Surf's up

One thing that never fails to leave me feeling awestruck is when I consider that astronomers figured out the shape and size of the Milky Way Galaxy while residing inside it.

I mean, think about it.  Imagine you're a tiny being (with a telescope) sitting on a raindrop near one edge of a huge cloud, and your task is to try to measure the distances and positions of enough other raindrops to make a good guess about the size and shape of the entire cloud.  That's what the astronomers have accomplished -- enough to state with reasonable confidence that we're in one of the arms of a barred spiral galaxy.

If ever there was an image you need to study in detail, this is it.  Take a look at the original, close up.  The Solar System is in the Orion Arm, directly down from the center of the galaxy.  The thing that blew me away is the circle marked "Naked Eye Limit" -- literally every star you have ever seen without the use of a telescope is in that little circle.  [Image licensed under the Creative Commons Pablo Carlos Budassi, Milky Way map, CC BY-SA 4.0]

What's even more astonishing is that the stars making up the Milky Way (and every other galaxy) are moving.  Not fast enough, on that kind of a size scale, that the map will be inaccurate any time soon; but fast enough to be measurable from here on Earth.  In fact, it was anomalies in galactic rotation curves -- the plot of the orbital speed of stars around the center of the galaxy, as a function of their distance from the center -- that clued in the brilliant astrophysicist Vera Rubin that there was (far) more matter in galaxies than could be seen, leading to the bizarre discovery that there is five times more dark matter (matter that only interacts via gravity) than there is the ordinary matter that makes up you, me, the Earth, the Sun, and the stars.

All of this makes the new study out of the European Space Agency even more incredible.  New data from the Gaia Telescope has found that the entire Milky Way is rippling as it rotates, a little like the fluttering of a Spanish dancer's frilly skirt.  The period of this wave-like motion is on the order of ten thousand light years, and it appears to affect the entire galaxy.

The astrophysicists are still trying to figure out what's causing it.

"What makes this even more compelling is our ability, thanks to Gaia, to also measure the motions of stars within the galactic disc," said lead author Eloisa Poggio, an astronomer at the Istituto Nazionale di Astrofisica (INAF) in Italy.  "The intriguing part is not only the visual appearance of the wave structure in 3D space, but also its wave-like behavior when we analyze the motions of the stars within it."

The discovery hinged on the use of standard candles, something you may be familiar with if you've read any cosmology.  Calculating distances of astronomical objects is tricky, for the same reason that it's difficult to tell how far away a single light is at night.  If the light seems bright, is it intrinsically bright (and perhaps quite distant), or are you looking at something that is dimmer, but close to you?  The only way to calculate astronomical distances is to use the small number of objects for which we know the intrinsic brightness.  The two most common are Cepheid variables, stars for which the oscillation period of luminosity is directly related to their brightness, and type 1a supernovas, which always have about the same peak luminosity.  Between these two, astrophysicists have been able to measure the changing positions of stars as the ripple of the wave passes them.

So the stars in our galaxy are riding the cosmic surf, and at the moment no one knows why.  One possibility is that this is a leftover gravitational effect from a collision with a dwarf galaxy some time in the distant past -- a little like the ripples from dropping a pebble into a pond lasting long after the pebble has come to rest on the bottom.  But the truth is, it will take further study to figure out for sure what's causing the wave.

Me, I find the whole thing staggering.  To think that only a little over a hundred years ago, there were still astronomers arguing (vehemently) that the only galaxy in the universe was the Milky Way, and all of the other galaxies were merely small local nebulae.  The last century has placed us into a universe vaster than the ancients could ever have conceived -- and I have no doubt that the next century will astonish us further, and in ways we never could have imagined.

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The edges of knowledge

The brilliant British astrophysicist Becky Smethurst said, "The cutting edge of science is where all the unknowns are."  And far from being a bad thing, this is exciting.  When a scientist lands on something truly perplexing, that opens up fresh avenues for inquiry -- and, potentially, the discovery of something entirely new.

That's the situation we're in with our understanding of the evolution of the early universe.

You probably know that when you look out into space, you're looking back into time.  Light is the fastest information carrier we know of, and it travels at... well, the speed of light, just shy of three hundred thousand kilometers per second.  The farther away something is, the greater the distance the light had to cross to get to your eyes, so what you're seeing is an image of it when the light left its surface.  The Sun is a little over eight light minutes away; so if the Sun were to vanish -- not a likely eventuality, fortunately -- we would have no way to know it for eight minutes.  The nearest star other than the Sun, Proxima Centauri, is 4.2 light years away; the ever-intriguing star Betelgeuse, which I am so hoping goes supernova in my lifetime, is 642 light years away, so it might have blown up five hundred years ago and we'd still have another 142 years to wait for the information to get here.

