Stars Are REALLY Warm-hearted

“I don’t understand, profesora. The Sun’s fuel is hydrogen. The books say when the Sun runs out of fuel it will eject much hydrogen and collapse to a white dwarf. So it didn’t run out of fuel, yes?”

“That’s an excellent question, Maria. Your simple sketch of layered zones is adequate for a stable star like our Sun is now. When things go unstable we need to pay more attention to dynamic details like mass, pressure and diffusion. The numbers matter.”

“I had that the fusion zone is 30% up from the center, and the top of the radiation zone is at 70%.”

“Yes, but percentages of a straight line don’t really give us a feel for the volumes and masses. Volumes grow as the radius cubed. The Sun’s core, the part inside your 30% radius, holds (30%)3 which is less than 3% of the Sun’s volume. The convection shell on the outside is also 30% thick, but that zone accounts for ⅔ of the star’s volume.”

“But not ⅔ of the mass, I think. The core is the most dense, yes?”

“Truly. The core is <chuckle> at the core of the matter. It’s obviously under compression from all the mass above it, but there’s a subtler and more important reason. The Sun’s internal temperatures are so high that everything acts like an ideal gas, even near the center. Once you’re beneath the convection zone, the only transport mechanism is diffusion influenced by gravity. Helium nuclei weigh four times what hydrogen nuclei do. Helium and heavier things tend to sink toward the center, hydrogen tends to float upward. What effect does that have on the core’s composition?”

“The core is heavy with much helium, not as much hydrogen.”

“Good. Now, what’s next above the core?”

“The fusion zo– Oh! The place where there’s enough hydrogen to do the fusing.”

“If the temperature and pressure are right. That turns out to be a delicate balance. Too much heat makes that region expand, average distance between atoms increases and that slows down the fusion reaction. Too much pressure slows diffusion which then slows the reaction by hindering hydrogen’s entry and helium’s exit. Too little heat or too little pressure do the opposite. Now you know why the fusion zone is so narrow in our diagrams, only about 10% of a radius.”

“No fusion in the other layers?”

“Less than 1% of the total. So we’ve got nearly all the heat in the star coming from hydrogen‑to‑helium fusion in this diffusion‑controlled gaseous reaction zone buried deep in the star.”

“Ah! Now I see. It is wrong to say the star dies because it runs out of fuel. There is still much hydrogen in the upper zones, but the diffusion doesn’t let enough enter the fusion zone. That is why the fire goes out. What happens then?”

“It mostly depends on the star’s mass. Really big ones have a sequence of deeper, hotter fusion layers in their core, forming heavier and heavier atoms all the way down to iron. Each layer is diffusion‑limited, of course, and the whole thing is like a stack of Jenga blocks supported by heat coming from below. If reaction in any layer overruns its fuel delivery then it stops producing heat. The whole stack collapses violently to form a neutron star or a black hole. Nearby infalling atoms collide and radiate in an exponential heat‑up. But the stars are many millions of kilometers across. The outermost layers don’t have time to fall all the way in. Their imploding gases slam into gases exploding from the collapse zone — BLOOEY! — there’s a nova spewing hydrogen and stardust across the Universe.”

“That is how our Sun will die?”

“No, it’s too small for such violence so it’s fated for a gentler old age. Five billion years from now its core will be mostly carbon and oxygen. Fuel delivery won’t be able to sustain further fusion reactions. The radiation and convection layers will simply settle inward, releasing enough gravitational potential energy to start hydrogen fusion in an expanding cool red shell outside the core.”

“Hee-hee — no lo va la nova, profesora, the nova doesn’t go.

  • Thanks to Victoria, who asked the question.

~~ Rich Olcott

Layer Upon Layer

“Excuse me, profesora, you wanted me to come to your office?”

“Yes, Maria. Come in, please. I wanted to have a chat with you before you give your class presentation tomorrow.”

“I am a little nervous about it.”

“I thought you might be. I wanted to help with that. I’ll start by saying that your English language skills have gotten much better than you give yourself credit for. Better yet, you’ll be speaking before friends who want you to succeed. I’m sure you’ll do fine. I think if we go over your material together you’ll be more confident. Come open your laptop on my desk where we can both see it. Now bring up your first slide.”

“Yes, profesora. Already you know that the title of my presentation is ‘The Structure of The Sun.’ I only have one slide, this one, that shows a slice of a star like our Sun.”

“How did the star get that way?”

“It condensed from a galactic gas cloud that was mostly hydrogen. I plan to talk about that with waving of the hands because a good picture of it needs to be in motion and I don’t know how to do that yet.”

“Fair enough, just don’t skip over it. Beginnings are important. Now talk me through your diagram.”

