Bridging the Great Divide

One of the main things that separates scientists from the rest of us is that they notice things we very likely just take for granted.

Gregor Mendel started in the research that eventually would uncover the four fundamental laws of inheritance when he noticed that some traits in pea plants seemed to skip a generation.  Percy Spencer was messing around with vacuum tubes, and noticed that in a certain configuration, they caused a chocolate bar in his pocket to melt -- further inquiry led to the invention of the microwave oven.  French physicist Henri Becquerel discovered radioactivity when he accidentally ruined some photographic plates with what turned out to be a chunk of uranium ore.  Alexander Fleming saved countless lives with the discovery of penicillin -- found because he wondered why a colony of mold on one of his culture plates seemed to be killing the bacteria near it.

I consider myself at least a little above average, savvy-wise, but I don't have that ability -- to look at the world and think, "Hmm, I wonder why that happened?"  Mostly I just assume "that's the way it is" and don't consider it much further, a characteristic I suspect I share with a lot of people.  In this vein, here's some research about something I've known about since I first started reading junior books on astronomy, when I was maybe ten years old, and never thought was odd -- or even worth giving any thought to.

There's a strange gap, something astronomers call "The Great Divide," between Mars and Jupiter.  The distance between Mars and Jupiter is over twice as great as the diameter of the entire inner Solar System.  In that gap is a narrow band called the Asteroid Belt -- and not a hell of a lot else.

Even more peculiar, when you think about it (which as I said, I didn't), is why inside of the Great Divide all the planets are small, dense, and rocky, and outside of it the planets are low-density gas giants (I do remember being shocked by the density thing as a kid, when I read that Saturn's overall density is lower than that of water -- so if you had a swimming pool big enough, Saturn would float).

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

The problem with these sorts of observations, though -- even if you stop to wonder about them -- is that until very recently, we pretty much had a sample size of one Solar System to work with, so there was no way to tell if any particular feature of ours was odd or commonplace.  Even now, with the discovery of so many exoplanets that it's estimated there are a billion in our galaxy alone, we only have tentative information about the arrangement of planets around stars, to determine if there's any sort of pattern there, such as the apparent one in our neck of the woods.

Well, it looks like the physicists may have explained the Great Divide in our own Solar System and the compositional difference of the planets on either side of it in one fell swoop.  A team from the Tokyo Institute of Technology and Colorado University has found that the Great Divide may be a relic of a ring of material that formed around the early Sun, and then was pulled apart and essentially "sorted" by the gravitational pulls of the coalescing planets.

The authors write:

We propose... that the dichotomy was caused by a pressure maximum in the disk near Jupiter’s location... One or multiple such—potentially mobile—long-lived pressure maxima almost completely prevented pebbles from the Jovian region reaching the terrestrial zone, maintaining a compositional partition between the two regions. We thus suggest that our young Solar System’s protoplanetary disk developed at least one and probably multiple rings, which potentially triggered the formation of the giant planets.

And once the process started, it accelerated, pulling dense, rocky material inward and lightweight, organic-chemical-rich material outward, resulting in a gap -- and an outer Solar System with gas giants surrounding an inner Solar System with small, terrestrial worlds.

"Young stellar systems were often surrounded by disks of gas and dust," said Stephen Mojzsis of Colorado University, who co-authored the paper, which appeared in Nature.  "If a similar ring existed in our own Solar System billions of years ago, it could theoretically be responsible for the Great Divide, because such a ring would create alternating bands of high- and low-pressure gas and dust.  Those bands, in turn, might pull the Solar System's earliest building blocks into several distinct sinks -- one that would have given rise to Jupiter and Saturn, and another Earth and Mars.

"It is analogous to the way the Continental Divide in the Rocky Mountains causes water to drain one way or another.  That's similar to how this pressure bump would have divided material in the early Solar System...  But that barrier in space was not perfect.  Some outer Solar System material still climbed across the divide.  And those fugitives could have been important for the evolution of our own world...  Those materials that might go to the Earth would be those volatile, carbon-rich materials.  And that gives you water.  It gives you organics."

And ultimately, it gives the Earth life.

Now, it bears keeping in mind that we can't generalize from this to other star systems.  There have already been dozens of "hot Jupiters" discovered, gas giants that orbit close in to their host star; the wonderful astrophysicist Dr. Becky Smethurst mentioned just last week in her monthly "Night Sky News" video the discovery of an ultra-low-density "super-puff planet" that orbits so close that the physicists are scratching their heads trying to explain how the planet's light, fluffy atmosphere doesn't get blown away entirely.  But the Mojzsis et al. paper seems to have taken a big step forward in explaining the configuration of planets in our own immediate neighborhood.

All based on an observation most of us knew about, and very likely few of us had ever thought to question.

All of which brings to mind the wonderful quote by Hungarian biochemist Albert von Szent-Györgyi -- "Discovery consists of seeing what everyone has seen, and thinking what nobody has thought."

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

The ancient immigrant

It's sometimes hard to fathom that only a hundred years ago, there was still a spirited argument going on in the astronomical community over whether the Milky Way was the only galaxy in the universe -- and that the other "nebulae" might be merely small-ish features lying in the outskirts.

The center of the "Milky Way is all there is" faction was the famous astronomer Harlow Shapley, who was their spokesperson in the 1920 "Great Debate" with Heber Doust Curtis, who believed the "nebulae" (or at least some of them) were very distant galaxies more or less like our own.  Neither man came away from the Debate convinced of the other's reasoning, but the whole affair was conclusively settled a few years later when Edwin Hubble discovered Cepheid variables in the Andromeda Galaxy.  Cepheid variables are a curious type of star that experience a regular periodicity in brightness, and the brilliant astronomer Henrietta Swan Leavitt had showed that their periods of variability were related in a straightforward way to their intrinsic brightness.  Because of this, they can be used as standard candles -- just as you can estimate how far away a motorcycle is at night if you compare how bright the headlight appears to be with how bright you know it actually is.

