Hit by the firehose

An impending astronomical event has brought to the general attention a phenomenon called a nova.  Misleadingly named after the Latin word for "new," a nova isn't a new star at all -- it's an old star (some of them very old) that suddenly flares up and becomes visible to the naked eye.  The nova in question is T Coronae Borealis, a star that is ordinarily around an apparent magnitude of +10 (making it far too faint to see without a telescope), but every eighty or so years flares up to a magnitude of around +2, becoming easily visible for a short time before fading to its original unimpressive luminosity.

Novas of this type are double star systems.  One member of the pair is a white dwarf -- the white-hot core of a Sun-like star at the end of its life -- and the other one is usually a giant.  What happens is that the super-dense white dwarf gradually siphons off gas from its larger but less dense partner, and as the gas falls onto the white dwarf's surface it heats and compresses, finally becoming hot enough to fuse into helium.  This releases more heat energy still, and causes a runaway chain reaction, resulting in the flare-up.  But the total amount of hydrogen available isn't really that great -- it's only a shell of material on the surface -- so the reaction runs out of steam, and the pair settles down again until enough more gas is siphoned off to trigger another flash.

T Coronae Borealis is due -- overdue, according to some astrophysicists -- for a blaze-up.  So those of you in the Northern Hemisphere, watch for this "new star" -- it's something you'll likely never get another chance to see.

The reason the topic comes up is some new data from the Hubble Space Telescope about novas in another galaxy -- M87, a supergiant elliptical galaxy in the constellation Virgo.  

[Image licensed under the Creative Commons Ngc1535, M87 Galaxy from the Mount Lemmon SkyCenter Schulman Telescope courtesy Adam Block, CC BY-SA 4.0]

M87 became famous because it was the galaxy whose massive central black hole became the first ever to be photographed.  Since then, it's been studied extensively, and the most recent information we've learned about it is downright puzzling.

Most black holes are surrounded by an accretion disk -- a violent whirlpool of gas spiraling down toward the event horizon.  As it spins, the ionized atoms release energy in the form of x-rays; some of them are accelerated enough to escape completely.  The result is a narrow jet of plasma and electromagnetic radiation, aligned with the poles of the black hole's magnetic field.

Especially with a supermassive black hole like the ones at the center of galaxies, having the jet aimed at you personally would be a very bad thing.  Anything less than a thousand light years away would be deep fried.  Even farther away, the effects of the plasma stream would be devastating.

And what the recent study found is that stars that are hit by this blast of radiation are much more likely to go nova -- and no one is really sure why.

"There's something that the jet is doing to the star systems that wander into the surrounding neighborhood. Maybe the jet somehow snowplows hydrogen fuel onto the white dwarfs, causing them to erupt more frequently," said astrophysicist Alec Lessing of Stanford University, who co-authored the study, in an interview with Science Daily.  "But it's not clear that it's a physical pushing.  It could be the effect of the pressure of the light emanating from the jet.  When you deliver hydrogen faster, you get eruptions faster.  Something might be doubling the mass transfer rate onto the white dwarfs near the jet."

The bottom line is, the astrophysicists are not sure why it's happening, but some interaction between the jet and the stars caught in it is making candidate stars "pop off like camera flashes."

I guess it's not surprising that when you put two of the most violent astronomical phenomena together -- the massive hydrogen bomb of novas, and the giant firehose of plasma from a supermassive black hole -- they behave in surprising ways.  The astrophysicists will be working their models trying to figure out just what exactly is going on here.

And for those of you who are worriers, M87 and its accompanying jets of radiation are a comfortable 53 million light years away.  Even our own galactic core is 26,000 light years away, and its radiation jets are aimed in a direction almost exactly ninety degrees away from us; the Solar System lies in one of the outer spiral arms, which are arrayed pretty much in a flat plane perpendicular to the rotational and magnetic axis of the galaxy.

So this phenomenon is certainly awe-inspiring, but it's not dangerous.  At least not to us.  As far as any inhabited planets caught in the outflow, well... good luck to them.