“It starts in the middle ¿see the fusion zone? where protons, that’s hydrogen atoms without their electrons, are squeezed together to release energy and make alpha particles, that’s helium atoms without their electrons. The protons have the same charge so they push each other away, but they are beneath many kilometers of mass that push them together. Also, the temperature is very hot, tens of millions of degrees. Hot atoms move fast, so when the protons are pushed together it happens with enough force and speed .. sorry, I need a word, superar?”

“Overcome.”

“Thank you. The protons are pushed together with enough force and speed to overcome the charge barrier. The actual reactions are complicated. At the end there is an alpha particle, four times heavier than a proton, and there is much more energy than the overcoming used up. The fusion zone makes heat and the heavy alpha particles fall down into the ash zone. The heat must go somewhere. Already the center is hotter so the new heat goes upward into the radiation zone.”

“And it’s called that because…?”

“Because atom motion is so, mm, frantic?”

“Good word.”

“… So frantic that there’s no moving in the same direction together, no convection like when steam rises over boiling water. Heat can only travel by convection, conduction or radiation. If there is no convection, moving heat must go neighbor‑to‑neighbor by conduction which is collision or by radiation which is photons jumping between atoms again and again until they escape. I have read that one photon’s energy can take 10000 years to cross the radiation zone.”

“So how is the next zone different?”

“It is much higher up from the center, nearly ¾ of the way to the surface. The pressure is 100 times less than in the fusion zone. The atoms have more room to move around together and form winds to carry the heat up by convection. But they can’t only go up, they have to come down, too, and that’s why my drawing has loops.”

“Is there a name for the loops?”

“Oh, yes, they are called Bénard cells and they’re very much like what I see looking into a pot of water just before it boils.”

“What’s the orange above the convection zone?”

“That’s the part of the Sun that we see, the photosphere that emits light in a continuous spectrum. The Fraunhofer lines, the dark lines in the astronomer’s spectrum, are the shadows of atoms high in in the photosphere that absorb only certain colors. I was surprised to learn how narrow the photosphere is, not even 0.02% of the Sun’s radius. Anyway, that’s my presentation, but now I have a question. The Sun’s fuel is hydrogen. The books say when the Sun runs out of fuel it will eject much of its hydrogen mass and collapse to a white dwarf. So it didn’t run out of fuel, yes?”

~~ Rich Olcott

Generation(s) of Stars

“How’re we gonna tell, Mr Moire?”

“Tell what, Jeremy?”

“Those two expanding Universe scenarios. How do we find out whether it’s gonna be the Big Rip or the Big Chill?”

“The Solar System will be recycled long before we’d have firm evidence either way. The weak dark energy we have now is most effective at separating things that are already at a distance. In the Big Rip’s script a brawnier dark energy would show itself first by loosening the gravitational bonds at the largest scale. Galaxies would begin scattering into the voids between the multi‑galactic sheets and filaments we’ve been mapping. Only later would the galaxies themselves release their stars to wander off and dissolve when dark energy gets strong enough to overcome electromagnetism.”

“How soon will we see those things happen?”

“If they happen. Plan on 188 billion years or so, depending on how fast dark energy strengthens. The Rip itself would take about 2 billion years, start to finish. Remember, our Sun will go nova in only five billion years so even the Rip scenario is far, far future. I prefer the slower Chill story where the Cosmological Constant stays constant or at least the w parameter stays on the positive side of minus‑one. Weak dark energy doesn’t mess with large gravitationally‑bound structures. It simply pushes them apart. One by one galaxies and galaxy clusters will disappear beyond the Hubble horizon until our galaxy is the only one in sight. I take comfort in the fact that our observations so far put w so close to minus‑one that we can’t tell if it’s above or below.”

“Why’s that?”

“The closer (w+1) approaches zero, the longer the timeline before we’re alone. We’ll have more time for our stars to complete their life cycles and give rise to new generations of stars.”

“New generations of stars? Wow. Oh, that’s what you meant when you said our Solar System would be recycled.”

“Mm-hm. Think about it. Back when atoms first coalesced after the Big Bang, they were all either hydrogen or helium with just a smidgeon of lithium for flavor. Where did all the other elements come from? Friedmann’s student George Gamow figured that out, along with lots of other stuff. Fascinating guy, interested in just about everything and good at much of it. Born in Odessa USSR, he and his wife tried twice to defect to the West by kayak. They finally made it in 1933 by leveraging his invitation to Brussels and the Solvay Conference on Physics where Einstein and Bohr had their second big debate. By that time Gamow had produced his ‘liquid drop‘ theory of how heavy atomic nuclei decay by spitting out alpha particles and electrons. He built on that theory to explain how stars serve as breeder reactors.”

“I thought breeder reactors are for turning uranium into plutonium for bombs. Did he have anything to do with that?”