So the discovery of Cepheids in Andromeda gave Hubble a way of figuring out how far away it is from us, and it turns out not to be a small cloud in the fringes of our own galaxy, but an "island universe" of its own that's actually slightly larger than the Milky Way, and 2.5 million light years away.  And of course, Hubble and others went on to discover red shift and the expanding universe, and better telescopes showed that there are billions of galaxies out there, some of them so far away that the light they emit has been traveling for most of the age of the known universe in order to get here.

A cool postscript is that Shapley, confronted with this evidence, admitted defeat, and went on to make major contributions to galactic astronomy.  It's what I love about science; it self-corrects, and the best scientists look on these reversals as opportunities rather than embarrassments.  (Although Shapley did allow himself a moment of rueful laughter at his own error, calling Hubble's paper on Cepheids in Andromeda as "the letter that destroyed my universe."

In any case, we now live in a cosmos so much vaster and richer and stranger than the one they knew a century ago -- one can only imagine what more we'll know a hundred years from now.

These musings come up because of a wonderful piece of research out of the University of Chicago, where a group of undergraduate astronomy students were assigned by their teacher, Alex Ji of the Sloan Digital Sky Survey, to analyze recent SDSS data for anything anomalous, and they found something astonishing; an ancient star that not only appears to date from the very early universe, but has migrated here from the Large Magellanic Cloud, a star cluster than neighbors the Milky Way.

Astronomers can make a shrewd guess about how old a star is based upon its metallicity -- what proportion of its makeup is anything other than hydrogen and helium.  (Astronomers confusingly call all other elements metals, which must annoy the hell out of the chemists.)  Since all of the hydrogen, and a good fraction of the helium, were formed during the Big Bang -- and virtually all of the other elements have been created since then through nuclear fusion and energetic events like supernovae and neutron star collisions -- the quantity of metals in a star tells you how many cycles of birth and death occurred prior to the star's formation.  The Sun, for example, is fairly metal-rich, and is probably a third- or fourth-generation star; its contents were enriched from the activity of previous generations of stars.  (It's still not as high in metals as the bizarre Przybylski's Star, which is so anomalously high in rare heavy elements that there's a credible case to be made that it was seeded by technological aliens.)

The newly-discovered star, however is the opposite; it's called SDSS-J0715-7334, and from its spectrum it appear to be almost entirely hydrogen and helium.  Even relatively lightweight elements like carbon are so rare in it that they're well-nigh undetectable.  In fact, its total metallicity is 0.005% of the Sun's -- making it by far the most metal-poor star ever detected.

Me, I wonder how this star lived as long as it has.  Most of the universe's first-generation stars have long since exhausted their fuel and either collapsed into white dwarfs or else flared out as supernovae.  How has this one persisted all this time?

Weirder still, the star didn't start out in the Milky Way.  Using its current motion, the students calculated that it originated from outside of our galaxy, and backtracked its path to the Large Magellanic Cloud.

Okay, I'm super impressed.  These are a bunch of undergraduates, for cryin' out loud.  As an undergraduate, I was mostly focused on eating pizza and hanging out with my friends and earning grades that were at least high enough not to get me kicked out of the university.  These young people?

They're getting their names in the author line on papers in Nature Astronomy.

"These students have discovered more than just the most pristine star." said Juna Kollmeier, the Director of SDSS.  "They have discovered their inalienable right to physics.  Surveys like SDSS and Gaia make that possible for students of all ages everywhere on Earth and this example shows that there is still plenty of room for discovery."

So that's our cool story for the day.  An ancient immigrant from another star cluster.  Further evidence of Carl Sagan's evocative words: "Somewhere, something incredible is waiting to be known."

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

View of a cataclysm

As an example of what I wrote about yesterday -- that the universe is amazing enough without having to make shit up to embellish it -- today I want to tell you about one of the latest discoveries from the James Webb Space Telescope.

First, a bit of background.

I've written here before about gamma-ray bursters -- the phenomenon that one astronomer described as "second only to the Big Bang as the most energetic phenomenon known."  They ordinarily last between a few seconds and a couple of minutes, and during that time release more energy than the Sun will in its entire ten-billion-odd-year-long life.  Interestingly, the cause is unknown.  Various models have suggested the phenomenon might result from two neutron stars spiraling into one another, a stellar hypergiant undergoing core collapse, or energy release from a magnetar.  Or, possibly, more than one of the above.

We simply don't know.

Whichever it turns out to be, you would not want to be looking down the gun barrel of one of these things when it went off.  It's thought that a hundred or so light years would be what is terrifyingly known as the "kill zone."  Farther off, though, it could still be catastrophic; there's some suspicion that the Late Ordovician Mass Extinction -- one of the "Big Five" mass extinctions, and second only to the Permian-Triassic "Great Dying" event in terms of magnitude -- was caused by a nearby gamma ray burst.

Fortunately, there's nothing close to us that looks capable of doing this.  All of the ones we've observed have been in other galaxies, where they register as blips in the gamma ray region of the spectrum on powerful telescopes, and pose no threat to us here.  Which is a good thing, because heaven knows we have enough else to worry about at the moment.

Anyhow, that's all background.  An astrophysicist at Rutgers University was analyzing data collected by the James Webb Space Telescope last July, and discovered something mind-boggling -- a gamma-ray burster called GRB 250702B, located in a galaxy eight billion light years away.  But its distance, and the fact that we could see it from that far away, isn't the wildest thing about it.  You remember how I said that most gamma-ray bursters have a duration of between a few seconds and a minute or two?  And during that time they exceed the Sun's entire lifetime energy output?

