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

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?

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.

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.

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: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.

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?