View Full Version : Light from neutron stars


Shenzhou
01-27-05, 01:39 PM
It's my understanding that electrons play a major role in the emission of light from hot black bodies. Could somebody elaborate or correct me because I'm puzzled as to how - if this is the case - a neutron star can shine?

Maddad
01-28-05, 03:37 PM
You may be mixing two separate ideas. The black hole is black because within its event horizon light cannot escape. The neutron star is not black because its gravitational field, while extremely strong, does not reach that of the black hole. Light is able to escape the neutron star, so we are capable of seeing it.

Yuriy
01-28-05, 04:19 PM
Shenzhou,
You are right: the major source of light (and/or X-rays and gamma-quanta) from "hot black stars", like pulsars (I like this definition - they indeed are very hot and very ... dark, because of absence of any essential isotropic shining), indeed is caused by the synchrotrionic iradiation of electrons along pulsars magnetic axis.

Shenzhou
01-29-05, 04:45 AM
No, I'm not confusing black holes with neutron stars. I'm not talking about gravity, but about black body radiation from neutron stars.

Yuriy, does that mean that we can't see neutron stars unless they're pulsars and their beam sweeps across the Earth? What is isotropic shining? Is synchrotron radiation the only radiation that they emit?

Edit: Neutron stars can be seen even when the Earth is not within the cone of their jet. There's a picture here (http://imagine.gsfc.nasa.gov/docs/science/know_l1/neutron_stars.html) but what is the mechanism by which they emit radiation (black body radiation, not synchrotron radiation) if they do not contain any electrons?

Yuriy
01-29-05, 05:58 AM
Shenzhou,

What is isotropic shining?
Shining with the same intensity in any direction. Like any usual stars does, as Sun does.

Yuriy, does that mean that we can't see neutron stars unless they're pulsars and their beam sweeps across the Earth? Is synchrotron radiation the only radiation that they emit?
No, it does not. Because pulsar actually have irradiation in any direction, but it is absolutely miserable in comparison with synchrotronic one along its magnetic axis.
Synchrotronic radiation appears due to deceleration of the electrons (or any other charged particles): when charge suddenly changes direction of its motion or its speed it irradiates a huge amount of photons. Recall the polar shining in sky of Earth. It happens just due to effects I mentioned.
Because pulsars have a huge magnetic field its synchrotronic irradiation of electrons is also huge.
Yes, we would not see them as pulsars if by accident their beam would not sweep across the Earth. We actually see a lot less pulsars than nature has them exactly by this reason.

Shenzhou
01-29-05, 06:24 AM
I understand about the necessity of having a neutron star's jet aligned with the Earth for it to be recognized as a pulsar, but this isn't what I'm talking about. You said that neutron stars radiate in all diretions. I thought as much, but I want to know the mechanism behind this radiation. I am assuming that it's temperature-dependent black body radiation - like that emitted from stars and incandescent light bulbs. The thing is, I thought that such emission involved electrons (photons being the manifestation of an oscillating electromagnetic field). Neutron stars, by definition, consist entirely of neutrons. So what's the origin of this "isotropic" light?

Does my mistake lie in my understanding of the process by which hot bodies emit light?

Yuriy
01-29-05, 10:29 AM
Shenzhou,
Any pulsar is far from having only the "black body" spectrum: there are a lot electrons, protons. No matter that gravitation there is huge, the neutrons still experienced a beta-decay at a measurable rate. So, there is even some anti-neutrino irradiation too. Charged particles collide and accelerate into fields of each other; so, there is a lot of gamma-quanta irradiation too. All those processes keep spectrum of radiation of pulsars far away of a simple black body spectrum, in principle... Real rates of each type of radiation I do not remember...
As you understand, I answer your questions "as I remember". I do not do your work for you: searching Internet for process you like to now in details. Do it, ask Internet for key words "pulsars, irradiation, electrons, protons" and you will find a lot very good explanations and newest facts on it... I only give you a guiding path to right understanding, not full and exact theory and facts... If you will find there something tough or suspicious ask here we will discuss it together…

Shenzhou
01-29-05, 02:31 PM
Yuriy,
I think you're misunderstanding me when I say "black body radiation". I'm not talking about gravity or black holes, but rather the electromagnetic radiation emitted by an ideal absorber and emitter. See this website:

http://galileo.phys.virginia.edu/classes/252/black_body_radiation.html

What puzzles me is the process by which neutron stars emit black body radiation given the necessity for charged particles, which I thought that neutron stars lacked.

