http://news.discovery.com/space/exotic-electroweak-star-predicted.html

Thanks a lot! My english is improving but there are still many errors – so correction always helps. And “enaugh” is a word I’m spelling wrong many times, I don’t know why…..

@ Tom Marking

I have no “real” answer for you, but I think that most neutron stars have masses of about 2 solar masses or more. I don’t know if it’s impossible to have a neutron star lighter than the earth, but I think this is VERY unlikely.

If anyone knows, are there any known cases of binary pulsars where the combined mass of the two pulsars is less than 2.8 solar masses? Also, what would determine the lower limit for the mass of a neutron star? Could you have one with the mass of the earth or smaller?

I’ve being doing some more checking and, yes, you’re right, but you have misspelled a couple of words more than once in your posts: it should be weird, not “weired”, and it should be enough, not “enaugh.”

Excuse me for being a nitpicker, but I used to be a proofreader — old habits die hard!

The “upper mass limit” that you mentioned, this is known as the Tolman-Oppenheimer-Volkoff (TOV) limit; there is some speculation that a quark star might be created if it has a density of between 2 to 3 solar masses, but any star of over 5 solar masses will inevitably collapse due to gravitational forces forming a black hole.

Yes, it does. If you intend to correctly model pulsar formation you will need a supercomputer, and even then, you are likely to be wrong because we don’t have a good model. For example, the EOS (Equation Of State) for neutron stars which links temperature, density, pressure, etc. together is pretty much not known to any high degree of accuracy. Also, angular momentum is not conserved since mass is ejected from the star during the supernova event.

Isn’t the radiation from pulsars synchrotron radiation caused by charged particles rotating around magnetic fields. The frequency of emission is:

Freq = B * q / (2 * pi * m)

B is the magnetic field strength in teslas
q is the charge of the particle in coulombs
m is the mass of the particle in kilograms

So for very high frequency radiation such as gamma rays the magnetic field is intense. The region where B is very large is close to the magnetic poles of the pulsar. X-rays will be generated farther out. Ultraviolet radiation still farther out, visible light even farther, and so on. Now, because the magnetic field lines curve around the geometry may be such that the gamma ray emitting region is aligned with our line of sight but the visible light emitting region is not.

Or another alternative is that the gamma ray emitting region has lots of charged particles which allow it to emit strongly whereas the visible light emitting region is relatively depleted in charged particles. So there are lots of explanations that could explain the data. Finding out which explanation is correct is, of course, more tricky.

You are both right that there is that threshold of 1.4 solar masses. But it doesn’t apply to neutron stars because there is a BIG difference in the formation process between a neutron star and a white dwarf which is limited by the Chandrasekhar limit.
Also during a type-B-Supernova (or core-collapse-supernova as those guys are called, too, when neutron stars or black holes form) things are really weired, so there is no need for a neutron star to be “so massive”.

If a core of a star burns out of fuel, that is it burns silicium to iron (not to mention that this process only takes few days – very short time!!), then iron-core finally collapses in a very short amount of time (some milli-seconds). The pressure is so high that the electrons are pressed into the protons to form a neutron and send out a neutrino. Although neutrinos do not interact very willingly with matter the density of the surrounding material is high enaugh that the cross section of neutrino-partical-interaction is rather big. So the neutrinos crash into the surrounding material and blow it off – the supernova explodes and leaves behind the degenerated core containing only neutrons.
But the degeneracy depends on neutrons and not on electrons which means that we have another upper mass limit, but not a lower limit. A neutron star can actually be lighter than 1.4 solar masses due to the different formation processes.

The Chandrasekhar limit is for electron degeneracy which does not apply for neutron stars but for white dwarfs. Those guys form rather non-violent compared to a supernova. If a light-weight star (like the sun) runs out of fuel the core contracts (very slowly ) and settles finally when the electrons degenerate. The outer layers of the star have been shed throughout the lifetime of the star leaving behind the core.