This is true even of close objects, of course.  You never see anything as it is; you always see it as it was.  Because right now my sleeping puppy is a little closer to me than the rocking chair, I'm seeing the chair a little further in the past than I'm seeing him.  But the fact remains, neither of those images are of the instantaneous present; they're ghostly traces, launched at me by light reflecting off their surfaces a minuscule fraction of a second ago.

Now that we have a new and extremely powerful tool for collecting light -- the James Webb Space Telescope -- we have a way of looking at even fainter, more distant stars and galaxies.  And as Becky Smethurst put it, "In the past four years, JWST has been taking everything that we thought we knew about the early universe, and how galaxies evolve, and chucking it straight out of the window."

In a wonderful video that you all should watch, she identifies three discoveries JWST has made about the most distant reaches of the universe that still have yet to be explained: the fact that there are many more large, bright galaxies than our current model would predict are possible; that there is a much larger amount of heavy elements than expected; and the weird features called "little red dots" -- compact assemblages of cooler red stars that exhibit a strange spectrum of light and evidence of ionized hydrogen, something you generally only see in the vicinity hot, massive stars.

Well, she might have to add another one to the list.  Using data from LOFAR (the Low Frequency Array), a radio telescope array in Europe, astrophysicists have found bubbles of electromagnetic radiation surrounding some of the most distant galaxies, on the order of ten billion light years away.  This means we're seeing these galaxies (and their bubbles) when the universe was only one-quarter of its current age.  These radio emissions seem to be coming from a halo of highly-charged particles between, and surrounding, galaxy clusters, some of the largest structures ever studied.

[Image credit: Chandra X-ray Center (X-ray: NASA/CXC/SAO; Optical: NASA/ESA/STScI; Radio: ASTRON/LOFAR; Image Processing: NASA/CXC/SAO/N. Wolk)

"It's as if we've discovered a vast cosmic ocean, where entire galaxy clusters are constantly immersed in high-energy particles," said astrophysicist Julie Hlavacek-Larrondo of the Université de Montréal, who led the study.  "Galaxies appear to have been infused with these particles, and the electromagnetic radiation they emit, for billions of years longer than we realized...  We are just scratching the surface of how energetic the early Universe really was.  This discovery gives us a new window into how galaxy clusters grow and evolve, driven by both black holes and high-energy particle physics."

Every once in a while I'd have a student tell me, in some disdain, "I don't know why we have to learn science when it could all be proven wrong tomorrow."  My response to that is that science's ability to self-correct is a strength, not a weakness.  How is desperately hanging on to your prior understanding when you're presented with new evidence a good thing?   People like to be sure of everything, but really, are we ever?  Nothing is ever absolutely settled; we sometimes kid ourselves that we've found The Answer, but that's honestly a response born of a combination of insecurity and the desire not to think about the matter any more.

Richard Feynman, in his wonderful book The Pleasure of Finding Things Out, summarized this brilliantly:

There is no learning without having to pose a question.  And a question requires doubt.  People search for certainty.  But there is no certainty.  People are terrified — how can you live and not know?  It is not odd at all.  You only think you know, as a matter of fact.  And most of your actions are based on incomplete knowledge and you really don't know what it is all about, or what the purpose of the world is, or know a great deal of other things.  It is possible to live and not know.

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The empty galaxy

A couple of weeks ago, I began a post with a quote from physicist Albert Michelson in which he confidently claimed that everything in physics was pretty well settled -- in 1894.  Right before the discovery of the relativity and quantum mechanics would shake science to its foundations.

I read yet another paper just yesterday highlighting the inadvertent irony of Michelson's statement, and which once again shows us that we are very far from understanding everything there is to understand.  This one was about the accidental discovery of a galaxy that has an extremely odd characteristic.

It appears to have no stars whatsoever.

The object, dubbed J0613+52, is about ten times less massive than the Milky Way -- so smaller than your typical galaxy, but still pretty damn huge, weighing in at about two billion solar masses.  But the entire thing is made up of diffuse gas and dust -- no stars at all.

Because of this, it has an extremely low luminosity.  It was only discovered because of a mistake -- the astronomers at the Green Bank Observatory were trying to aim it elsewhere, but had mistyped the coordinates -- but when the telescope focused on the spot, they saw a blip of hydrogen spectral emission lines in what appeared to be an empty region of space.  More detailed study of the spot found that the emission lines were coming from a huge but faint dust cloud that was on the scale of galaxies mass-wise but seemed to have undergone no star formation.

"It’s likely there is a decent amount of dark matter present as well," said Karen O’Neil, senior scientist at Green Bank, who led the research.  "But lingering uncertainties about the dark galaxy’s exact physical size make associated dark-matter estimates hazy at best...  J0613+52 is completely isolated, with no neighboring galaxy closer than 330 million light-years or so; our own Milky Way, in fact, appears to be the object’s closest-known companion.  In these void areas of the universe, gas should be too diffuse to form any galaxy-like object.  Clearly that’s not quite true."