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Ripples in the cosmic pond

Springboarding off yesterday's post, about a mysterious flare-up of Sagittarius A* (the supermassive black hole at the center of the Milky Way galaxy), today we have an even more momentous discovery -- a background thrum of gravitational waves from supermassive black holes in orbit around each other.

Gravitational waves are created when massive objects accelerate through space.  They're actually pulsed fluctuations in the fabric of space-time that propagate out from the source at the speed of light.  The idea has been around for a long time; English mathematician Oliver Heaviside proposed them all the way back in 1893.  Once Einstein wrote his paradigm-overturning paper on relativity in 1915, Heaviside's proposal gained a solid theoretical underpinning.

The problem was detecting them.  They're tiny, especially at large distances from the source; and the converse difficulty is that if you were close enough to the source that they were obvious, they'd be big enough to tear you to shreds.  So observing from a distance is the only real option.

[Image licensed under the Creative Commons ESO/L. Calçada/M. Kornmesser, Artist’s impression of merging neutron stars, CC BY 4.0]

The result is that it took a hundred years to get direct evidence of their existence.  In 2015 the LIGO (Laser Interferometer Gravitational Wave Observatory) successfully detected the gravitational waves from the merger of two black holes.  The whirling cyclone of energy as they spun around their center of mass, then finally coalesced, caused the space around the detector to oscillate enough to trigger a shift in the interference pattern between two lasers.  The physicists had finally seen the fabric of space shudder for a moment -- and in 2017, the accomplishment won the Nobel Prize for Rainer Weiss, Kip Thorne, and Barry Barish.

Now, though, a new study at the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has found a whole different kind.  Instead of the sudden, violent, there-and-gone-again waves seen by LIGO, NANOGrav has found a background "hum" in the universe -- the stirring of spacetime because of the orbiting of supermassive black holes around each other.

The accomplishment is made even more astonishing when you find out how long the wavelengths of these waves are.  Frequency is inversely proportional to wavelength, so the "nanohertz" part of the name of the observatory might have given you a clue.  The gravitational waves detected by NANOGrav have wavelengths measured in light years.  So how in the hell do you detect a wave in which -- even traveling at the speed of light -- the trough of the wave doesn't hit you until a year after the crest?

The way they did it is as clever as it is amazing.  Just as you can see a pattern of waves if you look across the surface of a pond, the propagation of these gravitational waves should create a ripple in space that affects the path of any light that travels through them.  The scientists at NANOGrav measured the timing of the light from pulsars -- the spinning remnants of collapsed massive stars, that because of their immense mass and breakneck rotational speed flash on and off with clocklike precision.  And sure enough, as the waves passed, the contraction and expansion of the fabric of space in between caused the pulsars to seem to speed up and slow down, by exactly the amount predicted by the theory.

"The Earth is just bumping around on this sea of gravitational waves," said astrophysicist Maura McLaughlin, of West Virginia University, who was on the team that discovered the phenomenon.

It's a little overwhelming to think about, isn't it?  Millions of light years away, two enormous black holes are orbiting around a common center of gravity, and the ripples that creates in the cosmic pond flow outward at the speed of light, eventually getting here and jostling us.  Makes me feel very, very small.

Which, honestly, is not a bad thing.  It's always good to remember we're (very) tiny entities in a (very) large universe.  Maybe it'll help us not to take our day-to-day worries quite so seriously.

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Blink of an eye

I know I often write about strange and unexpected discoveries out there in the universe in the context of how much more we have to learn, but even by those standards, the discovery announced by astrophysicists at the Massachusetts Institute of Technology this week was bizarre.

Most everyone knows about black holes -- stellar remnants that have collapsed to a density that they warp space/time into a closed surface.  The usual way this is put is that their gravitational pull is so strong even light isn't fast enough to escape, but that's not really that accurate; light is massless and therefore photons exert (and feel) no gravitational pull.  However, they are constrained to follow the lines of space/time they're traveling through, just as cars can take whatever road their drivers choose, but are still constrained to stay on the surface of the Earth's sphere.