Yuriy
01-29-05, 03:05 PM
Shenzhou,
I understood you very well. It is you who mixed notions...
Black body radiation is very specific spectrum of radiation that is created ... by black bodies. Notion "Black body" is very special: it is body that absorbs all falling radiation and emits only some very specific spectrum that is totally defined by the thermodynamic equilibrium of radiation inside that body. Good approximation of "black body" is a thermostatic box with small hole on its one wall. The radiation from this hole very well represents spectrum of "black body". This spectrum depends only upon temperature at what this "black body" is kept. So, you can not have any "black body" spectrum from any star, but some of them can have spectrum close to such. Particularly, spectrum of neutron stars (not focused radiation along magnetic field's axis!) is close to such one, BUT IT IS NOT "BLACK BODY'S" SPECTRUM. Your words

What puzzles me is the process by which neutron stars emit black body radiation ... is wrong assertion about neutron stars: they do not emit black body radiation in any sense!
P.S. And I mentioned gravity only to catch your attention that betta-becay of neutrons under strong gravitational field is changing dramatically, so, any consideration of this process as a SOURCE of RADIATION, should keep it in mind... Remember your question about what radiation pulsars create? Particularly - (anti)neutrinos ... Simply we can not detect them yet...

geodesic
01-29-05, 04:05 PM
...charged particles, which I thought that neutron stars lacked.The structure of a neutron star is actually more complicated than just being composed of neutrons:
http://en.wikipedia.org/wiki/Neutron_star

superluminal
01-29-05, 10:12 PM
Yuriy, just a small point. Earlier you said:


Synchrotronic radiation appears due to deceleration of the electrons (or any other charged particles): when charge suddenly changes direction of its motion or its speed it irradiates a huge amount of photons. Recall the polar shining in sky of Earth. It happens just due to effects I mentioned.

Are you talking about the auroras? If so, I was taught that they are due to energetic charged particles of solar origin colliding with and exciting rare nobel gasses high in the upper atmosphere, causing them to fluoresce. Also wouldn't it take much more energy and at a much higher energy gradient than that contained in earth's feeble magnetic field to cause particles to emit any significant synchrotron radiation?

Yuriy
01-30-05, 01:22 AM
Superluminal,
What I did is I brought to ones attention the fact that charged particles are trapped in a magnetic field so they mostly get be screwed on the magnetic field’s lines. The Polar Auroras become possible exactly because of charged particles (mostly protons and electrons in so called Radiation belts of Earth), being trapped by the Earth magnetic field, are “falling” on the Earth atmosphere and causing Northern Lights. For us was important not the mechanism of appearance of visible light, but fact of motion of charges in the magnetic field. Now imagine situation in pulsar: super high magnetic field, which will cause trapping of every moving charge. Therefore all electrons and protons in pulsars should be in constant motions, screwed on the magnetic lines, i.e. there are strong fluxes of the charged particles to the poles of star. What happens with those charges in vicinity of poles? It very much depends on structure of pulsar. They can collide with iron core, or turn along stars surface, or some more complex case… In any case they will experience a huge longitudinal and transversal accelerations and consequently – will irradiate a huge flux of photons (mostly due to two major mechanisms: due to as we call it in Russian – “brake irradiation”, and/or due to so called cyclotron irradiation. Because speeds of charges in pulsars can be relativistic ones, this last will be actually a synchrotron irradiation).
Two small notices:
1. “Solar charged particles” are essential only because they are the most important source of charged particles in Radiation Belts of Earth. Auroras will appear no matter what is source of charged particles in those Belts.
2. Excitation of “Nobel” gases important for the “coloring” of Auroras, not for their existence…

superluminal
01-30-05, 10:49 AM
Yuriy,
I don't care what the context of your statement was. It was incorrect and misleading. A simple "Yes you are correct and I was wrong" would have done. Your post, again:


Synchrotronic radiation appears due to deceleration of the electrons (or any other charged particles): when charge suddenly changes direction of its motion or its speed it irradiates a huge amount of photons. Recall the polar shining in sky of Earth. It happens just due to effects I mentioned.

This clearly states that you attribute auroras to "synchrotronic" radiation. Synchrotron radiation plays no role in auroral displays.

As to your two notices:


1. “Solar charged particles” are essential only because they are the most important source of charged particles in Radiation Belts of Earth. Auroras will appear no matter what is source of charged particles in those Belts.


Obvious. All I said was that Earth's auroras are due to particles of solar origin.


