All that glitters

If you own anything made of gold, take a look at it now.

I'm looking at my wedding ring, made of three narrow interlocked gold bands.  It's a little scratched up after twenty-two years, but still shines.

Have you ever wondered where gold comes from?  Not just "a gold mine," but before that.  If you know a little bit of physics, it's kind of weird that the periodic table doesn't end at atomic number 26.  The reason is a subtle but fascinating one, and has to do with the binding energy curve.

The vertical axis is a measure of how tightly the atom's nucleus is held together.  More specifically, it's the amount of energy (in millions of electron-volts) that it would take to completely disassemble the nucleus into its component protons and neutrons.  From hydrogen (atomic number = 1) up to iron (atomic number = 26), there is a relatively steady increase in binding energy.  So in that part of the graph, fusion is an energy-releasing process (moves upward on the graph) and fission is an energy-consuming process (moves downward on the graph).  This, in fact, is what powers the Sun; going from hydrogen to helium is a jump of seven million electron-volts per proton or neutron, and that energy release is what produces the light and heat that keeps us all alive.

After iron, though -- specifically after an isotope of iron, Fe-56, with 26 protons and 30 neutrons -- there's a slow downward slope in the graph.  So after iron, the situation is reversed; fusion consumes energy, and fission releases it.  This is why the fission of uranium-235 generates energy, which is how a nuclear power plant works.

It does generate a question, though.  If fusion in stars is energetically favorable, increasing stability and releasing energy, up to but not past iron -- how do the heavier elements form in the first place?  Going from iron to anywhere would require a consumption of energy, meaning those will not be spontaneous reactions.  They need a (powerful) energy driver.  And yet, some higher-atomic-number elements are quite common -- zinc, iodine, and lead come to mind.

Well, it turns out that there are two ways this can happen, and they both require a humongous energy source.  Like, one that makes the core of the Sun look like a wet firecracker.  Those are supernova explosions, and neutron star collisions.  In fact, a while back, two astrophysicists -- Szabolcs Marka of Columbia University and Imre Bartos of the University of Florida -- found evidence that the heavy elements on the Earth were produced in a collision between two neutron stars, on the order of a hundred million years before the Solar System formed.

This is an event of staggering magnitude.  "If you look up at the sky and you see a neutron-star merger a thousand light-years away," Marka said, "it would outshine the entire night sky."

What apparently happens is when two neutron stars -- the ridiculously dense remnants of massive stellar cores -- run into each other, it is such a high-energy event that even thermodynamically unfavorable (energy-consuming) reactions can pick up enough energy from the surroundings to occur.  Then some of the debris blasted away from the collision gets incorporated into forming stars and planets.  And here we are, still with tons of lightweight elements, but a surprisingly high amount of heavier ones, too.

But how do they know it wasn't a nearby supernova?  Those are far more common in the universe than neutron star collisions.  Well, the theoretical yield of heavy elements is known for each, and the composition of the Solar System is far more consistent with a neutron star collision than with a supernova.  And as for the timing, a chunk of the heavy isotopes produced are naturally unstable, so decaying into lighter nuclei is favored (which is why heavy elements are often radioactive; the products of decay are higher on the binding energy curve than the original element was).  Since this happens at a set rate -- most often calculated as a half-life -- radioactive isotopes act like a nuclear stopwatch, analogous to the way radioisotope decay is used to calculate the ages of artifacts, fossils, and rocks.  Backtracking that stopwatch to t = 0 gives an origin of about 4.7 billion years ago, or a hundred million years before the Solar System coalesced.

So next time you look at anything made of heavier elements -- gold or silver or platinum, or (more prosaically) the zinc plating on a galvanized steel pipe -- ponder for a moment that it was formed in a catastrophically huge collision between two neutron stars, an event that released more energy in a few seconds than the Sun will produce over its entire lifetime.  Sometimes the most ordinary things have a truly extraordinary origin -- something that never fails to fascinate me.

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Strange attractors

Dear Readers,

I am going to be taking a short break from Skeptophilia, so this will be my last post for a week and a half.  Lord willin' an' the creek don't rise, as my grandma used to say, I'll be back at it on Monday, January 27.

cheers,

Gordon

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I've always found the concept of the Strong Anthropic Principle wryly amusing.

The idea here is that something (usually a benevolent deity) fine-tuned the universe in just such a way to be hospitable for us -- for having forces perfectly balanced to hold matter together without causing a runaway collapse, for having gravitational pull strong enough to form stars and planets, for having electromagnetic forces of the right magnitude to generate the chemical reactions that ultimately led to organic molecules and life, and so on.

To me, this argument ignores two awkward facts.  First, of course our universe has exactly the right characteristics to generate and support life; if it didn't, we wouldn't be here to consider the question.  (This is called the "Weak Anthropic Principle," for obvious reasons.)  Second, though -- the Strong Anthropic Principle conveniently avoids the fact that a large percentage of the Earth, and damn near one hundred percent of the universe as a whole, is completely and unequivocally hostile to us, and probably to just about any living thing out there.

