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

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...

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...

Re:Yes, but...

[Patrik_AKA_RedX]

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

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: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.

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: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.

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...

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.

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?