Neutron star question (again)

[DarrenS]

I've read somewhere, maybe on this very board, that it is thought that some neutron stars orbit each other in pairs, or orbit a regular star, with a period of just a few seconds? Also, I've heard that pulsars are thought to be neutron stars rotating incredibly quickly, pulsing at a fraction of a second in some cases, like a radio frequency lighthouse.

My question is, what the heck would one of these things look like if you approached it from a 'safe' distance? (assume there is a safe distance from which you can observe it) I can't imagine something that huge rotating so quickly.

[scr4]

The most famous binary pulsar is the PSR 1913+16 (http://astrosun.tn.cornell.edu/courses/astro201/psr1913.htm), but the orbital period is 7.75 hours. It still loses measureble amounts of orbital energy in the form of gravitational waves. (In fact, this is considered the first concrete proof of the existance of gravitational waves, worthy of a Nobel prize.) As the pair loses energy they will come closer, and the orbital period will become shorter, until they finally collide. That should happen in about 300 million years. I don't have the numbers with me but by the time the orbital period has shortened to a few seconds, I think it's only minutes or seconds away from collision. You don't want to be anywhere close when that happens.

According to this list of binary pulsars (http://www.johnstonsarchive.net/relativity/binpulstable.html), the shortest orbital period observed is 95 minutes.

[DarrenS]

I was sure I'd seen that there were pulsars with an orbital period of the order of seconds.

This site (http://antwrp.gsfc.nasa.gov/apod/ap951122.html) says that the Crab Nebula Pulsa flashes about 30 times every second - doesn't this mean that the central pulsar must be rotating incredibly quickly?

[scr4]

Yes, the Crab pulsar rotates 30 times a second. But rotational period has nothing to do with orbital period. The earth rotates once a day and orbits the sun once a year.

[DarrenS]

Thanks for the clarification, scr4 - yes, I was straying dangerously close to mixing up the concept of rotational period with that of orbital period. I understand they are unrelated - in fact, I believe that on Venus the rotational period is longer than the orbital period - the day is longer than the year.

Still, I would love to see what a relatively big object like a pulsar (ok, I know it's smaller than the sun - but a big thing anyway?) rotating at 30 times per second would look like from a reasonable distance.

[Whack-a-Mole]

Originally posted by DarrenS
Still, I would love to see what a relatively big object like a pulsar (ok, I know it's smaller than the sun - but a big thing anyway?) rotating at 30 times per second would look like from a reasonable distance.

It probably wouldn't look like much. Spin a glass with water in it and get the water moving rapidly. Does that look interesting? Ok...it's a pretty flawed analogy but honestly I don't think you'd notice much about the spin although observing a Neutron Star up 'close' would probably be a pretty cool thing in its own right.

As for being big a Neutron Star is actually pretty small. About ten miles or so in diameter. Big on a human scale but pretty darn small overall (smaller than many cities). Its fast rotational period is easy to understand in principle. Just like an ice skater spinning faster when they pull their arms in so to does a star increase in speed when it shrinks. Our sun has a diameter of something like 860,000 miles...those are some mighty long arms to pull in so you can see where the incredible rotational speed comes from (and in fact most Neutron Stars come from suns larger than our sun...longer arms yet).

[scotth]

DarrenS, not only is it smaller than the Sun, it is smaller than the Earth. In fact, without doing the actual math, I would expect by the time a neutron start approached 50 miles in diameter it would have enough mass to collapse into a black hole instead.

What you would probably see is a featureless ball some 10 miles or so across spinning very quickly. Of course, it is hard to tell if a "featureless" ball is spinning. As long as you stayed out of the arc of the magnetic poles rotation, you could probably get as close as 1 AU pretty safely.

I don't know exactly how wide or narrow the spread of radiation from a neutron star is, but as long as you stay clear of that and orbit at a distance that is far enough away to avoid dangerous amounts of tidal forces, you should be able to look at it (with a telescope).

[Whack-a-Mole]

Originally posted by scotth
In fact, without doing the actual math, I would expect by the time a neutron start approached 50 miles in diameter it would have enough mass to collapse into a black hole instead.


???

Not sure I get that. A neutron star is the last stage of star before you get to a black hole (unless Quark Stars (http://www.nature.com/nsu/020408/020408-8.html) turn out to be real...at something like 7 miles in diameter). Basically I thought a Neutron Star didn't have quite enough matter to continue its collapse to a black hole.

I wonder if the binary scr4 mentioned would become a black hole when the two stars eventually collide? THAT would be cool to watch although I agree you wouldn't want to be anywhere near that thing when that happened (and by near I think a lightyear might be too close but I couldn't say for sure).

[scotth]

Originally posted by Whack-a-Mole


???

Not sure I get that. A neutron star is the last stage of star before you get to a black hole (unless Quark Stars (http://www.nature.com/nsu/020408/020408-8.html) turn out to be real...at something like 7 miles in diameter). Basically I thought a Neutron Star didn't have quite enough matter to continue its collapse to a black hole.


A neutron star has a pretty well established density. What I was saying is, that a neutron star has a maximum possible geometric size because..... As the neutron star increases in diameter it is increasing in mass. There is an upper limit on the mass which imposes an upper limit on the size if the density is constant.

[Whack-a-Mole]

Ahh...I get you now scotth. I was thinking you somehow meant that once a star passed 50 miles in diameter while shrinking it was destined to be a black hole. I see what you're saying now is if you added enough mass to grow the Neutron Star to 50 miles in diameter then it'd have enough mass to collapse into a black hole (although I'm not sure how that'd work because as you added mass density would increase so while you'd be growing the Neutron Star you'd be shrinking it at the same time...into a Qurak Star perhaps).

[scotth]

I don't think density would increase (at least not much)... for it to increase further it would have to turn into something else like a quark star (if they exist) or a blackhole.

[bryanmcc]

A neutron star would probably look like a giant ball bearing. It would be almost completely smooth -- the highest "mountain" on its surface would probably be a thousandth of a millimeter -- and, as has been stated, it would be spinning increadibly fast but that would be hard to tell visually. I'm guessing that its density would make it reflect most light, giving it the appearance of polished metal, but couldn't really say for sure. Initially it would be hot enough to be glowing white, but would eventually cool off over hundreds of millions of years.

-b

[scotth]

I've thought about the shiny aspect too and didn't really get anywhere.

If the neutron star actually had exposed neutons (neutrons stars are expect to have some thickness of normal matter on their surfaces), I can't imagine how light would interact with it. Light only couples with charge carrying particles. Neutrons don't have a charge. Would it be transparent? Doesn't seem likely to me, but I don't know.

The normal matter on the surface is what would really be seen, and I guess we would have to know what it was specifically to know its optical properties.

[Omphaloskeptic]

Originally posted by scotth
I've thought about the shiny aspect too and didn't really get anywhere.

If the neutron star actually had exposed neutons (neutrons stars are expect to have some thickness of normal matter on their surfaces), I can't imagine how light would interact with it. Light only couples with charge carrying particles. Neutrons don't have a charge. Would it be transparent? Doesn't seem likely to me, but I don't know.
I thought this at first, too, but sadly I don't think it's true. Neutrons do have a charge quadrupole moment, so photons and neutrons can interact. Neutronium isn't transparent.

[scotth]

Originally posted by Omphaloskeptic

I thought this at first, too, but sadly I don't think it's true. Neutrons do have a charge quadrupole moment, so photons and neutrons can interact. Neutronium isn't transparent.

Really? Does this arise from the charge of it's (the newtron) constituent parts becoming visible (to other particles) at very close range?


I've only really been through QED in any detail, so the behavior of things inside the nucleus is still pretty hazy.

[The Bad Astronomer]

I did some work on a pulsar that is orbited by a white dwarf. The name is UV1820-30, for those keeping score at home. The white dwarf whips around the NS every 11 minutes.

As I was looking at the data, and asking the scientist in charge about it, I began to realize what I was seeing. The ultraviolet light increases every 11 minutes, and it's because the part of the WD facing the NS comes into our view: that part of the WD gets heated by the NS and emits UV photons.

I swear, the hair on the back of neck stood up. Here was a white dwarf, probably about as massive as the Sun, being tossed around by a neutron star like it was a toy on a string. Not only that, but the white dwarf-- some of which are already incredibly hot-- was getting heated by the neutron star as if it were facing an oven; which, in reality it was.

The Universe is a fascinating place, full of stories. Amazing.

[scotth]

Dude, you need to do a "Good Astronomy" book. You could put short little stories in it like that.