I hope I got these things right I didn’t looked it up, but this is what I remembered.

I still think it is amazing. Gravitation is always known to be “weak”, the weakest force at all. But finally it can overcome everything else if you don’t pay attention

It’s a pretty simple calculation really.

L = I * omega = I * (2*pi / T)

where
L is the angular momentum in kilogram – meter^2 per second
I is the moment of inertia in kilogram – meter^2
omega is the angular velocity in radians per second
T is the period in seconds

For a sphere of uniform density:

I = 0.4 * M * R^2

where
M is the mass in kilograms
R is the radius in meters

So combining equations we have:

L = 0.8 * pi * M * R^2 / T

For the sun:
M = 1.99E30 kilograms
R = 6.96E8 meters
T = 25 days = 2.16E6 seconds
L = 1.12E42 kilogram – meter^2 per second

The important point to remember is that L is conserved whether the sun contracts or expands so we have:

T = 0.8 * pi * M * R^2 / L

If the sun were to shrink to the size of a neutron star (i.e., radius = 12 kilometers) then it’s rotation period would be:

T = 6.43E-4 seconds

So it would rotate 1,555 times per second which is somewhat higher but still comparable to the fastest pulsars that have been measured.

Hope this helps.

I’m not sure about neutron stars but this has definitely been observed with white dwarfs that rotate – they can be over the Chandrasekhar limit of 1.4 solar masses.

http://en.wikipedia.org/wiki/Champagne_Supernova_(astronomy)

“However, the progenitor of SN 2003fg reached two solar masses before exploding, more massive than thought possible. The primary mechanism invoked to explain how a white dwarf can exceed the Chandrasekhar mass is unusually rapid rotation; the added support effectively increases the critical mass. An alternative explanation is that the explosion resulted from the merger of two white dwarfs.”

Since no one has caught this one, I thought I might mention it. I thought there was a minimum limit on the mass of a neutron star of 1.4 solar masses, not 1 solar mass. It’s called the Chandrasekhar limit. If the mass is below 1.4 solar masses then the object is supported by electron degeneracy pressure and becomes a white dwarf, not a neutron star.

To expand on Jose, no. All light, from radio to gamma travels at the same speed. A black hole will trap all light, a neutron star will trap no light. I’m not positive, but it may be possible to have a gravitationally caused red shift. In that case you’d have the opposite: no gamma rays visible on the outside.

….. I thought neutron stars had to have a minimum mass above the Chandrasekhar Limit of 1.44 solar masses. Or have I got something wrong?

I suspect that the answer will turn out to be something like what Rob Keown suggested: the gamma ray beam is slightly wider than the radio beam, and we’re juuuuuust catching the edge of the beam.

But any way you fry this, it is still cool.

(grabs coffee and ingests large quantity)

GAry 7

I was just wondering, if there could be a neutron star who is too massive to be stable (so the Pauli principle cannot achieve enaugh pressure against gravitation) but gains is stability through its rotation.

Anne, I think, too, that it would be quite unstable. But in theory such a neutron star could be out there – and if only for a short time…. ah, neutron stars, wounderful weired objects

If it could, would it be the stiffest material (theoretically) possible?

If it could remain compacted, it would make one heck of an inertial energy storage device,,,with a compact core massing 100 kg, the core would be really small. Could drive my (electrically powered) Blazer 10,000 miles on a charge.

Just looking for a SciFi story idea,,,

Gary 7

Gravity is not constant, and this pulsar is able to hold everything back but gamma rays.
There, I said it. Now let the ‘gasping’ begin!

“Science without religion is lame, religion without science is blind.”
- Albert Einstein, “Science, Philosophy and Religion: a Symposium”, 1941

Believe it or not, you can still have both. It doesn’t have to be black or white. Now Einstein wasn’t known to be a religious man, but I think he said it well with that one quote. Thats why some of us are still in our right minds.