It's one of those hostile bits that got me thinking about the whole issue today, because astronomers recently observed a phenomenon called a fast radio burst in our own galaxy -- a mere thirty thousand light years away -- and the thing that produces it is not only bizarre in the extreme, but is something that we're very, very lucky not to be any closer to.

The beast that produces this is called a magnetar, and appears to be a rapidly-spinning neutron star, with a mass of two to three times that of the Sun but compressed into a sphere only about twenty kilometers in diameter.  This means that the surface gravitational attraction is astronomical (*rimshot*).  Any irregularities in the topography would be crushed, giving it a smooth surface with a relief less than that of a brand-new billiard ball.

The most bizarre thing about magnetars, however, is the immense magnetic field that gives them their name.  Your typical magnetar has an average magnetic field flux density of ten billion Teslas -- on the order of a quadrillion times the field strength of the Earth.  This is why they are, to put it mildly, really fucking dangerous.  The article in Astronomy about the discovery explained it graphically (if perhaps using slightly more genteel language):

The magnetic field of a magnetar is about a hundred million times stronger than any human-made magnet.  That’s strong enough that a magnetar would horrifically kill you if you got within about 620 miles (1,000 km) of it.  There, its insanely strong magnetic field would pluck electrons from your body’s atoms, essentially dissolving you.

This brought up a question in my mind, though; magnetic fields of any kind are made by moving electrical charges -- so how can a neutron star (made, as one would guess, entirely of neutrons) have any magnetic field at all, much less an "insanely strong" one?   Turns out I'm not the only one to ask this question, as I found out when I did some digging and stumbled on the Q-and-A page belonging to Cole Miller, Professor of Astronomy at the University of Maryland.  Miller says the reason is that not all of the particles in a neutron star are neutrons.  While the structure as a whole is electrically neutral, about ten percent of the total mass is made up of electrons and protons that are free to move.  Take those charged particles and whirl them around hundreds of times per second, and you have a magnetic field that is not only insanely strong, but really fucking dangerous.

This all comes up because of the observation of a thirty-millisecond-long fast radio burst coming from within our galaxy.  All the others that have been detected were in other galaxies, and the distances involved (not to mention how sporadic they are, and how quickly they're over) make them difficult to explain.  But this comparatively nearby one gave us a load of new information -- especially when a second burst came from the same magnetar a few days later.

[Image licensed under the Creative Commons ESO/L. Calçada, Artist’s impression of the magnetar in the extraordinary star cluster Westerlund 1, CC BY 4.0]

Astronomers and astrophysicists are still trying to explain the phenomenon, including odd features of this particular one such as its relative faintness.  As compared to bursts from other galaxies this one was a thousand times less luminous.  Why is still a matter of conjecture.  Is it because bursts this weak occur in other galaxies, but from this distance would be undetectable?  Is it because the distant galaxies are much younger (remember, looking out in space is equivalent to looking back in time), so stronger bursts only happen early in a galaxy's evolution?  At this point, we don't know.  As Yvette Cendes, author of the Astronomy article, put it:

It is far too early to draw a firm conclusion about whether this relatively faint FRB-like signal is the first example of a galactic fast radio burst — making it the smoking gun to unlocking the entire FRB mystery.  And there are also still many preliminary questions left to answer.  For example, how often do these fainter bursts happen?  Are they beamed so not all radiation is equally bright in all directions?  Do they fall on a spectrum of FRBs with varying intensities, or are they something entirely new?  And how are the X-ray data connected?

As usual with science, the more we know, the more questions we have.

In any case, here we have a phenomenon that's cool to observe, but that you wouldn't want to be at all close to.  Not only do we have the magnetic field to worry about, but the burst itself is so energetic that anything nearby would get flash-fried.

So "the universe is fine-tuned to be congenial to us" only works if you add, "... except for the 99.9% of it that is actively trying to kill us."  Not that this makes it any less magnificent, but it does make you feel a little... tiny, doesn't it?  Probably a good thing.  Humans do stupid stuff when they start thinking they're the be-all-end-all.

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NEW!  We've updated our website, and now -- in addition to checking out my books and the amazing art by my wife, Carol Bloomgarden, you can also buy some really cool Skeptophilia-themed gear!  Just go to the website and click on the link at the bottom, where you can support your favorite blog by ordering t-shirts, hoodies, mugs, bumper stickers, and tote bags, all designed by Carol!

Take a look!  Plato would approve.

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The interstellar lighthouse

It's funny the questions you don't think to ask.  You find out something, accept it without any objections, and only later -- sometimes much later -- you stop and go, "Okay, hang on a moment."

That happened to me just yesterday, about a topic most of us don't ponder much, and that's the peculiar astronomical object called a neutron star.  It was on my mind not by random chance -- even I don't just sit around and say, "Hmm, how about those neutron stars, anyway?" -- but because of some interesting research (about which I'll tell you in a bit).

