Cloud collision

Contrary to what the medieval church wanted you to believe, the Earth is in constant motion.

They went to enormous lengths to stand by the principle that we're the center of the universe, motionless, while everything revolves around us in perfect circles.  Arrogant attitude, that.  Also wildly wrong.  Not only is the apparent motion of the stars at night caused by our own rotation, the stars aren't in quite the same positions at a given time one night as compared to the next because we're revolving around the Sun.

Then along came Kepler, and showed that even the "perfect circles" part was wrong; the planets and their moons orbit in ellipses, not circles, some of them quite eccentric (the mathematicians' word for the degree to which an ellipse deviates from a circle).

It's even worse than that.  The Earth's axis precesses, wobbling like a spinning top, drawing out a circle in the sky once every twenty-six thousand years.  So Polaris, hasn't always been the pole star, and at some point won't be any longer.  This fact was discovered by the Greek astronomer Hipparchus, but although the aforementioned church fathers loved the Greek philosophers -- they were especially fond of Aristotle -- they were also excellent at ignoring evidence that challenged their own worldviews, so Hipparchus's studies of axial precession were brushed aside.  The thirteenth century Persian polymath Nasir al-Din al-Tusi studied astronomical records and came up with a value very close to our currently accepted precession rate, but the church fathers didn't much listen to the Muslims, either, so it wasn't until eighteenth century French mathematician Jean le Rond d'Alembert said, "No, really, guys, this precession thing is real" that people in the western world started to accept it.

The path of apparent precession of the pole star. The bright star at the bottom is Vega, which was the pole star twelve thousand years ago (and will be again in fourteen thousand years). [Image licensed under the Creative Commons Tauʻolunga, Precession N, CC BY-SA 2.5]

But even that's not the end of it, because the Sun (and the rest of the Solar System) are in the edge of one of the spiral arms of the Milky Way, and are traveling at about 230 kilometers per second in orbit around the galactic core.  This is a good clip -- it's only a bit under a thousandth of the speed of light -- but even so, the galaxy is so enormous it will take about 225,000,000 years to complete one orbit.  Put another way, the last time the Solar System was in this spot was the early Triassic Period -- right at the beginning of the "Age of Dinosaurs."

It's this last motion that's what brings the topic up today, because a team led by Boston University astronomer Merav Opher has just found that the motion of the Sun and planets around the galactic center swept it through two successive clouds of cold gas and dust, hitting one about seven million years ago and another a little over two million years ago.  The clouds, which from our current perspective are in the constellation of Lynx, provided enough resistance that the heliosphere -- the region of space dominated by the outward pressure of material thrown off by the Sun -- shrank to the point that the planets were exposed to the dust of the interstellar medium.  This caused a spike of supernova-generated isotopes like iron-60 and plutonium-244 in cosmic dust trapped in sediments and ice layers here on Earth.

Opher's team found this cosmic dust in every place of those ages they looked.  It was the fingerprint of a collision -- between the Solar System and a pair of clouds.

It's an open question what effect that had on the Earth.  The collisions happened just as our hominid ancestors were moving their way out of the African savanna, so any additional flux of cosmic rays from being outside the heliopause didn't seem to do us any harm.  But it's a cool reminder that although we feel like the Earth is solid and unmoving beneath our feet, it's actually being spun around the universe like a little kid on the Tilt-o-Whirl.

But finally, there's even another layer on top of all the above, because the Milky Way and the entire Local Group are moving toward something called the "Great Attractor" at six hundred kilometers per second, over twice as fast as the Solar System's orbital velocity around the galactic center.  Presumably this is because of some sort of gravitational effect, but what sucks is that although we know the general direction where the Great Attractor is located, we don't even know what's there because it's directly on the opposite side of the center of our own galaxy.  In other words, we can't see where we're headed because the Milky Way is in the way.  

What it's in the way of remains to be seen.

So yeah.  The medieval church fathers were kind of spectacularly wrong.  The more we've learned, the weirder the universe gets, and the farther from the center of anything we appear to be.  It's better this way, though, because it gives us constant reminders of how grand and magnificent the universe is -- even if the inevitable consequence is a reminder of how tiny we are by comparison.

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

The firehose

There's some weird stuff going on with M87.

M87 is a supergiant elliptical galaxy in the constellation of Virgo.  It was discovered and catalogued in 1781 by French astronomer Charles Messier -- the "M" designation in many of the brightest nebulae and galaxies comes from their listing in the Messier catalogue -- but the telescopes of his time weren't good enough to make out much detail.  Even through better telescopes it looks like an uninteresting fuzzy blob, mostly because it's 53 million light years away.

This belies its magnitude.  It contains over a trillion stars, and is orbited by around fifteen thousand globular clusters (compare this to the Milky Way's paltry two hundred or so), and has a ginormous black hole at its center with a mass 2.4 billion times that of the Sun.  It is this black hole that you undoubtedly remember from the famous photographs in March of 2021:

[Image licensed under the Creative Commons Event Horizon Telescope, A view of the M87 supermassive black hole in polarised light, CC BY 4.0]

So this is impressive enough as is.  But then the astronomers and astrophysicists starting noticing that the black hole itself was behaving... oddly.

Three weeks ago, a team led by Yuzhu Cui of Shanghai Jiao Tong University published a paper in Nature showing that the black hole at the center of M87 was not only spinning (which isn't at all unusual; most black holes spin) but was precessing.  If you've ever played with a gyroscope, you've seen precession; get it started spinning, and for a little bit it'll stand upright, but then it starts to wobble, and its spin axis traces out a cone that gets wider and wider as the spin rate goes down because of friction.  The Earth precesses, with a period of about 26,000 years, meaning that Polaris wasn't the North Star a few thousand years ago, nor will it be a few thousand years in the future.  Twelve thousand years ago, the North Star was the bright star Vega in the constellation Lyra, made famous as the home of the benevolent aliens in the brilliant movie Contact.