Robert Minchin, of the National Radio Astronomy Observatory in New Mexico, heard O'Neil present the findings at last week's meeting of the American Astronomy Society, and was obviously impressed.  "I think it’s definitely a real detection," Minchin said.  "It does look like a primordial object.  It’s a bit like discovering a living dinosaur and having it there to study."

Artist's depiction of J0163+52 [Image credit: STScI POSS-II (starfield); additional illustration by NSF/GBO/P.Vosteen]

What puzzles me is that J0613+52 is only ("only") 330 million light years away, so not even close to being the farthest galaxy we've seen.  The universe as a whole is forty times older than the light we're seeing from this bizarre empty galaxy, so you'd think it'd have had plenty of time to form stars from all that hydrogen gas.  Instead, it seems to be a relatively homogeneous dust cloud.  You have to wonder, what's keeping it that way?  Gravity is relentless and inexorable -- the current models indicate that even tiny anisotropies (unevenness) in the mass distribution will result in the denser regions gaining mass at the expense of the less dense regions, resulting in clumps of matter that eventually coalesce into stars.

For a dust cloud that massive to last over twelve billion years without forming stars is somewhere beyond peculiar.

It may be that I'm missing something, here.  (Okay, given that I'm not an astrophysicist, it's certain that I'm missing something.)  But even with my no-more-than-basic understanding of astronomy, this object seems really peculiar.

As is the fact that it was discovered accidentally because one of the astronomers had entered a typo in the coordinates.

I'm sure the astronomers are going to be busy looking at the empty galaxy and trying to figure out what it is, and also looking for others.  Given its extremely low luminosity, and the fact that we found it by basically aiming a big telescope at a random spot in the sky, you have to wonder how many other similar structures there are.

I'll end with the words spoken by Hamlet, which have been quoted many times before but seem apposite: "There are more things in Heaven and Earth, Horatio, than are dreamt of in your philosophy."

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A glimpse of a monster

Ever heard of TON 618?

I hadn't until a few days ago, which is surprising considering my dual fascination with (1) astronomy, and (2) things that are huge and violent and could kill you.  TON 618 is a quasar, a luminous active galactic nucleus common in the universe's distant past.  Thankfully the cosmos has settled down a bit, because these things are so energetic their light is still visible today from billions of light years away.

Even by quasarian (I just made that word up, you should find a way to incorporate it into your daily speech) standards, TON 618 is impressive.  It showed up in a 1957 survey of faint blue stars, but its intense red shift indicated it was extremely distant and therefore a lot brighter than it looked.  It was entry #618 in the Tonantzintla Catalogue, a list of stars described in the bulletin of the Tonantzintla and Tacubaya Observatories in Mexico, and that's what gave it its rather unassuming name.

Once you start looking into this thing, though, you find it's anything but unassuming.

First, it's huge.  The black hole at the center of TON 618 is forty billion times more massive than the Sun.  If you put it where the Sun is, the entire Solar System would be inside its event horizon -- in fact, its event horizon is estimated at forty times the orbit of the planet Neptune.

Because of this, it has an impossibly high gravitational field, and is the center of a turbulent infall of matter.  This unfortunate gas and dust, as it accelerates toward its inevitable doom, is compressed and heated, emitting enough light to make TON 618 one of the most luminous objects in the known universe.  If the above comparisons weren't enough to blow your mind, TON 618 is estimated to have a luminosity of 4 x 10^40 watts -- about 140 trillion times brighter than the Sun.

It's also what is known as a Lyman-alpha blob.  This is another astronomical creature I just learned about, only found in the early universe (and therefore at this point, very far away).  The name comes from its extremely high emission of the Lyman alpha emission line of hydrogen, which has only been used as a tool for astronomers in recent years; it is so strongly absorbed by the Earth's atmosphere that it is essentially invisible to ground-based telescopes.  With the advent of orbiting telescopes like the Hubble, Kepler, and James Webb Space Telescopes, astronomers are finding more and more Lyman-alpha emitters in the distant (i.e. early) universe, but the debate goes on about what those emissions mean -- and why they aren't seen in nearby objects.

[Image licensed under the Creative Commons J.Geach/D.Narayanan/R.Crain, Computer simulation of a Lyman-alpha Blob, CC BY 4.0]

The most fascinating question about all this is where -- and what -- are quasars now?  The surmise is that for the most part they've settled down to become quiet, ordinary galactic nuclei.  But what about monsters like TON 618?  It's on the order of ten thousand times more massive than Sagittarius A*, the black hole at the center of the Milky Way; so if this eventually evolved into a galactic center, it would have to be one big-ass galaxy.

To put it in quantitative scientific terms.

