Advance notice

Today's science news involves something called the Chandrasekhar Limit.

Stars live for most of their lives in an equilibrium between two forces; the inward pull of their own gravity, and the outward pressure from the heat generated by fusion in their cores.  As long as there is plenty of hydrogen left to power fusion, those forces are equal and opposing, and the star is stable.

When the hydrogen is depleted, though, the balance shifts.  The core cools, and the gravitational collapse resumes.  This, however, heats things up -- recall the "ideal gas law" from high school chemistry, and that temperature and pressure are inversely proportional -- and the star begins to fuse the helium "ash" left over from hydrogen burning into carbon.  Eventually that runs out, too, and the process repeats -- carbon to oxygen and silicon, and on up the scale until finally it gets to iron.  At that point, there's nowhere to go; after iron, fusion begins to be an endothermic (energy-requiring) reaction, and the star is pretty much out of gas.

What happens at this point depends on one thing: the star's initial mass.  For a star the size of the Sun, the later stages liberate enough energy to balloon the outer atmosphere into a red giant, and when the final collapse happens, it blows off that atmosphere into a wispy bubble called a planetary nebula.  

The Cat's Eye Nebula (NGC 6543) [Image is in the Public Domain courtesy of NASA]

What's left at the center is the exposed core of the star -- a white dwarf, still glowing from its residual heat.  It doesn't collapse further because its mass is held up by electron degeneracy pressure -- the resistance of electrons to occupying the same quantum state, something known as the Pauli Exclusion Principle.  But it's no longer capable of fusion, so it will simply cool and darken over the next few billion years.

For heavier stars -- between two and ten times the mass of the Sun -- electron degeneracy is not sufficient to halt the collapse.  The electrons are forced into the nuclei of the atoms, and what's left is a densely-packed glob of neutrons called, appropriately enough, a neutron star.  So much energy is liberated by this process that the result is a supernova; the atmosphere is blown away completely, and the collapsed core, which is made of matter dense enough that a teaspoonful would weigh as much as Mount Everest, spins faster and faster because of the Law of Conservation of Angular Momentum, in some cases reaching speeds of thirty rotations per second.  This whirling stellar core is called a pulsar.

For stars even larger than that, though, the pressure of neutron star matter isn't enough to stop the gravitational collapse.  In fact, nothing is.  The supernova and subsequent collapse lead to the formation of a singularity -- a black hole.

So that's the general scheme of things, but keep in mind that this is the simplest case.  Like just about everything in science, reality is more complex.

Suppose you had an ordinary star like the Sun, but it was in a binary system.  The Sun-like star reaches the end of its life as a white dwarf, as per the above description.  Its partner, though, is still in stable middle age.  If it's close enough, the dense, compact white dwarf will begin to funnel material away from its partner, siphoning off the outer atmosphere, and -- this is the significant part -- gaining mass in the process.

Artist's conception of the white dwarf/main sequence binary AE Aquarii [Image is in the Public Domain courtesy of NASA]

The brilliant Indian physicist Subrahmanyan Chandrasekhar figured out that this process can only go on so long -- eventually the white dwarf gains enough mass that its gravity exceeds the outward pressure from electron degeneracy.  At a mass of 1.4 times that of the Sun -- the Chandrasekhar Limit -- the threshold is reached, and the result is a sudden and extremely violent collapse and explosion called a type 1a supernova.  This is one of the most energetic events known -- in a few seconds, it liberates 10^44 Joules of energy (that's 1, followed by 44 zeroes).

So this is why I got kind of excited when I read a paper in Nature Astronomy about a binary star system only 150 light years away made of two white dwarf stars, which are spiraling inward and will eventually collide.

Because that would be the type 1a supernova to end all type 1a supernovas, wouldn't it?  No gradual addition of little bits of mass at a time until you pass the Chandrasekhar Limit; just a horrific, violent collision.  And 150 light years is close enough that it will be a hell of a fireworks show.  Estimates are that it will be ten times brighter than the full Moon.  But at that distance, it won't endanger life on Earth, so it'll be the ideal situation -- a safe, but spectacular, event.

The two stars are currently orbiting their common center of mass at a distance of about one-sixtieth of that between the Earth and the Sun, completing an orbit every fourteen hours.  Immediately before collision, that orbital period will have dropped to the frantic pace of one revolution every thirty seconds.  After that...

... BOOM.

But this was the point where I started thinking, "Hang on a moment."  Conservation of energy laws suggest that to go from a fourteen-hour orbit with a radius of around 2.5 million kilometers, to a thirty-second orbit with a radius of close to zero, would require an enormous loss of energy from the system.  That kind of energy loss doesn't happen quickly.  So how long will this process take?

And there, in the paper, I found it.

This spectacular supernova isn't going to happen for another 23 billion years.

This was my expression upon reading this:

I don't know about you, but even in my most optimistic moments I don't think I'm going to live for another 23 billion years.  So this whole thing gives new meaning to the phrase "advance notice."

You know, I really think y'all astrophysicists need to step up your game, here.  You get our hopes up, and then say, "Well, of course, you know, astronomical time scales..."  Hell, I've been waiting for Betelgeuse to blow up since I was like fifteen years old.  Isn't fifty years astronomical enough for you?

And now, I find out that this amazing new discovery of two madly-whirling white dwarf stars on an unavoidable collision course is going to take even longer.  To which I say: phooey.

I know your priority isn't to entertain laypeople, but c'mon, have a heart.  Down here all we have to keep our attention is the ongoing fall of civilization, and that only gets you so far.  Back in the day, stuff like comets and supernovas and whatnot were considered signs and portents, and were a wonderful diversion from our ancestors' other occupations, such as starving, dying of the plague, and being tortured to death by the Inquisition.  Don't you think we deserve a reason to look up, too?  In every sense of the phrase?

So let's get a move on, astrophysicists.  Find us some imminent stellar hijinks to watch.  I'll allow for some time in the next few months.  A year at most.  I think that's quite generous, really.