I first learned about these odd beasts when I took a class called Introduction to Astronomy at the University of Louisiana.  The professor, Dr. Whitmire, explained them basically as follows.

Stars are stable when there's a balance between two forces -- the outward pressure from the heat generated in the core, and the inward pull because of the gravity exerted by the star's mass.  During most of a star's life, those two are in equilibrium, but when the core exhausts its fuel, the first force diminishes and the star begins to collapse.  With small stars like the Sun, the collapse continues until the mutual repulsion of the atoms' electrons becomes a sufficient force to halt it from shrinking further.  This generates a white dwarf.

In a star between 10 and 29 times the mass of the Sun, however, the mutual electric repulsion isn't strong enough to stop the collapse.  The matter of the star continues to fall inward until it's only about ten kilometers across -- a star shrunk to the diameter of a small city.  This causes some pretty strange conditions.  The matter in the star becomes unimaginably dense; a teaspoon of it would have about the same mass as a mountain.  The pressure forces the electrons into the nuclei of the atoms, crushing out all the space, so that what you have is a giant electrically-neutral ball -- effectively, an enormous atomic nucleus made of an unimaginably huge number of neutrons.

The first neutron star ever discovered, at the center of the Crab Nebula [Image is in the Public Domain, courtesy of NASA/JPL]

The immense gravitational pull means that the surface of a neutron star is the smoothest surface known; any irregularities would be flattened out of existence.  (It's worth mentioning that even the Earth is way smoother than most people realize.  The distance between the top of Mount Everest and the bottom of the Marianas Trench is less, as compared to its size, than the topographic relief in a typical scratch on a billiard ball.)

So far, so good.  But it was the next thing Dr. Whitmire told us that should have made me pull up short, and didn't until now -- over forty years later.  He said that as a neutron star forms, the inward collapse makes its rotational speed increase, just like a spinning figure skater as she pulls in her arms.  Because of the Conservation of Angular Momentum, this bumps up the rotation of a neutron star to something on the order of making a complete rotation thirty times per second.  A point on the surface of a typical neutron star is moving at a linear speed of about one-third of the speed of light.

Further, because neutron stars have a phenomenally large magnetic field, this creates two magnetic "funnels" on opposite sides of the star that spew out jets of electromagnetic radiation.  And if these jets aren't aligned with the star's spin axis, they whirl around like the beams of a lighthouse.  A neutron star that does this, and appears to flash on and off like a strobe light, is called a pulsar.

This was the point when the red flags should have started waving, especially since I majored in physics and had taken a class called "Electromagnetism."  One of the first things we learned is that Scottish physicist James Clerk Maxwell discovered that magnetic fields are generated when charged particles move.  So how can a neutron star -- composed of electrically-neutral particles -- have any magnetic field at all, much less one so huge?  (The magnetic field of a typical neutron star is on the order of ten million Tesla; by comparison, one of the largest magnetic fields ever generated in the laboratory is a paltry sixteen Tesla, but was still enough to levitate a frog.)

The answer is a matter of conjecture.  One possibility is that even though a neutron star is neutral overall, there is some separation of charges within the star's interior, so the whirling of the star still creates a magnetic field.  Another possibility is that since neutrons themselves are composed of three quarks, and those quarks are charged, neutrons still have a magnetic moment, and the alignment of these magnetic moments coupled with the star's rotation is sufficient to give it an overall enormous magnetic field.  (If you want to read more about the answer to this curious question, the site Medium did a nice overview of it a while back.)

So it turns out that neutron stars aren't the simple things they appeared to be at first.  Not that this is much of a surprise; a recurring theme here at Skeptophilia is that nature always seems to turn out to be more complicated than we expected.  What brought this up in the first place was yet another anomalous observation about neutron stars, described in a series of papers I ran across in Astrophysical Journal Letters.  The conventional wisdom was that a neutron star's magnetic field would be oriented along an axis (which, as noted above, may not coincide perfectly with the star's spin axis).  This means that it would behave a bit like an ordinary magnet, with a north pole and a south pole on geometrically opposite sides.

That's what astronomers thought, until they found a pulsar with the euphonious name J0030+0451, 1,100 light years away in the constellation of Pisces.  Using the x-ray jets from the pulsar -- which should be aligned with its magnetic field -- they mapped the field itself, and found something extremely strange.

Instead of two jets, aligned with the poles of the magnetic field, J0030+0451 has three -- and they're all in the southern hemisphere.  One is (unsurprisingly) at the southern magnetic pole, but the other two are elongated crescents at about sixty degrees south latitude.

To say this is surprising is an understatement, and the astronomers are still struggling to explain it.

"From its perch on the space station, NICER [the Neutron star Interior Composition Explorer] is revolutionizing our understanding of pulsars," said Paul Hertz, astrophysics division director at NASA Headquarters in Washington.  "Pulsars were discovered more than fifty years ago as beacons of stars that have collapsed into dense cores, behaving unlike anything we see on Earth."