Around a black hole, the fabric of space is so distorted that it's a bit like an infinitely-deep well.  Once inside the black hole's event horizon, there's no escape.  Weirder still, since not only space but time is affected by such a mass, to an outside observer watching an object falling into a black hole, it would seem to take an infinite amount of time.  The closer it got, the slower it would move, until it finally paused, forever, on the surface of the event horizon.  (Due to the vagaries of general relativity, this wouldn't help the hapless space traveler falling into a black hole.  Time would run at the regular speed for him/her, and in a very finite amount of time, the spaceship and everything and everyone in it would get ripped to shreds by the tidal forces exerted by the black hole's mass.)

[Image licensed under the Creative Commons User:Alain r, BH LMC, CC BY-SA 2.5]

It's this last bit that's germane to this week's announcement.  Black holes are black (duh), even light can't escape, so how do we see them?  It's because of material that's falling toward them.  Black holes tend to form accretion disks of stuff circling the central singularity at a high rate of speed, and the acceleration of this disk causes it to emit x-rays.  (In fact, the first black hole ever observed, Cygnus X-1, was given that name because it was the first x-ray source found in the constellation Cygnus.)

So we see the black hole by its brilliant x-ray "corona."  Generally, the larger the black hole, the bigger the accretion disk and the brighter it is; the center of the Milky Way, the object called Sagittarius A*, has a radius of 22 million kilometers, and parts of its accretion disk are being whirled about at thirty percent of the speed of light.

1ES 1927+654 is another such supermassive galactic nucleus, but its behavior is even weirder than our own.  In March 2018 it flashed -- its luminosity suddenly jumped by forty percent.  Keep in mind that these things are already phenomenally luminous, so such a jump is stupendous by anyone's estimate.  "This was an AGN [active galactic nucleus] that we sort of knew about, but it wasn't very special," said MIT astronomer Erin Kara.  "Then they noticed that this run-of-the-mill AGN became suddenly bright, which got our attention, and we started pointing lots of other telescopes in lots of other wavelengths to look at it."

That's why Kara and her team were watching when 1ES 1927+654 suddenly -- and completely -- disappeared.

Put more accurately, it became undetectable.  Something had apparently vaporized its corona completely, causing the x-ray emissions to stop.  The most amazing thing is how fast it happened -- an astronomical blink of an eye.  "We expect that luminosity changes this big should vary on timescales of many thousands to millions of years," said Kara.  "But in this object, we saw it change by 10,000 over a year, and it even changed by a factor of 100 in eight hours, which is just totally unheard of and really mind-boggling."

Here's what they think happened.

Something disrupted the accretion disk, possibly a large star that got caught in the black hole's gravitational well and was shredded by tidal forces as it approached.  That's a lot of material to throw into the accretion disk at once, and the star's own gravity destabilized the disk.  As an analogy, imagine a small whirlpool, like water going down a drain.  If you pour something like a dye into it slowly, it gets pulled in and incorporated smoothly.  But drop a gallon of dye into the whirlpool suddenly, and it disrupts the whirlpool completely, turning it into turbulent chaos.

That's what is thought to have happened here.  The shredding of the star is what created the flash we detected two years ago, then as the remnants plunged into the accretion disk, it blew the disk apart, and a lot of the material simply dropped into the black hole.  The acceleration of the material around the black hole is what causes the corona -- so when that's gone, the x-rays stop, and the black hole becomes undetectable.

The astronomers believe that the black hole's luminosity will "turn back on" as material around it begins to whirl around again, but the honest truth is that no one knows what it'll do next.  "We want to keep an eye on it," Kara said.  "It's still in this unusual high-flux state, and maybe it'll do something crazy again, so we don't want to miss that."

So that's our "you thought outer space was weird before" story for today.  Once again illustrating that we really are only on the beginning of our journey toward understanding the universe we live in.  If you keep your eyes on the stars, you will never lack for something to fascinate and startle you.

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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!]