Exploding Neutron Star

Mick Ohrberg writes "According to NASA News, scientists at NASA and CITA are watching a neutron star (4U 1820-30, 25,000 light years from Earth) explode. Or rather - watch an explosion happen just a few miles above the surface of this immensely dense body. What happens is that matter (mostly helium) from a companion star is by the gravity of the neutron star and collected on the surface until a layer is formed and sufficient pressure is generated. This will cause the helium to fuse into carbon and other elements, releasing enormous amounts of energy in the X-ray band. The event was caught using NASA's Rossi X-ray Timing Explorer. More details can be found here."

Comments

Additional Linkage

[brownpau]

CG animation and screencaps here:
http://www.gsfc.nasa.gov/topstory/2004/0220 stardis k.html

Re:Additional Linkage

[Anonymous Coward]

http://www.gsfc.nasa.gov/topstory/2004/0220stardis k.html

Really cool story.

[kniLnamiJ-neB]

Astronomical Phenomenae make the best /. stories. Last week there was the black hole chowing down on the star, and now we're blowing stuff up.

Not from Helium Fusion

[Rob Riggs]

Ballantyne and his colleague... observed a "superburst." These are much more rare than ordinary, helium-powered bursts and release 1000 times more energy. Scientists say superbursts are caused by a buildup of nuclear ash in the form of carbon from the helium fusion.

This was a burst from carbon fusion. The ash from the helium fusion process.

Can some astro-phys whiz tell me why there can be a buildup of atomic matter on the nuetron star? How can the baryons remain in atomic nuclei and not get incorporated as nuetrons into the nuetron star directly?

Re:Not from Helium Fusion

[RobertB-DC]

Can some astro-phys whiz tell me why there can be a buildup of atomic matter on the nuetron star? How can the baryons remain in atomic nuclei and not get incorporated as nuetrons into the nuetron star directly?

I'm not a astrophysicist (and I'm not even going to make up an acronym for the fact), but it seems like you could liken the situation to a planet, say Jupiter. At Jupiter's core, there's a dense ball of something. But out in the visible layers, we have wispy gas. When Galileo entered the planet, it didn't turn immediately into the sort of matter in the core. For that matter, we're on the surface of a planet with a molten iron interior, but I'm not melting (yet).

From what I've read about neutron stars, they're not undifferentiated neutrons all the way out, anyway. As I recall, they're suspected of having a crust of iron -- the end-of-the-line element in nucleosynthesis.

It's not a big step to go from there to an "atmosphere" of super-dense helium that builds up over time until the pressure is enough to spark nuclear fusion. Then, you have the leftover carbon layered on top of the iron, with more helium piling on top of it.

After a certain number of He fusion explosions, you'd have a lot of super-dense carbon... and the next He explosion sparks the C to fuse. Eventually, it seems like you end up with Fe ash, and you're back where you started.

It's too bad such cool events are so long ago and far away. On the other hand, if they were nearer and more recent, we'd fry before we could enjoy the view...

Yes, but...

[Pumpernickle]

This will cause the helium to fuse into carbon and other elements, releasing enormous amounts of energy in the X-ray band.

Yes, but how much of that is diamond?

Re:Yes, but...

[Patrik_AKA_RedX]

Probably none of it. It's probably in a plasma state rather than solid.

Re:Yes, but...

[pclminion]

Who says there's no such thing as solid plasma?

Some amount of energy...

[ControlFreal]

From the article:

It poured out more energy in three hours than the sun does in 100 years

Given that the sun produces about 3.8e+26 Watt, and that a year contains about 3.15e+7 seconds, the explosion comes down to a total energy release of about 1.1e+36 Joules.

Still, this is puny compared with a gamma-ray burst: in 60 seconds, that yields about 10e+45 Joules.

Re:Some amount of energy...

[ControlFreal]

Ah crap: that's supposed to be 1e+45 Joules...

Isn't this called a Nova?

[Vellmont]

Or have the terms changed? Not to be confused with the very different (and vastly more powerful) super nova.

Re:Isn't this called a Nova?

[Mick Ohrberg]

A neutron star is a star that already has gone nova (basically meaning "new", since supernovae appeared as "new" stars in the heavens in the good old days). The neutron star is basically the core remains of a star that through a supernova explosion has shedded all the outer gas layers (forming nebluae), and all that is left is the heavy core.

Our own sun is much to small to form a neutron star. When it shedds the outer gas layers it will form a white dwarf that eventually will turn into a cold iron ball. Larger stars go supernova and the core turns neutron star. Larger stars still may have a core so large and dense that its own gravity causes it to collapse - black holes.

I am not aware of any other uses for the term "nova" than in "supernova".

Re:Isn't this called a Nova?