Of course, there's no way to find out for sure.  When you look into the distance, you're also looking into the past, because the light that reaches your eyes (or telescope) took a finite length of time to arrive.  So you're always seeing things as they were, not as they are, and the farther away something is, the further back in time you're looking.  We're seeing TON 618 as it was about 10.8 billion years ago -- there's no way to know what, or where, it is now.

But that doesn't stop it from being an astonishing object.  The more sophisticated our instruments get, and the more detailed our scientific knowledge, the more weird and wonderful and magnificent the universe becomes.  

Even so, I'm glad that TON 618 -- whatever it is -- is located at a safe distance.  As fascinating as it is, it wouldn't make a good neighbor.

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Newton modified

Back in the 1970s and 1980s, astrophysicist Vera Rubin discovered something odd about the rates at which stars were revolving around their home galaxies; the stars in the outer reaches of the galaxy were orbiting as quickly as the ones nearer to the center.

Called the "flat rotation curve problem," this observation flies in the face of an astronomical principle that's been known since the seventeenth century, which is Kepler's Third Law.  Kepler's Third Law states that for bodies orbiting the same center of gravity, the square of the orbital period (time taken for the object to make a single orbit) is proportional to the cube of the average distance between the object and the center of gravity.  Put more simply, the farther out an orbiting object is, the slower it should be moving.  This law holds beautifully for the planets, asteroids, and comets in the Solar System.

Unfortunately, when Rubin looked at galactic rotation rates, she found that Kepler's Third Law appeared not to hold.  What it looked like was that there was a great deal more mass in the galaxy than could be seen, and that mass was spread out in some kind of invisible halo surrounding it.  That additional mass would account for the flatness of the rotation curves.

It was forthwith nicknamed dark matter.

The calculations of Rubin and others showed that the amount of dark matter was not insignificant.  Current estimates place it at around 27% of the total mass of the universe.  Only 5% is baryonic (ordinary) matter, so the matter we can't see outweighs ordinary matter by over a factor of five.  (The other 68% is the even weirder and more elusive dark energy, about which we know next to nothing.)

The problem is, every experiment designed to directly detect dark matter has resulted in zero success.  Whatever it is, it seems not to interact with ordinary matter at all other than via its gravitational pull.  These repeated failures drew rueful comparisons between dark matter and the luminiferous aether, the mysterious substance through which light waves were alleged to propagate.  The aether was proposed back in the nineteenth century because it was hard to imagine how light waves moved through a vacuum unless it had a medium -- what, exactly, was waving?  The existence of aether was conclusively disproven by the elegant Michelson-Morley experiment, which showed that unlike any other kind of wave, the speed of light waves seemed to be invariant regardless of the direction of motion of the observer.  It remained for Albert Einstein to explain how that could possibly be -- and to figure out all the strange and counterintuitive outcomes of this phenomenon, with his Special and General Theories of Relativity.

More than one modern physicist has surmised that dark matter might similarly be the result of a fundamental misunderstanding of how gravity works -- and that we are just waiting for this century's Einstein to turn physics on its head by pointing out what we've missed.

Enter Israeli physicist Mordehai Milgrom.

Milgrom is the inventor of MoND (Modified Newtonian Dynamics), a model which -- like the Theories of Relativity -- proposes that the explanation for the anomalous observations is not that there's some unseen and undetectable substance causing the effect, but that our understanding of how physics works is incomplete.  In particular, Milgrom says, there needs to be a modification to the equations of motion at very small accelerations, such as the ones experienced by stars orbiting in the outer reaches of galaxies.

With those modifications, the orbital rates make perfect sense.  No dark matter needed.

The Whirlpool Galaxy [Image licensed under the Creative Commons NASA/ESA/JPL/Hubble Heritage Team & C. Violette, M51 (2), CC BY-SA 4.0]

As with relativity -- and any other time someone has claimed to overturn a long-established paradigm -- MoND hasn't achieved anywhere near universal acclaim.  The Wikipedia article on it (linked above) states, gloomily, "no satisfactory cosmological model has been constructed from the hypothesis."  And it does lack the blindingly bright insight of Einstein's models, where taking the "problem of the seeming invariance of the speed of light" and turning it into the "axiom of the actual invariance of the speed of light" triggered a shift in our understanding that has since passed every empirical test ever designed.  Compared to Einstein's model, MoND almost seems like "Newton + an add-on," with no particularly good explanation as to why high accelerations obey Newton's laws but low ones don't.  (Of course, there's a parallel here to Einstein, as well -- at low speeds, Newton's laws are accurate, while at near-light speeds, Einsteinian effects take over.  So maybe Milgrom is on to something after all.)

After all, it's not like the other option -- dark matter -- has much going for it experimentally.