“By the start of the war he was a US citizen as well as a top-flight nuclear theorist but they kept him away from the Manhattan Project. That undoubtedly was because of his Soviet background. During the war years he taught university physics, consulted for the Navy, and thought about how stars work. His atom decay work showed that alpha particles could escape from a nucleus by a process a little like water molecules in a droplet bypassing the droplet’s surface tension. For atoms deep inside the Sun, he suggested that his droplet process could work in reverse. He calculated the temperatures and pressures it would take for gravity to force alpha particles or electrons into different kinds of nuclei. The amazing thing was, his calculations worked.”

“Wait — alpha particles? Where’d they come from if the early stars were just hydrogen and helium?”

“An alpha particle is just a helium atom with the electrons stripped off. Anyway, with Gamow leading the way astrophysicists figured out how much of which elements a given star would create by the time it went nova. Those elements became part of the gas‑dust mix that coalesces to become the next generation of stars. We may have gone through 100 such cycles so far.”

“A hundred generations of stars. Wow.”

~~ Rich Olcott

Galaxies Fluffy And Faint

Cathleen’s at the coffee shop’s baked goods counter. “A lemon scone, please, Al.”

I’m next in line. “Lemon sounds good to me, too. It’s a warm day.”

The Pinwheel Galaxy, NGC 5457
Credit: ESA/Hubble

“Sure thing, Sy. Hey, got a question for you, Cathleen, you bein’ an Astronomer and all. I just saw an Astronomy news item about a fluffy galaxy and they mentioned a faint galaxy. Are they the same and why the excitement?”

“Not the same, Al. It’ll be easier to show you in pictures. Sy, may I borrow Old Reliable?”

“Sure, here.”

“Thanks. OK, Al, here’s a classic ‘grand design‘ spiral galaxy, NGC 5457, also known as The Pinwheel. Gorgeous, isn’t it?”

“Sure is. Hey, I’ve wondered — what does ‘NGC‘ stand for, National Galaxy Collection or something?”

“Nope. The ‘G‘ doesn’t even stand for ‘Galaxy‘. It’s ‘New General Catalog‘. Anyway, here’s NGC 2775, one of our prettiest fluffies. Doesn’t look much like the Pinwheel or Andromeda, does it?”

NGC 2775
Credit: NASA / ESA / Hubble / J. Lee / PHANGS-HST Team / Judy Schmidt

“Nah, those guys got nice spiral arms that sort of grow out of the center. This one looks like there’s an inside edge to all the complicated stuff. And it’s got what, a hundred baby arms.”

“The blue dots in those ‘baby arms’ are young blue stars. They’re separated by dark lanes of dust just like the dark lanes in classic spirals. The difference is that these lanes are much closer together. The grand design spirals are popular photography subjects in your astronomy magazines, Al, but they’re only about 10% of all spirals. I’ll bet your news item was about 2775 because we’re just coming to see how mysterious this one is.”

“What’s mysterious about it?”

“That central region. It’s huge and smooth, barely any visible dust lanes and no blue dots. It’s bright in the infra‑red, which is what you’d expect from a population of old red stars. In the ultra‑violet, though, it’s practically empty — just a small dot at the center. UV is high‑energy light. It generally comes from a young star or a recent nova or a black hole’s accretion disk. The dot is probably a super-massive back hole. but its image is just a tiny fraction of the smooth region’s width. With a billion red stars in the way it’s hard to see how the black hole’s gravity field could have cleaned up all the dust that should be in there. Li’l Fluffy here is just begging for some Astrophysics PhD candidates to burn computer time trying to explain it.”

NGC 1052-DF2
Credit: NASA, ESA, and P. van Dokkum (Yale University)

“What about Li’l Faint?”

“That’s probably this one, NGC 1052-DF2. Looks a bit different, doesn’t it?

“I’ll say. It’s practically transparent. Is it a thing at all or just a smudge on the lens?”

“Not a smudge. We’ve got multiple images in different wavelength ranges from multiple observatories, and there’s another similar object, NGC 1052-DF4, in the same galaxy group. We even have measurements from individual stars and clusters in there. The discovery paper claimed that DF2 is so spread out because it lacks the dark matter whose gravity compacts most galaxies. That led to controversy, of course.”

“Is there anything in Science that doesn’t? What’s this argument?”

“It hinges on distance, Sy. The object is about as wide as the Milky Way but we see only 1% as many stars. Does their mass exert enough gravitational force to hold the structure together? There’s a fairly good relationship between a galaxy’s mass and its intrinsic brightness — more stars means more emitting surface and more mass. We know how quickly apparent brightness drops with distance. From other data the authors estimated DF2 is 65 lightyears away and from its apparent brightness they back‑calculated its mass to be just about what you’d expect from its stars alone. No dark matter required to prevent fly‑aways. Another group using a different technique estimated 42 lightyears. That suggested a correspondingly smaller luminous mass and therefore a significant amount of dark matter in the picture. Sort of. They’re still arguing.”