And if you come up with something good, I might even forgive you for getting my hopes up about something amazing that won't happen for another 23 billion years.

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A celestial do-si-do

A common -- although, as it turns out, completely understandable -- error is to say that the Earth and other planets orbit the Sun.

No, I'm not recommending a return to the geocentric model, where the Earth is at the center of the universe and everything orbits around it in perfect circles, as decreed by the Almighty at the moment of creation (which, of course, was six thousand years ago).  The inaccuracy I'm referring to is much smaller than that -- but is still significant.

Instead of saying "the planets orbit the Sun," the more precise way to state it is that the planets and the Sun all orbit their common center of gravity.  Newton's Third Law describes how every force exerted creates an equal and opposite force -- so just as the Sun is pulling on the Earth, the Earth is pulling on the Sun.  The result is that both are in a dance around the system's center of gravity.  Given the Sun's vastly larger mass, their mutual center of gravity is well inside the Sun, so to say "the Earth orbits the Sun" is a sufficiently close approximation to account for what we observe on a daily basis.

The effect is big enough, though, that this is one of the ways that exoplanets have been discovered -- mostly in nearby systems, where it's easier to see.  A star with an unseen companion gets pulled around as they orbit their common center of gravity, so from our perspective it looks like the star has a slight wobble.  As the wobble is bigger if the planet has a larger mass, this technique has been used mostly to find exoplanets that are gas giants, like Jupiter and Saturn, which are big enough to sling their host star around more effectively.

Sometimes, though, looking for a stellar wobble results in discovering something else -- an invisible object much too massive to be a planet, in a celestial do-si-do with a star.

That was the subject of a paper published this week in The Open Journal of Astrophysics, describing research led by Kareem El-Badry of Caltech.  The team found 21 stars with heavy but invisible companions, which from their size appear to be neutron stars, the collapsed, ultra-dense cores left behind by giant stars after they exhaust their fuel.

The curious thing is that prior to the formation of a neutron star, the giant star went supernova -- so why didn't that colossal explosion completely blow away the Sun-like star it's paired with?  The simple answer is we don't know.  "We still do not have a complete model for how these binaries form," El-Badry said.  "In principle, the progenitor to the neutron star should have become huge and interacted with the solar-type star during its late-stage evolution.  The huge star would have knocked the little star around, likely temporarily engulfing it.  Later, the neutron star progenitor would have exploded in a supernova, which, according to models, should have unbound the binary systems, sending the neutron stars and Sun-like stars careening in opposite directions...  The discovery of these new systems shows that at least some binaries survive these cataclysmic processes even though models cannot yet fully explain how."

If El-Badry et al.'s research bears up, it will be the first time neutron stars have been detected purely by their gravitational effects.

So that's today's cool news from science.  A stellar dance between a Sun-like star and a collapsed, super-dense neutron star.  And I love that El-Badry ends with the words, "... models cannot yet fully explain how."  Focus on the word "yet."  These are the sorts of things that push science forward -- some unexplained observation that makes scientists scratch their heads.  As Isaac Asimov put it, "The most exciting phrase to hear in science, the one that heralds new discoveries, is not 'Eureka!', but '... that's funny.'"

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Vanishing act

In Madeleine L'Engle's seminal young-adult fantasy novel The Wind in the Door, there's something that is making the stars go out.

Not just stop shining, but disappear entirely.  Here's the scene where the protagonist, Meg Murry, first witnesses it happening:

The warm rose and lavender of sunset faded, dimmed, was extinguished.  The sky was drenched with green at the horizon, muting upwards into a deep, purply blue through which stars began to appear in totally unfamiliar constellations.

Meg asked, "Where are we?"

"Never mind where.  Watch."

She stood beside him, looking at the brilliance of the stars.  Then came a sound, a violent, silent, electrical report, which made her press her hands in pain against her ears.  Across the sky, where the stars were clustered as thickly as in the Milky Way, a crack shivered, slivered, became a line of nothingness.

Within that crack, every star that had been there only a moment ago winked out of existence.

A central point in the story is that according to the laws of physics, this isn't supposed to happen.  Stars don't just vanish.  When they end their lives, they do so in an obvious and violent fashion -- even small-mass stars like the Sun swell into a red giant, and eventually undergo core collapse and blow off their outer atmospheres, creating a planetary nebula.  

The Cat's Eye Nebula [Image is in the Public Domain courtesy of NASA/JPL and the ESO]

Larger stars end their lives even more dramatically, as supernovas which lead to the formation of a neutron star or a black hole depending on how much matter is left over once the star blows up.

Well, that's what we thought always happened.

A study out of the University of Copenhagen has found that like in A Wind in the Door, sometimes stars simply... vanish.  A team of astrophysicists has found that instead of the usual progression of Main Sequence > Giant or Supergiant > BOOM! > White Dwarf, Neutron Star, or Black Hole, there are stars that undergo what the astrophysicists are (accurately if uncreatively) calling "complete collapse."  In a complete collapse, the gravitational pull is so high that even considering the power of a supernova, there's just not enough energy available for the outer atmosphere to achieve escape velocity.  So instead of exploding, it just kind of goes...

... pfft.

Unlike what Meg Murry witnessed, though, the matter that formed those stars is still there somewhere; the Law of Conservation of Matter and Energy is strictly enforced in all jurisdictions.  The star that was the focus of the study, VFTS 243, is part of a binary system -- and its companion star continued in its original orbit around their mutual center of mass without so much as a flutter, so the mass of its now-invisible partner is still there.  But the expected cataclysmic blast that usually precedes black hole formation never happened.