[Noren]

This is sort of a higher-powered version of a nova, which is hydrogen fusing at a white dwarf star.

Re:Isn't this called a Nova?

[QuantumFTL]

I am not aware of any other uses for the term "nova" than in "supernova".

Along with the ordinary use of "nova" there is also something called a hypernova. Think of it as a supernova's big brother.

I was privileged enough to be at the colloquium where Hans Bethe unveiled his theory about hypernovae and gamma ray bursts. You can find an interesting paper on hypernova at Cornell's arxiv.org.

Cheers,
Justin Wick

P.S. The papers done the research group I'm in at Cornell talk about things similar to this accretion process. They can be found here.

Re:Isn't this called a Nova?

[mbrother]

There are many types of "nova" if you include everything with that name. There are supernova, dwarf nova, hypernova, etc. The garden variety nova is from hydrogen fusion on the surface of a white dwarf star, normally in a binary system with mass transfer. Dwarf nova happen in the accretion disks in binary systems. Supernova can happen in single, massive stars at the end of their lives (type II), or in white dwarf binary systems when enough mass transfer makes the white dwarf collapse, probably to form a neutron star (type I). I'll go ahead and plug my novel Star Dragon (see my webpage link) which takes place in the dwarf nova system SS Cygni and explains some of this in a work of fiction.

Re:Isn't this called a Nova?

[casavet]

Back some time after SN1987A, I was fortunate enough to be at a seminar where Hans Bethe presented some results of SuperNova explosion computer modelling. It boiled down to the computer model not being able to get the SN to explode, it always succumbed to gravitational collapse. IIRC, Prof. Bethe pointed out 2 possible issues with the model:
1. To simplify the physics of the situation, the model assumed a symettrical(sp?) body and explosion (current SN research blows that view out of the water), and
2. No accounting for the neutrino/matter interaction and energy transfer - under normal conditions the neutrino interaction can be ignored without concern, but this may not be true during SN explosion - I still haven't heard any feedback on this issue.

I must say that I'm impressed that he's still around, and doing interesting work!!!

onion model

[barakn]

The strength of gravity at the neutron star's surface isn't enough to squeeze material there into the exotic nuclear material people associate with neutron stars. The pressure increases with depth, though, and so things at depth can be squeezed to densities where they transform. This leads to a layered structure for a neutron star.

The outermost layer (ignoring ash layers), the outer crust, is about .3 km of of heavy nuclei (Fe-56) and free electrons near the surface and heavier nuclei deeper in, all at densities less than 4*10^11 g/cc. At greater densities, neutron drip begins. This forms the .6 km inner crust of heavy nuclei (Kr-118), a superfluid of free neutrons, and relativistic degenerate electrons. At still greater densities (>2*10^14 g/cc), all the nuclei have dissolved, and so the innermost 9.7 km truly is like one giant atomic nucleus with superfluid neutrons, superfluid superconducting protons, and relativistic degenerate electrons, though there may be more exotic particles like pions in the core at densities > 4*10^14 g/cc.

As noted, lighter elements can accrete on top of the outer crust until the point where their own weight causes pressures and densities sufficient enough for fusion. BANG!

Re:onion model

[hcg50a]

Right.

The density of the collapsed, degenerate-matter object (ie., the neutron star) is enormously greater than the density of the normal-matter crust, that the crust behaves almost like a very thin and very dense "atmosphere" above the neutron star.

As pointed out, when enough debris accumulates in this crust, all sorts of interesting things can happen:

a) Some of it fuses into higher elements (as reported in the article). This fusion releases tremendous amounts of energy.

b) Some of it undoubtedly collapses into degenerate matter, releasing tremendous amounts of energy.

In fact, probably a) or b) can kick off the other process as well.

It should also be noted that neutron stars are left-over cores of supernovae. A supernova occurs when the normal-matter core of a very large star suddenly collapses into a degenerate object.

This collapse is on an astronomical scale: Something about the size and mass of the sun collapses into an object 10 miles across, with the same mass.

(The sun itself is too small for this to happen).

The collapse results in a star blowing off most of its mass in an enormous implosion-explosion.

The neutron star is what's left over. If it's massive enough, an event horizon forms around the neutron star, turning it into a black hole.

Re:onion model

[RobertB-DC]

The neutron star is what's left over. If it's massive enough, an event horizon forms around the neutron star, turning it into a black hole.

That concept sparked a question... since the pressure increases with depth, what happens when the pressure at the very center crosses the line between degenerate matter and a singularity?

In other words, is there any way to conceive of a black hole at the center of a neutron star?

Or would the following events occur in rapid succession:

* Some pocket of swirling neutrons near the center gets dense enough to develop an event horizon.

* Any bit of matter close to the event horizon "falls" in, leaving a gap behind it (and a flash of radiation, right?).