“But why does it exist at all?”

“That’s another question.”

~~ Rich Olcott

  • Thanks to Oriole for suggesting this topic.

A Star’s Tale

It’s getting nippy outside so Al’s moved his out‑front coffee cart into his shop. Jeremy’s manning the curbside take‑out window but I’m walking so I step inside. Limited seating, of course. “Morning, Al. Here’s my hiking mug, fill ‘er up with high‑test and I’ll take a couple of those scones — one orange, one blueberry. Good news that the Governor let you open up.”

“You know it, Sy. Me and my suppliers have been on the phone every day. Good thing we’ve got long‑term relationships and they’ve been willing to carry me but it gets on my conscience ’cause they’re in a crack, too, ya know?”

“Low velocity of money hurts everybody, Al. Those DC doofuses and their political kabuki … but don’t get me started. Hey, you’ve got a new poster over the cash register.”

“You noticed. Yeah, it’s a beaut. Some artist’s idea of what it’d look like when a star gets spaghettified and eaten by a black hole. See, it’s got jets and a dust dusk and everything.”

“Very nice, except for a few small problems. That’s not spaghettification, the scale is all wrong and that tail-looking thing … no.”

Artist’s impression of AT2019qiz. Credit: ESO/M. Kornmesser
Under Creative Commons Attribution 4.0 International License

“Not spaghettification? That’s what was in the headline.”

“Sloppy word choice. True spaghettification acts on solid objects. Gravity’s force increases rapidly as you approach the gravitational center. Suppose you’re in a kilometer-long star cruiser that’s pointing toward a black hole from three kilometers away. The cruiser’s tail is four kilometers out. Newton’s Law of Gravity says the black hole pulls almost twice as hard on the nose as on the tail. If the overall field is strong enough it’d stretch the cruiser like taffy. Larry Niven wrote about the effect in his short story, Neutron Star.”

“The black hole’s stretching the star, right?”

“Nup, because a star isn’t solid. It’s fluid, basically a gas held together by its own gravity. You can’t pull on a piece of gas to stretch the whole mass. Your news story should have said ‘tidal disruption event‘ but I guess that wouldn’t have fit the headline space. Anyhow, an atom in the star’s atmosphere is subject to three forces — thermal expansion away from any gravitational center, gravitational attraction toward its home star and gravitational attraction toward the black hole. The star breaks up atom by atom when the two bodies get close enough that the black hole’s attraction matches the star’s surface gravity. That’s where the scale problem comes in.”

Al looks around — no waiting customers so he strings me along. “How?”

“The supermassive black hole in the picture, AT2019qiz, masses about a million Suns‑worth. The Sun‑size star can barely hold onto a gas atom at one star‑radius from the star’s center. The black hole can grab that atom from a thousand star‑radii away, about where Saturn is in our Solar System. The artist apparently imagined himself to be past the star and about where Earth is to the Sun, 100 star‑radii further out. Perspective will make the black hole pretty small.”

“But that’s a HUGE black hole!”

“True, mass‑wise, not so much diameter‑wise. Our Sun’s about 864,000 miles wide. If it were to just collapse to a black hole, which it couldn’t, its Event Horizon would be about 4 miles wide. The Event Horizon of a black hole a million times as massive as the Sun would be less than 5 times as wide as the Sun. Throw in the perspective factor and that black circle should be less than half as wide as the star’s circle.”

“What about the comet‑tail?”

“The picture makes you think of a comet escaping outward but really the star’s material is headed inward and it wouldn’t be that pretty. The disruption process is chaotic and exponential. The star’s gravity weakens as it loses mass but the loss is lop‑sided. Down at the star’s core where the nuclear reactions happen the steady burn becomes an irregular pulse. The tail should flare out near the star. The rest should be jagged and lumpy.”

“And when enough gets ripped away…”

“BLOOEY!”

~~ Rich Olcott

  • Thanks to T K Anderson for suggesting this topic.
  • Link to Technical PS — Where Do Those Numbers Come From?.

The Fourth Brother’s Quest

Newt Barnes is an informed and enthusiastic speaker in Cathleen’s “IR, Spitzer and the Universe” memorial symposium. Unfortunately Al interrupts him by bustling in to refresh the coffee urn.

