Dark shadows

After yesterday's rather depressing post about politics, today we're going to turn our eyes away from the troubled and turbulent Earth and out to the skies.

"The more we look, the more we see" sounds like a tautology, but it the realm of the sciences, it isn't.  Sometimes it takes training, and careful examination of what's in front of you, even to know exactly what it is you're looking at.

This is especially true in astronomy.  Consider that in only four hundred years, we've gone from:

• stars being equidistant points of light on a sphere with the Earth at the center;
• to recognizing that stars are, in fact, not all the same distance away from us, and their apparent motion comes from the combination of Earth's rotation and its circling the Sun;
• to realizing that even the nearest stars are incredibly far away;
• to discovering that the Sun is a star -- and the stars are suns -- and they're all made of more or less the same stuff;
• to the shocked understanding that galaxies are millions of light years away, are composed of billions of stars -- and there are trillions of galaxies, almost all of which are rushing away from us at breakneck speeds.

Along the way, we've discovered hundreds of different celestial objects and phenomena, some of which are positively mind-boggling, and many of which we still have yet to explain completely.

The topic comes up because of an article I read yesterday by astronomer Phil Plait.  I discovered Plait a few years ago because of his excellent website Bad Astronomy (about myths and misconceptions concerning the skies).  I've also read several of his books, and he's an excellent example of a scientist who is also highly skilled at bringing cutting-edge science to us interested laypeople.  (I especially recommend Death from the Skies!, about which writer Daniel H. Wilson said, "Reading this book is like getting punched in the face by Carl Sagan.  Frightening, yet oddly exhilarating.")

In any case, Plait's article is entitled "What Is Inside Our Galaxy's Darkest Places?", and is about dust clouds.  I knew at least a little about celestial dust clouds, which are thought to be the raw materials that can eventually collapse to form stars and planets, something I touched on in a post last week.  But there was a lot in the article that was new to me -- and intriguingly weird.

The dark dust clouds Plait describes are called Bok globules, after astronomer Bart Bok who studied them, and there are estimated to be millions of them in our galaxy alone.  And "dark" is something of an understatement; the dust and gas they contain reduces the intensity of any light coming through them by a factor of fifteen trillion.  The result is that they look like a black, starless blotch in the sky.  I was immediately reminded of the Black Thing from Madeleine L'Engle's A Wrinkle in Time:

"That shadow out there."  Calvin gestured.  "What is it?  I don't like it."

"Watch," Mrs. Whatsit commanded.

It was a shadow, nothing but a shadow.  It was not even as tangible as a cloud.  Was it cast by something?  Or was it a thing in itself?

The sky darkened.  The gold left the light and they were surrounded by blue, blue deepening until where there had been nothing but the evening sky there was now a faint pulse of a star, and then another and another and another.  There were more stars than Meg had ever seen before.

"The atmosphere is so thin here," Mrs. Whatsit said, as though in answer to her unasked question, "that it does not obscure your vision as it would at home.  Now look.  Look straight ahead."

Meg looked.  The dark shadow was still there.  It had not lessened or dispersed with the coming of night.  And where the shadow was, the stars were not visible. 

Of course, Bok globules are just dust clouds, not the distilled essence of evil.

I hope.

Barnard 68, a Bok globule about five hundred light years from Earth [Image credit: European Southern Observatory]

But the thing that amazed me the most about these dust clouds was how little matter they actually contain, and yet how good they are at blocking light.  Plait tells us that they average about a million molecules per cubic centimeter -- which seems like a lot until you find out that air at sea level contains ten trillion trillion molecules per cubic centimeter.  But despite their thinness, if you put the Sun a half-light-year away from the Earth -- so, only a little more than ten percent of the distance to the nearest star --and put a typical Bok globule in between, the Sun's light would be so attenuated it wouldn't be visible to the naked eye.

Which is why I started with "the more you look, the more you see."  Or -- more accurately, in this case -- the more you look, the more you realize how much we might not be seeing.

In any case, I don't want to steal any more of Plait's thunder, because you should all read his article, which is wonderful fun (and is linked above).  And if you're on Bluesky, subscribe to him, because his posts are awesome.

So that's today's cool new thing I learned about the universe.  Which is also valuable because it takes my mind off what's happening down here.  All in all, things seem to look up when I do.

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A wolf, a disk, and a lighthouse

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

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

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

[Image credit: European Southern Observatory]

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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