It appears that we still have a way to go to fully explain how they work.  But that's how it is with the entire universe, you know?  No matter where we look, we're confronted by mysteries.  Fortunately, we have a tool that has proven over and over to be the best way of finding answers -- the collection of protocols we call the scientific method.  I have no doubt that the astrophysicists will eventually explain the odd magnetic properties of pulsars.  But the way things go, all that'll do is open up more fascinating questions -- which is why I've said many times that if you're interested in science, you'll never run out of things to learn.

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A wolf, a disk, and a lighthouse

Because here in the United States, many Americans are looking at tomorrow's election the way a man walking in a railway tunnel sees the headlights of an approaching train, today I'd like to direct your attention away from the Earth entirely, into the cold, desolate voids of outer space.

Which, all things considered, seem like a pretty congenial place by comparison.

In the past week we've had three cool astronomical discoveries announced, highlighting the exciting fact of how much more we have left to learn about the universe in which we live.  The first comes from the European Southern Observatory, which got some fantastic new images of a nebula in the constellation Scorpio called the Dark Wolf Nebula, which (fitting to its name) they released on Halloween:

[Image credit: European Southern Observatory]

The Dark Wolf, and other dark nebulae -- such as the famous Coalsack Nebula in the constellation Crux -- are aggregations of dust and gas that shroud stars behind them.  They're far from being passive light-blockers, however; dark nebulae are often the sites of rapid star formation, as the material collapses into clumps and fusion starts.  Once this occurs, the radiation pressure from the newly-formed stars blows away the extra dust, revealing the newborn star cluster, such as what we see now in the Orion Nebula and the Pleaides.

The second study is a bit of a puzzle, and involves the star Vega, a bright star in the constellation Lyra easily visible in the Northern Hemisphere at this time of year.  Vega is only 25 light years away, and was made famous as the origin of the alien signal in the movie Contact, which remains my all-time favorite movie.

Vega is a young A-class blue-white star about twice the Sun's mass, forty times brighter, and almost 4,000 C hotter (surface temperature).  Because of its luminosity and proximity, it's one of the most intensively-studied stars in the sky, and a recent announcement by NASA (based on data from the Hubble and James Webb Space Telescopes) indicate that it's got a feature that's peculiar by any standards -- and suggest that one scene in Contact was downright prescient.

In the movie, astronomer Ellie Arroway intercepts a transmission from an advanced technological species which contains instructions on how to build a device that warps space and time, allowing a passenger to cross interstellar distances and drop in for a visit.  When Arroway (after many twists and turns and setbacks) ends up taking a ride in the device, it brings her to Vega, where she sees a massive debris disk -- but no planets.

And that's exactly what Hubble and the JWST found.  Having a debris disk isn't at all unusual; after all, current models indicate that planet formation occurs by gravitational clumping from a flat disk surrounding the parent star (much as stars coalesce from dust and gas in dark nebulae).  But what's strange is that Vega's disk is almost entirely homogeneous, made up of a circular sheet of similar-sized particles.  No planets at all.

"Between the Hubble and Webb telescopes, you get this very clear view of Vega," said team member Andras Gáspár of the University of Arizona. " It's a mysterious system because it's unlike other circumstellar disks we've looked at.  The Vega disk is smooth, ridiculously smooth."

There appears to be a trend toward gradually decreasing size at the edges of the disk, thought to be because radiation pressure tends to blow small particles outward more efficiently than larger ones.  But other than that, the disk is relatively featureless, which is something not seen in other stars of similar ages and characteristics, such as Fomalhaut in the constellation Piscis Australis.

"Given the physical similarity between the stars of Vega and Fomalhaut, why does Fomalhaut seem to have been able to form planets and Vega didn't?" said team member George Rieke, also of the University of Arizona.  "What's the difference?  Did the circumstellar environment, or the star itself, create that difference?  What's puzzling is that the same physics is at work in both."

The last story will appeal to anyone who likes to think about the extremes which nature can sometimes achieve, and has to do with something that's pretty astonishing all by itself -- neutron stars.  Neutron stars form from the gravitational core collapse of a star greater than about 1.4 solar masses; the outer atmosphere gets blown away in a supernova, and the core falls inward, overcoming electrostatic repulsion and electron degeneracy pressure, which has the effect of crushing electrons into atomic nuclei, forming (in essence) a gigantic ball of neutrons.

This means neutron stars are some of the densest known objects.  A matchbox-sized chunk of a typical neutron star would weigh three billion tonnes.  But they have another wild characteristic, which is why the topic comes up today; most of them rotate like crazy.

The reason is conservation of angular momentum -- the same reason that a spinning figure skater increases her rotational speed as she brings her arms inward.  When a neutron star collapses, this reduces its effective radius (what physicists call the moment of inertia), and the rate of rotation increases to compensate.

When the neutron star is emitting jets of radiation, this creates an effect like the beams from a lighthouse -- which is how we get pulsars.

