Scary Astronomical Phenomena

[Ray Moscow]

I spotted this on Bad Astronomy. I think the term "Holy Sh*t!" pretty well sums up the topic:

OK, so maybe we can be a *little* frightened. (http://blogs.discovermagazine.com/badastronomy/2009/06/18/ok-so-maybe-we-can-be-a-little-frightened/)

In December 2004, the magnetar SGR 1806-20 underwent such a starquake. In one-tenth of a second the subsequent blast released something like 2 times 10^46 ergs of energy — equal to about 50 trillion times the Sun’s output during that same period.

Holy crap.

This star sits about 50,000 light years from the Earth: literally halfway across the Milky Way galaxy from us. Yet, even from that forbidding distance, this titanic event was able to physically affect the Earth. It compressed our magnetic field and partially ionized our atmosphere, causing it to puff up measurably.

Mind you, it was 500 quadrillion kilometers (300 quadrillion miles) from us at the time.

Unfortunately there's another one of these things somewhat closer to us.

[LoneWolf]

Yeah, I read that post. So this thing has an actual crust, like a planet?

And something 300 quadrillion miles away having that kind of affect? DAMN! When I think about some of the stuff out in the universe I am in awe. But if I think about it too much I get a little scared. Our planet is so insignificant, let alone ourselves.

[Ray Moscow]

The thought I had was that all life on earth could be wiped out, and the universe (if there are other minds out there to think about it) would no more notice than we might notice stepping on an ant.

[Garrett]

In December 2004, the magnetar SGR 1806-20 underwent such a starquake.

This star sits about 50,000 light years from the Earth: literally halfway across the Milky Way galaxy from us.

I'd think that rather than that star undergoing a starquake in 2004, it must've happened around 47996 BC.

[Alex]

What (theoretically) will happen to neutron stars or magnetars? Does their superdensity somehow resist neutron decay?

[Berthold]

What (theoretically) will happen to neutron stars or magnetars? Does their superdensity somehow resist neutron decay?
There are even much smaller units of matter in which neutrons do not decay. :D :wave:

[lpetrich]

Neutron stars' bulk stabilizes their neutrons. They also have some protons among their neutrons, but these protons are accompanied by electrons, which must also be present in the neutron stars' bulk. Otherwise, that bulk will be electrically-charged enough to attract electrons into it. If the protons start becoming too abundant, then the electrons will become too abundant, which will force up their kinetic energies enough to make new electron-proton combinations more massive than neutrons.

This happens because electrons follow Fermi-Dirac statistics, being only one per quantum state. For an average separation of l, then their average wavelength must also be l, and their average momentum h/l. The neutron-proton mass difference is about 1.3 MeV, meaning that the electrons must have a total energy of about that. This is about 2 1/2 times greater than the electron's rest mass of about 0.511 MeV, meaning that the electrons will be relativistic. Thus, their energies are about hc/l, and l must be about 10-12 m. By comparison, nucleons are about 10-15 in size, meaning that there is about one electron-proton pair per billion neutrons.

[Alex]

Ipetrich: I've stared at your words in ignorant awe.

From my schoolboy physics, all I understand about neutron stars is that they're superdense objects held together by gravity. I also happen to know that a "liberated" neutron outside the atomic nucleus will decay into a proton, an electron, and an antineutrino.

Does a mass containing nothing but neutrons exist indefinitely or will it decay "eventually"?

[Berthold]

Ipetrich: I've stared at your words in ignorant awe.

From my schoolboy physics, all I understand about neutron stars is that they're superdense objects held together by gravity. I also happen to know that a "liberated" neutron outside the atomic nucleus will decay into a proton, an electron, and an antineutrino.

Does a mass containing nothing but neutrons exist indefinitely or will it decay "eventually"?
As far as my limited knowledge goes, a neutron star (at least its core) is in a state of aggregation comparable to atomic nuclei.

On things like "indefinitely" and "eventually", have a look here (http://en.wikipedia.org/wiki/Proton_decay). Although the article is about protons, the theory is for baryons in general.

[lpetrich]

Ipetrich: I've stared at your words in ignorant awe.

From my schoolboy physics, all I understand about neutron stars is that they're superdense objects held together by gravity. I also happen to know that a "liberated" neutron outside the atomic nucleus will decay into a proton, an electron, and an antineutrino.
That's correct for a free neutron, but neutrons inside of nuclei can be stabilized by the protons in those nuclei. This would be because decaying would produce a nucleus with too many protons packed into its volume. Each individual proton would be forced into a smaller individual volume, forcing up its kinetic energy and making it energetically unfavorable for a neutron to decay into one.