This one lasted for seven hours.

That, my friends, is what the astrophysics community refers to as "a metric fucktonne of energy."  I can't even wrap my brain around how humongous this thing was, and I have a bachelor's degree in physics, so you'd think I'd be able to handle big numbers.

In my defense, neither, apparently, can the astrophysicists.  Eliza Neights, of NASA's Goddard Space Flight Center, said it was like "nothing we've ever seen before."  And whatever it was, it's left behind nothing much visible to study.  "In such vibrant and unprecedented detail, we see just one very large galaxy with a dust lane," said Huei Sears of Rutgers, who led the study.  "The galaxy has such complex structure that it's not a hundred percent clear if there's anything left to see of the explosion, but if there is, it's really faint."

Artists' conception of GRB 250702B [Image credit: NOIRLab/NSF/AURA/M. Garlick]

One suggestion is that this outburst was the result of a tidal disruption event -- a massive star, or possibly a neutron star, being ripped to shreds as it spirals into a black hole.

Because that's not a terrifying scenario to think about.

But the fact is, the scientists are struggling to explain what could have caused a cataclysm of this magnitude.  It doesn't fit with known models, and there's the exciting possibility that in order to account for it, we might be in the realm of "new physics."

In any case, here's a nice example of the fact that we don't need to add anything fringe-y to the universe to make it weird and scary and astonishing.  Real science does that just fine on its own.

I mean, I don't know how you could even dream up something wilder than a seven-hour-long energetic burst that makes the Sun look like a wet firecracker.  All I can say is that when Shakespeare talked about "there are more things in Heaven and Earth... than are dreamt of in your philosophy," he was not engaging in hyperbole.

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

Hellscape

In the Star Trek episode "The Savage Curtain," the intrepid crew of the Enterprise visit the planet Excalbia.  I forget why, because the planet was completely covered by churning seas of lava, so it wasn't exactly a great site for an away mission.  But when they get there, they find that there's one spot that's hospitable, and in fact has Earth-like conditions, by which I mean the typical Star Trek landscape of sand, styrofoam rocks, and scraggly vegetation.  It turns out that the livable area was created by some superpowerful aliens to provide a spot where Captain Kirk could have a battle involving Abraham Lincoln, Genghis Khan, and various other historical and not-so-historical figures to find out whether good is actually stronger than evil.

Okay, put that way, I know the plot sounds pretty fucking ridiculous, but don't yell at me.  I didn't write the script.

In any case, I was reminded of Excalbia when I read about a new study out of Oxford University using data from the James Webb Space Telescope.  The team looked at a recently-discovered exoplanet, L 98-59 d, which orbits a red dwarf star only thirty-five light years away, and found that it's unlike any other exoplanet we've thus far studied.

It's about 1.6 times the mass of the Earth, but for a planet its size has a fairly low density, and spectroscopic data has shown an atmosphere rich in an element you usually don't find -- sulfur.  Venus's atmosphere has some sulfuric acid, but L 98-59 d had a great deal more, mostly in the form of the toxic and vile-smelling hydrogen sulfide.  This is over a surface that appears, like Excalbia, to be largely molten.

Being chemically reactive, you wouldn't expect hydrogen sulfide to be long-lived in a planet's atmosphere, and it's sufficiently lightweight that stellar activity should readily blow it out into space.  Some process, therefore, must be generating it as fast as it's consumed either by being blasted out of the atmosphere or chemically reacted and then drawn down by convection of the liquid rock surface.  Apparently, something about the mechanics of a deep, silicate-rich mantle is causing the entrapment and release of the huge amounts of sulfur we see in the atmosphere, but how that works is still a mystery.  And given how far outside the norm L 98-59 d is -- or, what our models suggested was the norm -- it makes you wonder what else might be out there.

"This discovery suggests that the categories astronomers currently use to describe small planets may be too simple," said Harrison Nicholls, who was lead author on the paper.  "While this molten planet is unlikely to support life, it reflects the wide diversity of the worlds which exist beyond the Solar System.  We may then ask: what other types of planet are waiting to be uncovered?"

And that's considering the number of strange planetary types we already had in the exoplanet zoo, which include bizarre hycean planets, hydrogen-rich water worlds; chthonian planets, the cores of what used to be "hot Jupiters" that had their atmospheres stripped by stellar wind; and tidally-locked eyeball planets, with such extreme differences in temperature between their light and dark sides that they experience continuous blistering superhurricanes at the light/dark boundary.  That the sulfurous hellscape of L 98-59 d isn't something the astrophysicists had even thought up -- well, let's just say that what Carl Sagan called the "Encyclopedia Galactica" might be a lot longer, and weirder, than we'd ever dreamed.

So that's the cool news from the astronomers for the week.

Oh, and by the way, good turned out to be stronger than evil, although while finding that out Abraham Lincoln got assassinated again, which was kind of a shame.  On the positive side, Genghis Khan and Kahless the Unforgettable and various other execrable individuals went down to ignominious and well-deserved defeat.  Captain Kirk unsurprisingly got his shirt ripped open and gained valuable opportunities to show off his chest, but despite that the superpowerful aliens decided they'd gotten their answer and let the Enterprise and its crew go.  So in case you're wondering about philosophical questions like the relative power of good and evil, Star Trek solved it all in forty-five minutes, not counting commercial breaks.

Maybe we should turn Kirk et al. loose on whether intelligence always beats stupidity, because at the moment that one seems to be an open question.

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

Worlds in collision

In 2021, University of Washington astronomer Anastasios (Andy) Tsanidakis was reviewing data collected by the European Space Agency's Gaia Mission, and discovered something really strange.