And MoND just got a significant leg up with an observation of the behavior of star clusters that was the subject of a paper in Monthly Notices of the Royal Astronomical Society last week.  In open star clusters, as new stars ignite it produces an outward push that can blow away material (including other stars), creating two "tidal tails" that precede and trail the cluster as it moves through space.  According to Newtonian dynamics (with or without dark matter), the two tails should have about the same mass.

"According to Newton's laws of gravity, it's a matter of chance in which of the tails a lost star ends up," explains Dr. Jan Pflamm-Altenburg of the Helmholtz Institute of Radiation and Nuclear Physics at the University of Bonn.  "So both tails should contain about the same number of stars.  However, in our work we were able to prove for the first time that this is not true: In the clusters we studied, the front tail always contains significantly more stars nearby to the cluster than the rear tail."

This peculiar observation fits the predictions of MoND much better than it does the predictions of the Newtonian model.

"The results [of simulations using MoND] correspond surprisingly well with the observations," said Ingo Thies, co-author of last week's paper.  "However, we had to resort to relatively simple computational methods to do this.  We currently lack the mathematical tools for more detailed analyses of modified Newtonian dynamics."

So the matter is very far from settled.  What's certain is that, similar to the physicists' situation in the late nineteenth century with regards to the behavior of light, there's something significant we're missing.  Whether that's some odd form of matter that doesn't interact with anything except via gravity, or because we've got the equations for the laws of motion wrong, remains to be seen.

And of course, after that, we still have dark energy to explain.  I think the physicists are going to be busy over the next few decades, don't you?

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An encounter with Charybdis

At the center of our seemingly tranquil galaxy, there's a black hole massive enough that it significantly warps spacetime, swallows any matter that gets close enough, and in the process emits truly colossal amounts of radiation.  Named Sagittarius A*, it was discovered in 1954 because of its enormous output in the radio region of the spectrum.  [N. B.  Throughout this post, when I refer to the black hole's radiation output, I am not of course talking about anything coming from inside its event horizon; that's physically impossible.  But the infalling matter that gets eaten by it does emit electromagnetic radiation before it takes its final plunge and disappears forever.  Lots of it.]

This thing is a real behemoth, at an estimated four million times the mass of the Sun.  There is a lot of interstellar dust between it and us -- after all, when you're looking at the constellation of Sagittarius, you're looking down a line going directly along the plane of the galaxy toward its center -- but even without the dust, it wouldn't be all that bright.  Most of its output isn't in the visible light region of the spectrum.  This doesn't mean it's dim in the larger sense; not only are there the radio waves that were the first part of its signal detected, but it has enormous peaks in the gamma and x-ray part of the spectrum as well.

Earlier this month, the European Southern Observatory released the first actual photograph of Sagittarius A*:

[Image licensed under the Creative Commons EHT Collaboration, EHT Sagittarius A black hole, CC BY 4.0]

How could something that enormous form?  We have a pretty good idea about how massive stars (over ten times the mass of the Sun) become black holes; when their cores run out of fuel, the gravitational pull of its mass collapses it to the point that the escape velocity at its surface exceeds the speed of light.  At that point everything that falls within its event horizon is there to stay.

But we're not talking about ten times more massive than the Sun; this thing is four million times more massive.  Where did all that matter come from -- and how did it end up at the center of not only our galaxy, but every spiral galaxy studied?

A step was taken in our understanding of galactic black hole formation by a team of astronomers at the University of North Carolina - Chapel Hill, in a paper that appeared this week in The Astrophysical Journal.  It's long been known that most large galaxies are attended by an array of dwarf galaxies, such as the Milky Way's Small and Large Magellanic Clouds.  (Which, unfortunately, are only visible in the Southern Hemisphere.  This is why they're named after Magellan.  Typical of the Eurocentric approach to naming stuff; clearly indigenous people knew about the Magellanic Clouds long before Magellan ever saw them.)  It's also known that because of the gravitational pull of the larger galaxies, the smaller ones eventually collide with them and merge into a single galaxy.  In fact, that even happens to big galaxies; gravity has a way of winning, given enough time.  The Milky Way and the Andromeda Galaxy, which are about the same size, will eventually come together into a single blob of stars, but what its final shape will be is impossible to predict.

As an aside, there's no need to worry about this.  First, it's not going to happen for another four and a half billion years.  Second, when galaxies (of any size) collide, there are relatively few actual stellar collisions.  Galaxies are mostly empty space, and when they merge the stars that comprise them mostly just pass each other without incident.

But not the black holes at their centers.  Those, being the center of mass of the entire aggregation, eventually slam together in a collision with a magnitude that's impossible to imagine.  And the team at UNC found out that this is one of the ways that galactic black holes become so large; they discovered that even dwarf galaxies have central black holes, and when they get swallowed up, that mass gets added to the central black hole of the larger galaxy.

