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Magnetars Don’t Suck

One bit of astronomical news caught my attention recently not only because of the the science behind it but how the various news outlets treated it.

Some were saying that this was a new mystery about black hole formation, others were saying that a mystery has been solved. I’m siding with the “case-closed” interpretation but you can decide for yourself. Here’s what you need to know.

A recent study led by Dr. Ben Ritchie of Open University, describes a very special type of neutron star called a magnetar which is essentially an incredibly magnetic neutron star.

Conventional wisdom holds that when a star of 10 to 25 solar masses collapses at the end of its life after a supernova, the result is a city-sized neutron star.

This process is actually pretty interesting….With no stellar furnace fighting against the inward pull of gravity, everything falls towards the center of mass. All the in-falling matter forces the electrons to fill all the lower, previously available shells around each atom. Once these shells are filled, no more squeezing can be done if the remaining mass of the star was about 1 and 1/2 solar masses or less. This pressure preventing further collapse is called electron degeneracy and it arises because electrons cannot occupy the same quantum state (pauli exclusion principle). What remains is called a white dwarf.

If however, there is more than 1.5 solar masses, then electron degeneracy isn’t strong enough and all the electrons and protons are actually forced together creating more and more neutrons. This process stops due to another type of pressure called neutron degeneracy. That means that neutrons must obey the pauli exclusion principle just like electrons (If you had more mass however, even neutron degeneracy couldn’t stop further collapse and you end up with a black hole).

Ok, so now we have a neutron star and since these stars are essentially pure nuclear matter, they are so dense that a thimbleful would weigh on the order of millions of tons.

Once in awhile, neutron stars also have an incredibly powerful magnetic field around them. This is the rare and wonderful magnetar.

When I say powerful, I mean it’s crazy strong. We’re talking about a magnetic field on the order of ten giga-teslas which is quadrillions of times more powerful earth’s puny magnetic field of only 30–60 tiny microteslas.

This magnetic field wouldn’t only be deadly to robots getting too close; it could easily kill a flesh and blood person even if he were 1000 km away. The water in your body is diamagnetic meaning that is creates its own magnetic field in opposition to an externally applied magnetic field. These battling fields then would just tear apart your body apart rather efficiently I would think.

Back to the news item; what astronomers recently discovered was a magnetar in the Westerlund 1 star cluster, about 16,000 light years from Earth. This cluster is pretty amazing. Many of its stars are as bright as almost a million Suns and some have a diameter two thousand times that of our Sun. Some are so massive and bright that if the earth were nearby, it would look like our sky was filled with 100s of stars as bright as the full moon. Shades of Asimov’s Nightfall story huh?

There is one magnetar in this cluster that was especially odd because it had apparently formed from a parent star that had 40 solar masses. How then could it become a magnetar and not a black hole? Something was wrong about our theories about black holes or our ideas about stellar evolution.
It looks like the latter is the answer.

Dr. Ben Ritchie and his colleagues claim that this magnetar once had a companion star meaning that it was in a binary star system. This other star then likely siphoned off the mass of this big star until it weighed enough to become a neutron class star instead of a black hole class sucking machine.

So the take-away here is that it looks like binary star systems can play a much greater role in stellar evolution than we previously thought.

I thought you might like to know that.

5 comments to Magnetars Don’t Suck

  • Fergus Gallagher

    Magnetic fields require a generating current. Any ideas how neutrons can do that?

  • Fergus Gallagher

    Sorry – I was lazy. Google is (now) my friend.

  • DrMonkey

    This is fascinating stuff, and it made me realise that I haven’t really read much concrete about the collapse of stars into their various descendants since my early teen years.

    I definitely recommend reading up on what’s going on with elementary the particles during this collapse (I just did, and feel a lot more educated for half an hour of reading). My new-found understanding of electrons and protons being “forced together” is far more interesting than I would have thought from the simple phrase: protons and neutrons are both made of 3 quarks, the difference is that protons have two up quarks and one down, whereas neutrons have one up and two down. In neutron star formation, one of the up quarks interacts with a W- boson and changes its “flavour” to down. The boson is produced when an electron degenerates, producing the boson and an electron neutrino. This means that as it collapses, the neutron star will be emitting electron neutrinos – it seems that these take away the energy that would otherwise prevent collapse beyond electron degeneracy.

  • JasonWilson

    I do so enjoy a interstellar quantum mechanical story of sub atomic bonding and mass loss when I hear one.
    I wonder how they knew this magnetar was “formed from a parent star that had 40 solar masses.”
    My real question (and I may be exposing myself as a real noob here)is where did all this extra mas end up? I imagine the star siphoning off the extra mass had to be a bit heavier than 40 solar masses to do the trick. So is it realistic to think that this magnetars old compadre is now likely a black whole?
    If not, why?
    And really, whats with the neutrally charged sup atomic particle producing such a strong magnetic field when you get on the of a sh** tone (give of take a whole hell of a lot)together, anyway?

  • johntheplumber

    My physics master got worked up when all I asked was why did they call the north pole north.

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