An ordinary, Sun-like star called Gaia20ehk -- eleven thousand light years away in the constellation Puppis -- had, up until 2016, a nearly flat energy output.  This is more or less what our Sun would look like from that distance; yes, there are minor fluctuations, but (fortunately for us) it's pretty stable over short time intervals.

Then... well, here it is in Tsanidakis's words: "The star's light output was nice and flat, but starting in 2016 it had these three dips in brightness," he said.  "And then, right around 2021, it went completely bonkers.  I can't emphasize enough that stars like our Sun don't do that.  So when we saw this one, we were like 'Hello, what's going on here?'"

The chaotic fluctuations in energy output were across the electromagnetic spectrum, but strongest in the infrared region.  And stranger still, a more detailed analysis showed that the peculiar behavior was not from the star itself, but because there was -- suddenly -- a huge, irregular debris cloud surrounding it.  This rock and dust eclipsed the star's light, but some of it was apparently radiating itself, accounting for the wild yo-yoing in the infrared.  "The infrared light curve was the complete opposite of the visible light," Tzanidakis said.  "As the visible light began to flicker and dim, the infrared light spiked.  Which could mean that the material blocking the star is hot -- so hot that it's glowing in the infrared."

Tsanidakis and his team figured out that there was only one phenomenon that fit all the observations; two of Gaia20ehk's planets had collided with each other.

"It's incredible that various telescopes caught this impact in real time," Tzanidakis said.  "There are only a few other planetary collisions of any kind on record, and none that bear so many similarities to the impact that created the Earth and Moon.  If we can observe more moments like this elsewhere in the galaxy, it will teach us lots about the formation of our world."

Artist's rendition of the collision of the two planets in the Gaia20ehk system [Image credit: A. Tsanidakis et al.]

Tsandiakis and his colleagues are particularly interested in watching how this all plays out, because -- as he mentioned -- it is very similar to the process that is thought to have formed the Moon.  The collision between the proto-Earth and a Mars-sized planet astronomers call Theia, something like 4.5 billion years ago, triggered the remelting of the entire combined mass; the energy of the collision sheared off a chunk of Theia, which collapsed into what would eventually become the Moon.  Now that we've actually seen something similar happening in another star system, astronomers will be on the lookout for more events like this.

"How rare is the event that created the Earth and Moon?  That question is fundamental to astrobiology," said James Davenport, senior author of the paper, which was published three days ago in Astrophysical Journal Letters.  "It seems like the Moon is one of the magical ingredients that makes the Earth a good place for life.  It can help shield Earth from some asteroids, it produces ocean tides and weather that allow chemistry and biology to mix globally, and it may even play a role in driving tectonic plate activity.  Right now, we don't know how common these dynamics are.  But if we catch more of these collisions, we'll start to figure it out."

Tsanidakis explains that while collisions are probably common in the early history of a stellar system, they can still occur in systems with stable, middle-aged stars like Gaia20ehk.  Near passes by other stars, or by rogue exoplanets, could destabilize planetary orbits, causing one of the system's planets either to be ejected, or (in this case) gradually to spiral inward.  This could explain the three dips in brightness that was his first clue something odd was happening -- they represent grazing passes as the two planets' orbits overlapped more and more.  But eventually, they got close enough that there was a head-on impact, and all hell broke loose.

Considering the quantity of data that missions like Gaia produce, I find it astonishing that Tsanidakis and his colleagues even picked up on it.  You have to wonder what other wonders might be hidden in the enormous hauls from JWST, Hubble, and (soon) the Vera Rubin Telescope.  Fortunately, a sharp-eyed astronomer caught this one, and as a result we've learned a huge amount about exoplanetary collisions.

It's staggering to think about.  The awe-inspiring vistas we're seeing through our best telescopes are only now being studied and analyzed, and who knows what else the astronomers will find?

All from following astrophysicist Neil deGrasse Tyson's adjuration -- "Keep looking up."

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

The slingshot

Coming right on the heels of yesterday's post about a star so large the astrophysicists are at a loss to explain how it even exists, today we have...

... a supermassive black hole moving so fast it seems to be exceeding the escape velocity of the entire galactic cluster.

The paper about the discovery, by Yale University astronomer Pieter van Dokkum et al., appeared last week in Astrophysical Journal Letters, and its findings are hard to summarize without lapsing into superlatives.  Data from the James Webb Space Telescope identified a large, rapidly-moving object from its bow shock -- the compression waves surrounding a projectile as it moves through a medium (picture the pile-up of water and resulting waves preceding a boat as it moves across the surface of a lake).  But an analysis of this particular bow shock demonstrated something incredible; the object creating it was ten million times the mass of the Sun -- thus, a supermassive black hole -- and it was moving at an estimated three hundred kilometers a second.

For reference, this is over two hundred times faster than the muzzle velocity of a rifle bullet.

A map of the JWST data that led to the discovery [Image credit van Dokkum et al.]

Amongst the many cool things about this discovery is that there is a higher-than-expected number of very young stars in the wake of this thing.  Apparently, the compression caused by the black hole is triggering gas cloud collapse and star formation as it passes.

What could give something this massive that much momentum?  The quick answer is "no one knows for sure," but a good candidate is a galactic merger.  Two colliding galaxies represent a quantity known to astrophysicists as "a shitload of kinetic energy," and the slingshot effect -- where two moving objects pass close enough to each other that there's a transfer of momentum, causing one to slow down and the other to accelerate -- could be responsible.  It might be that this was once the black hole at the center of a galaxy, but the collision caused it to swing around an even more massive black hole from the other galaxy, resulting in its being jettisoned -- not just from the combined mass of the merger, but from the entire galactic cluster.

