Exploding Neutron Star 35
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."
Additional Linkage (Score:5, Informative)
http://www.gsfc.nasa.gov/topstory/2004/022
Re:Additional Linkage (Score:3, Informative)
Really cool story. (Score:2, Funny)
Not from Helium Fusion (Score:4, Interesting)
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 (Score:5, Informative)
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 (Score:5, Informative)
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 (Score:5, Informative)
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 (Score:4, Interesting)
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 (Score:3, Interesting)
I don't believe so. IANAastrophysicist, but I can repeat the popular treatment that I read once without too many errors (I hope).
Re:onion model (Score:2)
Yes, but... (Score:2, Funny)
Yes, but how much of that is diamond [slashdot.org]?
Re:Yes, but... (Score:4, Informative)
Re:Yes, but... (Score:2)
Some amount of energy... (Score:5, Interesting)
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 [about.com], 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 [bham.ac.uk]: in 60 seconds, that yields about 10e+45 Joules.
Re:Some amount of energy... (Score:5, Informative)
Ah crap: that's supposed to be 1e+45 Joules...
Isn't this called a Nova? (Score:3, Interesting)
Re:Isn't this called a Nova? (Score:4, Informative)
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".
It's one kind of nova, not a supernova (Score:5, Interesting)
Re:It's one kind of nova, not a supernova (Score:2)
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 (Score:2)
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 doe
Re:Isn't this called a Nova? (Score:5, Informative)
Along with the ordinary use of "nova" there is also something called a hypernova [nasa.gov]. 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 [arxiv.org] 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 [cornell.edu].
Re:Isn't this called a Nova? (Score:1)
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
Re:Isn't this called a Nova? (Score:5, Informative)
Re:Isn't this called a Nova? (Score:5, Informative)
Don't Forget... (Score:5, Informative)
Re:Don't Forget... (Score:3, Informative)
Or is it the other way around...
MOD PARENT UP - Definitions of now (Score:4, Interesting)
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 [slashdot.org] on this.
Re:MOD PARENT UP - Definitions of now (Score:4, Interesting)
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 (Score:3, Funny)
How are you "fine"?
Best-case scenario is that the
Re:MOD PARENT UP - Definitions of now (Score:2)
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.