“You’re always building models. Stone circles. Cathedrals. Pipe-organs. Adding machines. I got no idea why I’m here now, you know that? But if the run goes off tonight, you’ll have finally managed the real thing.”
“I don’t know what you’re talking about.”
“That’s ‘you’ in the collective. Your species.”
— William Gibson, Neuromancer
Sometime in the next 100 days, a star will explode.
The star’s name is T Coronae Borealis, and normally you can’t see it without a telescope: it’s too far away. But when it explodes, you’ll be able to see it just fine. It won’t be the brightest star in the sky, or anything like that. But it will be a reasonably bright star — “second magnitude”, if you’re an astronomer or a nerd — in a place where there was no star before.
It won’t last, of course. The new star — “nova” is the term, which of course just means “new” in Latin — will shine for a few days, then gradually fade back into obscurity.
Maybe you’ve heard of a supernova? Okay, so this isn’t that. This is it’s less spectacular little cousin, the plain and simple nova. A nearby supernova would light up the sky, potentially glowing as bright as the full Moon. This will just be a middling bright star that will (to our eyes) appear from nowhere and then, over a few days or weeks, fade away.
What’s actually happening… Okay, so long ago there was a binary star system, two stars in a close orbit. Then one of the stars got old and died. But it left behind its dense, dead core — a white dwarf, it’s called. The white dwarf doesn’t generate new energy, but it still glows from leftover heat. And it’s very dense, so it has a fiercely strong gravitational field. Not as strong as a black hole, but strong enough that you’d want to avoid its immediate neighborhood.
Meanwhile the /other/ star is getting old too. And when stars get old, they swell up enormously, becoming “red giant” stars. And as the other star swells up, its outer layers expand until they reach the gravitational influence of the white dwarf. And then the outer layers of the star get sucked down into the dwarf. (Or, if you like, the dwarf begins to feed, vampirically, on its aging companion.)
With me so far? Okay, so all that star material, that blazing hot plasma that the dwarf is draining away? It doesn’t just disappear. It piles up on the white dwarf, forming a shell around it. And as years pass, it gets denser and hotter. It’s crushed by the dead star’s immense gravity, it can’t easily radiate its heat away, and more white-hot plasma is always pouring down, like a waterfall ten million miles high. It piles up and it piles up and eventually… it explodes: it hits the ignition temperature for nuclear fusion.
Nuclear fusion produces more heat, which causes more nuclear fusion. A runaway chain reaction spreads through the shell at nearly the speed of light. That shell of hot material — now many miles deep, and thousands of times denser than lead — detonates, pretty much all at once. It’s basically a hydrogen bomb the size of a planet. And — for a little while — it shines furiously bright.
Novas aren’t one-time things. They repeat, on cycles ranging from decades to millennia. That’s because dwarf’s dead core, impossibly dense and tough, survives the detonation. It survives and, after the explosion has passed and things have calmed down, it will begin once again to feed off the companion star. And so the cycle continues.
(If you want to be poetic about it: most white dwarfs are dead. This one is undead. It feeds off its companion for decades, and then it flares back to life. Real life, a proper star again, a raging nuclear furnace shouting defiance at the universe. But it can’t last. The fuel expires. The stolen life is used up; the flames die down. Imagine a vampire that’s allowed a few days of human life every century or so. That’s a nova.)
Astronomers are obviously pretty excited about this. But amateur astronomers are /particularly/ excited. Every night now, as sunset sweeps around the world, hundreds of telescopes turn towards that point in space. From back yards and hilltops and conveniently located parking lots, a small army of volunteer observers — there’s probably one in your neighborhood — watches intently. Got a telescope and a camera? Then you have a chance to win the lottery and be the first one to photograph the nova in the instant of detonation. And even if you don’t… you’re waiting for a nova! How cool is that?
— I said a nova was pretty small compared to a supernova, and that’s true. But “small compared to a supernova” is still a lot. If a nova took place in the Sun (it couldn’t, novas require a double star, but let’s just say), first you would burst into flames, and then Earth’s atmosphere would be stripped away, destroying all life. Like a lot of interesting astronomical events, it’s best viewed from a distance.
And T Coronae Borealis is comfortably distant: it’s around 2600 light years away. That means the light from it takes 2600 years to reach us. And that raises a couple of interesting points.
