Q & A – Neutron Stars

minisciencelogo-300pxAt the conclusion of each Mini-Science lecture, audience members submit their questions to the evening’s presenter, who answers as many as possible on the spot. Three of the unanswered questions are sent to the presenter for posting here. In addition, a quiz is held each week based on material from the lecture. Here are questions and the quiz from Prof. Victoria Kaspi’s lecture ‘Neutron Stars: Lighthouses of the Cosmos’ (April 1, 2009).

Q: Why is the speed of light the speed limit?

A: At first this question comes across to me like, “Why is a dog a dog?” Ignoring possible metaphysical interpretations of this question, I will assume the questioner is really asking, “Why does light travel at a finite speed, and why is it equal to `the speed of light,’ namely 300,000 kilometers per second. The simple answer is also to me, the true answer, which is simply “Because.” Physics is based on our observations of the Universe and it is first and foremost an experimentally verifiable fact that light travels at a finite speed, and that speed is equal to 300,000 km/s (roughly). (Actually, the latter is only the speed in a vacuum; in fact light travels slower in different media such as water, or glass.)

Perhaps the reader had a deeper question in mind, such as “Why can nothing travel faster than light?” Einstein grappled with this and one argument he gave relates to physics done on a train. Suppose you are on a windowless train moving at a fixed speed, with no decelerations or accelerations, and suppose the motion of the train is perfectly smooth so there are no bumps or jolts. Einstein argued that if you were doing a physics experiment on such a train, you would not be able to detect the motion of this train no matter what. In other words, there is no physics experiment that can be done that would reveal to you whether and how fast the train is moving relative to the ground, under these idealized assumptions. After some thought, it is likely that this postulate will seem reasonable to you.

But now imagine such an idealized train travelling as fast as light, and imagine you are on the train holding a mirror up to your face, and you are looking in the same direction that the train is moving. When the train is travelling at the speed of light, the reflection in the mirror must disappear because the light can never get back to your eyes. If this could be so, that right there would be an experiment that would reveal to you that the train was moving at the speed of light. So, if you accept the previous postulate that no such experiment can exist, this proves that the train cannot ever reach or exceed the speed of light.

I hope this answered your question…

Q: What happens to neutrons, protons, gluons in a neutron star? Are quarks just free?

A: The structure of the neutron star is actually quite complex. Near the surface, at the crust, there are real atoms, with protons and neutrons in the nuclei, with electrons orbiting (though the shapes of the orbits are elongated due to the intense magnetic field). As you move inward the density and pressure increase, and those atoms get squashed together. It is thought that there are multiple transitions in the geometry of the atoms with electrons actually starting to be shared among all nuclei (a sort of electron “sea”) and eventually the nuclei merging or “dripping” together. At this point it is actually preferred, due to particle physics rules, for neutrons to exist (i.e. protons and electrons merge to form neutrons) but some free electrons and protons are always around. As we get toward the core of the star where the density is much greater than that found in typical nuclei, we don’t know how matter behaves. Some researchers indeed believe there may be free quarks in the centres of neutron stars, but this hasn’t been proven.

For a nice web site on neutron-star structure, please see a nice introductory (though detailed) description by my colleague and old friend Cole Miller at http://www.astro.umd.edu/~miller/nstar.html

The wikipedia entry is good too: http://en.wikipedia.org/wiki/Neutron_star

Q: Since neutrons have no charge how does star rotation generate a magnetic field?

Great question! See the answer above. Even though the star is mainly neutrons, there are still some electrons and protons in the interior and these presumably generate the magnetic field.


Q: What rotates faster, the fastest-known millisecond pulsar or your kitchen blender?

A: The pulsar.

Q: Some stars, like “Cepheids” (about which you’ll undoubtedly hear in Prof. Rutledge’s lecture), pulsate because their radii get larger and smaller, with the entire stellar ‘surface’ getting brighter and dimmer. Is this why pulsars seem to pulsate?

A: No. Pulsars seem to pulsate because of their rotation. They have beams of radio waves (and other electromagnetic radiation) being emitted from their magnetic poles. Because the magnetic axis is not aligned with the rotation axis, we perceive a pulse each time the pulsar’s beam crosses our line of sight, like a lighthouse.

Q: How can finding even faster millisecond pulsars assist in our efforts to understand matter at extremely high densities?

Q: Different models for different behaviors of matter at ultrahigh densities (or different “equations of state”) predict different maximal rotation speeds for neutron stars. This is because, depending on the behaviour of matter in the stellar interior, the star can be more or less compact for the same mass. A very fast rotating star that is less compact will not be able to hold itself together as easily as a more compact star; rotating fast enough, the more extended star eventually flies apart. So by finding even one extremely fast rotation millisecond pulsars, we can prove that models that result in more extended neutron stars must be wrong. If we found a star rotating faster than 2000 times per second, we would rule out nearly every proposed equation of state… a very exciting possibility.

If you have any related comments or questions, please feel free to post them. We cannot promise a reply to every question but will answer what we can.

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