"We believe that the core of a star can collapse under its own weight, as happens to massive stars in the final phase of their lives," said Alejandro Vigna-Gómez, who co-authored the study.  "But instead of the contraction culminating into a bright supernova explosion that would outshine its own galaxy, expected for stars more than eight times as massive as the Sun, the collapse continues until the star becomes a black hole.  Were one to stand gazing up at a visible star going through a total collapse, it might, just at the right time, be like watching a star suddenly extinguish and disappear from the heavens.  The collapse is so complete that no explosion occurs, nothing escapes and one wouldn't see any bright supernova in the night sky.  Astronomers have actually observed the sudden disappearance of brightly shining stars in recent times.  We cannot be sure of a connection, but the results we have obtained from analyzing VFTS 243 has brought us much closer to a credible explanation."

You can see why I was immediately reminded of the scene in L'Engle's book.  And while I'm sure the answer isn't evil beings called Echthroi who are trying to extinguish all the light in the universe, the actual phenomenon is still a little on the unsettling side.

Once again showing that we are very far from understanding everything there is out there.  This sort of vanishing act has been high on the list of Things That Aren't Supposed To Happen.  It'll be interesting to see what the theorists propose with when they've had a shot at analyzing the situation, and if they can come up with some sort of factor that determines whether a massive star detonates -- or simply disappears.

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Witness to a crash

Well, thanks to my friend, the brilliant writer Gil Miller, I now have another reason to huddle under my blankie for the rest of the day.

We've dealt here before with a great many cosmic phenomena that you would seriously not want to get too close to.  Some of these sound like Geordi-Laforgian technobabble from Star Trek, but I promise all of them are quite real:

• supernovae
• quasars
• blazars
• fast radio bursts
• magnetars
• fast blue optical transients
• Wolf-Rayet stars
• gamma-ray bursters
• false vacuum collapse

From this,  you might come to the conclusion that I have a morbid fascination with astronomical phenomena that are big and scary and dangerous and can kill you.  This is not entirely incorrect; I would only modify it insofar as to add that I am also morbidly fascinated with geological phenomena (earthquakes, volcanoes, pyroclastic flows, lahars) and meteorological phenomena (hurricanes, tornadoes, lightning, microbursts) that are big and scary and dangerous and can kill you.

Call it a failing.

In any case, thanks to Gil's eagle-eyed facility for spotting cool recent research in science, I now have a new astronomical one to add to the list -- a luminous fast cooler.  This one provides the added frisson of being (as yet) unexplained -- although as you'll see, there's a possible explanation for it that makes it even scarier.

The research that uncovered the phenomenon was done by a team led by Matt Nicholl, astrophysicist at Queen's University Belfast, using data from ATLAS, the Asteroid Terrestrial-Impact Last Alert System (speaking of scary phenomena) telescope network in Hawaii, Chile and South Africa.  The event they discovered was (fortunately) nowhere near our own neighborhood; it was spotted in a galaxy two billion light years away.

What happened is that a completely ordinary, Sun-like star suddenly flared up by a factor of a hundred billion.  The first thought, of course, was supernova -- but this explosion's profile was completely different than that of a supernova, and stars the size of the Sun aren't supposed to go supernova anyhow.  Then, as if to add to the mystery, it cooled just as fast, fading by two orders of magnitude in only two weeks.  A month later, it was only at one percent of its peak brightness shortly after detonating (still, of course, considerably brighter than it had been).

The first question, of course, is "if it wasn't a supernova, what was it?"  And the answer thus far is "we're not sure."  So the researchers started trying to find other examples of the phenomenon, and uncovered two previously unrecognized events that matched the recent explosion's profile, one in 2009 and one in 2020.

But that still doesn't tell us how a perfectly ordinary star can suddenly go boom.  Nicholl says that the team has come up with only one possible hypothesis -- and it's a doozie.

"The most plausible explanation seems to be a black hole colliding with a star," Nicholl said.

Well, that's just all kinds of comforting.

Artist's conception of a black hole devouring a star [Image is in the Public Domain courtesy of NASA/JPL]

So it's all very well to say cheerily, "Hey, at least the Sun's not gonna go supernova, and we don't have any Wolf-Rayet stars nearby, and the nearest gamma-ray burster isn't pointed in our direction, and false vacuum collapse is really unlikely!  We're sitting here happily orbiting a highly stable star still in the prime of life, in a quiet corner of the galaxy!  What could go wrong?"

Apparently, what could go wrong is that a black hole could come swooping in out of nowhere and make the Sun explode.

Now, mind you, there are no black holes near us.  That we know of.  And chances are, we would, because even though they're black (thus the name), their influence on the matter around them is considerable.  The great likelihood is if there were a black hole headed for a crash with the Sun, you'd know about it plenty in advance.

Not that there's anything you could do about it, other than the time-honored maneuver of sticking your head between your legs and kissing your ass goodbye.

So thanks to Gil for making me feel even tinier and more fragile than I already did, which led me to share this delightful discovery with you.

Have a nice day.

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Kablooie

I'm kind of an excitable type.

I think that may be why I went into science.  The rigorous, evidence-basted methods of science were a nice antidote to the fact that my natural state is having my emotions swinging me around by the tail constantly.

Even after years of studying (and teaching) science, and twelve years of writing about it here at Skeptophilia Central, I still have the capacity for going off the deep end sometimes.  Which is what happened when I read a paper (a preprint, actually) from the Monthly Notices of the Royal Astronomical Society called "The Evolutionary Stage of Betelgeuse Inferred from its Pulsation Periods," by Hideyuki Saio (Tohoku University) and Devesh Nandal, Georges Meynet, and Sylvia Ekström (Université de Genève).

The constellation Orion.  Betelgeuse is in the upper left corner of the image.  [Image licensed under the Creative Commons Mouser, Orion 3008 huge, CC BY-SA 3.0]

First, a little background, before I get to the squee-inducing part.