2. Excitation of “Nobel” gases important for the “coloring” of Auroras, not for their existence…

Absolutely not true. Molecules of oxygen and nitrogen (correction - not "nobel gases") are excited (outer shell electrons) to higher energy states, and upon decaying back to their lower energy states give off visible photons, typically in red and green wavelengths. They exist only because there are atmospheric gasses present. Planets without atmospheres do not have auroras.

Addition:
Polar glows around pulsars, or whatever, my be due to synchrotron radiation from charged particles spiralling around intense magnetic field lines at very high speeds, but they are not auroras.

Yuriy: I know you know this. It is just for clarification.

Yuriy
01-30-05, 12:32 PM
superluminal,
Just for clarification and in fashion of your last post to me:
1. I do not care what you care or not care about.
2. I re-read my piosts in this thread and found that all my thought are correct and well explained, so , there is no need of any further explanations.
3. All your position is based on assumption that I was pretended in my analogy with Northern Light to give or gain ... of some explanation ... of mechanism of aurora lights on Earth. This assumption is wrong and I already explained it once here. All what this analogy should support was the similar type of motion of charged particles trapped by the magnetic field: why uniderectional, why from pole to pole, why synchrotronic. This is what the present thread is about...
4. BTW, "Noble gases", you have mentioned, indeed play a role in coloring Auroras (espesially argon) no matter what you think about it...
5. Nobody except you was connecting auroras with pulsars rotating beams, due to which we identifying these stars "pulsars". But, you'll be absolutely wrong if you will be thinking that on the pulsars auroras can not be realysed. But this thread is not about that...

superluminal
01-30-05, 01:01 PM
Yuriy,
If anyone else in this forum even remotely uses an analogy that is even microscopically off, you jump all over them. You made a statement of fact and it was wrong. You assume we are all stupid if we don't explicitly state every detail. Of course auroras could be present on pulsars or any object with magnetic fields, gasses, and charged particles. The primary gasses that contribute to auroral displays are molecular oxygen and nitrogen. We are not fools. Any other gasses could and certainly do play a role in aurora coloring. And I fully understand the difference between auroras and beams of radiation emanating from the poles of pulsars.

The problem is that you cannot gracefully admit an error. You are very arrogant. Are you ever wrong oh great one?

Xgen
01-30-05, 01:07 PM
It's my understanding that electrons play a major role in the emission of light from hot black bodies. Could somebody elaborate or correct me because I'm puzzled as to how - if this is the case - a neutron star can shine?

The processes that leads to irradiation of a black-body is different then the processes in neutron stars. But it is interesting question, is the same approach that had been applied to the black bodies can be applied to neutron stars? Maybe it had been done, if you found some work in the Net about it I will be glad to discuss it with you .

Anyway, the irradiation of black-body is described by the Planck equation and it is an irradiation of the photon-gas in the black-body, that is why it has a Bose-Einstein distribution. A gas from electrons or neutrons will have Fermi distribution. So may be you are not correct to say that electrons plays the major role in the black body distribution.

Yuriy
01-30-05, 01:32 PM
superluminal,
saying

The problem is that you cannot gracefully admit an error. You are very arrogant. Are you ever wrong oh great one?
you are mixing a small lie+a small verbal abuse+a little sarcasm... But I do not care... live as you can...
Answer is: "Several time in this forum I did errors, people pointed on them and my responce always was the same - Thanks, I already fixed it."
Error does not need discussion (if it is not an error that is more valuable than dozens of wright answers ... from textbooks. I met such errors a couple times in my life - each of them was a beginning of a new knowledge).
How I can be arrogant if I am spending so much time ... discussing some stuff with you? Arrogant people will not do that...

superluminal
01-30-05, 01:37 PM
Your response from the "Hypothetical Question" trhead:


you said in thread on pulsars?
Now here you have the same problem: the answer on your pathetic question

Need I say any more?

geistkiesel
01-31-05, 11:38 PM
You may be mixing two separate ideas. The black hole is black because within its event horizon light cannot escape. The neutron star is not black because its gravitational field, while extremely strong, does not reach that of the black hole. Light is able to escape the neutron star, so we are capable of seeing it.
Maddad, Hawking found that black holes are brilliant/shining . At the effective horizon of the BH 9off the surface) electron/positron pairs are produced with one being drawn ino the BH the other accelerated away. Also, Hawking, claims that the probability for v > c (amplitude (?)) for particles caught in a BH have a value > 0. How come the special relaivity theorists haven't tarred and feathered Hawking by now? Maybe it is because big dogs do the tar and feathering not the other way around.

Maddad, do you know if the photons generated from the neutron star are supposed to be from electron activity?

I dunno myself and frankly I question much of Hawking's physics.
Geistkiesel