After the noise subsides, Newt picks up his story. “As I was saying, it’s time for the Spitzer‘s inspirational life story. Mind you, Spitzer was designed to inspect very faint infra-red sources, which means that it looks at heat, which means that its telescope and all of its instruments have to be kept cold. Very cold. At lift-off time, Spitzer was loaded with 360 liters of liquid helium coolant, enough to keep it below five Kelvins for 2½ years.”

“Kelvins?”

“Absolute temperature. That’d be -268°C or -450°F. Very cold. The good news was that clever NASA engineers managed to stretch that coolant supply an extra 2½ years so Spitzer gave us more than five years of full-spectrum IR data.”

<mild applause>

“Running out of coolant would have been the end for Spitzer, except it really marked a mid-life transition. Even without the liquid helium, Spitzer is far enough from Earth’s heat that the engineers could use the craft’s solar arrays as a built-in sunshield. That kept everything down to about 30 Kelvins. Too warm for Spitzer‘s long-wavelength instruments but not too warm for its two cameras that handle near infra-red. They chugged along just fine for another eleven years and a fraction. During its 17-year life Spitzer produced pictures like this shot of a star-forming region in the constellation Aquila…”

NASA/JPL-Caltech/Milky Way Project.

The maybe-an-Art-major goes nuts, you can’t even make out the words, but Newt barrels on. “Here’s where I let you in on a secret. The image covers an area about twice as wide as the Moon so you shouldn’t need a telescope to spot it in our Summertime sky. However, even on a good night you won’t see anything like this and there are several reasons why. First, the light’s very faint. Each of those color-dense regions represents a collection of hundreds or thousands of young stars. They give off tons of visible light but nearly all of that is blocked by their dusty environment. Our nervous system’s timescale just isn’t designed for capturing really faint images. Your eye acts on photons it collects during the past tenth of a second or so. An astronomical sensor can focus on a target for minutes or hours while it accumulates enough photons for an image of this quality.”

“But you told us that Spitzer can see through dust.”

“That it can, but not in visible colors. Spitzer‘s cameras ignored the visible range. Instead, they gathered the incoming infrared light and separated it into three wavelength bands. Let’s call them long, medium and short. In effect, Spitzer gave us three separate black-and-white photos, one for each band. Back here on Earth, the post-processing team colorcoded each of those photos — red for long, green for medium and blue for short. Then they laid the three on top of each other to produce the final image. It’s what’s called ‘a falsecolor image’ and it can be very informative if you know what to look for. Most published astronomical images are in fact enhanced or colorcoded like this in some way to highlight structure or indicate chemical composition or temperature.”

“What happened after the extra extra years?”

“Problems had just built up. Spitzer doesn’t orbit the Earth, it orbits the Sun a little bit slower than Earth does. It gets further away from us every minute. It used to be able to send us its data almost real-time, but now it’s so far away a 2hour squirt-cast drains its batteries. Recharging the batteries using Spitzer‘s solar arrays tilts the craft’s antenna away from Earth — not good. Spitzer‘s about 120° behind Earth now and there’ll come a time when it’ll be behind the Sun from us, completely out of communication. Meanwhile back on Earth, the people and resources devoted to Spitzer will be needed to run the James Webb Space Telescope. NASA decided that January 30 was time to pull the plug.”

Cathleen takes the mic. “Euge, serve bone et fidélis. Well done, thou good and faithful servant.”

~~ Rich Olcott

A Tale of Four Brothers

Jim hands the mic to Cathleen, who announces, “Bio-break time. Please be back here in 15 minutes for the next speaker. Al will have fresh coffee and scones for us.” <a quarter-hour later> “Welcome back, everyone, to the next session of our ‘IR, Spitzer and the Universe‘ memorial symposium. Our next speaker will turn our focus to the Spitzer Space Telescope itself. Newt?”

“Thanks, Cathleen. Let’s start with a portrait of Spitzer. I’m putting this up because Spitzer‘s general configuration would fit all four of NASA’s Great Observatories…

A NASA artist’s impression of Spitzer against an IR view of the Milky Way’s dust

“Each of them was designed to be carried into space by one of NASA’s space shuttles so they had to fit into a shuttle’s cargo bay — a cylinder sixty feet long and fifteen feet in diameter. Knock off a foot or so each way to allow for packing materials and loading leeway.”

<voice from the crowd> “How come they had to be in space? It’d be a lot cheaper on the ground.”

“If you’re cynical you might say that NASA had built these shuttles and they needed to have some work for them to do. But the real reasons go back to Lyman Spitzer (name sound familiar?). Right after World War II he wrote a paper listing the benefits of doing Astronomy outside of our atmosphere. We think Earth’s atmosphere is transparent, but that’s only mostly true and only at certain wavelengths. Water vapor and other gases block out great swathes of the infrared range. Hydrogen and other atoms absorb in the ultraviolet and beyond. Even in the visible range we’ve got dust and clouds. And of course there’s atmospheric turbulence that makes stars twinkle and astronomers curse.”

