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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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.

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