1) I am sure you have more of those.
2) I am sure you could get more from people in your field.
3) Their fascinating.
4) You might get a bunch of the everyday joe's to read it.
5) You would entertain them with short little stories, digestable by about anyone.
6) It would sneak some true scientific facts and how they are discovered into their minds. (What a dirty trick.)
7) It might open a couple of minds to how amazing this universe really is and jumpstart that slumbering investigative nature.

Who else but you could publish the "Good Astronomy" book? And, it would pose such a good counter example to all the things covered in the first book.

BTW, I have spotted your name in the national media quite a number of times since your book has come out. You are getting dangerously close to famous.

[Khan]

So NS itself doesn't give off the pulses when the WD isn't blocking it? The radiation from the NS causes the WD to "light up" and give off radiation bursts when it's on the side of the NS opposite the observer?

In other words, it's kinda like the reverse of a satellite dish?

[Triskadecamus]

However degenerate the ordinary matter is on the surface of a neutron star, it will have at its outermost surface at least some matter able to reflect, and even absorb and emit normal light.

During the lifetime of a neutron star matter will accrete to it's surface, so even as it "eats" normal matter, converting it to neutronium, some film of degenerate normal matter, and perhaps even an atmosphere of denser elements in plasma form will remain. Such an atmosphere would of course be measured in millimeters from its surface, in all likelihood. Still, with a surface, and potential atmosphere of normal matter, what you would see is a dimly glowing ball.

But what of the fact that its light is red shifted from its escape trajectory? Gravity on the surface of a medium sized neutron star must be measured in billions of G. Ordinary light emitted at visual frequencies upwards faces a huge gravity well, although it must escape eventually. But what does it look like, when it gets out to more flat space? Obviously too dim to be seen at interstellar distances, but what about at Mars orbit distances?

The same thing about infalling light, in reverse. Your standard profile of starlight gets accelerated, that is, blue shifted the entire way in, having an excitation potential of hard gamma, and X rays by the time it hits the matter on the surface. Then think about the magnetic fields whipping by at many per second rates, each swing with the magnetic power of a whole star.

Probably quite a light show, and not all of it gets whipped out through the magnetic the poles, and surely some must be ordinary light, or higher energy stuff that gravity heterodynes into visible ranges. Yeah, probably scenic wonders for traveling spacemen, if their shields can take it.

Book me a flight! I'm game in as far as Mars orbit, if the engineering department thinks the old girl will handle it. Engage!

Tris
----------------------------
"In my opinion, there's nothing in this world, Beats a '52 Vincent, and a red headed girl." ~ Richard Thompson ~

[DarrenS]

Thanks for all the replies - this is fascinating stuff and it is great to have access to intelligent people debating it. There are a few comments on what would be the macroscopic physical properties of Neutronium, which I find equally fascinating. (This latter point has been discussed before - search the archives for 'Neutronium')

scotth: I totally agree that Bad Astronomer should write a book. I love the idea of putting astronomy into human terms - where, I realize it borders on science fiction. Questions exactly like this one - given appropriately advanced technology (ok, magic) - what would it be like to approach a black hole? A neutron star ? A neutron star orbitting a black hole? A binary star system? I understand how important is the study of the properties of the CMB and hypothetical properties of dark matter for example, but I am way more enthralled by descriptions such as Bad Astronomer's, above. Or the statement that someone once posted on here (might have been BA or Chronos) about how if you happened to be standing in the corona of the sun, you'd die of hypothermia - because even though the average temperature is very high, the heat density (hope that's the right term) is very low.

Are there any books like that already out there?

[scr4]

scotth: I totally agree that Bad Astronomer should write a book.

For those who don't know, he's already written one. (http://www.amazon.com/exec/obidos/ASIN/0471409766/qid=1002324596/sr=1-3/ref=sr_1_0_3/103-7192648-5486217/badastronomy) (I think scotth was suggesting he write a second one.)

[scotth]

Originally posted by scr4


For those who don't know, he's already written one. (http://www.amazon.com/exec/obidos/ASIN/0471409766/qid=1002324596/sr=1-3/ref=sr_1_0_3/103-7192648-5486217/badastronomy) (I think scotth was suggesting he write a second one.)

Quite right.

And in case it wasn't clear (it seemed clear last nite).... The book should be a collection of small personal discovery stories like the one he shared in this thread. More of his, and other ones collected from colleages.