“So he wanted to put telescopes above all that.”

“Absolutely. He leveraged his multiple high-visibility posts at Princeton, constantly promoting government support of high-altitude Astronomy. He was one of the Big Names behind getting NASA approved in the first place. He lived to see the Hubble Space Telescope go into service, but unfortunately he died just a couple of years before its IR companion was put into orbit.”

“So they named it after him?”

“They did, indeed. The Spitzer was the fourth and final product of NASA’s ‘Great Observatories’ program designed to investigate the Universe from beyond Earth’s atmosphere. The Hubble Space Telescope was first. It was built to observe visible light but it also gave NASA experience doing unexpected inflight satellite repairs. <scattered chuckles in the audience. The maybe-an-Art-major nudges a neighbor for a whispered explanation.> The Atlantis shuttle put Hubble into orbit in 1990. Thirty years later it’s still producing great science for us.”

<The maybe-an-Art-major yells out> “And beautiful pictures!”

“Yes, indeed. OK, a year later Atlantis put Compton Gamma Ray Observatory into orbit. Its sensors covered a huge range of the spectrum, about twenty octaves as Jim would put it, from hard X-rays on upward. In its nine years of life it found nearly 300 sources for those high-energy photons that we still don’t understand. It also detected some 2700 gamma ray bursts and that’s something else we don’t understand other than that they’re way outside our intergalactic neighborhood.”

“Only nine years?”

“Sad, right? Yeah, one of its gyroscopes gave out and NASA had to bring it down. Some people fussed, ‘It’ll come down on our heads and we’re all gonna die!‘ but the descent stayed under control. Most of the satellite burned up on re-entry and the rest splashed harmlessly into the Indian Ocean.”

<quiet snuffle>

“Cheer up, it gets better. A month and a half after Compton‘s end, the Columbia shuttle put Chandra X-Ray Observatory into orbit. Like Hubble, Chandra‘s still going strong and uncovering secrets for us. Chandra was first to record X-rays coming from the huge black hole at the Milky Way’s core. Chandra data from the Bullet Cluster helped confirm the existence of dark matter. Thanks to Chandra we understand Jupiter’s X-ray emissions well enough to steer the Juno spacecraft away from them. The good stuff just keeps coming.”

“Thanks, that helps me feel better.”

“Good, because it’s time for the Spitzer‘s inspirational life story.”

~~ Rich Olcott

Fierce Roaring Beast

A darkish day calls for a fresh scone so I head for Al’s coffee shop. Cathleen’s there with some of her Astronomy students. Al’s at their table instead of his usual place behind the cash register. “So what’s going on with these FRBs?”

She plays it cool. “Which FRBs, Al? Fixed Rate Bonds? Failure Review Boards? Flexible Reed Baskets?”

Jim, next to her, joins in. “Feedback Reverb Buffers? Forged Razor Blades?
Fennel Root Beer?”

I give it a shot. “Freely Rolling Boulders? Flashing Rapiers and Broadswords? Fragile Reality Boundary?”

“C’mon, guys. Fast Radio Bursts. Somebody said they’re the hottest thing in Astronomy.”

Cathleen, ever the teacher, gives in. “Well, they’re right, Al. We’ve only known about them since 2007 and they’re among the most mystifying objects we’ve found out there. Apparently they’re scattered randomly in galaxies all over the sky. They release immense amounts of energy in incredibly short periods of time.”

“I’ll say.” Vinnie’s joins the conversation from the next table. “Sy and me, we been talking about using the speed of light to measure stuff. When I read that those radio blasts from somewhere last just a millisecond or so, I thought, ‘Whatever makes that blast happen, the signal to keep it going can’t travel above lightspeed. From one side to the other must be closer than light can travel in a millisecond. That’s only 186 miles. We got asteroids bigger than that!'”

“300 kilometers in metric.” Jim’s back in. “I’ve played with that idea, too. The 70 FRBs reported so far all lasted about a millisecond within a factor of 3 either way — maybe that’s telling us something. The fastest way to get lots of energy is a matter-antimatter annihilation that completely converts mass to energy by E=mc².  Antimatter’s awfully rare 13 billion years after the Big Bang, but suppose there’s still a half-kilogram pebble out there a couple galaxies away and it hits a hunk of normal matter. The annihilation destroys a full kilogram; the energy release is 1017 joules. If the event takes one millisecond that’s 1020 watts of power.”

“How’s that stand up against the power we receive in an FRB signal, Jim?”

“That’s the thing, Sy, we don’t have a good handle on distances. We know how much power our antennas picked up, but power reception drops as the square of the source distance and we don’t know how far away these things are. If your distance estimate is off by a factor of 10 your estimate of emitted power is wrong by a factor of 100.”