The nebula surrounding the pulsar PSR B1509-58, which glows because of the radiation jets from the neutron star [Image is in the Public Domain courtesy of NASA]

And now, a team at the Technological University of Denmark has found a neutron star with a spin rate of an almost unimaginable 716 rotations per second, putting it in a tie for the fastest spinning astronomical object known.

"We were studying thermonuclear explosions from this system and then found remarkable oscillations, suggesting a neutron star spinning around its centre axis at an astounding 716 times per second," said Gaurava K. Jaisawal, first author on the study, which was published last week in the Astrophysical Journal.  "If future observations confirm this, the 4U 1820-30 neutron star would be one of the fastest-spinning objects ever observed in the universe, matched only by another neutron star called PSR J1748-2446."

So those are our cool discoveries in outer space for today.  And now, I suppose that we should reluctantly turn our attention back to the planet we live on.  If you live in the United States, please please please vote tomorrow.  If you live elsewhere, you might direct a prayer to whatever deity you happen to favor.  I know I've been a disbeliever for a good long while, but hell, at this point we need all the help we can get.

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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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A rare firecracker

A layperson might be excused if looking up at the night sky, (s)he concluded that other than slight variations in color and brightness, one star is pretty much like another.

I can't deny they look that way.  Even with the best telescope available to amateur astronomers, stars are featureless points of light.  About all we earthbound amateurs could discern that might clue us in to stars' wide variety of features, compositions, and behaviors is that some (many of them, in fact) are in binary or multiple star systems, and that a few fluctuate in brightness at regular intervals.  (Variable stars, in fact, were known to the ancients, and because in general our ancestors felt that the heavens should be eternal and changeless, they were viewed with great suspicion; one of the best-known, in fact, is Algol, which comes from the Arabic words for "the ghoul's head.")

First with the Hubble Space Telescope, and now with the James Webb Space Telescope, we've finally gotten the first direct photographs of stars showing any kind of detail (and the first direct photographs of exoplanetary systems).  But astrophysical data collection, often in regions of the electromagnetic spectrum the eye can't see, has given us more information about the wild variety of stars out there -- many of which are only now beginning to be understood.

Take for example the binary star system with the euphonious name CPD-29-2176, located a bit over eleven thousand light years away.  This pair is so strange that its characteristics are thought to match only one in every ten billion star systems, meaning there are probably only ten or so of them in the entire Milky Way.  (Fortunate, then, that one is close enough to study.)  First discovered from its x-ray signature by NASA's Neil Gehrels Swift Observatory and later studied by the SMARTS 1.5-meter Telescope, CPD-29-2176 is a kilonova progenitor system -- a pair of stars in which one is destined to blow up.

Artist's impression of CPD-29-2176  [Image courtesy of CTIO/NOIRLab/NSF/AURA/J. da Silva]

The mechanism is a little like a type 1a supernova, in which a white dwarf is in a close orbit with a larger main-sequence star.  The white dwarf, a dense, hot stellar nucleus of a moribund star, slowly draws off material from it partner through its intense gravitational pull, creating a whirlpool of accreting matter.  This, however, can only go on so long; once the white dwarf exceeds the Chandrasekhar limit, about 1.4 solar masses, it suddenly collapses.  The temperature skyrockets, and the former white dwarf becomes a supernova intense enough to blow the companion right out of orbit.

Here, though, the dynamics are a bit different.  If a supernova is a "holy shit!" event, a kilonova is more of a "meh."  What apparently is happening is the two stars are already so close that one is losing material to the other at a colossal rate.  The result: once the losing star burns through its fuel, at which point it should undergo the collapse/explosion cycle, there won't be enough fuel left to spike its temperature much.  It will trigger a kilonova (also called an ultra-stripped supernova), which is to an actual supernova what a wet firecracker is to a nuclear bomb.

What's even more interesting is that the same fate is predicted for the companion star; ultimately, what will be left is two neutron stars whirling around a common center of gravity, eventually falling inward and coalescing.  The release of gravitational potential energy by the merger will tear the stars apart -- stunning this could happen to objects so dense -- and the resulting debris, highly enriched in heavy elements, will be dispersed to the cosmos.

As astonishing as it sounds, all of the heavy elements -- the gold and silver in our jewelry, the mercury in our thermometers (well, old ones, at least), the uranium in our nuclear power plants, the rare earth elements in our computers -- were created in the cores of dying stars.  (If you want to learn more about this astonishing process, I did a piece here at Skeptophilia about it a couple of years ago.)

While a kilonova isn't going to be anything spectacular to watch from here on Earth, it's a rara avis indeed in the galactic zoo.  

Every time I read about some new astronomical discovery, it highlights for me how much more complex the universe is than the ancients dreamed.  Their point sources of light on crystal spheres, driven by deities and heavenly powers, miss the true intricacy of the cosmic clockwork by light years.  How delighted Galileo and Copernicus and Eratosthenes would be to know what we know -- to get a glimpse of a universe so vast, and so diverse, that it far surpasses the famous quote by Shakespeare -- "There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy."