[Whack-a-Mole]

Just as a thought experiment:

Assume you could strip away the outer layers of a Neutron Star and expose the neutronium underneath (and assume it would remain neutronium...I'm guessing that's the proper term for that material).

Would the Neutron Star be dark? That is, can neutronium glow? My understanding is you need electrons changing orbits to create light. In neutronium the electrons are all crammed into the nucleus of atoms so they can't be jumping about.

[scotth]

Originally posted by Whack-a-Mole
Just as a thought experiment:

Assume you could strip away the outer layers of a Neutron Star and expose the neutronium underneath (and assume it would remain neutronium...I'm guessing that's the proper term for that material).

Would the Neutron Star be dark? That is, can neutronium glow? My understanding is you need electrons changing orbits to create light. In neutronium the electrons are all crammed into the nucleus of atoms so they can't be jumping about.

Hah, busted Whack-a-Mole not reading the whole thread... (Just messin' with ya)... but that was my question above, just phrased a bit different.If the neutron star actually had exposed neutons (neutrons stars are expect to have some thickness of normal matter on their surfaces), I can't imagine how light would interact with it. Light only couples with charge carrying particles. Neutrons don't have a charge. Would it be transparent? Doesn't seem likely to me, but I don't know.

and the response:
I thought this at first, too, but sadly I don't think it's true. Neutrons do have a charge quadrupole moment, so photons and neutrons can interact. Neutronium isn't transparent.

[Whack-a-Mole]

Originally posted by scotth
Hah, busted Whack-a-Mole not reading the whole thread... (Just messin' with ya)... but that was my question above, just phrased a bit different.

I don't think so. I read your thread and the response and it seemed like it was asking how neutronium reflects light. I'm asking whether it can produce (or emit) light. It's weird to think of an incredibly hot ball being as dark as midnight which is what I was getting at.

[scotth]

Originally posted by Whack-a-Mole


I don't think so. I read your thread and the response and it seemed like it was asking how neutronium reflects light. I'm asking whether it can produce (or emit) light. It's weird to think of an incredibly hot ball being as dark as midnight which is what I was getting at.

Missed that finer point.... I had considered those in the same thought process in the past... I was considering that all interaction with light (by basis of a charged particle) would be solved together or not.

[cdhostage]

Wait a second. Assume that we have a sphere of nothin' but neutronium - let's assume there's no normal matter getting in our sightline. It has been a neutronium star for a relatively short amount of time (yes, I know it would attract anything nearby and convert most of it to neutronium w/ a layer of normal plasma - quiet). It is very very hot. I have no idea how hot, but the neutrons are probably bumping against eachother as vigourously as do the particles in the center of a star.

It is spinning at a fast rate.

What effects does this object have on its surroundings and what does it radiate?

[scotth]

I would love to understand this process as well. I feel certain that I will be hitting the books some more to prepare myself to understand the answer.

It is one thing to have an answer, and another to actually understand that answer.

I don't think I am really ready to understand it yet. If I was, I could probably deduce it for myself.

For example: "Neutrons do have a charge quadrupole moment"

Ok, but I don't undertand how.

[JS Princeton]

Neutron stars radiate in the radio. That's what a pulsar is.

[scotth]

Originally posted by JS Princeton
Neutron stars radiate in the radio. That's what a pulsar is.

No problem with that statement....

But, here is my confusion. Radio is still photons. Photons (to my understanding) only interact with charged particles. Neutrons have no charge.

If your statement is true (and I don't doubt that it is), it reveals to me that my understanding of photon-particle interactions is incomplete.

My first guess would be that the photos could interact with sub atomic particles that make up a neutron. All of which have a (fractional) charge. But, I still feel I have more studying to do to really grasp how this might happen or if indeed this is the correct answer.

[Whack-a-Mole]

Originally posted by JS Princeton
Neutron stars radiate in the radio. That's what a pulsar is.

So, by this can I assume (with the understanding of the dangers ass/u/me [mostly me] entails) that a naked Neutron Star would look black in appearance to my unaided eye? Maybe applying a color is wrong (although I'd be interested to know that too) I mean more that it won't illuminate anything for my mortal eyes to perceive.

[scotth]

Originally posted by Whack-a-Mole


So, by this can I assume (with the understanding of the dangers ass/u/me [mostly me] entails) that a naked Neutron Star would look black in appearance to my unaided eye? Maybe applying a color is wrong (although I'd be interested to know that too) I mean more that it won't illuminate anything for my mortal eyes to perceive.

