Geminga, a nearby pulsar

[eburacum45]

For the OA project, I'm making a Celestia model of a nearby neutron star, Geminga; the excellent pulsar models by Cham are an inspiration, but I wanted to see if I could make one from first principles. But I've got a question. Should the central object be flattened by its rotation?

here is a movie (2.5 mb) of the work in progress
http://www.orionsarm.com/movies/Geminga.wmv
this movie is slowed down, somewhat; the real star rotates about 4 times a second.
http://en.wikipedia.org/wiki/Geminga

All the models and graphics I've seen of pulsars and neutron stars show the central object as a near-perfect sphere; however, such a fast spinning object should surely be flattened into a highly oblate spheroid.

My question is, should pulsars be modelled as oblate spheroids? In other words is neutronium fluid enough to become distorted by centrifugal force? It would be easy, using Celestia's settings, to model such an object as an M+M shaped spheroid, although I would have to guess the degree of oblateness (unless that information is available somehow).

[ajtribick]

Values calculated for a pulsar mass of 1.4 solar masses and a radius of 10 km. (These kind of figures seem to be fairly commonly-used for neutron stars)

Magnitude of the gravitational acceleration = GM/r^2 = 1.9E+12 m/s^2
Magnitude of the centrifugal acceleration = omega^2*r = 4*pi^2*r/P^2 = 2.5E+04 m/s^2

Centrifugal force is approximately 75 million times weaker than gravity.

[Cham]

The shape of a real pulsar is a complicated matter, and isn't just a result of the gravitational and centrifugal forces. The magnetic force and the induced electric field are acting in a subtle way and may alter the shape significantly. This is a fascinating subject. However, I don't have much precise information about this, so consider the pulsars as perfect spheres.

[eburacum45]

My daughter's boyfriend (bright lad; only 16) suggested to me that gravity would be very much greater than centrifugal force on these objects.

And it looks like he was right.

Thanks, people; the fact that gravity is on the order of 75 million times greater than centrifugal force would mean that a pulsar is a practically perfect sphere, barring any magnetic effects.

[selden]

"Starquakes" on pulsars cause changes in their rotational periods. They've been successfully modeled as collapsing oblate surfaces.

See http://books.google.com/books?id=ksBuB_ ... TIGjxvBSp8

[ajtribick]

This raises the question of what kind of displacement is involved in a starquake.

For example, if we assume the star is uniformly dense (bad assumption, but this is a kind of back-of-the-envelope calculation), dropping the outer millimetre of the star by one millimetre would involve an energy release of about 10^33 J, or the amount of energy that the Sun releases in about 50 days.

How energetic are these starquakes?

[Reiko]

I don't know the answer to your question but that was cool!

[fsgregs]

Speculation regarding the characteristics of neutron stars seems to be confusing in many ways. Based upon what I have understood, a neutron star is the compressed core of a supernova explosion and is the densest visible object in the universe other than a black hole. As such, its gravity is ENORMOUS (as reported above). I do not recall the source, but I recall reading that the gravity is sooooo strong that a neutron star would not bulge, even when spinning. Further, neutrons would not have any ability to pile up in hills and valleys ... there would be no pits, holes, bumps or even pimples of any kind. It would just be perhaps the smoothest object in the universe, with an outer atomic layer of neutrons laid down like a solid film of oil.

Likewise, since the internal core temperature of a supernova going off is reported to be in excess of 500,000,000o C, than I would assume the resulting neutron star would also be incredibly HOT ... perhaps in excess of that temperature. As such, would not its surface "color" glow far far above the visible range? It would not just be violet-hot, it would have to be X-ray or gamma hot! Thus, what would a neutron star "look like" through a telescope or by naked eye? It seems to me its surface would not be tinted blue. That is the wavelength given off by objects at about 35,000 K, far, far, far below the theoretical surface temperature of a neutron star.

I have read that it takes billions of years for neutron stars and white dwarfs to cool down. If this is true, then would we see Geminga or any other neutron star at all in visible light? Would they not be a kind of X-ray glowing, rapidly spinning, whirling demon of death, giving off deadly radiation far above the visible range ... emitting crushing gravity and whopping magnetic fields ... just waiting for the unsuspecting to come by

There is also the question of texture. If it is that hot, would it appear granular? I thought neutron stars are made of neutrons ... a kind of super-super hot spherical solid ball that would probably appear as smooth as a ball of glowing glass.

Just my thoughts, but ...

Frank

[eburacum45]

Actually I would refer interested parties to this topic over at Bautforum;
http://www.bautforum.com/questions-answ ... stars.html
there the member Timb finds that very young, millisecond pulsars would be reasonably oblate, despite their huge gravity; and after a re-think, Grant Hutchison agrees with him.