos, and becomes the parent material for forming planets.  It is, in fact, how most of the carbon, oxygen, and nitrogen in our bodies were formed.

As Carl Sagan famously said, "We are made of starstuff."

A type 1a supernova remnant [Image is in the Public Domain courtesy of NASA/JPL]

But ZTF J1813+4251 isn't headed for such a dramatic exit -- eventually the white dwarf will pull away the outer layers of the partner star's atmosphere, and after that the two will just spiral around each other wildly for a few million years, gradually cooling and slowing from their current frenetic pace.  So maybe "cataclysmic" isn't the right word for this pair; their crazy tarantella will simply wind down, leaving two cold clumps of stellar ash behind.

Honestly, if I were a star, I think I'd rather go out with a bang.

****************************************