Sagittarius A* sits in the middle of the whirling vortex of stars, like the sea monster Charybdis in Greek mythology, sucking down anything that comes close enough -- including, apparently, other black holes.  The celestial fireworks with a collision between two large black holes, such as the ones in the Milky Way and Andromeda, must release a fantastic amount of energy.

Wouldn't that be something to see?

From a safe distance, of course.

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Collision of galaxies

When I was an undergraduate at the University of Louisiana, I and several of my friends were blown away by the original series Cosmos, written and narrated by Carl Sagan.

Monday mornings, we gathered in the student lounge, eagerly discussing whatever mind-blowing filigree of physics had been the subject of that week's episode.  I still recall one of the ones that made the biggest impression on me -- the tenth episode, "The Edge of Forever," which included, among many other things, wonderful simulations of the motion of stars within a galaxy, and what happens when two galaxies collide.  (You can watch a clip of it here.)  The simulations were (at the time) state-of-the-art, and certainly enough to blow the mind of a sophomore physics student like myself; what struck me most was that galaxies aren't rigid, and their constituent stars don't "hang together," but move independently around the massive black hole at the galactic core.  This can settle down into a shape that seems pretty stable -- such as the spiral pattern of the Milky Way -- or it can destabilize, flinging stars out into space, exploding the galaxy and scattering its pieces across hundreds of thousands of light years.

Sagan, of course, put it best: "A galaxy is a fluid made of a billion suns, all bound together by gravity."

When galaxies collide, it disrupts both completely; at the same time, collisions between the stars themselves are extremely uncommon.  However big the stars are, they're still minuscule with respect to the galaxies that contain them.  It's like the atoms writ large, isn't it?  The seemingly solid matter around you is made up of tiny charged particles interacting through the force of electromagnetism, but in between those particles is... nothing.  Matter is mostly empty space, and only seems solid because you're feeling the mutual repulsion of the electrons in your fingers and the electrons on the surface of whatever you're touching.  Likewise, most of interstellar space is very close to nothing, and the galaxies themselves are made up of particles (stars) interacting through a different force (gravity), and separated by vast, empty voids.

Makes you almost think that the pagans might have been on to something with their dictum of "As above, so below."

Map of the Milky Way, as it would look from above the galactic disk [Image licensed under the Creative Commons 鄭興武和馬克 裡德（Mark J. Reid）銀河系棒和旋臂結構遺產性巡天(BeSSeL)項目組/南京大學/哈佛-斯密松天體物理中心., Milky Way large, CC BY-SA 4.0]

This topic, and my reminiscences of Cosmos, come up because of a paper in Nature Astronomy last week called "Evidence for a Vast Prograde Stellar Stream in the Solar Vicinity," by a team led by astronomer Lina Necib of the California Institute of Technology.  What this paper tells us is something stunning; there is a streamer of stars in the Milky Way that started out somewhere else, and collided with our galaxy.  Rather fortunately, apparently the angle and velocity with which the streamer hit were more or less the same direction the original galaxy was turning, so these stars simply got sucked in, like some bits of debris going down a whirlpool.

The streamer has been named Nyx, after the Greek goddess of the night.  250 stars have been identified as being part of Nyx.  "The two possible explanations here are that they are the remnants of a [galactic] merger, or that they are disk stars that got shaken into their new orbits because of a collision with the disk of the Milky Way," said study lead author Lina Necib, in an interview with CNN.  The likelihood, though, is the former, something that is expected to be confirmed by chemical analysis of the constituent stars.  "Galaxies form by swallowing other galaxies," Necib said. "We've assumed that the Milky Way had a quiet merger history, and for a while it was concerning how quiet it was because our simulations show a lot of mergers.  Now, we understand it wasn't as quiet as it seemed.  We're at the beginning stages of being able to really understand the formation of the Milky Way."

I think it's stunning that we can figure out this sort of thing at all -- that 250 out of the estimated 250 billion stars in the Milky Way started out somewhere else in the universe.  I think that's pretty damn impressive.  "This particular structure is very interesting because it would have been very difficult to see without machine learning," Necib said.  "I think we reached a point in astronomy where we are not data limited anymore.  This project is an example of something that would have not been possible a few years ago, the culmination of developments in data with Gaia, high resolution simulations, and machine learning methods."

How pleased and amazed Carl Sagan would have been.  He went a long way toward bringing the wonders of the universe, from the largest scales to the smallest, to laypeople.  He certainly blew the minds of me and my friends, and that was back in 1980.  Necib's comment, that we're still at the beginning of being able to understand the formation of galaxies, tells us that we have a long way still to go -- and that many, many more eye-opening discoveries are sure to come our way in the next years.

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This week's Skeptophilia book of the week is for anyone fascinated with astronomy and the possibility of extraterrestrial life: The Sirens of Mars: Searching for Life on Another World, by Sarah Stewart Johnson.

