Celestia Forums

I'm an engineer, not a physicist. So please forgive my simple question.

Since pulsars are very tiny stars, I would assume that their light is emitted omni-directionally. If so, then as they spin, I would expect minimal variation in their intensity. However, based on the amount of change in magnitude, it almost seems as if the star is black on one side and light on the other -- thus making it very obvious that it's spinning.

Help me out here. Why does a pulsar pulse?

I'm a physicist, not an engineer. So please forgive my complex answer.

A pulsar is basically a huge pack of neutrons. A giant atomic nucleus surrounded by an atmosphere of ionised gaz.

Neutrons are caracterised by a magnetic moment, even if its charge is zero. That's because a neutron is made up of three quarks, which are charged and moving very fast. So inside a neutron star, many neutrons align their magnetic moment, a bit like a ferromagnet material.

So a neutron star may have a net magnetic moment. Most neutron stars are rotating around their center of mass (spin angular momentum). So the magnetic moment is spinning. This is a first cause of radiation, because according to Maxwell theory, a variable magnetic moment should produce some electromagnetic radiation (radio frequency, or higher frequency, depending of the rotation rate). But there is a second cause of radiation : the ionised gaz around the neutron star. Many free electrons in the atmosphere are trapped in the intense magnetic field generated by the star's magnetic moment. When moving in circles, the accelerated electrons produces some electromagnetic radiation which may be released as visible light. The neutron star (now a "pulsar") is behaving a bit like a lighthouse.

Is my answer obscure enough ?

That's not bad. But I'm still just a stupid engineer.

Your explanation would imply (to me, anyway) that the star's magnetic moment would tend to fluxuate unpredictably. Wouldn't that obscure the period of the pulse?

Anonymous wrote:Your explanation would imply (to me, anyway) that the star's magnetic moment would tend to fluxuate unpredictably. Wouldn't that obscure the period of the pulse?

No. The magnetic moment is just rotating. It is the generator (or the source) of the radiation. In the very long run, the pulsar's rotation rate is slowly decreasing, because of the loss of radiation energy.

A pulsar is just like a magnet bar rotating at a fast rate, thus emitting some light from the poles.

OK. Got it now. Thanks, Cham.

Of course, Cham's explanation is still extremely simplified.

Most pulsars emit radiation in extremely narrow beams, apparently from the regions around their magnetic poles. There are several different emmission mechanisims operating, including heating of ionized gasses as they impact in the polar regions, and synchrotron radiation as they're accelerated outward by the magnetic fields. Some pulsars radiate most of their energy in radio frequencies, others in X-Rays. A search of the Web will locate many detailed descriptions of the physics involved. The January issue of "Sky and Telescope" includes an article on magnetars, which are similar.

Neutrons stars, pulsars and magnetars are absolutly fascinating objects. They produce an intense magnetic field, they are spinning very fast, they are extremely dense (inconceivable density), they generate a strong relativistic gravitational field (general relativity is king here), there are all sorts of radiation patterns surrounding them, and they represent probably the last state of matter before the black hole state. They aren't too big compared to the human dimensions (few km radius).

Pulsars should be represented in Celestia, and we have just a few of them. I'm having a great hope in your efforts, Selden, to make a descent 1500 pulsars file, with a good marker script. We should discuss more about the color assigned to them.

I suggest that the colors should represent the distance uncertainties (if available). Blue means "safe", red means very uncertain. What do you think ?

Cham,

I think the Mark script(s) need to set the markers to different colors depending on what information the viewer wants to see. Marking the different types of pulsars, for example, or relating the color to the rotation speed, or perhaps to the density of pulsars in the region.

One thing I find interesting is that if I squint at the pulsar marks, I can see what look like outlines of the arms of the galaxy that are near us. Appropriate color coding might make them more obvious.

Yes. Different markers scripts may be very usefull.

By the way, once you have a .cel script running (I can't use celx scripts on the OS X version of Celestia), how do you unmark everything at once ?

unmarkall {}

selden wrote:I think the Mark script(s) need to set the markers to different colors depending on what information the viewer wants to see. Marking the different types of pulsars, for example, or relating the color to the rotation speed, or perhaps to the density of pulsars in the region.

What kind of pulsar types are you thinking ? And I don't believe density of pulsars in the region could be usefull. It will be misleading in Celestia.

I think the only usefull information to be shown as colors in Celestia are the rotation rate, pulsar types (?), and distance uncertainties.

Different people find different kinds of information useful. Seeing that a particular display isn't useful can be a learning experience, too.

Can you make those different scripts ?

I suggest that you give a dark color to all pulsars on the two "offending" planes, or maybe just don't mark them.

We shall see...

*ahem*

Selden

what is the signification of the colors in your pulsar script ? I understand it's related to "citations", but what more ?

Cham,

The pulsars marked with different colors come from different catalogs. In principle one could figure out from the color of a particular mark which catalog that pulsar originally was listed in.

The pulsar database contains values which were obtained from about 135 different published catalogs. The program that I wrote to translate the data into Celestia's format assigned a random color to each of the catalog designations in the database. (It was easy to do it randomly, but hard to do it in some ordered way.) I didn't bother to create an output file to show which color was associated with which catalog.

I did it this way because I wanted to see if the pulsars with strange distances all were from one catalog or were from different catalogs. If their marks all were the same color (from the same catalog) then I was going to try to find out what was different about that particular catalog. However, as you can see by their different colors, they're from different catalogs.

Careful reading of the documentation revealed that the distances of most pulsars were calculated from the dispersion seen in their radio signals and from a model of the amount of dispersion caused by the interstellar medium, which is different in different directions and at different distances. Apparently pulsars with very poor or unknown dispersion values (and thus with very poor distance estimates) were assigned distances corresponding to +/- 1.76 kpc above and below the galactic plane. No real pulsars are in those locations, so the 100 or so pulsars that have very poor distance estimates are easy to separate from the others.

Thanks for the explanations Selden.

Yes, the colors associated to various catalogs is a good idea.

This addon is great because we can feel the galactic arms with it. But is it real, or just a pattern created by the observer's eye and associated to a selection limitation ? How can we know ?