[Image licensed under the Creative Commons Tauʻolunga, Precession N, CC BY-SA 2.5]

So precession of a spinning body isn't that unusual, either, but considering the angular momentum of a 2.4 billion solar mass object, it's kind of surprising that the M87 black hole is precessing fast enough to be observable from 53 million light years away.  But it is -- and its period of precession is only eleven years!

This means that the fountain of radiation and debris being shot out along its spin axis is flailing around like the jet from a loose firehose.  

Then, a new paper -- still in the preprint stages -- has added another bizarre twist.  A team of astrophysicists led by Michael M. Shara, Curator of Astrophysics for the American Museum of Natural History, has found that wherever that wildly-precessing jet nozzle is aimed, there's a higher rate of stars going nova.  Novae are explosions less violent than supernovae (those actually blow the unfortunate star to smithereens); they seem to occur mostly when white dwarf stars accrete matter from nearby dust clouds or by stealing it from a binary star partner, triggering instability and a sudden flare-up.  Here, though, the mechanism isn't understood.  Whether the jet of debris from the black hole is compressing the stars that get in the way and triggering detonation, or if it's simply that the material itself is getting caught by white dwarfs and causing the novae, isn't known.

But it's quite a mental image, isn't it?  A careening jet from a spinning supermassive black hole blasts away at stars in its path, and makes them blow up.

Leaves me feeling glad we live in the tranquil outer reaches of our own galaxy.  I know the Milky Way has its own massive black hole at the center, but out here in quiet stellar suburbia, we're pretty insulated from all that craziness.

I'm perfectly happy hearing about the wild gyrations of M87 -- from a safe vantage point 53 million light years away.

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

This is the dawning of the Age of... Capricorn?

(One of a series of reposts, for your enjoyment while I'm on vacation.  First posted in January 2011.)

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

The hottest news today, for those who believe that their personalities, destinies, and love lives are controlled by the positions of distant planets relative to arbitrary patterns of even-more-distant stars, is: you're not the astrological sign you think you are.

The ancient Greeks are the ones who are responsible for a lot of the names we use for constellations today.  They looked up into the night sky, probably after having tanked up on ouzo and retsina, and instead of seeing what most of us do -- a completely random arrangement of stars -- they saw patterns that reminded them of people, animals, and objects from their myths and folk tales.  Thus we have a vague, wandery curve of faint stars that is Draco the Dragon, a pair of bright stars that is Canis Minor the Little Dog, a crooked zigzag that is Cassiopeia the Celestial Queen, and a little group of six stars that is Waldo the Sky Wombat.

Okay, I made the last one up.  But some of them are equally weird.  There's Coma Berenices, "Berenice's Hair;" Fornax the Furnace; Volans the Flying Fish; for people who like things simple and obvious, Triangulum the Triangle; and for people in the southern hemisphere who like things simple and obvious, Triangulum Australe the Southern Triangle. 

Even earlier, astronomers during the Babylonian times had noticed that the sun and the planets seemed to trace a path against the stars, and that path is the zodiac.  The twelve zodiac constellations are the ones that the sun seems to move through, as the earth travels around the sun; and your sign is supposed to be the constellation in which the sun seemed to reside at the moment of your birth.

But now, astronomers with the Minnesota Planetarium Society have released a bombshell.  Because the Earth's axis precesses, the constellations of the zodiac aren't lined up the way they were during the time of the ancient Greeks.  Precession happens because the Earth wobbles like a top as it spins, and the axis of the earth traces out a circular path every 26,000 years (meaning that Polaris won't be the North Star forever).  As a result, the whole zodiac has tipped by about ten degrees, and most likely you aren't the sign you think you are -- you are the one immediately preceding it, or possibly even the one before that.

Worse news still if you're a Sagittarius; not only are you not a Sagittarius, your sign is likely to be a constellation that isn't even part of the standard zodiac.  During Greek times, the zodiac actually passed briefly through the constellation Ophiuchus, the Snake Handler, but because thirteen seemed an unpropitious number for the zodiac constellations, and also because "Ophiuchus" sounds like the scientific name of an intestinal parasite, they threw it out.  Now, however, because of the precession of the Earth, the zodiac spends a lot longer in Ophiuchus, and it's no longer possible to ignore it.  So if you were a Sagittarius, you're probably now an Ophiuchus, and might want to consider a career as a herpetologist, or at least a snake charmer.

And I guess I'm not really a Scorpio.  This is too bad.  I kind of liked being a Scorpio.  They're supposed to be deep, intense, passionate, secretive, and a little dangerous, which I always thought was cool.  Now, I guess I'm a Virgo, which means I'm weak, stubborn, and petulant.  So I've gone from being James Bond to being George Costanza.  It figures.

Of course, I console myself with the knowledge that astrology is pretty silly anyhow; one has to wonder why anyone ever found it plausible that the fact that Saturn was in Capricorn at the moment of your birth is why you like cottage cheese.  (Okay, I made that up because I don't feel like researching what it really means if Saturn is in Capricorn.  But my point stands.)  Right now, I'm mostly curious to see what the astrologers will do -- if they will revise their astrological charts to reflect the actual positions of the sun and planets relative to the stars, or if they'll keep doing what they've always done.

My money is on the latter.  I'm guessing that they'll figure that they've never worried about a minor issue like whether their predictions have any basis in reality, so why start now?