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.

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Explosions in Andromeda

We've learned a lot about our own galaxy by studying our "sister galaxy," Messier-31, better known as the Andromeda Galaxy.  It's situated about 2.5 million light years away, so from our perspective looks to the naked eye like little more than a smudge of light in the night sky.

[Image is in the Public Domain, courtesy of NASA/JPL]

I remember when I was a kid and first grasped how far away Andromeda is.  Like many people of my generation, I was captivated by the original Star Trek.  In the episode "By Any Other Name," Kirk et al. are confronted by some aliens called Kelvans who come from the Andromeda Galaxy, and want to hijack the Enterprise to get back home.  Now, recall that because of warp drive, the intrepid space-farers of the United Federation of Planets are tooling about on a weekly basis, zipping from planet to planet, covering light-years of distance in mere hours.  So it was a bit of a shock -- to me, at least -- that at maximum warp, it would take three hundred years to reach the Andromeda Galaxy.

So far, in fact, that the Kelvans propose to reduce most of the crew to little geometric solids to save on food, lessen the likelihood of rebellion, have at least some of them still alive upon arrival, and also to reduce the number of extras the show's producers had to hire.

Of course, Kirk saves the day and they end up returning to our galaxy, kindly offering to leave the Kelvans on an uninhabited planet all their own.  Who could resist that?

In any case, I was blown away by how far away the Andromeda Galaxy is, not to mention the fact that the writers of Star Trek got that bit right given their extensive history of playing fast-and-loose with physics, despite Scotty's repeated admonition that ye canna change the laws thereof.  Everyone knows the stars in our own galaxy are far away; but this is an entirely different order of magnitude of distance.

Considering how far away we are from it, if you have a good enough telescope, it's surprising how spectacular it is.  Like our own, it's a spiral galaxy, so the disadvantage of being situated inside the thing we're trying to study has been ameliorated by the fact that there's a similar one right next door.  It's home to a trillion stars.

And there are some interesting ones.  Just last month, there was a paper in Nature about the discovery of a peculiar object called a recurrent nova that I had never heard of before.   A team of researchers found that this object, with the euphonious name M31N 2008-12a, is a white dwarf being circled by a small, dim star.  This pairing is resulting in some seriously cool behavior, which I'm glad we're observing from a safe 2.5 million light years away.

What's happening is this.  The white dwarf, which is the core of a collapsed star about the size of our Sun, has such a high gravitational pull that it's siphoning off material from its companion.  When the gas and dust approach the surface of the white dwarf, it's heated and compressed so much that the hydrogen component fuses into helium.  This releases so much energy that it causes an explosion, blowing away the top layer of the dust into space.

What's amazing is that these explosions are happening about once a year, and have been going on for a million years.  This has left a shell of dust 400 light years across.   But what's more fascinating still is that it can't go on forever.  Despite the explosions, the white dwarf is gradually gaining mass at the expense of its companion.  Once its mass gets to about 1.4 times the mass of the Sun -- the Chandrasekhar Limit -- the gravitational pull will exceed the outward pressure exerted by the atoms in the star, and it will collapse.  That collapse will trigger further fusion, of helium into carbon, carbon into oxygen, and so forth, and the energy produced by that will trigger one of the brightest events in the universe, a Type 1a Supernova.

Cool enough already, but wait till you hear the rest.  The fusion triggered by the explosion is what creates virtually all the heavier elements in the periodic table.  So a sizable fraction of the atoms in your body were formed during the first few seconds of a colossal stellar explosion.  We are, as Carl Sagan trenchantly remarked, truly made of star-stuff.

Oh, and the parts of the exploding white dwarf not blown away into space, to seed future planets and stars and life forms, are blown inward so hard that the electrons are forced into the atomic nuclei, resulting in, basically, a big ball o' neutrons.  This takes the remaining mass of the star and compresses it into a sphere about ten kilometers across, generating a substance so dense that a matchbox-sized piece of it would weigh three billion tons.

Like I said.  Good thing we're out here at a safe distance.  Sucks for the Kelvans, though.

The one disappointing thing is that the paper in Nature says that although the recurrent nova is still firing off once a year, the cataclysmic final explosion isn't going to happen for another forty thousand years, give or take a year or two.  So unfortunately, we won't be around to see it.  Unless some alien race shows up and turns us into geometric solids and sits us on a shelf, reawakening us just before the cosmic show starts.

But I suppose that's too much to hope for.

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You can't get on social media without running into those "What Star Trek character are you?" and "Click on the color you like best and find out about your personality!" tests, which purport to give you insight into yourself and your unconscious or subconscious traits.  While few of us look at these as any more than the games they are, there's one personality test -- the Myers-Briggs Type Indicator, which boils you down to where you fall on four scales -- extrovert/introvert, sensing/intuition, thinking/feeling, and judging/perceiving -- that a great many people, including a lot of counselors and psychologists, take seriously.

In The Personality Brokers, author Merve Emre looks not only at the test but how it originated.  It's a fascinating and twisty story of marketing, competing interests, praise, and scathing criticism that led to the mother/daughter team of Katharine Briggs and Isabel Myers developing what is now the most familiar personality inventory in the world.

Emre doesn't shy away from the criticisms, but she is fair and even-handed in her approach.  The Personality Brokers is a fantastic read, especially for anyone interested in psychology, the brain, and the complexity of the human personality.

[If you purchase the book from Amazon using the image/link below, part of the proceeds goes to supporting Skeptophilia!]