The question that naturally comes up is "what if it was headed toward us?"  Well, to start with, it's not; just a glance at the map of the bow shock should tell you that.  Second, its light has a red shift of 0.96, putting it at about a billion light years away, so even if it was, it wouldn't be anything you or I would have to fret about in our lifetimes.

On the other hand, what if there was a black hole like this headed our way?  Being black (as advertised), would we see it coming before the Earth was messily devoured?  The answer is "almost certainly;" not only would there be the effects of the compression waves heating up the gas ahead of it, causing it to emit radiation, there'd be the fact that massive black holes cause gravitational lensing -- they bend and distort the light of objects behind them.  If we were looking down the barrel of a black hole headed our way, we'd see this as an optical effect called an Einstein-Chwolson ring:

[Image licensed under the Creative Commons ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre, G. Anselmi, T. Li, CC BY-SA IGO 3.0, Close-up of the Einstein ring around galaxy NGC 6505 ESA506346, CC BY-SA 3.0 IGO]

[Nota bene: black holes that are not moving toward us also cause gravitational lensing and Einstein-Chwolson rings; it'd be the combination of the lensing effect and the heating of the gas in front of the black hole that would tell us it was heading in our direction.]

Given astronomical distances, though, we still wouldn't have to worry about anything in our lifetimes.  It might be bad news for our possible descendants a hundred million years from now, but there are way worse problems to concern ourselves with in the interim.  And in any case, even if there was a supermassive black hole headed our way that was due to arrive either a hundred million years from now or a week from next Tuesday, there'd be absolutely nothing we could do about it.  Altering the trajectory of a something with ten million times the mass of the Sun, traveling at three hundred kilometers per second, gives new meaning to the word "unfeasible."  The only option, really, would be to stick your head between your legs and kiss your ass goodbye.

But like I said, the one van Dokkum et al. discovered isn't going to be a problem, even millions of years from now.  It's something we can goggle at from a safe distance.  A massive bullet flying through space, leaving a spangle of new stars in its wake.  Yet another example of how endlessly awe-inspiring the universe is -- and the more we find out about it, the more wonderful it gets.

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

Off the charts

Way back around 1910, Danish astronomer Ejnar Hertzsprung and American astronomer Henry Norris Russell independently found a curious pattern when they did a scatterplot correlation between stars' luminosities and temperatures.

The graph, now called the Hertzsprung-Russell Diagram in their honor, looks like this:

[Image licensed under the Creative Commons Richard Powell, HRDiagram, CC BY-SA 2.5]

Most stars fall on the bright swatch running from the hot, bright stars in the upper left to the cool, dim stars in the lower right; the overall trend for these stars is that the lower the temperature, the lower the luminosity.  Stars like this are called main-sequence stars.  (If you're curious, the letter designations along the top -- O, B, A, F, G, K, and M -- refer to the spectral class the star belongs to.  These classifications were the invention of the brilliant astronomer Antonia Maury, whose work in spectrography revolutionized our understanding of stellar evolution.)

There is also a sizable cluster of stars off to the upper right -- relatively low temperatures but very high luminosities.  These are giants and supergiants.  In the other corner are white dwarfs, the exposed cores of dead stars, with very high temperatures but low luminosity, which as they gradually cool slip downward to the right and finally go dark.

So there you have it; just about every star in the universe is either a main-sequence star, in the cluster with the giants and supergiants, or in the curved streak of dwarf stars at the bottom of the diagram.

Emphasis on the words "just about."

One star that challenges what we know about how stars evolve is the bizarre Stephenson 2-18, which is in the small, dim constellation Scutum ("the shield"), between Aquila and Sagittarius.  At an apparent magnitude of +15, it is only visible through a powerful telescope; it wasn't even discovered until 1990, by American astronomer Charles Bruce Stephenson, after whom it is named.

Its appearance, a dim red point of light, hides how weird this thing actually is.

When Stephenson first analyzed it, he initially thought what he was coming up with couldn't possibly be correct.  For one thing, it is insanely bright, estimated at a hundred thousand times the Sun's luminosity.  Only its distance (19,000 light years) and some intervening dust clouds make it look dim.  Secondly, it's enormous.  No, really, you have no idea how big it is.  If you put Stephenson 2-18 where the Sun is, its outer edge would be somewhere near the orbit of Saturn.  You, right now, would be inside the star.  Ten billion Suns would fit inside Stephenson 2-18.

If a photon of light circumnavigated the surface of the Sun, it would take a bit less than fifteen seconds.  To circle Stephenson 2-18 would take nine hours.

This puts Stephenson 2-18 almost off the Hertzsprung-Russell Diagram -- it's in the extreme upper right corner.  In fact, it's larger than what what stellar evolution says should be possible; the current model predicts the largest stars to have radii of no more than 1,500 times that of the Sun, and this behemoth is over 2,000 times larger.

Astronomers admit that this could have a simple explanation -- it's possible that the measurements of Stephenson 2-18 are overestimates.  But if not, there's something significant about stellar evolution we're not understanding.

Either way, this is one interesting object.

There's also a question about what Stephenson 2-18 will do next.  Astrophysicists suspect it might be about to blow off its outer layers and turn either into a luminous blue variable or a Wolf-Rayet star (the latter are so weird and violent I wrote about them here a while back).  So it may not be done astonishing us.

As far as the scientists, they love peculiar puzzles like this.  Contrary to the picture many people have, of scientists being stick-in-the-mud conservatives who do nothing but prop up the current dominant paradigm, the vast majority of scientists absolutely live for having their prior notions being challenged, because that's when new avenues for understanding open up.

As the brilliant polymath 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.'"

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

With a whimper

The death of massive stars, ten or more times the mass of the Sun, is thought to have a predictable -- if violent -- trajectory.

During most of their lifetimes, stars are in a relative balance between two forces.  Fusion of hydrogen into helium in the core releases heat energy, which increases the pressure in the core and generates an outward-pointing force.  At the same time, the inexorable pull of gravity generates an inward-pointing force.  For the majority of the star's life, the two are in equilibrium; if something makes the core cool a little bit, gravity wins for a while and the star shrinks, increasing the pressure and thus the rate of fusion.  This heats the core up, increasing the outward force and stopping the collapse.

Nice little example of negative feedback and homeostasis, that.  Stars in this long, relatively quiescent phase are on the "Main Sequence" of the famous Hertzsprung-Russell Diagram:

[Image licensed under the Creative Commons Richard Powell, HRDiagram, CC BY-SA 2.5]

Once the hydrogen fuel starts to deplete, though, the situation shifts.  Gravity wins once again, but this time there's not enough hydrogen-to-helium fusion to counteract the collapse.  The core shrinks, raising the temperature to hundreds of millions of degrees Kelvin -- enough to fuse helium to carbon.  This release of energy causes the outer atmosphere to balloon outward, and the star becomes a red supergiant -- the surface is cool (and thus reddish), but the interior is far hotter than the core of our Sun.

Two famous stars -- Betelgeuse (in Orion) and Antares (in Scorpio) are in this final stage of their lives.

Here's where things get interesting, because the helium fuel doesn't last forever, either.  The carbon "ash" left behind needs an even higher temperature to fuse into oxygen, nitrogen, and heavier elements, which happens when the previous process repeats itself -- further core collapse, followed by further heating.  But this can't go on indefinitely.  When the fusion reaction starts to generate iron, the game is up.  Iron represents the turnaround point on the curve of binding energy, where fusion stops being an exothermic (energy-releasing) reaction and becomes endothermic (energy-consuming).  At that point, the core can't respond with anything to support the pull of gravity, and the entire star collapses.  The outer atmosphere rebounds off the collapsing core, creating a shockwave called a core-collapse (type II) supernova, releasing in a few seconds as much energy as the star did during its entire life on the main sequence.  What's left afterward is a super-dense remnant -- either a neutron star or a black hole, depending on its mass.

Well, that's what we thought happened.  But now a paper in Science describing the collapse of a supergiant star in the Andromeda Galaxy has suggested there may be a different fate for at least some massive stars -- that they may go out not with a bang, but with a whimper.

The occurrence that spurred this discovery was so underwhelming that it took astronomers a while to realize it had happened.  A star began to glow intensely in the infrared region of the spectrum, and then suddenly -- it didn't anymore.  It seemed to vanish, leaving behind a faintly glowing shell of dust.  Kishalay De, lead author of the paper, says what happened is that we just witnessed a black hole forming without a supernova preceding it.  The core ran out of fuel, the outer atmosphere collapsed, and the star itself just kind of... winked out.

"This has probably been the most surprising discovery of my life," De said.  "The evidence of the disappearance of the star was lying in public archival data and nobody noticed for years until we picked it out...  The dramatic and sustained fading of this star is very unusual, and suggests a supernova failed to occur, leading to the collapse of the star’s core directly into a black hole.  Stars with this mass have long been assumed to always explode as supernovae.  The fact that it didn’t suggests that stars with the same mass may or may not successfully explode, possibly due to how gravity, gas pressure, and powerful shock waves interact in chaotic ways with each other inside the dying star."

It's honestly unsurprising that we don't have the mechanisms of supernovae and black hole formation figured out completely.  They're not frequent occurrences.  The most recent easily visible supernova in the Milky Way was all the way back in 1604 -- "Kepler's Supernova," as it's often called.  Since then we've seen them occur in other galaxies, but that means from here they're invisible to the naked eye, and often difficult to study even with powerful telescopes.

But I will say that the whole thing has me worried.  Betelgeuse is predicted to run out of fuel soon, and all my life I've been waiting for it to explode violently (yes, yes, I know that "soon" to an astrophysicist means "some time in the next hundred thousand years).  If it just decides to go pfft and vanish one night, I'm gonna be pissed.

Oh, well, as my grandma used to tell me, wishin' don't make it so.  But still.  Life down here on Earth has been pretty damn distressing lately, can't we have just one nice thing?

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

The tide is high

The list of confirmed exoplanets now exceeds six thousand.  Considering the fact that the three main ways they're detected -- direct measure of stellar wobbles, transit photometry, and Doppler spectroscopy -- all require either that the host star be close, that the planets be massive, or that the planetary orbit be aligned just right from our perspective, or all three, it's almost certain that there are vast numbers of exoplanets going undetected.

All of which bodes well for those of us who would love for there to be extraterrestrial life out there somewhere.

On the other hand, of the exoplanets we've found, a great many of them are inhospitable to say the least, and some of them are downright bizarre.  Here are a few of the weirder ones:

• TrES-2b, which holds the record as the least-reflective planet yet discovered. It's darker than a charcoal briquet.  This led some people to conclude that it's made of dark matter, something I dealt with here at Skeptophilia a while back.  (tl:dr -- it's not.)
• CoRoT-7b, one of the hottest exoplanets known.  Its composition and size are thought to be fairly Earth-like, but it orbits its star so closely that it has a twenty-day orbital period and surface temperatures around 3000 C.  This means that it is likely to be completely liquid, and experience rain made of molten iron and magnesium.
• PSR J1719−1438, a planet orbiting a pulsar (the collapsed, rapidly rotating core of a giant star), and therefore somehow survived its host star going supernova.  It has one of the fastest rates of revolution of any orbiting object known, circling in only 2.17 hours.
• V1400 Centauri, a planet with rings that are two hundred times wider than the rings of Saturn.  In fact, they dwarf the planet itself -- the whole thing looks a bit like a pea in the middle of a dinner plate.
• BD+05 4868 Ab, in the constellation of Pegasus.  Only 140 light years away, this exoplanet is orbiting so close to its parent star -- twenty times closer than Mercury is to the Sun -- that its year is only 30.5 hours long.  This proximity roasts the surface, melting and then vaporizing the rock it's made of.  That material is then blasted off the surface by the stellar wind, so the planet is literally evaporating, leaving a long, comet-like trail in its wake.