First, T Coronae Borealis has already exploded. It exploded almost 2600 years ago. The light of that explosion has been traveling through space for all that time. Now it has traveled 99.99% of the way to us. Almost here! It will arrive, at last, sometime in the next hundred days.
Second, the star exploded around 600 BC. Which by coincidence, happens to be roughly the time when Middle Eastern and Greek astrologers started applying advanced mathematics to the sky, attempting to predict eclipses and the movement of the planets. (The twelve constellations of the Zodiac seem to date from around this time, though their first recorded appearance is a bit later.)
Here’s the thing: for centuries — millennia — humans looked to the sky and tried to use the sky to impose order on the messy, chaotic, unpredictable mess of the human world. Will the rains come on time? Will the peasants rebel? Should we prepare for famine? Will there be war? Astrologers tried to grasp the unknowable future by turning to the cool, bright, ever-turning sky.
In the end, that didn’t work. The human world remains chaotic. The sky can’t predict us.
But we got a consolation prize: we can predict the sky.
{ 7 comments }
Alan White 06.16.24 at 5:20 am
You have an uncanny knack for eloquently capturing the wonder of the skies. Please keep posting!
John Q 06.16.24 at 6:07 am
Thanks for this lucid explanation, Doug.
My own attempts to observe exciting astronomical phenomena, starting with Halley’s comet, have mostly been unsuccessful, so I will leave it to some other lucky observer to sight the nova.
steven t johnson 06.16.24 at 6:37 pm
Love the post.
All young stars are hydrogen bombs, though. That’s the quibble, sorry I’m compulsive. Following may still be interesting? Or boring? Maybe too elementary, caution is advised.
[Gravitational compression of hydrogen gas heats it to the point where fusion begins in a chain reaction that produces incredible energy. As ages go by the hydrogen is slowly used up and the most common fusion product, helium, accumulates. If the mass of the star is great enough (it usually is if I remember correctly) gravitational compression again heats the helium to a high enough temperature to begin “burning” the helium.
This higher temperature pushes the outer layers of the star outward. The expansion also cools the surface. This is the red giant phase the companion star is undergoing, by the way. As time and fusion go on, further fusion products, metals, are produced so long as energy is released by the fusion chain reaction.
But when iron begins to predominate in the star, the chain reaction slows, then stops. Iron is the threshold where fusion of atoms is a net energy producer. Fusion of heavier atoms needs energy. The companion star’s infalling matter provides the energy for more fusion.
The blast of the nova spews all the heavier elements into space where they are eventually, after astronomically long time is captured by gravity in other stars. (By the way, all stars blast out a few heavier atom, but again during a nova the rate is once again astronomically higher.)
Stars that have accumulated metals (astronomers seem to call every other element than hydrogen and maybe helium metals, which is not the high school chemistry definition) thus were called Generation II stars. Heavier elements are also captured by forming planets, which in what are called rocky planets like Earth, can comprise the bulk of the matter. So-called gas giants were, maybe still are, believed to be largely hydrogen and helium.
Rocky planets are believed to mostly closer to the star because ultimately the heavy stuff sinks to the bottom of the gravity well of the star, while hydrogen and helium, being light is pushed further away by the radiation pressure. Also, hydrogen and helium are more apt to escape the inner planets because, being closer to the star, they are relatively hotter, providing the energy for the atoms to escape in a process analogous to evaporation. The point of the old radiometers you may have seen in high school is to show the existence of radiation pressure.
The cosmic abundances of the elements stems from the different probabilities of fusion. The burning in stars is not chemical combustion, but one similarity is that some atoms are more apt to fuse and produce. Working out the chain of steps, bears some similarity to working out the intermediate processes in chemical reactions. And modeling the relative abundances and probabilities of intermediates when there are alternate pathways is also an issue. The study of meteorites thus is very much about studying stars as well as planets.]
Peter Erwin 06.16.24 at 8:30 pm
But when iron begins to predominate in the star, the chain reaction slows, then stops. Iron is the threshold where fusion of atoms is a net energy producer. Fusion of heavier atoms needs energy. The companion star’s infalling matter provides the energy for more fusion.