* The unimaginable pressure pushes more and more matter into the growing singularity, and the event horizon grows.

* At some point, the whole neutron star collapses in a massive burst of gamma radiation, leaving behind a black hole, still sucking matter from its binary twin.

An alternative would be that the nascent black hole's radiation pressure would somehow keep the rest of the neutron star's core from falling into it. But at this point, I am so beyond my depth, I may as well be talking about the physics of the Kryptonite to be found under the iron crust.

Re:onion model

[Tackhead]

> The outermost layer (ignoring ash layers), the outer crust, is about .3 km of of heavy nuclei (Fe-56) and free electrons near the surface and heavier nuclei deeper in, all at densities less than 4*10^11 g/cc. At greater densities, neutron drip begins. This forms the .6 km inner crust of heavy nuclei (Kr-118), a superfluid of free neutrons, and relativistic degenerate electrons. At still greater densities (>2*10^14 g/cc), all the nuclei have dissolved, and so the innermost 9.7 km truly is like one giant atomic nucleus with superfluid neutrons, superfluid superconducting protons, and relativistic degenerate electrons, though there may be more exotic particles like pions in the core at densities > 4*10^14 g/cc.

(Sound physics, eminently plausible)

...but I still can't help but say "And we thought tunelling through the ice on Europa was tough!"

I use science because it works. I like science because it describes a universe full of wonders that put theologians to shame.

Impossible Things I'd Like To Do:

1) Visit a neutron star.
2) Stand on the surface of a planet that was flung from the orbit of a red giant during a close pass by a supermassive black hole at the center of a galaxy. Look up. Watch the star being devoured. Enjoy the gravitational lensing effects.
3) Go back in time and see what quasars looked like while they were forming.

Re:onion model

[Anonymous Coward]

The key idea here is degeneracy. Pauli and Heisenberg conspire to keep the neutrons from coming too close together: no two particles can occupy the same quantum state.

As you compact them closer together your certainty of their location increases, and so your certainty of their velocity decreases. The denser the neutron gas, the higher the average velocity of the neutrons must be. Once the neutron gas is relativistic, the star becomes unstable and we get a very, very rapid collapse. The exact point at which this happens is not well known; for the collapse of a star supported by degenerate electrons it's 1.44 solar masses, but we don't know the neutron as well as we do the electron.

A neutron star collapses to a black hole very, very quickly indeed, and it takes forever to do it. But that's another story :-)

It's one kind of nova, not a supernova

[Spamalamadingdong]

A supernova is the event which creates the neutron star in the first place. At least one kind of nova is associated with neutron stars; I am not enough of an astrophysics geek to be sure if the term is also associated with flares from unstable normal stars, but I would suspect so (anything that brightens enough to be a "new star" would be a nova in the old nomenclature).

Our own sun is much to small to form a neutron star. When it shedds the outer gas layers it will form a white dwarf that eventually will turn into a cold iron ball.

It'll be a carbon ball, not iron; the Sun does not have enough mass to begin carbon fusion and create signficant amounts of heavier elements. It will begin the red-giant phase when it starts fusing helium (which happens when hydrogen fusion no longer generates enough heat to keep the core from contracting further) and die when it runs out.

Re:It's one kind of nova, not a supernova

[SmackCrackandPot]

It will begin the red-giant phase when it starts fusing helium

This is one thing I don't understand about red giant stars. How can a star expand in size, if it's mass remains the same? If less heat is generated (which drives the convection currents to the outer layers on the surface), then the star should shrink rather than expand?

Re:It's one kind of nova, not a supernova

[Spamalamadingdong]

This is something I don't understand extremely well myself, but I'll hand-wave and hope that I don't get anything very wrong or confuse people either.

The size of the star (the radius of the photosphere) is set in part by the power needing to be radiated; the more power the core is cranking out, the hotter the stars outer layers get and the more they expand. Much of the mass of the star is in the core, so the reduced pressure from the expanded outer layers (they are farther away from the core so gravity doesn't pull on them as hard) does not affect the core's conditions much.

As for why the core cranks out more power when it starts burning helium, it takes higher pressures and temperatures to ignite helium. The core will contract until there is enough pressure to resist further contraction, and in a star too small to form a neutron star this either comes from heat or degeneracy pressure. A hotter core means more transferred out, so the end of hydrogen fusion results in contraction, heating, ignition of helium fusion and expansion of the outer layers from the greater heat output. Helium fusion does not release as much energy as hydrogen fusion, so the greater output is not sustained anywhere near as long; the red-giant phase of a star is much shorter than the "main sequence" phase.

Yes, a star at 2000 K radiates a lot less per square meter than a star at 5700 K like our sun, but a star with the radius of the orbit of Venus (57 million miles) has a lot more radiating surface than a star with a radius of 400,000 miles.