Johnson is a planetary scientist at Georgetown University, and is also a hell of a writer.  In this book, she describes her personal path to becoming a respected scientist, and the broader search for life on Mars -- starting with simulations in the most hostile environments on Earth, such as the dry valleys of central Antarctica and the salt flats of Australia, and eventually leading to analysis of data from the Mars rovers, looking for any trace of living things past or present.

It's a beautifully-told story, and the whole endeavor is tremendously exciting.  If, like me, you look up at the night sky with awe, and wonder if there's anyone up there looking back your way, then Johnson's book should be on your reading list.

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

Ripples in the cosmic pond

I know I've said this before, so at the risk of ringing the changes on this once too many times: I find it endlessly fascinating how much we can figure out about the universe, sitting here on this little speck of rock circling a mediocre star in the arm of an average galaxy.

Three papers came out last week in Nature Astronomy that each individually might bowl you over with the scale of things; put together, they're kind of staggering. First we have a paper by a team led by Tiantian Yuan, of Australia’s ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D).  Entitled "A Giant Galaxy in the Young Universe with a Massive Ring," this research looks at a very unusual object -- a collisional ring -- that is eleven billion light years away.

Meaning we're seeing it as it was eleven billion years ago.

"It is a very curious object that we’ve never seen before," Yuan said, in a press release.  "It looks strange and familiar at the same time.  It is making stars at a rate fifty times greater than the Milky Way.  Most of that activity is taking place on its ring – so it truly is a ring of fire."

"The collisional formation of ring galaxies requires a thin disk to be present in the ‘victim’ galaxy before the collision occurs," added Kenneth Freeman of the Australian National University, who co-authored the paper.  "The thin disk is the defining component of spiral galaxies: before it assembled, the galaxies were in a disorderly state, not yet recognizable as spiral galaxies.  In the case of this ring galaxy, we are looking back into the early universe by eleven billion years, into a time when thin disks were only just assembling.  For comparison, the thin disk of our Milky Way began to come together only about nine billion years ago.  This discovery is an indication that disk assembly in spiral galaxies occurred over a more extended period than previously thought."

Yes, you read that right.  The images of this object are pictures of something that existed two billion years before the Milky Way formed.

The second paper is about an object that is even older and father away than the ring galaxy.  Titled "A Cold, Massive, Rotating Disk Galaxy 1.5 Billion Years After the Big Bang," by a team led by Marcel Neeleman of the Max Planck Institute for Astronomy, is estimated to be 12.3 billion light years away -- so this structure is not only the oldest disk galaxy ever observed, it also gives us incredible new data on the way galaxies in general form.

The authors write:

Massive disk galaxies like the Milky Way are expected to form at late times in traditional models of galaxy formation, but recent numerical simulations suggest that such galaxies could form as early as a billion years after the Big Bang through the accretion of cold material and mergers.  Observationally, it has been difficult to identify disk galaxies in emission at high redshift in order to discern between competing models of galaxy formation...  The detection of emission from carbon monoxide in the galaxy yields a molecular mass that is consistent with the estimate [that the galaxy's mass is] about 72 billion solar masses.  The existence of such a massive, rotationally supported, cold disk galaxy when the Universe was only 1.5 billion years old favours formation through either cold-mode accretion or mergers, although its large rotational velocity and large content of cold gas remain challenging to reproduce with most numerical simulations.

So the astrophysicists are going to be sifting through that data for quite some time.

The third, "The Recurrent Impact of the Sagittarius Dwarf on the Star Formation History of the Milky Way," by a team led by Tomás Ruiz-Lara of the Instituto de Astrofísica de Canarias, describes something pretty amazing about our own galaxy; the main disc is orbited by a dwarf galaxy (the Sagittarius Dwarf in the title) which has an elliptical orbit, so it at regular intervals pierces the disc of the Milky Way -- causing eddies that trigger a huge spike in star production.

"You have the Milky Way in equilibrium, mostly calm, and then when Sagittarius passed it was like throwing a stone in a lake," said Ruiz-Lara, in an interview with New Scientist.  "It created these ripples in the galaxy’s density, so some areas became more dense and started forming stars more efficiently...  Maybe without Sagittarius the solar system wouldn’t exist.  The timing works out, but there is no way for us to know for sure."

[Image is in the Public Domain courtesy of NASA, ESA, and The Hubble Heritage Team STScI/AURA]

Gradually the dwarf galaxy is losing energy and its orbit is pulled in tighter and tighter, making these collisions and bursts of star formation more frequent.  "It’s getting closer and closer, little by little over time, and in the end it will merge with the Milky Way," Ruiz-Lara said.

Not to worry, that won't happen for another billion-odd years.  So no need to run and see if your stellar collision insurance is paid up.