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A black widow pirouette

It's difficult to talk about neutron stars without lapsing into superlatives.  They're the collapsed remnants of large stars -- those that start out at about eight solar masses or larger -- and form because when the star exhausts its fuel at the end of its life, the ongoing battle between the outward pressure from the heat from the core and the inward pull of gravity from the star's mass goes out of equilibrium.  Gravity wins.  The outer layers of the star fall inward, and the increase in pressure spikes the temperature to an estimated billion degrees Celsius.  This creates a massive explosion -- a supernova -- releasing 10^44 joules of energy.

I don't know about you, but I have a hard time even wrapping my brain around a number that big.  Suffice it to say that the explosion of a supernova releases in a few minutes as much energy as the Sun will produce in its entire ten-billion-year life.

These unimaginable pressures jam together atomic nuclei that otherwise would never have overcome the electrostatic repulsion (all nuclei have a net positive charge, and like charges repel), producing pretty much every element in the periodic table heavier than nitrogen.  

So as bizarre as it seems, the oxygen you breathe, the calcium in your bones, the sodium and chlorine you sprinkle on your food at dinnertime, the iron in your blood, the silicon in your window glass, the gold and silver in your jewelry -- all were created in the unimaginable violence of stellar collapse and explosion.

Nota bene: This is an oversimplification; not only are there several types of supernovas which vary some in output, there are other phenomena, like the merger of neutron stars (more on that in a moment), that can create heavy elements and disperse them through the cosmos -- but it'll do as a first-order approximation.  If you're curious about breaking it down further, the following table represents a finer-grained analysis of where all the elements come from:

[Image licensed under the Creative Cosmos Cmglee, Nucleosynthesis periodic table, CC BY-SA 3.0]

As astrophysicist Carl Sagan put it, "We are made of star stuff...  Our ancestors worshipped the Sun, and they were far from foolish.  It makes sense to revere the Sun and the stars, for we are their children."

The reason this comes up because of a recent discovery that adds a new weird twist to the behavior of neutron stars.  About three thousand light years away is what's left of a triple-star system.  Multiple-star systems aren't that uncommon; a while back I wrote here about one of the most peculiar ones known, Algol (in the constellation Perseus).  The newly-discovered one, though, called ZTF J1406+1222, has an additional layer of strangeness; not only are two of the components neutron stars, they're close enough that they're whirling around their common center of gravity so fast that they complete their orbits in only sixty-two minutes.  In fact, they're close enough that the heavier of the two is siphoning off material from the lighter one.  Stars like this are called black widow binaries, from the unfortunate habit of female black widow spiders eating their mates.

The most astonishing thing about all this is to consider how much force it would take to pull material from a neutron star.  The collapse of the core during its formation was so powerful that it basically crushed the electrons of the constituent atoms into the nucleus, raising its density so high that it's estimated that one teaspoon full of neutron star material would weigh as much as a mountain.

That's the stuff that's being ripped from the surface of the lighter member of the pair.

What about star number three?  The third companion is a much smaller stellar remnant, a white dwarf, that has a ten thousand year orbital period -- almost as if it's edging carefully around its violently spinning friends, keeping at a safe distance while the inner two tear each other apart.

It's unknown how this mad pirouette will end.  The surmise is eventually the two will merge, but what happens then?  If the combined mass is high enough (estimated at about twenty solar masses), then even the neutron/neutron repulsion, mediated by the strong nuclear force, would be insufficient to overcome the gravitational pull, and the collapse will resume -- forming a black hole.  It's also possible that the inertia of such huge masses being accelerated so quickly will rip the two apart completely, flinging neutron star material -- which, once it cools and settles down, would be the aforementioned heavy elements -- across the area of space it's sitting in.

So that's our mind-boggling cosmic tale for today.  It's easy to forget, here on the (comparatively) placid Earth, the unimaginable violence that happens out there in space.  Not only that -- the same violence is what created most of the atoms that make up ourselves and all of the everyday objects we're surrounded with.

When you think about it, there's nothing about the universe we live in that isn't extraordinary.

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Strange attractor

I've always found the concept of the Strong Anthropic Principle wryly amusing.

The idea here is that something (usually a benevolent deity) fine-tuned the universe in just such a way to be hospitable for us -- for having forces perfectly balanced to hold matter together without causing a runaway collapse, for having gravitational pull strong enough to form stars and planets, for having electromagnetic forces of the right magnitude to generate the chemical reactions that ultimately led to organic molecules and life, and so on.

To me, this argument ignores two awkward facts.  First, of course our universe has exactly the right characteristics to generate and support life; if it didn't, we wouldn't be here to consider the question.  (This is called the "Weak Anthropic Principle," for obvious reasons.)  Second, though -- the Strong Anthropic Principle conveniently avoids the fact that a large percentage of the Earth, and damn near 100% of the universe as a whole, is completely and unequivocally hostile to us, and probably to just about any living thing out there.