Stars exist in a state of tension between two forces -- the inward pull of gravity and the outward pressure from the heat produced by fusion in the core.  At the very beginning of their lives, stars form from a loose cloud of mostly hydrogen gas that collapses under its own attractive gravitational force.  That collapse increases the pressure and temperature, and -- if the initial cloud was big enough -- eventually they rise high enough to trigger the fusion of hydrogen atoms into helium.  This is a (very) energy-releasing reaction -- physicists call such reactions exothermic -- and that energy pushes outward, balancing the inward pull of gravity.  The star goes into equilibrium.

But there's not an infinite supply of hydrogen.  The hydrogen fuel in the core is eventually exhausted, so fusion slows down.  The temperature drops, as does the outward pressure, so -- for a while -- gravity wins.  The star collapses, heating the core up, until the temperature and pressure become sufficient to fuse the helium "ash" in the core into carbon.  (This process, incidentally, is where the carbon in the organic molecules in our bodies comes from; Carl Sagan was spot-on in saying "We are made from star stuff.")

Helium fusion is also exothermic, so once again, the star goes into equilibrium.  But then the helium runs out, and the collapse resumes until the pressure and temperature are high enough to fuse carbon into oxygen. 

Then oxygen into silicon.  Then silicon into iron.

Two things are important here.  The first is that each of the reactions -- from hydrogen fusing into helium through silicon fusing into iron -- produces less energy than the one before it but requires higher temperatures and pressures to make it happen.  The second is that something happens when you pass that final reaction, which is that any subsequent fusion into heavier elements is an endothermic, or energy-consuming, reaction.

So when the silicon is used up, and the star's core is made mostly of iron, there's pretty much nowhere to go.  The gravitational collapse picks up again, and there is no "next reaction" that might produce energy to balance it.  So the collapse continues until finally there's such a tremendous temperature spike that the entire star goes kablooie.

This is called a supernova, and it releases more energy in a few seconds than the star liberated in the entire rest of its life.  The unimaginable pressures do fuse some of the iron in the core into those heavier elements, despite the energy required, and that's where all the elements on the periodic table with atomic numbers higher than 26 come from, from the gold in our jewelry to the silver in our coinage and the copper in our electrical wires.

With me so far?  Because there's one more thing I haven't told you.

Each stage in a star's life takes much less time than the one before it.

The hydrogen to helium stage lasts millions to billions of years.  (The Sun is in the hydrogen-burning stage, and is estimated to have another five billion years to go.)  Higher-mass stars have higher pressures and temperatures, and consume their fuel at a greater rate, but we're still talking tens to hundreds of millions of years.  Helium-to-carbon lasts maybe a million years; carbon-to-oxygen, we're talking decades.

After that, it's pretty much a ticking time bomb with a very short fuse.

Now for the punch line: the Saio et al. paper suggests that the pulsation periods of the red supergiant star Betelgeuse indicate that it is nearing the end of the carbon burning stage.  So we might actually have a shot at seeing one of the brightest stars in the sky go supernova in our lifetimes.

This paper has even the scientists flipping out.  One of my favorite science vloggers, astronomer Becky Smethurst of Oxford University, did a YouTube video about this paper and you could tell she was barely keeping it together.  Ordinarily, whenever you hear about anything impressive in sciences like astronomy and geology -- such as a supernova or gamma-ray burster, or the Yellowstone Supervolcano erupting or the East African Rift Zone tearing Africa apart -- the scientists will respond with a deep sigh and a monotone "as we've explained many times before, blah blah blah astronomical/geological time scales blah blah blah."

Now, though, the astronomers are actually acting like this is the real deal.  (And in fact, if Saio et al. are right, Betelgeuse has probably already blown itself to smithereens; at six-hundred-odd light years away, we just haven't gotten the memo yet.)

When this happens, it's gonna be spectacular.  A supernova that close will be bright enough to read by at night, most likely for months, and will be easily visible during the day.  The happy news is that it's not close enough to do us any damage; a supernova under twenty-five light years away could be catastrophic, doing nasty stuff like blowing away the atmosphere.  (Fortunately, there are no supernova candidates anywhere near that close to us.)  Betelgeuse will just create some amazing fireworks, as well as permanently changing the contour of the familiar constellation of Orion.

So my opinion is: bring on the supernova.  We could use a little excitement down here.

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Forensic geology

I've been interested in rocks since I was a kid.  My dad was a rockhound -- more specifically, a lapidary, who made jewelry from such semiprecious stones as turquoise, agate, and jasper.  The high point of my year was our annual trip to Arizona and New Mexico, when we split our time between searching for cool rocks in the canyons and hills of the southwestern desert and pawing through the offerings of the hundreds of rock shops found throughout the region.

Besides the simple beauty of the rocks themselves, it fascinated me to find out that with many rocks, you could figure out how and when they formed.  A lot of the gem-quality rocks and minerals my dad was looking for -- malachite, azurite, and opal amongst them -- are created by slow precipitation of layers of minerals from supersaturated water; others, such as lapis lazuli, rhodonite, and garnets form when metal-bearing rocks are metamorphosed by contact with magma far underground.

[Image licensed under the Creative Commons Olga Semiletova, Минералы горных пород, Creative Commons Attribution-Share Alike 4.0 International license]

Once I found out that the "when" part was also often knowable, through such techniques as radioisotope dating and stratigraphy, it was always with a sense of awe that I held pieces of rock in my hand.  Even around where I live now, where there are few if any of the lovely gem-quality stones you find in the southwest, there's still something kind of mind-boggling about knowing the layers of limestone and shale that form the bedrock here in upstate New York were formed in the warm shallows of a warm ocean during the Devonian Period, on the order of four hundred million years ago.

But if you think that's impressive, wait till you hear about the research out of the University of Johannesburg that was published in the journal Icarus last week.

The research centered around a stone in the desert of western Egypt called Hypatia, given the name by Egyptian geologist Aly Barakat in honor of the brilliant, tragic polymath whose career was cut short when she was brutally murdered by a mob on the orders of Cyril, bishop of Alexandria.  (The aftermath, although infuriating, is typical of the time; Hypatia was largely forgotten, while Cyril went on to be canonized as a saint by the Roman Catholic Church.)  The stone, fittingly considering Hypatia's contributions to astronomy, turns out to be extraterrestrial in origin, later falling as a meteorite to the surface of the Earth.