Today, though, we're going to look at some recent research about a planet that should be near the top of the "Weirdest Exoplanets Known" list.  It's 55 Cancri Ae, the innermost of four (possibly six; two additional ones are suspected but unconfirmed) planets around the star 55 Cancri A, a K-type orange star a little over forty light years away.  55 Cancri Ae orbits its host star twice as close as Mercury does the Sun, making a complete ellipse around it in only a bit under three days.  This means that like CoRoT-7b and BD+05 4868 Ab, it's crazy hot.

This is where some new research comes in.  A presentation at an exoplanet conference in Groningen, Netherlands last week considered a puzzling feature of 55 Cancri Ae -- a measure of its heat output shows odd, non-cyclic fluctuations that don't seem to be in sync with its orbital period (or anything else).  The fluctuations aren't small; some of them have approached a 1,000 C difference from peak to trough.  They were first detected ten years ago, and physicists have been at a loss to account for the mechanism responsible.

But now, we might have an explanation -- and it's a doozy.  Models developed by exoplanet astrophysicist Mohammed Farhat of the University of California - Berkeley found that the anomalous temperature surges could be explained as moving hotspots.

Which sounds pretty tame until you read Farhat's description of what this means.  We're talking about a planet close in to a star not much smaller than the Sun, being whirled around at dizzying speeds.  This means it's experiencing enormous tidal forces.  The planet itself is so hot it's probably liquid down to its core.  Result: tidal waves of lava several hundred meters high, moving at the speed of a human sprinter.

The presentation definitely got the attendees' attention.  "This is right in the sweet spot of something that is interesting, novel, and potentially testable," said planetary astronomer Laura Kreidberg, of the Max Planck Institute for Astronomy.  "I had this naïve idea that lava flows were too slow-moving to have an observable impact, but this new work is pointing otherwise."

The whole thing reminds me of the planet Excalbia from Star Trek, from the episode "The Savage Curtain," which was completely covered by churning seas of lava -- except for the spot made hospitable by some superpowerful aliens so Captain Kirk could have a battle involving Abraham Lincoln, Genghis Khan, and various other historical and not-so-historical figures to find out whether good was actually stronger than evil.

Put that way, I know the plot sounds pretty fucking ridiculous, but don't yell at me.  I didn't write the script.

In any case, I doubt even the Excalbians would find 55 Cancri Ae hospitable.  But it is fascinating.  It pushes the definition of what we even consider a planet to be -- a sloshing blob of liquid rock with lava waves taller than a skyscraper.  Makes me thankful for the calm, temperate climes of Earth.

The universe is a scary place, sometimes.

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

The oddest star in the galaxy

I'll start today with a quote (often misquoted) from William Shakespeare -- more specifically, Hamlet, Act I, Scene 5:

Horatio:

O day and night, but this is wondrous strange!

 Hamlet:

And therefore as a stranger give it welcome.
There are more things in heaven and earth, Horatio,
Than are dreamt of in your philosophy.

 Horatio and Hamlet, of course, are talking about ghosts and the supernatural, but it could equally well be applied to science.  It's tempting sometimes, when reading about new scientific discoveries, for the layperson to say, "This can't possibly be true, it's too weird."  But there are far too many truly bizarre theories that have been rigorously verified over and over -- quantum mechanics and the General Theory of Relativity jump to mind immediately -- to rule anything out based upon our common-sense ideas about how the universe works.

That was my reaction while watching a YouTube video about an astronomical object I'd never heard of -- Przybylski's Star, named after its discoverer, Polish-born Australian astronomer Antoni Przybylski.  The video comes from astronomer David Kipping's channel Cool Worlds Lab, which looks at cutting-edge science -- and tantalizing new data about the universe we live in.  (You should subscribe to it -- you won't be sorry.)  Przybylski's Star is 355 light years from Earth, in the constellation of Centaurus, and is weird in so many ways that it kind of boggles the mind.

It's classified as a Type Ap star.  Type A stars are young, compact, luminous, and very hot; the brightest star in the night sky, Sirius, is in this class.

The "p" stands for "peculiar."

[Image licensed under the Creative Commons Vizzualizer, Przybylski's Star, CC BY-SA 4.0]

Przybylski's Star rotates slowly.  I mean, really slowly.  Compared to the Sun, which rotates about once every 27 days, Przybylski's Star rotates once every two hundred years.  Most type A stars rotate even faster than the Sun; in fact, a lot of them rotate so quickly that the light from their receding hemisphere and that from their approaching hemisphere experience enough red-shift and blue-shift (respectively) to smear out their spectral lines, making it impossible for us to tell exactly what they're made of.

It's a good thing that didn't happen with Przybylski's Star, because the strangest thing about it is its composition.  This star has a spectral signature so anomalous that its discoverer initially thought that his measurements were crazily off.

"No star should look like that," Przybylski said.

You probably know that most ordinary stars are primarily composed of hydrogen, and of the bit that's not hydrogen, most of it is helium.  Hydrogen is the fuel for the fusion in the core of the star, and helium is the product formed by that fusion.  Late in their life, many stars undergo core collapse, in which the temperatures heat up enough to fuse helium into heavier elements like carbon and oxygen.  Most of the rest of the elements on the periodic table are generated in supernovas and in neutron stars, a topic I dealt with in detail in a post I did about six years ago.