It's one of those hostile bits that got me thinking about the whole issue today, because astronomers just last week observed a phenomenon called a fast radio burst in our own galaxy -- a mere thirty thousand light years away -- and the thing that produces it is not only bizarre in the extreme, but is something that we're very, very lucky not to be any closer to.

The beast that produces this is called a magnetar, and appears to be a rapidly-spinning neutron star, with a mass of two to three times that of the Sun but compressed into a sphere only about twenty kilometers in diameter.  This means that the surface gravitational attraction is astronomical (*rimshot*).  Any irregularities in the topography would be crushed, giving it a smooth surface with a relief less than that of a brand-new billiard ball.

The most bizarre thing about magnetars, however, is the immense magnetic field that gives them their name.  Your typical magnetar has an average magnetic field flux density of ten billion Teslas -- on the order of a quadrillion times the field strength of the Earth.  This is why they are, to put it mildly, really fucking dangerous.  The article in Astronomy about last week's discovery explained it graphically (if perhaps using slightly more genteel language):

The magnetic field of a magnetar is about a hundred million times stronger than any human-made magnet.  That’s strong enough that a magnetar would horrifically kill you if you got within about 620 miles (1,000 km) of it.  There, its insanely strong magnetic field would pluck electrons from your body’s atoms, essentially dissolving you.

This brought up a question in my mind, though; magnetic fields of any kind are made by moving electrical charges -- so how can a neutron star (made, as one would guess, entirely of neutrons) have any magnetic field at all, much less an insanely strong one?  Turns out I'm not the only one to ask this question, as I found out when I did some digging and stumbled on the Q-and-A page belonging to Cole Miller, Professor of Astronomy at the University of Maryland.  Miller says the reason is that not all of the particles in a neutron star are neutrons.  While the structure as a whole is electrically neutral, about ten percent of the total mass is made up of electrons and protons that are free to move.  Take those charged particles and whirl them around hundreds of times per second, and you have a magnetic field that is not only insanely strong, but really fucking dangerous.

This all comes up because of last week's observation of a thirty-millisecond-long fast radio burst coming from within our galaxy.  All the others that have been detected were in other galaxies, and the distances involved (not to mention how sporadic they are, and how quickly they're over) make them difficult to explain.  But this comparatively nearby one gave us a load of new information -- especially when a second burst came from the same magnetar a few days later.

[Image licensed under the Creative Commons ESO/L. Calçada, Artist’s impression of the magnetar in the extraordinary star cluster Westerlund 1, CC BY 4.0]

As this observation was only made last week, astronomers and astrophysicists are still trying to explain it, including odd features such as its relative faintness.  As compared to bursts from other galaxies this one was a thousand times less luminous.  Why is still a matter of conjecture.  Is it because bursts this weak occur in other galaxies, but from this distance would be undetectable?  Is it because the distant galaxies are much younger (remember, looking out in space is equivalent to looking back in time), so stronger bursts only happen early in a galaxy's evolution?  At this point, we don't know.  As Yvette Cendes, author of the Astronomy article, put it:

It is far too early to draw a firm conclusion about whether this relatively faint FRB-like signal is the first example of a galactic fast radio burst — making it the smoking gun to unlocking the entire FRB mystery.  And there are also still many preliminary questions left to answer.  For example, how often do these fainter bursts happen?  Are they beamed so not all radiation is equally bright in all directions?  Do they fall on a spectrum of FRBs with varying intensities, or are they something entirely new?  And how are the X-ray data connected?

As usual with science, the more we know, the more questions we have.

In any case, here we have a phenomenon that's cool to observe, but that you wouldn't want to be at all close to.  Not only do we have the magnetic field to worry about, but the burst itself is so energetic that anything nearby would get flash-fried.

So "the universe is fine-tuned to be congenial to us" only works if you add, "... except for the 99.9% of it that is actively trying to kill us."  Not that this makes it any less magnificent, but it does make you feel a little... tiny, doesn't it?  Probably a good thing.  Humans do stupid stuff when they start thinking they're the be-all-end-all.

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This week's Skeptophilia book recommendation is about a phenomenal achievement; the breathtaking mission New Horizons that gave us our first close-up views of the distant, frozen world of Pluto.

In Alan Stern and David Grinspoon's Chasing New Horizons: Inside the Epic First Mission to Pluto, you follow the lives of the men and women who made this achievement possible, flying nearly five billion kilometers to something that can only be called pinpoint accuracy, then zinging by its target at fifty thousand kilometers per hour while sending back 6.25 gigabytes of data and images to NASA.

The spacecraft still isn't done -- it's currently soaring outward into the Oort Cloud, the vast, diffuse cloud of comets and asteroids that surrounds our Solar System.  What it will see out there and send back to us here on Earth can only be imagined.

The story of how this was accomplished makes for fascinating reading.   If you are interested in astronomy, it's a must-read.