But "extraterrestrial" is a big place, as it were.  Where exactly did it form?  Chemical tests on the rock found that it didn't match the composition of any known asteroid or comet; then, the mystery deepened when it was found to contain nickel phosphide, which has never been found on any solid material tested in the entire Solar System.

Further tests only made the rock seem more anomalous.  Silicon, second only to oxygen as the most common element in the Earth's crust (a little over 28%, to be exact), was almost absent, as were calcium, chromium, and manganese; on the other hand, there was far more iron, sulfur, phosphorus, copper, and vanadium than you'd expect.  The ratios were far off not only from rocks in our Solar System, they didn't match the composition of interstellar dust, either.

The researchers decided to go at it from the other direction.  Instead of trying to find another sample that matched, they looked at what process would create the element ratios that Hypatia has.  And they found only one candidate that matched.

A type 1a supernova.

Type 1a supernovas occur in binary star systems, when one of the stars is relatively low mass (on the order of the Sun) and ends its life as a super-compact white dwarf star.  White dwarf stars have an upper limit on their mass (specifically about 1.4 times the mass of the Sun) called the Chandrasekhar limit, after Nobel Prize winning astronomer Subrahmanyan Chandrasekhar.  The reason is that at the end of a star's life, when the outward pressure caused by the fusion in the core drops to the point that it can't overcome the inward pull of gravity from the star's mass, it begins to collapse until some other force kicks in to oppose it.  In white dwarf stars, this occurs when the mutual repulsion of electrons in the star's constituent atoms counterbalances the pull of gravity.  In stellar remnants more than 1.4 times the mass of the Sun, electrostatic repulsion isn't powerful enough to halt the collapse.  (The other two possibilities, for progressively higher masses, are neutron stars and black holes.)

In binary stars, when one of the members becomes a white dwarf, the gravitational pull of its extremely compact mass begins to siphon material from its companion.  This (obviously) increases the white dwarf's mass.  Once it passes the Chandrasekhar limit, the white dwarf resumes its collapse.  The temperature of the white dwarf skyrockets, and...

... BOOM.

The whole thing blows itself to smithereens.  Fortunately for us, really; a lot of the elements that make up the Solar System were formed in violent events such as the various kinds of supernovas.  But the models of the relatively rare type 1a (only thought to happen once or twice a century in a typical galaxy of a hundred billion stars) generate a distinct set of elements -- and the percent composition of Hypatia matches the prediction perfectly.

So this chunk of rock in the Egyptian desert was created in the cataclysmic self-destruction of a white dwarf star, probably long before the Solar System even formed.  Since then it's been coursing through interstellar space, eventually colliding with our obscure little planet in the outskirts of the Milky Way.

When I was twelve, holding a piece of billion-year-old limestone from the Grand Canyon, little did I realize how much more amazing such origin stories could get.

I think the real Hypatia would have been fascinated.

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Kaboom!

Sometimes I have to tell y'all about something not because it's weird or controversial, but purely because it's cool.

Today we're going to look at some very recent research into what happens during a supernova.  The difficulty with studying supernovas is (1) they're rare, (2) it's not easy to generate comparable conditions in the lab, and (3) if you're close enough to them to get a really good look, you'll probably get vaporized and never even get to the peer review stage with your research.  That a supernova is cataclysmic hardly even bears mention; exactly how cataclysmic is hard even to wrap your brain around.  In a short time -- days to weeks -- a supernova releases more energy than the Sun will over its entire ten billion year lifetime.  The energy release of a typical supernova is 100 quintillion yottajoules.

Being a former science teacher, I thought I knew my metric prefixes, but yotta- was one I didn't know.  Kilo-, mega-, tera- I know; on the other end, micro-, nano-, and pico-.  But yotta- I had to look up.  Turns out it's one septillion; one followed by twenty-four zeroes.  So a supernova releases 100 quintillion of those.

Suffice it to say that 100 quintillion yottajoules is a lottajoules.

[Image licensed under the Creative Commons ESO/L. Calçada, The material around SN 1987A, CC BY 3.0]

Astonishingly, scientists have now been able to recreate in the lab a burst that for a tiny fraction of a second has the energy release of a supernova.  It's so fast that it doesn't blow the lab to smithereens, but it's still pretty spectacular.  Physicist Hye-Sook Park of the Lawrence Livermore National Laboratory accomplished this by amplifying and then focusing 192 lasers on a tiny target, which implodes and then explodes violently, mimicking for one ten-billionth of a second the conditions that occur the moment a supernova goes kaboom. 

The result is a fantastically energetic plasma -- stream of charged particles -- that can pick up 1,000 trillion electron volts of energy, which (for comparison) is about five hundred times the energy that the particles in the largest accelerators have.  This shock wave of charged particles generates a rapidly fluctuating magnetic field.  "The result is a complex feedback process of jostling particles and fields, eventually producing a shock wave," said team member Anatoly Spitkovsky of Princeton University.  "This is why it’s so fascinating.  It’s a self-modulating, self-controlling, self-reproducing structure.  It’s like it’s almost alive.”

One of the things that Park's group was looking for was a sign of the Weibel instability -- a pileup of charged particles moving in opposite directions, which was predicted to happen as the outflowing plasma from the supernova met the plasma of the interstellar medium.  The instability occurs because the outward surge isn't uniform, so it breaks into streams that interlace with inward-streaming interstellar plasma like the your fingers when you clasp your hands together.  These streamers are moving charges -- i.e., an electric current.  And any electric current generates a magnetic field, which then acts to reinforce the positions, directions, and speeds of the particle streams.

Which is exactly what Park's experiment did.