White dwarfs are the remnants of stars that never got as far as making iron. Most of them are made of a mix of carbon and oxygen; some have some neon, others are mostly helium. (The fusion of helium tends to produce a mixture of carbon and oxygen; the fusion of carbon produces a mixture of magnesium and neon.) What actually stops the fusion is the core of the star becoming degenerate: the density is high enough — and the temperature low enough — that the core can resist the inward pull of gravity by the incredible pressure produced by degenerate electrons. So the core stops contracting and never reaches the densities and temperatures necessary for the later stages of nuclear fusion that would produce things like silicon and iron. In really massive stars (more than about ten times the Sun’s mass), the temperature in the core is so high that degeneracy never sets in, and so the density (and temperature) keep increasing until you get to later stages of fusion.
The blast of the nova spews all the heavier elements into space …
The blast of the nova spews a mixture of (unburned) hydrogen and helium (some accreted from the companion star, some formed in the nuclear burning). The “metals” making up the bulk of the white dwarf (again, mostly carbon and oxygen) mostly stay where they are.
Stars that have accumulated metals (astronomers seem to call every other element than hydrogen and maybe helium metals, which is not the high school chemistry definition) thus were called Generation II stars.
It’s mostly the other way around: so-called <a href=”https://en.wikipedia.org/wiki/Stellar_population>Population I stars are younger and (mostly) metal-rich, while Population II stars are older and (mostly) metal-poor. (Think of the numbers as denoting vague ages, rather than sequential “generations”.) In truth, this kind of language is a bit passé, in part because you can have young stars that are metal-poor (typically in star-forming dwarf galaxies) and old stars that are metal-rich (lots of stars in the central regions of massive galaxies are like that). Though the term “Population III” is still used to refer to the (so-far unobserved) first generation of stars.
(Yes, called anything heavier than helium a “metal” is kind of silly; we’ve been doing that since the 1950s, I’m afraid, and are unlikely to stop anytime soon.)
steven t johnson 06.17.24 at 7:57 pm
Thanks for the correction of my error in reversing Generations II and I. My bad.
On the point about the all the heavier elements being spewed into space, the error was my customary bad writing. I was thinking of all the “metals” found in planets ultimately come from stars. Typically, that was not at all clear. But it is entirely true that they “mostly stay where they are.” I don’t know of any cosmological models that find the original, Big Band nucleosynthesis producing anything other than a small percentage of lithium. Astronomically speaking, planets verge on being negligible in mass in comparison to stars though. A quick google of cosmic abundance and the first table coming up estimates hydrogen and helium being about 98% of the known universe. In that sense, the odd custom of astronomers using the word “metals” the way they have maybe isn’t so odd?
And yes, the different kinds of white dwarfs, much less other stars that weren’t even on the main sequence is not explored. But I suspect I’ve been boring enough without going into the glories of the Hertzsprung-Russell diagram.
oldster 06.18.24 at 11:49 am
I grew up reading popular science, in the middle of the previous century, and so learned that novas were exploding stars. It sounds like the march of science has revised that, so that exploding stars are now supernovae, and the term nova is restricted to these cyclical efflorescences given off by one member of a binary pair. The change in terminology is fine with me — sometimes new uses of words indicate that science has progressed. But it’s also a marker to me of how out-of-date I have become with respect to some basic science.
I was somewhat more aware of the discovery of exoplanets, which followed something like the template of the old joke about an actor’s career. When I was a child, people speculated that it might be possible for other stars to have planets. Then the first exoplanet was discovered, and the discovery was earth-shaking. Now there are so many documented cases that they are treated like lint accumulating in pockets. This too, in its way, is scientific progress.
Peter Erwin 06.21.24 at 9:51 am
@ oldster:
I grew up reading popular science, in the middle of the previous century, and so learned that novas were exploding stars. It sounds like the march of science has revised that, so that exploding stars are now supernovae, and the term nova is restricted to these cyclical efflorescences given off by one member of a binary pair.
The term “supernova” actually dates to the early 1930s, after the realization that some “novae” (from the Latin stellar nova = “new star”) were much, much brighter than others. The modern theoretical understanding dates mostly to the 1950s. (But it’s understandable that what was then fairly new science wouldn’t get communicated very accurately in popular science accounts.)
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