In fact, in proton-rich nuclei, instead of neutrons decaying into protons, protons decay into neutrons -- they do positron beta decay or capture orbiting electrons.

Does a mass containing nothing but neutrons exist indefinitely or will it decay "eventually"?
In the absence of baryon-number-violating decays, a neutron star should exist indefinitely.

As far as my limited knowledge goes, a neutron star (at least its core) is in a state of aggregation comparable to atomic nuclei.
Yes, it's more-or-less a giant atomic nucleus with a mass about 1 to 1.4 times the mass of the Sun.

On things like "indefinitely" and "eventually", have a look here (http://en.wikipedia.org/wiki/Proton_decay). Although the article is about protons, the theory is for baryons in general.
That's baryon-number-violating decay, which is VERY slow, if it happens at all. Most GUT's, however, predict such decays, though they are suppressed by GUT energy scales being something like 1016 times a proton/neutron's mass.

What we were talking about here were weak-interaction decays, which conserve baryon number.

[LoneWolf]

Ipetrich: I've stared at your words in ignorant awe.

From my schoolboy physics, all I understand about neutron stars is that they're superdense objects held together by gravity. I also happen to know that a "liberated" neutron outside the atomic nucleus will decay into a proton, an electron, and an antineutrino.

Does a mass containing nothing but neutrons exist indefinitely or will it decay "eventually"?
As far as my limited knowledge goes, a neutron star (at least its core) is in a state of aggregation comparable to atomic nuclei.

On things like "indefinitely" and "eventually", have a look here (http://en.wikipedia.org/wiki/Proton_decay). Although the article is about protons, the theory is for baryons in general.

Interesting. So the decay of subatomic particles like protons is purely hypothetical.

[Ray Moscow]

Another scary one: Eta Carinae (http://en.wikipedia.org/wiki/Eta_Carinae)

Very large stars like Eta Carinae use up their fuel very quickly because of their disproportionately high luminosities. Eta Carinae is expected to explode as a supernova or hypernova some time within the next million years or so. As its current age and evolutionary path are uncertain, it could explode within the next several millennia or even in the next few years. LBVs such as Eta Carinae may be a stage in the evolution of the most massive stars; the prevailing theory now holds that they will exhibit extreme mass loss and become Wolf-Rayet Stars before they go supernova, if they are unable to hold their mass to explode as a hypernova.[18]

The thing is, we're pretty close to it.

It is possible that the Eta Carinae hypernova or supernova could affect Earth, about 7,500 light years away, but would not likely affect terrestrial humans directly, who will be protected from gamma rays by the atmosphere, as well from some other cosmic rays by the magnetosphere. The damage would likely be restricted to the upper atmosphere, the ozone layer, spacecraft, including satellites, and any astronauts in space, although a certain few claim that radiation damage to the upper atmosphere would have catastrophic effects as well.

[Puck]

Ipetrich: I've stared at your words in ignorant awe.

He can have that effect, can't he? ;) I'm often in awe, too.

[LoneWolf]

Ipetrich: I've stared at your words in ignorant awe.

He can have that effect, can't he? ;) I'm often in awe, too.

Lauren knows too much for his own damn good.

:)

I love you Loren. Of course I'm a bit drunk at the moment.

[Loren Pechtel]

What (theoretically) will happen to neutron stars or magnetars? Does their superdensity somehow resist neutron decay?

I don't think it's so much that they resist decay as that the extreme pressure makes it an equilibrium reaction rather than a one-way reaction. You get just as much electron + proton -> neutron as neutron decay.

[Brother Daniel]

Interesting. So the decay of subatomic particles like protons is purely hypothetical.
No. That article is talking about protons specifically (not subatomic particles in general). Moreover, its introduction is misleading. As lpetrich noted, protons in a proton-rich nucleus do decay* (either by positronic beta decay or by electron capture). What is purely hypothetical is whether there are other ways for a proton to decay -- ways that could apply to free protons (i.e. typical hydrogen nuclei) for example.

*footnote: The energy of the electrical repulsion between protons adds mass to the nucleus. In a proton-rich nucleus, this added mass can be greater than the mass difference between a neutron and a proton. Hence protons want to turn into neutrons in this case -- for the same reason that neutrons otherwise want to turn into protons.

[Berthold]

And all I intended with that link was a hint as to the relative meaning (and caution required in use) of "eventually". :rolleyes: