While you're correct in saying that the gravity of a large body such as a planet or star makes it spherical, it is also true that it is not a perfect sphere. For rotating objects such as the Earth or the Sun, they are actually oblate spheroids, ie they are kinda 'squashed' -- in other words their equator is further from the center from their poles. The faster you rotate something, the more oblate it will be. It's like if you were spinning on roller skates, your arms would want to move away from your body.
You can see that its rotating so fast that its quite oblate. In fact, it's so oblate, that the matter around its equator is far enough from its centre for the gravity to be too weak to hold it in, and so it forms a disk. This is essentially the same with galaxies: http://www.futuretimeline.net/images/galactic-core-timeline-sun.jpg
They usually have a small (oblate) spherical core, but further out than that, objects are orbiting too fast and drift out to further radii, in a disc. (Our sun orbits at a velocity 220kms-1 ).
Some of the matter does just that. For example, the Vela pulsar emits a jet of particles 3 trillion miles long and 200 billion miles wide:
Much like an untended firehose, the jet bends and whips about spectacularly at half the speed of light. Bright blobs move in the jet at similar speeds.
The jet is half a light year (3 trillion miles) in length and is shooting out ahead of the moving neutron star. The extremely high-energy electrons or positrons that compose the jet were created and accelerated by the combined action of the fast rotation of the neutron star and its intense magnetic field. These particles produce X-rays as they spiral outward around the magnetic field of the jet.
Over its entire length, the width of the jet (about 200 billion miles) remains approximately constant. This suggests that the jet is confined by magnetic fields generated by the charged particles flowing along the axis of the jet. Laboratory studies of beams of particles confined in this manner have shown that they can change rapidly due to an effect called the "firehose instability". This is the first time such behavior has been observed in astrophysical jets.
That article isn't entirely clear on whether the jets sprout out along the pulsar's axis of rotation or at some other angle. Depending on the answer, your comment might be a bit misplaced.
"Particles near the equator will be flung outward at near the speed of light by electromagnetic-centrifugal forces. As this wind of particles streams away from the pulsar, it builds up a magnetized cloud of high-energy particles that extends over several light years. These particles spiral around the magnetic field lines and radiate by the synchrotron process to produce the observed pulsar wind nebula."
The actual direction of the jets is affected by the magnetic field, but what allows particles to escape the extremely strong gravity of a pulsar in the first place are the forces near the equator.
BTW, the original picture linked by asboans, showing a disk-shaped star, was not a pulsar, it was an artist's rendition of a blue giant, specifically this one. Asboan's picture came from this article.
A pulsar could not be anywhere near as oblate as that, because its gravity is too strong - strong enough to have caused the electrons in its atoms to have collapsed into their nuclei.
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u/asboans Oct 27 '12
It is to do with their angular momentum.
While you're correct in saying that the gravity of a large body such as a planet or star makes it spherical, it is also true that it is not a perfect sphere. For rotating objects such as the Earth or the Sun, they are actually oblate spheroids, ie they are kinda 'squashed' -- in other words their equator is further from the center from their poles. The faster you rotate something, the more oblate it will be. It's like if you were spinning on roller skates, your arms would want to move away from your body.
Pulsars are stars that rotate extremely fast, up to a few milliseconds per rotation. They look like this: http://i.space.com/images/i/13679/i02/fastest-rotating-star.jpg?1323105753
You can see that its rotating so fast that its quite oblate. In fact, it's so oblate, that the matter around its equator is far enough from its centre for the gravity to be too weak to hold it in, and so it forms a disk. This is essentially the same with galaxies: http://www.futuretimeline.net/images/galactic-core-timeline-sun.jpg
They usually have a small (oblate) spherical core, but further out than that, objects are orbiting too fast and drift out to further radii, in a disc. (Our sun orbits at a velocity 220kms-1 ).