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

All that glitters

If you own anything made of gold, take a look at it now.

I'm looking at my wedding ring, made of three narrow interlocked gold bands.   It's a little scratched up after almost eighteen years, but still shines.

Have you ever wondered where gold comes from?  Not just "a gold mine," but before that.  If you know a little bit of physics, it's kind of weird that the periodic table doesn't end at 26.  The reason is a subtle but fascinating one, and has to do with the binding energy curve.

The vertical axis is a measure of how tightly the atom's nucleus is held together.  More specifically, it's the amount of energy (in millions of electron-volts) that it would take to completely disassemble the nucleus into its component protons and neutrons.  From hydrogen (atomic number = 1) up to iron (atomic number = 26), there is a relatively steady increase in binding energy.  So in that part of the graph, fusion is an energy-releasing process (moves upward on the graph) and fission is an energy-consuming process (moves downward on the graph).  This, in fact, is what powers the Sun; going from hydrogen to helium is a jump of seven million electron-volts per proton or neutron, and that energy release is what produces the light and heat that keeps us all alive.

After iron, though -- specifically after an isotope of iron, Fe-56, with 26 protons and 30 neutrons -- there's a slow downward slope in the graph.  So after iron, the situation is reversed; fusion would consume energy, and fission would release it.  This is why the fission of uranium-235 generates energy, which is how a nuclear power plant works.

It does generate a question, though.  If fusion in stars is energetically favorable, increasing stability and releasing energy, up to but not past iron -- how do the heavier elements form in the first place?  Going from iron to anywhere would require a consumption of energy, meaning those will not be spontaneous reactions.  They need a (powerful) energy driver.  And yet, some higher-atomic-number elements are quite common -- zinc, iodine, and lead come to mind.

Well, it turns out that there are two ways this can happen, and they both require a humongous energy source.  Like, one that makes the core of the Sun look like a wet firecracker.  Those are supernova explosions, and neutron star collisions.  And just last week, two astrophysicists -- Szabolcs Marka of Columbia University and Imre Bartos of the University of Florida -- found evidence that the heavy elements on the Earth were produced in a collision between two neutron stars, on the order of a hundred million years before the Solar System formed.

This is an event of staggering magnitude.  "If you look up at the sky and you see a neutron-star merger 1,000 light-years away," Marka said, "it would outshine the entire night sky."

What apparently happens is when two neutron stars -- the ridiculously dense remnants of massive stellar cores -- run into each other, it is such a high-energy event that even thermodynamically unfavorable (energy-consuming) reactions can pick up enough energy from the surroundings to occur.  Then some of the debris blasted away from the collision gets incorporated into forming stars and planets -- and here we are, with tons of lightweight elements, but a surprisingly high amount of heavier ones, too.

But how do they know it wasn't a nearby supernova?  Those are far more common in the universe than neutron star collisions.  Well, the theoretical yield of heavy elements is known for each, and the composition of the Solar System is far more consistent with a neutron star collision than with a supernova.  And as for the timing, a chunk of the heavy isotopes produced are naturally unstable, so decaying into lighter nuclei is favored (which is why heavy elements are often radioactive; the products of decay are higher on the binding energy curve than the original element was).  Since this happens at a set rate -- most often calculated as a half-life -- radioactive isotopes act like a nuclear stopwatch, analogous to the way radioisotope decay is used to calculate the ages of artifacts, fossils, and rocks.  Backtracking that stopwatch to t = 0 gives an origin of about 4.7 billion years ago, or a hundred million years before the Solar System coalesced.

So next time you look at anything made of heavier elements -- gold or silver or platinum, or (more prosaically) the zinc plating on a galvanized steel pipe -- ponder for a moment that it was formed in a catastrophically huge collision between two neutron stars, an event that released more energy in a few seconds than the Sun will produce over its entire lifetime.  Sometimes the most ordinary things have a truly extraordinary origin -- something that never fails to fascinate me.

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In the midst of a pandemic, it's easy to fall into one of two errors -- to lose focus on the other problems we're facing, and to decide it's all hopeless and give up.  Both are dangerous mistakes.  We have a great many issues to deal with besides stemming the spread and impact of COVID-19, but humanity will weather this and the other hurdles we have ahead.  This is no time for pessimism, much less nihilism.

That's one of the main gists in Yuval Noah Harari's recent book 21 Lessons for the 21st Century.  He takes a good hard look at some of our biggest concerns -- terrorism, climate change, privacy, homelessness/poverty, even the development of artificial intelligence and how that might impact our lives -- and while he's not such a Pollyanna that he proposes instant solutions for any of them, he looks at how each might be managed, both in terms of combatting the problem itself and changing our own posture toward it.

It's a fascinating book, and worth reading to brace us up against the naysayers who would have you believe it's all hopeless.  While I don't think anyone would call Harari's book a panacea, at least it's the start of a discussion we should be having at all levels, not only in our personal lives, but in the highest offices of government.