And not to overload you with superlatives, but the speed of these streams is staggering -- 1,500 kilometers a second, enough to circle the Earth twice in a little less than a minute.  Because the magnitude of the magnetic field is dependent on the current speed, this produces huge field strengths -- in that one ten-billionth of a second, it created a magnetic field of 30 tesla, which is twenty times the strength of the magnetic field in an MRI machine.

One of the coolest things about all this is how excited Park gets when one of her experiments succeeds.  "I still clearly remember the time when I was seeing something nobody’s seen," she said.  "When things go well, everyone nearby knows.  I've heard people say, 'We can hear Hye-Sook screaming.'"

This kind of excitement in discovery is what science is all about -- the thrill of finding out something new about the universe.  It brings to mind the quote by the brilliant German chemist Carl Wilhelm Scheele: "It is the truth alone that we desire to know, and what a joy there is in discovering it."

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This week's Skeptophilia book-of-the-week is about our much maligned and poorly-understood cousins, the Neanderthals.

In Rebecca Wragg Sykes's new book Kindred: Neanderthal Life, Love, Death, and Art we learn that our comic-book picture of these prehistoric relatives of Homo sapiens were far from the primitive, leopard-skin-wearing brutes depicted in movies and fiction.  They had culture -- they made amazingly evocative and sophisticated art, buried their dead with rituals we can still see traces of, and most likely had both music and language.  Interestingly, they interbred with more modern Homo sapiens over a long period of time -- DNA analysis of humans today show that a great many of us (myself included) carry around significant numbers of Neanderthal genetic markers.

It's a revealing look at our nearest recent relatives, who were the dominant primate species in the northern parts of Eurasia for a hundred thousand years.  If you want to find out more about these mysterious hominins -- some of whom were our direct ancestors -- you need to read Sykes's book.  It's brilliant.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]

Eye of the storm

Ever heard of Wolf-Rayet stars?  They deserve more notice than they get, as one of the most violently energetic phenomena in the universe.  The fact that the name is not in common parlance -- when even the most scientifically-uninterested layperson has heard of supernovae and quasars and black holes -- is probably due to a combination of (1) their rarity, and (2) the fact that the ones that are visible to the naked eye are pretty unimpressive-looking at first glance.  Gamma Velorum and Theta Muscae, both of which are in the Southern Hemisphere and never visible where I live in upstate New York, are Wolf-Rayet stars that look completely ordinary until you check out their light spectra and find out that there's something really extraordinary going on.

The first thing that becomes apparent is that they are hot.  I mean, even by stellar standards.  Wolf-Rayet stars have a surface temperature between 30,000 K and an almost unimaginable 210,000 K.  (By comparison, the Sun's surface is about 5,700 K.)  These temperatures fuel an enormously strong stellar wind, which blows away almost all of the lightweight hydrogen in the outer layers, and also ionizes most of what is left -- predominantly oxygen, nitrogen, and carbon.  They're at the head of the list of potential gamma-ray bursters -- stars that undergo sudden collapse followed by a colossal explosion, resulting in a blast of gamma rays collimated into narrow beams along the star's rotational axis.  So having a Wolf-Rayet star's rotational axis pointed toward your planet would be like staring down the barrel of a loaded gun.

They're also beautiful.  At least from a distance.  The reason all this comes up is because of a paper last week in Monthly Notices of the Royal Astronomical Society about one that's been called "a stellar peacock" -- the star Apep, in the constellation Norma.  This Wolf-Rayet star has blown carbon-laden dust from its surface, which its high rotational speed swept into a pinwheel.

[Image licensed under the Creative Commons ESO/Callingham et al, The triple star system 2XMM J160050.7–514245 (Apep), CC BY 4.0]

The name Apep comes from Egyptian mythology -- Apep was the monstrous serpent who was the enemy of the god Ra.  Astronomer Joseph Callingham, one of the first to study Apep, thought the name was apt -- in his words it was "a star embattled within a dragon's coils."

All poetic license aside, the violent imagery is spot-on.  Wolf-Rayet stars eventually self-destruct, becoming black holes, but not until basically destroying anything unfortunate enough to be nearby.  So the bright spot at the center of Apep is the eye of a cosmic-scale storm.

Last week's paper, by a team led by University of Sydney student Yinuo Han, uses observational data from the Very Large Telescope in Chile to understand what is creating the spiral plumes.  The detail is phenomenal; in an interview with Science Daily, Han said, "The magnification required to produce the imagery was like seeing a chickpea on a table fifty kilometers away."

"[Wolf-Rayet stars] are ticking time bombs," said study co-author Peter Tuthill.  "As well as exhibiting all the usual extreme behavior of Wolf-Rayets, Apep's main star looks to be rapidly rotating.  This means it could have all the ingredients to detonate a long gamma-ray burst when it goes supernova."

It's hard to say anything about this group of stars without lapsing into superlatives.  "The speeds of the stellar winds produced are just mind-blowing," Han said.  "They are spinning off the stars at about twelve million kilometers an hour.  That's one percent the speed of light."

Fortunately for us, Apep is a safe 6,600 light years away, so it poses no danger to us.  If one was a lot nearer -- within 25 or so light years' distance -- it would be catastrophic.  The radiation bombardment could strip away the ozone layer, leaving the Earth's surface subject to massive irradiation.  There's decent evidence that some of the Earth's mass extinctions may have been caused by nearby supernovae (not necessarily Wolf-Rayets).  But to put your mind at ease, there aren't any supernovae candidates of any sort within what is rather terrifyingly called "the kill zone."

So that's a look at one of the most dangerous and beautiful phenomena in the universe.  I'm glad we're getting to see it, and find out a little bit about what makes it tick.

From a safe distance.

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This week's Skeptophilia book recommendation is brand new, and is as elegiac as it is inspiring -- David Attenborough's A Life on Our Planet: My Witness Statement and a Vision for the Future.

Attenborough is a familiar name, face, and (especially) voice to those of us who love nature documentaries.  Through series such as Our Planet, Life on Earth, and Planet Earth, he has brought into our homes the beauty of nature -- and its desperate fragility.