Don't Forget...

[dnahelix]

It really happened 25,000 years ago!

Re:Don't Forget...

[scharwenka]

No, it's really happening now. You see, time propagates at the speed of light!

Or is it the other way around...

How a neutron star collapses

[Tau Zero]

In other words, is there any way to conceive of a black hole at the center of a neutron star?

I don't believe so. IANAastrophysicist, but I can repeat the popular treatment that I read once without too many errors (I hope).

1. As the pressure on the matter in the neutron star increases, the velocity of the various particles (electrons, quarks) inside has to increase to resist it.
2. The increase in velocity also increases the mass-energy.
3. At some point the relativistic increase in the mass of the matter due to its greater velocity causes self-gravitation to equal the increased pressure. At this stage there is no possibility of pushing back any harder, and the core begins to collapse further.
4. This collapse never ends; it becomes a black hole.
5. There is not much of a burst of radiation. The matter falling into the BH is already at neutron-star densities and higher, and the free electrons and other charged particles scatter photons around the infalling matter rather than letting them travel in straight lines. The random-walk of photons carries them right into the BH along with the matter.

And I hope that wasn't too far off the mark.

MOD PARENT UP - Definitions of now

[ControlFreal]

The parent is right in that we see the explosion in our definition of now: remember, in relalivistic situations (i.e. anything happening either at speeds that are nonnegligible compared to the speed of light, or at distance scales that are large enough for propagation time to be nonneglibible on our time-scale of perception), there is no universal definition of "now": it's relative to each observer.

Please see my other comment on this.

Re:MOD PARENT UP - Definitions of now

[Richard Allen]

I read your other post, and though I find it interesting, I disagree.

Using your "see the sun" example, let's look at two scenarios.

1. It's Friday, August 20, 2004 at 4:30pm and you are watching Oprah. The sun explodes and (for sake of discussion) expands at the speed of light. By 4:38, you're dead.

2. It's Friday, August 20, 2004 at 4:30pm and you are watching Oprah. The sun explodes and (for sake of discussion) expands at the speed of light. At 4:31pm a commercial comes on and you get up to get a bag of Doritos. Fortunately for you, you've recently invented a space travel machine and your Doritos are in another Solar System. You immediately are worm-holed out of your living room. By 4:38, your television is toast, but you are fine.

In both scenarios, the sun explodes at the same time in relation to your existence in the living room. And, in both scenarios, the realization that it has exploded hits your television at the same time. But it one scenario, you escape "in the meantime". Therefore, to the Oprah watcher, the "now" was certainly different in each scenario.

Thoughts?

Re:MOD PARENT UP - Definitions of now

[Anonymous Coward]

I think when you returned to your living room through the wormhole, you would either be instantly vaporized, or end up in the vacuum of space, since, from your current location (dorito storage facility), you would have no idea that the sun had exploded.

Re:MOD PARENT UP - Definitions of now

[Tackhead]

> 2. It's Friday, August 20, 2004 at 4:30pm and you are watching Oprah. The sun explodes and (for sake of discussion) expands at the speed of light. At 4:31pm a commercial comes on and you get up to get a bag of Doritos. Fortunately for you, you've recently invented a space travel machine and your Doritos are in another Solar System. You immediately are worm-holed out of your living room. By 4:38, your television is toast, but you are fine.

How are you "fine"?

Best-case scenario is that the other solar system is Proxima Centauri.

And on Proxima, you're fine at 4:38. But unless it's gonna take you until December 2008 to get your Doritos, life's really gonna suck when you go back home to watch the rest of Oprah. (Why wouldn't you go back home after getting the Doritos? It's not like you have any reason to believe Sol exploded.)

On the other hand, compared to sitting around in Alpha Centauri to watch reruns of episodes featuring Y2K consultants desperately trying to restore their credibility, Al Gore on a debating stand, the 2000 election recounts, and 9/11... Well, maybe being vaporized at 4:39 when you get back to what was your living room is still an improvement.

On the third hand (handjobs from Centauri chicks rule), suppose you got distracted while in the kitchen (again I with the Centauri chicks). Four years spent munching on Doritos, getting three-fisted Centauri handjobs while you laugh at the reruns from early 2000 all the way up to the hilarious surprise finale of August 20, 2004 when all those Terran assholes, yes, even that bully in third grade and that skank who dumped you in college, fry! Well hey, that sounds pretty cool. Especially if there are Centauri chicks.

There's a hell of a universe next door. Let's go.

Re:MOD PARENT UP - Definitions of now

[dnahelix]

No, you are wrong.
Think of it this way; You do not see the Sun at all. You only see the photons that came from the sun. What you percieve is only an image, after all.