“Ballpark us.”

<sigh> “For a conservative estimate, say that next-nearest-neighbor galaxy is something like 1021 kilometers away. When the signal finally hits us those watts have been spread over a 1021-kilometer sphere. Its area is something like 1049 square meters so the signal’s power density would be around 10-29 watts per square meter. I know what you’re going to ask, Cathleen. Assuming the radio-telescope observations used a one-gigahertz bandwidth, the 0.3-to-30-Jansky signals they’ve recorded are about a million million times stronger than my pebble can account for. Further-away collisions would give even smaller signals.”

Looking around at her students, “Good self-checking, Jim, but for the sake of argument, guys, what other evidence do we have to rule out Jim’s hypothesis? Greg?”

“Mmm… spectra? A collision like Jim described ought to shine all across the spectrum, from radio on up through gamma rays. But we don’t seem to get any of that.”

“Terry, if the object’s very far away wouldn’t its shorter wavelengths be red-shifted by the Hubble Flow?”

“Sure, but the furthest-away one we’ve tagged so far is nearer than z=0.2. Wavelengths have been stretched by 20% or less. Blue light would shift down to green or yellow at most.”

“Fran?”

“We ought to get even bigger flashes from antimatter rocks and asteroids. But all the signals have about the same strength within a factor of 100.”

“I got an evidence.”

“Yes, Vinnie?”

“That collision wouldn’t’a had a chance to get started. First contact, blooie! the gases and radiation and stuff push the rest of the pieces apart and kill the yield. That’s one of the problems the A-bomb guys had to solve.”

Al’s been eaves-dropping, of course. “Hey, guys. Fresh Raisin Bread, on the house.”

~~ Rich Olcott

Friendly Resting Behemoths

Cube Roots

Cathleen steps into at Al’s for her morning coffee-and-scone.  “Heard you guys talking neutrinos so I’ll bet Al got you started with something about IceCube.  Isn’t it an awesome project?  Imagine instrumenting a cubic kilometer of ice, and at the South Pole!”

“Ya got me, Cathleen.  It knocked me out that anyone would even think of building it.  Where did the idea come from, anyhow?”

“I don’t know specifically, but it’s got a lot of ancestors, going back to the Wilson Cloud Chamber in the 1920s.”

“Oh, the cloud chamber!  Me and my brother did one for the Science Fair — used dry ice and some kind of alcohol in a plastic-covered lab dish if I remember right, and we set it next to one of my Mom’s orange dinner plates.  Spooky little ghost trails all over the place.”

“That’s basically what the first ones were.  An incoming particle knocks electrons out of vapor molecules all along its path.  The path is visible because the whole thing is so cold that other vapor molecules condense to form micro-droplets around the ions.  Anderson’s cloud chambers were good enough to get him a Nobel Prize for discovering the positron and muon.  But table-top devices only let you study low-energy particles — high-energy ones just shoot through the chamber and exit before they do anything interesting.”

“So the experimenters went big?”

“Indeed, Sy, massive new technologies, like bubble chambers holding thousands of gallons of liquid hydrogen or something else that reacts with neutrinos.  But even those experiments had a problem.”

“And that was…?”

FirstNeutrinoEventAnnotated 2
Adapted from public domain image
courtesy of Argonne National Laboratory

“They all depended on photography to record the traces.  Neutrino-hunting grad students had to measure everything in the photos, because neutrinos don’t make traces — you only find them by finding bigger particles that were disturbed just so.  The work got really intense when the astrophysicists got into the act, trying to understand why the Sun seemed to be giving off only a third of the neutrinos it’s supposed to.  Was the Sun going out?”

“Wait, Cathleen, how’d they know how many neutrinos it’s supposed to make?”

“Wow, Vinnie, you sure know how to break up a narrative, but it’s a fair question.  OK, quick answer.  We know the Sun’s mostly made of hydrogen and we know how much energy it gives off per second.  We’ve figured out the nuclear reactions it must be using to generate that energy.  The primary process combines four hydrogen nuclei  to make a helium nucleus.  Each time that happens you get a certain amount of energy, which we know, plus two neutrinos.  Do the energy arithmetic, multiply the number of heliums per second by two and you’ve got the expected neutrino output.”

“So is the Sun going out?”

“As usual, Al cuts to the chase.  No, Al, it’s still got 5 billion years of middle age ahead of it.  The flaw in the argument was that we assumed that our detectors were picking up all the neutrinos.”

“My mutations!”

“Yes, Vinnie.  Our detector technology at the time only saw electron neutrinos.  The Sun’s reactions emit electron neutrinos.  But the 93-million mile trip to Earth gave those guys plenty of time to oscillate through muon neutrino to tau neutrino and back again.  All we picked up were the ones that had gone through an integer number of cycles.”