At 93, Attenborough's A Life on Our Planet is a fitting coda to his lifelong quest to spark wonder in our minds at the beauty that surrounds us, but at the same time wake us up to the perils of what we're doing to it.  His message isn't all doom and gloom; despite it all, he remains hopeful, and firm in his conviction that we can reverse our course and save what's left of the biodiversity of the Earth.  It's a poignant and evocative work -- something everyone who has been inspired by Attenborough for decades should put on their reading list.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]

Deadly fireworks

I've always thought it would be amazingly cool to witness a supernova.

Imagine it.  Within a few hours, a dim, ordinary-looking star increases in luminosity until it outshines every other astronomical object in the sky except the Sun and Moon.  It's visible during the day and you can read by its light at night.  It's not a blink-and-you'll-miss-it phenomenon, either; the light from the massive explosion peaks rapidly but declines slowly.  Most supernovae will be visible for months, before dimming to near-invisibility, ending as neutron stars or black holes.

There are lots of candidates for what could be the next supernova, although don't get your hopes up; most of these fall into the "some time in the next million years" category.  Yeah, it could happen tomorrow, but I wouldn't put money on it.  Still, the list is sizable, and here are five of the best possibilities:

• Betelgeuse (720 light years away, in the constellation Orion).  This one got some serious press a few months ago because it suddenly started to decrease in brightness, and astronomers wondered if this was a prelude to an explosion.  What appears to have happened is that there was turbulence in the star's core that blew a cloud of dust from its surface, obscuring the star and making it appear to dim.  So we're still waiting for this red supergiant to explode, and probably will be for a while.
• IK Pegasi (154 light years away, in the constellation Pegasus).  IK Pegasi isn't well known because at an apparent magnitude of 6, it's not visible to the naked eye, but it bears mention as the nearest serious supernova candidate.  It's a double star -- a main-sequence star and a massive white dwarf orbiting a common center of mass.  As the main-sequence star evolves, it will become a red giant, with a radius large enough that its white dwarf companion will start suctioning matter from its surface.  When the white dwarf reaches what's called the Chandrasekhar Limit -- 1.4 solar masses -- it will explode cataclysmically as a Type 1a supernova.  This will not only be spectacular but potentially dangerous -- a topic we will revisit shortly.
• VY Canis Majoris (3,820 light years away, in the constellation Canis Major).  Another star not visible to the naked eye, VY Canis Majoris is a lot more spectacular than you'd think to look at it.  It's the largest star known, with a mass fifteen times that of the Sun, and a radius so large that if you put it where the Sun is, its surface would be about at the orbit of Jupiter (so we'd be inside the star).  This "hypergiant" is one of the most luminous stars in the Milky Way, and is only dim because it's so far away.  This one is certain to go supernova, probably some time in the next 100,000 years, and the remnants will collapse into a black hole.
• Eta Carinae (7,500 light years away, in the constellation Carina).  Eta Carinae is another huge star, with a radius twenty times that of the Sun, but what makes this one stand out is its bizarre behavior.  In 1837 it suddenly brightened to being one of the five brightest stars in the night sky, then over the next sixty years faded to the point that it was only visible in binoculars.  Detailed observations have shown that it blew out a huge cloud of material in "The Great Eruption," which is now the Homunculus Nebula.  It's a unique object, which makes it hard to predict its future behavior.  What seems certain is that it'll eventually explode, but there's no telling when that might occur.

The consensus amongst astronomers, however, is that the next likely supernova probably isn't on the list -- that it will be a previously-unknown white dwarf or an unremarkable-looking red giant.  We know so little about supernovas that it's impossible to predict them with any kind of accuracy.  And while this is an exciting prospect, we'd better hope that the next supernova isn't too close.

The Homunculus Nebula with Eta Carinae at the center [Image licensed under the Creative Commons ESA/Hubble, Cosmic Fireworks in Ultraviolet Eta Carinae Nebula, CC BY 4.0]

Not only do supernovas produce a lot of light, they generate a tremendous amount of radiation of other kinds, including cosmic rays.  A close supernova could produce enough cosmic rays to wipe out the ozone layer -- leading to a huge influx of ultraviolet light from the Sun, with devastating effects.

Scarily, this may have already happened in Earth's history.  One of the lesser-known mass extinctions occurred at the end of the Devonian Period, 359 million years ago.  Because it is poorly understood, and was dwarfed by the cataclysmic Permian-Triassic Extinction a little over a hundred million years later, it's not one you tend to read about in the paleontology-for-the-layperson books.  Even so, it was pretty significant, wiping out 19% of known families and 50% of known genera, including placoderms (armored fish), cystoids (a relative of the starfish), and graptolites (colonial animals not closely related to any living species).  Most striking were the collapse of reef-forming corals -- reefs didn't begin to form again on any significant scale until the Mesozoic Era, almost two hundred million years later -- and the near-complete wipeout of vertebrates.  The latter left no vertebrate species over a meter long (most of them were under ten centimeters), and again, it was millions of years before any kind of recovery took place.

Fortunately for us, it eventually did, because we're talking about our ancestors, here.

The cause of this catastrophe has been a matter of speculation, but a team led by Brian Fields, astrophysicist at the University of Illinois, may have found a smoking gun.  In a paper this week in Proceedings of the National Academy of Sciences, we find out that the most likely cause for the End-Devonian Extinction is a nearby supernova that caused the collapse of the ozone layer, leading to the Earth's surface being scorched by ultraviolet light.  This triggered a massive die-off of plants -- which had only recently colonized the land -- and worldwide anoxia.  

The result?  A mass extinction that hit just about every taxon known.

