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

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.

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...

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: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.

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.