“We changed technology, I take it?”

“Right again, Sy.  Instead of relying on nuclear reactions initiated by electron neutrinos, we went so spark chambers — crossed grids of very fine electrified wire in a box of argon gas.  Wherever a passing neutrino initiated an ionization, zap! between the two wires closest to that point.  Researchers could computerize the data reduction.  Turns out that all three neutrino flavors are pretty good at causing ionizations so the new tech cleared up the Solar Paradox, but only after we solved a different problem — the new data was point-by-point.  Working back from those points to the traces took some clever computer programming.”

“Ah, I see the connection with IceCube.  It doesn’t register traces, either, just the points where those sensors see the Cherenkov flashes.  It’s like a spark chamber grown big.”

“Cubic-kilometer big.”

~~ Rich Olcott

Naming the place and placing the name

“By the way, Cathleen, is there any rhyme or reason to that three-object object‘s funky name?  I’ve still got it on Old Reliable here.”

PSR J0337+1715

“It’s nothing like funky, Sy, it’s perfectly reasonable and in fact it’s far more informative than a name like ‘Barnard’s Star.’  The ‘PSR‘ part says that the active object, the reason anyone even looked in that system’s direction, is a pulsar.”

“And the numbers?”

“Its location in two parts.  Imagine a 24-hour clockface in the Solar Plane.  The zero hour points to where the Sun is at the Spring equinox.  One o’clock is fifteen degrees east of that, two o’clock is another fifteen degrees eastward and so on until 24 o’clock is back pointing at the Springtime Sun.  Got that?”

“Mm, … yeah.  It’d be like longitudes around the Earth, except the Earth goes around in a day and this clock looks like it measures a year.”

“Careful there, it has nothing to do with time.  It’s just a measure of angle around the celestial equator.  It’s called right ascension.

“How about intermediate angles, like between two and three o’clock?”

“Sixty arc-minutes between hours, sixty arc-seconds between arc-minutes, just like with time.  If you need to you can even go to tenths or hundredths of an arc-second, which divide the circle into … 8,640,000 segments.”

“OK, so if that’s like longitudes, I suppose there’s something like latitudes to go with it?”

“Mm-hm, it’s called declination.  It runs perpendicular to ascension, from plus-90° up top down to 0° at the clockface to minus-90° at the bottom.  Vivian, show Sy Figure 3 from your paper.”Right ascension and declination“Wait, right ascension in hours-minute-seconds but declination in degrees?”

“Mm-hm.  Blame history.  People have been studying the stars and writing down their locations for a long time.  Some conventions were convenient back in the day and we’re not going to give them up.  So anyway, an object’s J designation with 4-digit numbers tells you which of 13 million directions to look to find it.  Roughly.”

“Roughly?”

“That’s what the ‘J‘ is about.  If the Earth’s rotation were absolutely steady and if the Sun weren’t careening about a moving galaxy, future astronomers could just look at an object’s angular designation and know exactly where to look to find it again.  But it’s not and it does and they won’t.  The Earth’s axis of rotation wobbles in at least three different ways, the Sun’s orbit around the galaxy is anything but regular and so on.  Specialists in astrometry, who measure things to fractions of an arc-second, keep track of time in more ways than you can imagine so we can calculate future positions.  The J-names at least refer back to a specific point in time.  Mostly.  You want your mind bent, look up epoch some day.”

“Plane and ship navigators care, too, right?”

“Not so much.  Earth’s major wobble, for instance, shifts our polar positions only about 40 parts per million per year.  A course you plotted last week from here to Easter Island will get you there next month no problem.”

Old Reliable judders in my hand.  Old Reliable isn’t supposed to have a vibration function, either.  Ask her about interstellar navigation.  “Um, how about interstellar navigation?”Skewed Big Dipper

“Oh, that’d be a challenge.  Once you get away from the Solar System you can’t use the Big Dipper to find the North Star, any of that stuff, because the constellations look different from a different angle.  Get a couple dozen lightyears out, you’ve got a whole different sky.”

“So what do you use instead?”

“I suppose you could use pulsars.  Each one pings at a unique repetition interval and duty cycle so you could recognize it from any angle.  The set of known pulsars would be like landmarks in the galaxy.  If you sent out survey ships, like the old-time navigators who mapped the New World, they could add new pulsars to the database.  When you go someplace, you just triangulate against the pulsars you see and you know where you are.”

If they happen to point towards you! You only ever see 20% of them.  Starquakes and glitches and relativistic distortions mess up the timings.  Poor Xian-sheng goes nuts each time we drop out of warp.

~~ Rich Olcott