The idea that a supernova might have been to blame for the End-Devonian Extinction came from the presence of hundreds of thousands of plant spores in sedimentary rock layers that showed evidence of what appeared to be radiation damage.  This isn't conclusive, of course; the Fields et al. team is up front that this is only a working hypothesis.  What they'll be looking for next is isotopes of elements in those same rock layers that are only produced by bombardment with radiation, such as plutonium-244 and samarium-146.  "When you see green bananas in Illinois, you know they are fresh, and you know they did not grow here," Fields said, in an interview in Science Daily.  "Like bananas, Pu-244 and Sm-146 decay over time.  So if we find these radioisotopes on Earth today, we know they are fresh and not from here -- the green bananas of the isotope world -- and thus the smoking guns of a nearby supernova."

So as much as I'd love to witness a supernova in my lifetime, it'd be nice if it was one well outside of the terrifyingly-named "kill zone" (thought to be about 25 light years or so).  And chances are, there's nothing inside that radius we need to worry about.  If any of the known supernova candidates explode, we'll almost certainly be able to enjoy the fireworks from a safe distance.

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Fan of true crime stories?  This week's Skeptophilia book recommendation of the week is for you.

In The Poisoner's Handbook:Murder and the Birth of Forensic Medicine in Jazz Age New York, by Deborah Blum, you'll find out about how forensic science got off the ground -- through the efforts of two scientists, Charles Norris and Alexander Gettler, who took on the corruption-ridden law enforcement offices of Tammany Hall in order to stop people from literally getting away with murder.

In a book that reads more like a crime thriller than it does history, Blum takes us along with Norris and Gettler as they turned crime detection into a true science, resulting in hundreds of people being brought to justice for what would otherwise have been unsolved murders.  In Blum's hands, it's a fast, brilliant read -- if you're a fan of CSI, Forensics Files, and Bones, get a copy of The Poisoner's Handbook, you won't be able to put it down.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]

Cosmic dynamite

This seems to be a month for astronomical superlatives.  Two weeks ago I posted here at Skeptophilia about the discovery of the three most luminous quasars ever seen, and just last week a paper hit in Nature Astronomy showing that a supernova seen back in 2016 was the brightest and most powerful ever recorded, outshining the nearest competitor by a factor of ten.

In "An Extremely Energetic Supernova from a Very Massive Star in a Dense Medium" -- given scientists' usual efforts not to overplay their discoveries, the very title indicates how remarkable this is -- a team led by Matt Nicholl of the University of Birmingham describes a massive explosion of a star in a yet-unnamed galaxy four billion light years from us that released energy on the order of 10^52 ergs -- that's a 1 followed by fifty-two zeroes -- which is ten times higher than the previous record-holder.

The galaxy is a dwarf elliptical galaxy, and seems to be similar in structure to the Magellanic Clouds, the two dwarf galaxies near the Milky Way.  This supernova (SN2016aps) is something else again, outshining its entire host galaxy by a large margin.  "SN2016aps is spectacular in several ways," said Edo Berger, Harvard University astronomy professor and co-author on the paper, in a press release.  "Not only is it brighter than any other supernova we’ve ever seen, but it has several properties and features that make it rare in comparison to other explosions of stars in the universe...  The intense energy output of this supernova pointed to an incredibly massive star progenitor.  At birth, this star was at least a hundred times the mass of our Sun."

SN2016aps [Image from the Panoramic Survey Telescopes and Rapid Response System (Pan-STARRS)]

The reason that the star suffered such a colossal explosion seems to go back to events that preceded the final blast.  It's likely that the star itself was formed from the merger of two less massive stars, an event that caused a huge release of a shell of hydrogen gas from the combined star's surface.  When the core of the star(s) went supernova, the material blasted from the core collided with the shell of hydrogen, touching off an energetic shock wave of a size that beggars belief.

"Spectroscopic observations during the followup study revealed a restless history for the progenitor star,” said study lead author Matt Nicholl.  "We determined that in the final years before it exploded, the star shed a massive shell of gas as it violently pulsated.  The collision of the explosion debris with this massive shell led to the incredible brightness of the supernova.  It essentially added fuel to the fire."

The fact that this supernova is four billion light years away should be reassuring; not only is that a "far piece from here" (as my grandma used to describe anything more than about five miles away), but this means the explosion occurred four billion years ago.  The fact that there has never been anything seen on this magnitude from nearer to us (and therefore, that happened more recently) may mean not only that such events are extremely rare, but that they were more likely in the early universe than they are now.

Which is a relief.  As spectacular as this would be, seeing it from close range would be inadvisable.  Fans of the original Star Trek might remember the episode "All Our Yesterdays," wherein an entire planet's population jumped into the past to escape their host star's impending supernova, and the intrepid members of the Enterprise's away team get trapped in different pasts (of course), almost get killed and/or permanently stuck there (of course), and all get away with seconds to spare (of course).  The final moments -- the star blowing up, and the Enterprise hauling ass to get away -- is pretty dramatic, but underplays the actual magnitude of such an event.  Warp drive notwithstanding, being this close to a supernova would be a good way to get yourself vaporized.

Anyhow, that's our astronomical superlative of the week.  If April keeps going this way, you have to wonder what's next.  Me, I hope that it's a light show from Comet ATLAS, although sadly that's looking less and less likely.  Other than that?  We'll just have to keep our eyes on the skies.

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This week's Skeptophilia book recommendation of the week is brand new -- only published three weeks ago.  Neil Shubin, who became famous for his wonderful book on human evolution Your Inner Fish, has a fantastic new book out -- Some Assembly Required: Decoding Four Billion Years of Life, from Ancient Fossils to DNA.

Shubin's lucid prose makes for fascinating reading, as he takes you down the four-billion-year path from the first simple cells to the biodiversity of the modern Earth, wrapping in not only what we've discovered from the fossil record but the most recent innovations in DNA analysis that demonstrate our common ancestry with every other life form on the planet.  It's a wonderful survey of our current state of knowledge of evolutionary science, and will engage both scientist and layperson alike.  Get Shubin's latest -- and fasten your seatbelts for a wild ride through time.