Oct 17 APOD: Short Gamma-Ray Bursts

pauln · Post by **pauln** » Mon Oct 17, 2005 5:52 am

Okay, I feel kind of funny asking questions like these about an artist's conception of a cosmic event. I'm not trying to be picky... the pictures excited my curiosity and I seek enlightenment.

1) The two neutron stars are depicted as being basically spherical and devoid of any material exchange such as you would expect from coalescing stars. Does this reflect the neutrons being bound within the star by nuclear forces, which are very much stronger than gravity?

2) The stars are also depicted as being quite dark with brighter, reddish areas. Is it portrayed this way to indicate extreme gravitational red-shift from the emitted radiation?

All input would be gratefully accepted.

Cheers,
Paul.

makc · Post by **makc** » Mon Oct 17, 2005 6:32 am

Some previous input here, here and here (in reveresd order of appearence).

I was about to lock this topic (as it made third one on short GRBs), but it seems to be about today's APOD, so... let it be.

craterchains · Post by **craterchains** » Mon Oct 17, 2005 3:10 pm

Maybe we should have a "Ask the Artist" thread besides ask the Astronomer?

ta152h0 · Post by **ta152h0** » Mon Oct 17, 2005 4:33 pm

There was a one hour report on NASAtv about this with the last week where the actual scientists presented the data and answered questions. And " poof ", the picture appeared in APOD a couple days later. It appears APOD is sharing thru pictures the wonders of the universe and the purpose of the " discuss " forum is to further analyze subject pictures, along with teaching the unkowing some " knowing ".

ta152h0 · Post by **ta152h0** » Mon Oct 17, 2005 4:35 pm

and some spelling skills

S. Bilderback · Post by **S. Bilderback** » Tue Oct 18, 2005 12:48 am

From what I understand about neutron stars, the artist's concept could be a fair representation. They can vary is size and they start hot and cool over time. If they had any type of atmosphere it would be a very thin, dense layer and be extremely hot giving off high-energy radiation ionizing any surrounding gasses falling in. If I were to spend the time to create an image depicting a view of two neutron stars just before a collision, I would create a spectacular, sensationalized one - who wouldn't.

aif@comcast.net · Post by **aif@comcast.net** » Tue Oct 18, 2005 11:47 pm

solid bodies will break up once they pass a certain minimum distance (i think it is called the roche limit for moons and planets, named after the first astronomer to calculate it). the question is whether that would apply to neutron stars. how does neutron star degenerate matter behave? if it acts as ordinary solid matter, is it so dense that its attraction to itself makes the roche limit much smaller? or does it act as a gas/liquid, like the sun's corona or flares?

pauln · Post by **pauln** » Wed Oct 19, 2005 3:26 am

It's all very intriguing! There is a good (but possibly ageing) newbie intro to Neutron stars here:
http://www.astro.umd.edu/~miller/nstar.html

I'd read it previously and re-read it again before posting my original question here. Plenty of information about the internals and dynamics of neutron stars, but nothing specifically relating to colour/luminance/temperature relationships (ie the physical appearance of the neutron star), and nothing relating to gravitational interactions between neutron stars.

I waded through a lot of the "more input" suggested by makc and branched out into some of the other apparently promising threads... to no avail.

So I'm still left with the original questions...

Can neutron stars really be dark?
How strong are they? or maybe What is the Roche limit?

S. Bilderback · Post by **S. Bilderback** » Thu Oct 20, 2005 1:53 am

Your questions aren't easy to answer, a newly formed neutron star has a temp that can run in the millions degrees C, but that could be related to near by gasses being drawn in or the radiation given off as a remaining atmosphere collapses.

I haven't seen any research that would venture to put a value on the temp of the neutron core. There isn't enough known about most types of exotic matter, the neutron core could be at millions degrees C, or it could be that the density is so high that the neutrons cannot have any K-energy equaling absolute zero.

On the other end of the spectrum, there have been observations of old, very cold neutron stars, I would speculate that the appearance of neutron stars could very widely dependant on their age, size, and environment. On a close approach of two neutron stars, I doubt very much they would remain spherical.

Empeda · Post by **Empeda** » Thu Oct 20, 2005 8:31 am

My personal opinion is that the two neutron stars would have to be extemely close to each other in order to disturb one another - their immense gravity overcomes electron degeneracy pressure and therefore forms a star of neutrons held together by the strong nuclear force which is millions of times stronger than gravity.....

