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.

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