My point here is that if you look at the emission spectra of your average star, the spectral lines you see should mostly be the familiar ones from hydrogen and helium, with minuscule traces of the spectra of other elements.  The heaviest element that should be reasonably abundant, even in the burned-out cores of stars, is iron -- it represents the turnaround point on the curve of binding energy, the point where fusion into heavier elements starts consuming more energy than it releases.

So elements that are low in abundance pretty much everywhere, such as the aptly-named rare earth elements (known to chemists as the lanthanides), should be so uncommon as to be effectively undetectable.  Short-lived radioactive elements like thorium and radium shouldn't be there at all, because they don't form in the core of your ordinary star, and therefore any traces present had to have formed prior to the star in question's formation -- almost always, enough time that they should have long since decayed away.

The composition of Przybylski's Star, on the other hand, is so skewed toward heavy elements that it elicits more in the way of frustrated shrugs than it does in viable models that could account for it.  It's ridiculously high in lanthanides like cerium, dysprosium, europium, and gadolinium -- not elements you hear about on a daily basis.  There's more praseodymium in the spectrum of its upper atmosphere than there is iron.  Even stranger is the presence of very short-lived radioactive elements such as plutonium -- and actinium, americium, and neptunium, elements for which we don't even know a naturally-occuring nuclide synthesis pathway capable of creating them.

So where did they come from?

"What we’d like to know... is how the heavy elements observed here have come about," said astronomy blogger Paul Gilster.  "A neutron star is one solution, a companion object whose outflow of particles could create heavy elements in Przybylski’s Star, and keep them replenished.  The solution seems to work theoretically, but no neutron star is found anywhere near the star."

"[T]hat star doesn’t just have weird abundance patterns; it has apparently impossible abundance patterns," said Pennsylvania State University astrophysicist Jason Wright, in his wonderful blog AstroWright.  "In 2008 Gopka et al. reported the identification of short-lived actinides in the spectrum.  This means radioactive elements with half-lives on the order of thousands of years (or in the case of actinium, decades) are in the atmosphere...  The only way that could be true is if these products of nuclear reactions are being replenished on that timescale, which means… what exactly?  What sorts of nuclear reactions could be going on near the surface of this star?"

All the explanations I've seen require so many ad-hoc assumptions that they're complete non-starters.  One possibility astrophysicists have floated is that the replenishment is because it was massively enriched by a nearby supernova, and not just with familiar heavy elements like gold and uranium, but with superheavy elements that thus far, we've only seen produced in high-energy particle accelerators -- elements like flerovium (atomic number 114) and oganesson (atomic number 118).  These elements are so unstable that they have half-lives measured in fractions of a second, but it's theorized that certain isotopes might exist in an island of stability, where they have much longer lives, long enough to build up in a star's atmosphere and then decay into the lighter, but still rare, elements seen in Przybylski's Star.

There are several problems with this idea, the first being that every attempt to find where the island of stability lies hasn't succeeded.  Physicists thought that flerovium might have the "magic number" of protons and neutrons to make it more stable, but a paper released not long ago seems to dash that hope.

The second, and worse, problem is that there's no supernova remnant anywhere near Przybylski's Star.

The third, and worst, problem is that it's hard to imagine any natural process, supernova-related or not, that could produce the enormous quantity of superheavy elements required to account for the amount of lanthanides and actinides found in this star's upper atmosphere.

Which brings me to the wildest speculation about the weird abundances of heavy elements.  You'll never guess who's responsible.

Go ahead, guess.

There is a serious suggestion out there -- and David Kipping does take it seriously -- that an advanced technological civilization might have struck on the solution for nuclear waste of dumping it into the nearest star.  This explanation (called "salting"), bizarre as it sounds, would explain not only why the elements are there, but why they're way more concentrated in the upper atmosphere of the star than in the core.

"Here on Earth... people sometimes propose to dispose of our nuclear waste by throwing it into the Sun,” Wright writes.  “Seven years before Superman thought of the idea, Whitmire & Wright (not me, I was only 3 in 1980) proposed that alien civilizations might use their stars as depositories for their fissile waste.  They even pointed out that the most likely stars we would find such pollution in would be… [type] A stars!  (And not just any A stars, late A stars, which is what Przybylski’s Star is).  In fact, back in 1966, Sagan and Shklovskii in their book Intelligent Life in the Universe proposed aliens might 'salt' their stars with obviously artificial elements to attract attention."

A curious side note is that I've met (Daniel) Whitmire, of Whitmire & Wright -- he was a professor in the physics department of the University of Louisiana when I was an undergraduate, and I took a couple of classes with him (including Astronomy).  He was known for his outside-of-the-box ideas, including that a Jupiter-sized planet beyond the orbit of Pluto was responsible for disturbing the Oort Cloud as it passed through every hundred million years or so (being so far out, it would have a super-long rate of revolution).  This would cause comets, asteroids, and other debris to rain in on the inner Solar System, resulting in a higher rate of impacts with the Earth -- and explaining the odd cyclic nature of mass extinctions.

So I'm not all that surprised about Whitmire's suggestion, although it bears mention that he was talking about the concept in the purely theoretical sense; the weird spectrum of Przybylski's Star was discovered after Whitmire & Wright's paper on the topic.

Curiouser and curiouser.

So we're left with a mystery.  The "it's aliens" explanation is hardly going to be accepted by the scientific establishment without a hell of a lot more evidence, and thus far, there is none.  The problem is, the peculiar abundance of heavy elements in this very odd star remains unaccounted for by any science we currently understand.  The fact that Kipping (and others) are saying "we can't rule out the alien salting hypothesis" is very, very significant.

I'll end with another quote, this one from eminent biologist J. B. S. Haldane: "The universe is not only queerer than we imagine, it is queerer than we can imagine."

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