h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

"They can spin as fast as 600 times a second (or even faster) or as slow as five seconds."

Wouldn't it be better to say "They can spin several hundred times per second, or a slow as a few times per minute." Though, in fact, is there any limit either way?

For an *isolated* neutron star, the spin derives from the conservation of angular momentum of the surviving parts of the collapsed parent star. The parent star will have limits on its own rotational speed, otherwise it would tear itself apart, so this in turn sets limits for the neutron star.

However, if the neutron star has a lower mass companion star, it can subsequently get spun up to higher velocities during the process of mass accretion from the companion. This is how we get millisecond pulsars, and the "600 times a second" figure is simply the fastest we have observed. I don't think it's the case that a neutron star couldn't spin faster, but maybe there's no mechanism capable of getting it there...

I don't think there is a lower limit. The rotation of a neutron star will slow over time because of magnetic torques, or because of interactions with higher mass companions.

The article needs to replace "loose" with "lose" in the paragraph about pulsars. Also, I don't believe the bit about pulsars being more accurate than "any clock on earth". More accurate than the most accurate atomic clocks? How, then, are we able to detect glitches where the period changes by a few parts per million, and recovers after a few days?

Oh, and I agree with you about the wording!

OK, I've changed the wording and replaced 'loose' with 'lose'. Thanks very much for pointing this out.

The loose/lose confusion is one of my pet hates, and incredibly common.

How about the more technical aspects of my reply?

Now that I've actually done some research, rather than just spouting off the top of my head, I think the question of how a pulsar gets its spin is quite interesting. What I said about there being limits on the rotational speed of the donor star (I'm using the proper terms now!) was correct, but it turns out that this is still often sufficient to cause the neutron star to spin at its maximum possible rate (i.e any faster and it would start to shed mass from its equatorial region). But this isn't what we see; most young pulsars are spinning at well below this limit, which has been something of a mystery until quite recently. The theoretical picture which is now emerging is one of rapid spin-down over the first year or so of a pulsar's life, followed (sometimes) by a much slower spin-up to something approaching the theoretical limit. I think the article could discuss spin evolution without getting bogged down in technicalities. The conservation of angular momentum thing can be explained using the standard example of an ice-skater spinning faster as they pull in their arms.

The article seriously understates the density of a neutron star, claiming that a teaspoon of matter from one would weigh "a few thousand tons". In fact, the density of a neutron star approaches the density of a neutron: something like 1.57 * 10^12 kg / cm^3. So that teaspoon of matter would weigh more like 2000 million tonnes...

Of course, that's very difficult to imagine. I prefer the example I came across which said that if the period at the end of this sentence was made of neutron star stuff, it would weigh as much as the titanic (I haven't checked this is consistent with the previous figure, but I would if it was going into the article).

And the claim about pulsars being more accurate than any Earth clock is just wrong, I'm afraid.