I don't know for sure though.

makc · Post by **makc** » Thu Oct 20, 2005 11:46 am

Empeda wrote:...forms a star of neutrons held together by the strong nuclear force which is millions of times stronger than gravity.....

*cough isn't the whole idea of neutron star based on that nuclear forces which hold it from collapse are exactly equal to gravity? and the idea of black holes is that for massive objects gravity overcome those forces?

Empeda · Post by **Empeda** » Thu Oct 20, 2005 3:52 pm

It wouldn't be exact no - but I think I might be confusing myself here

let me think this through a sec *warning: train of thought coming up*...

The thing stopping the gravitational collapse of a neutron star is neutron degeneracy pressure which isn't the strong force - it's to do with quantum energy levels and spins and such like.

This degeneracy can be much greater than the gravity of the star - it is not an outward 'force' it's a pressure that prevents collapse.

I think what I was implying was that if a star has overcome electron degeneracy and collapsed into neutrons - the neutrons are sufficiently close enough for the strong force to come into play.

The strong force is attractive, not repulsive. So the strong force would glue the neutrons together - this force is millions of times stronger than gravity. Since the strong force has nothing to do with the 'balance' of the star, there's no reason why it sould equal its gravity.

Do you see how I'm thinking? Like I said, this is a train of thought of mine and could be completely wrong - if there's any compact object experts out there feel free to rip it to shreads....

pauln · Post by **pauln** » Thu Oct 20, 2005 9:46 pm

At this point, I'd like to say thanks very much to you guys for discussing the issue

Don't stop!

The issues you've raised are exactly the ones that caused me to post the topic in the first place. If the picture portrayed ordinary stars in collision then I'd have just shrugged it off as an inadequaely briefed artist! Of course the bodies would deform, and of course they wouldn't be dark (unless they were brown dwarfs).

But these are neutron stars, so I didn't know. I wondered if it were possible that extreme red-shift could possibly make them appear dark... I didn't think about ageing and cooling making them dark. Thanks S. Bilderback.

And now I'm learning about the issue of how rigid the neutron star is. Thanks Empeda and makc. It would seem to come down to the issue of whether the (almost) entire body of a neutron star is truly at nuclear density, and so bound by the strong nuclear force. Is that possible? Wouldn't that make it just a huge atomic nucleus? And I thought that as nuclei grow progressivly larger than Iron they become increasingly unstable and break down through fission. By that argument, shouldn't a neutron star fizz almost instantaneously?!?

Now I'm waaay off my own thread... sorry guys

S. Bilderback · Post by **S. Bilderback** » Fri Oct 21, 2005 12:35 am

That's a wonderful and closely related question about the state of matter in the core of a neutron star, it has me thinking on a new tangent. Recent developments of grand unification models have suggested that a neutron in a nucleus might decay with a half-life of 10^32 years, but a free neutron has a half-life of only about 10.3 minutes. With the exponentially large number of neutrons in a neutron star, what would happen to the energy of the decaying neutrons? The inner core of a neutron star might be very hot but trapped or the neutrons might be cold and in a stable state, never to decay.

When you figure it out, let me know!

Edited . . . Oooops!

makc · Post by **makc** » Fri Oct 21, 2005 7:05 am

S. Bilderback wrote:...a free proton has a half-life of only about 10.3 minutes.

*cough, *cough... I was just thinking about hydrogen...

Empeda · Post by **Empeda** » Fri Oct 21, 2005 2:12 pm

pauln wrote: And I thought that as nuclei grow progressivly larger than Iron they become increasingly unstable and break down through fission.

That's true - but that has a lot to do with the increase in PROTONS will over a distance greater than the strong force can operate (which is very very small) will repelled each other.

A neutron star is all neutron - i.e. not electrical charge. This is because the electron degeneracy of an atom has been overcome, and the electrons have been squashed into the protons - which form neutrons. So basically yes, neutron stars can be thought of as giant atomic nuclei - it's a comaprison I've heard many a time.

S. Bilderback · Post by **S. Bilderback** » Fri Oct 21, 2005 11:25 pm

Remember, it is only a theory that a neutron star is composed of only neutrons, we could all be wrong and it might be another state of matter or a mix of different matter-states all together.

Sorry, I like thinking outside the box - that's where the action is!

Empeda · Post by **Empeda** » Mon Oct 24, 2005 8:24 am

Very true.. sorry should point that out... The models are pretty convincing but it is still very much theory.

Starship Asterisk*

Oct 17 APOD: Short Gamma-Ray Bursts

Oct 17 APOD: Short Gamma-Ray Bursts

GRB

grb

shape of colliding neutron stars