It's kind of mind-boggling when you think about it, that a bunch of primates who were not so long ago loping around on the African savanna trying not to get eaten by lions have found a way to see into the farthest reaches of the universe.  Not to get cocky about it, but that's pretty spectacular.  Only a hundred years ago we didn't even know for sure how big or how old the universe is; today we're looking into the depths of space, and back in time almost as far is is physically possible.

It brings to mind the wonderful quote by Carl Sagan: "We are a way for the cosmos to know itself."

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This week's Skeptophilia book recommendation of the week is a fun one: acclaimed science writer Jennifer Ackerman's The Bird Way: A New Look at how Birds Talk, Work, Play, Parent, and Think.

It's been known for some years that a lot of birds are a great deal more intelligent than we'd thought.  Crows and other corvids are capable of reasoning and problem-solving, and actually play, seemingly for no reason other than "it's fun."  Parrots are capable of learning language and simple categorization.  A group of birds called babblers understand reciprocity -- and females are attracted to males who share their food the most ostentatiously.

So "bird brain" should actually be a compliment.

Here, Ackerman looks at the hugely diverse world of birds and gives us fascinating information about all facets of their behavior -- not only the "positive" ones (to put an human-based judgment on it) but "negative" ones like deception, manipulating, and cheating.  The result is one of the best science books I've read in recent years, written in Ackerman's signature sparkling prose.  Birder or not, this is a must-read for anyone with more than a passing interest in biology or animal behavior.

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

The black heart of M87

It's awfully easy to get discouraged lately.

The news seems to go from bad to worse every day.  Government corruption, terrorist attacks both here and overseas, every other news story a testament of the amazing ability of Homo sapiens to treat each other horribly.

So when we have a triumph, we should celebrate it.  Because as Max Ehrmann put it, in his classic poem "Desiderata:" "Many persons strive for high ideals; and everywhere life is full of heroism...  With all its sham, drudgery, and broken dreams, it is still a beautiful world."

We got a lovely example of that a couple of days ago, with the revelation that scientists had amassed enough data from the Event Horizon Telescope project to produce the first-ever photograph of a black hole.  It's at the heart of the massive elliptical galaxy M87, in the constellation of Virgo, and is about 54 million light years away.  With no further ado, here's the photo:

I think the left-hand one -- with less magnification -- is even more amazing, because it shows the black hole in context with its surroundings.  It looks more "real."  But in either case, they're stunning.  The glow comes from matter being sucked into the black hole, being heated up to the point of producing x-rays as a sort of electromagnetic death scream before dropping forever beyond the event horizon.  For the very first time, we're seeing an actual image of one of the most bizarre phenomena in nature -- an object so massive that it warps space into a closed curve, so that even light can't escape.

But even this isn't my favorite image that has come out of this study.

This is:

This is Katie Bouman, the MIT postdoc (soon to be starting as an assistant professor at CalTech) whose algorithm made the black hole image a reality.  Bouman responded to her sudden fame with modesty.  "No one of us could've done it alone," she said.  "It came together because of lots of different people from many backgrounds."

Which may well be, but no one is questioning her pivotal role in this groundbreaking achievement.  And what I love about the photograph above is that it perfectly captures the joy of doing science -- what Nobel Prize-winning physicist Richard Feynman called "the pleasure of finding things out."  We now have a window into a piece of the universe that was invisible to us before, and that ineffable feeling is clearly captured in her expression.

There have already been six papers written about this accomplishment, and that's only the beginning.  I find myself wondering what other obscure and fascinating astronomical phenomena Bouman's algorithm could be used to photograph -- and what we might learn from seeing them for real for the first time.  I also wonder what effect that will have on us ordinary laypeople.  The physicists may be comfortable living in the world of their mathematical models (I heard one physicist friend say, "The models are the reality; everything else is just pretty pictures"), but the rest of us need to be more grounded in order to understand.  So what if we were to see more than an artist's conception of things like pulsars, quasars, gamma-ray bursters, type 1-A supernovae?

Kind of a surfeit of wonder, that would be.  But what a way to be overwhelmed, yes?

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This week's Skeptophilia book recommendation is a fun one; Atlas Obscura by Joshua Foer, Dylan Thuras, and Ella Morton.  The book is based upon a website of the same name that looks at curious, beautiful, bizarre, frightening, or fascinating places in the world -- the sorts of off-the-beaten-path destinations that you might pass by without ever knowing they exist.  (Recent entries are an astronomical observatory in Zweibrücken, Germany that has been painted to look like R2-D2; the town of Story, Indiana that is for sale for a cool $3.8 million; and the Michelin-rated kitchen run by Lewis Georgiades -- at the British Antarctic Survey’s Rothera Research Station, which only gets a food delivery once a year.)

This book collects the best of the Atlas Obscura sites, organizes them by continent, and tells you about their history.  It's a must-read for anyone who likes to travel -- preferably before you plan your next vacation.

(If you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!)