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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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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Into the expanse

Last week, I did a post about dark matter and dark energy -- and how those could potentially drive a reworking of what we know about physics.  Today, there's another finding that is causing some serious head-scratching amongst the physicists:

The universe may be expanding faster than we thought.  Not by a small amount, either.  The difference amounts to about 9%.  Further, this means that the universe might also be younger than we'd thought -- by almost a billion years.

This rather puzzling conclusion is the result of work by a team led by Adam Riess, of Johns Hopkins University.  At issue here is the Hubble constant, the rate of outward expansion of spacetime.  It's not an easy thing to measure.  The usual method has been to use what are called standard candles, which need a bit of explanation.

The difficulty with accurately measuring the distance to the nearest stars is a problem that's been apparent for several centuries.  If two stars are equally bright as seen from Earth, it may be that they're shining at the same luminosity and are the same distance.  It's more likely, however, that they're actually at different distances, but the brighter one is farther away.  But how could you tell?

For the nearest stars, we can use parallax -- the apparent movement of the star as the Earth revolves around the Sun.  Refinements in this technique have resulted in our ability to measure a parallax shift of 10 microarcseconds -- one ten-millionth of 1/3600th of the apparent circumference of the sky.  This translates to being able to measure distances of up to 10,000 light years this way.

But for astronomical objects that are farther away, parallax doesn't work, so you have to rely on something that tells you the star's intrinsic brightness; then you can use that information to figure out how far away it is.  There are two very common ones used:

1. Cepheid variables.  Cepheids are a class of variable stars -- ones that oscillate in luminosity -- that have an interesting property.  The rate at which their brightness oscillates is directly proportional to its actual luminosity.  So once you know how fast it's oscillating, you can calculate how bright it actually is, and from that determine how far away it is.
2. Type 1a supernovae.  These colossal stellar explosions always result in the same peak luminosity.  So when one occurs in a distant galaxy, astronomers can chart its apparent brightness peak -- and from that, determine how far away the entire galaxy is.

A Cepheid variable [Image is in the Public Domain, courtesy of the Hubble Space Telescope]

So the standard candle method has allowed us to estimate the distances to other galaxies, you can combine that information with its degree of red shift (a measure of how fast it's moving away from us) to estimate the rate of expansion of space.

And here's where the trouble lies.  Previous measurements of the rate of expansion of space, made using information such as the three-degree microwave background radiation, have consistently given the same value for the Hubble constant and the same age of the universe -- 13.7 billion years.  Riess's measurement of standard candles in distant galaxies is also giving a consistent answer... but a different one, on the order of 12.8 billion years.

"It’s looking more and more like we’re going to need something new to explain this," Riess said.

John Cromwell Mather, winner of the 2006 Nobel Prize in Physics, was even more blunt.  "There are only two options," Mather said.  "1. We’re making mistakes we can’t find yet. 2. Nature has something we can’t find yet."

"You need to add something into the universe that we don’t know about,” said Chris Burns, an astrophysicist at the Carnegie Institution for Science.  "That always makes you kind of uneasy."

To say the least.  Throw this in with dark matter and dark energy, and you've got a significant piece of the universe that physicists have not yet explained.  It's understandable that it makes them uneasy, since finding the explanation might well mean that a sizable chunk of our previous understanding was misleading, incomplete, or simply wrong.

But it's exciting.  Gaining insight into previously unexplained phenomena is what science does.  My guess is we're awaiting some astrophysicist having a flash of insight and crafting an answer that will blow us all away, much the way that Einstein's insight -- which we now call the Special Theory of Relativity -- blew us away by reframing the "problem of the constancy of the speed of light."  Who this century's Einstein will be, I have no idea.

But it's certain that whoever it is will overturn our understanding of the universe in some very fundamental ways.

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I grew up going once a summer with my dad to southern New Mexico and southern Arizona, with the goal of... finding rocks.  It's an odd hobby for a kid to have, but I'd been fascinated by rocks and minerals since I was very young, and it was helped along by the fact that my dad did beautiful lapidary work.  So while he was poking around looking for turquoise and agates and gem-quality jade, I was using my little rock hammer to hack out chunks of sandstone and feldspar and quartzite and wondering how, why, and when they'd gotten there.

Turns out that part of the country has some seriously complicated geology, and I didn't really appreciate just how complicated until I read John McPhee's four-part series called Annals of the Former World.  Composed of Basin and Range, In Suspect Terrain, Rising from the Plains, and Assembling California, it describes a cross-country trip McPhee took on Interstate 80, accompanied along the way with various geologists, with whom he stops at every roadcut and outcrop along the way.  As usual with McPhee's books they concentrate on the personalities of the people he's with as much as the science.  But you'll come away with a good appreciation for Deep Time -- and how drastically our continent has changed during the past billion years.

[Note:  If you order this book using the image/link below, part of the proceeds will go to support Skeptophilia!]

Explosions in Andromeda

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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