I would assume that if it radiate in radio, that if it is hot enough it can also radiate in the visible spectrum as well.

I would also think that it was at least plausible that it may display a color in the light reflected/scattered by it..... or look like a mirror?

[DarrenS]

According to this (http://oposite.stsci.edu/pubinfo/PR/2000/35/pr.html) link, at least some Neutron stars aren't totally black:this is the closest and brightest of the few known isolated neutron stars Indeed, the fact that Hubble spotted it visually implies that the start is radiating at least some light in the visible spectrum. Also this is interesting. The object is very faint (26th magnitude or about 20 billion times fainter than the bright star Vega), and has a blue color.

[Whack-a-Mole]

Originally posted by DarrenS
Indeed, the fact that Hubble spotted it visually implies that the start is radiating at least some light in the visible spectrum.

Neutron Stars have material on their surface (degenerate matter I guess) that hasn't become neutronium yet. This stuff I would expect to be able to emit light. As previously mentioned though this surface layer is likely very thin so there isn't much to emit light even if what is there does so vigorously. My question was more on what you'd see if there were no outer, non-neutronium layer.

[scotth]

Originally posted by Whack-a-Mole


Neutron Stars have material on their surface (degenerate matter I guess) that hasn't become neutronium yet. This stuff I would expect to be able to emit light. As previously mentioned though this surface layer is likely very thin so there isn't much to emit light even if what is there does so vigorously. My question was more on what you'd see if there were no outer, non-neutronium layer.

I would expect several feet at least of (probably pretty dense) normal matter on the surface.

But you made the same point I was gonna, we were in a thought experiement concerning a "naked" neutron star, so the data wasn't strictly relevant.

[Omphaloskeptic]

Originally posted by scotth
Really? Does [the neutron quadrupole moment] arise from the charge of it's (the newtron) constituent parts becoming visible (to other particles) at very close range?

The short answer is yes. The details of baryon structure are still a field of research. The current models, AFAIK, don't agree with the observations very well, but that's the basic idea.

[Omphaloskeptic]

Originally posted by Whack-a-Mole
My question was more on what you'd see if there were no outer, non-neutronium layer.

I would expect (though I don't really know) that neutronium, like a normal material, has lots of modes above its ground state (like vibrational modes, etc.) that will be excited (in a thermal spectrum) if the neutronium is hot enough. I don't see any reason that there shouldn't be some decay modes with photoemission between some of these states, which is basically what thermal radiation is. Is there any reason this shouldn't happen?

[SPOOFE]

As the neutron star increases in diameter it is increasing in mass.
Hmm... actually, I came across a cite (http://www.astronomynotes.com/evolutn/s10.htm) a while ago - when studying neutronium for a debate - that said that degenerate matter - the stuff in White Dwarves and neutron stars - will decrease in diameter as it increases in mass. What's the real deal?

[scotth]

Originally posted by Omphaloskeptic


I would expect (though I don't really know) that neutronium, like a normal material, has lots of modes above its ground state (like vibrational modes, etc.) that will be excited (in a thermal spectrum) if the neutronium is hot enough. I don't see any reason that there shouldn't be some decay modes with photoemission between some of these states, which is basically what thermal radiation is. Is there any reason this shouldn't happen?

I was unaware that nuclear particles had "states". Electrons adopt states by being in different "orbits" around the nucleus. I don't know what is going on exactly with neutronium, but it would appear to me that most of what I know about normal matter wouldn't apply.

[scotth]

Originally posted by SPOOFE

Hmm... actually, I came across a cite (http://www.astronomynotes.com/evolutn/s10.htm) a while ago - when studying neutronium for a debate - that said that degenerate matter - the stuff in White Dwarves and neutron stars - will decrease in diameter as it increases in mass. What's the real deal?

Looking at the link:
Degenerate matter and neutronium are not the same thing. Degenerate is just highly compressed matter, atom still maintain their own identities.

Neutronium is a ball of neutrons. There should be zero empty space left in neutronium (or near zero). It really can't compress any more unless you actually start crushing the neutrons.

Granted, I could be wrong..... (clearly, I am at the limit and over the limit of my knowledge of physics at this point) But, it was my deduction that:

1) neutronium is nearly uniform in density.
2) If density is staying the same, adding mass will add size.
3) If we know an upper mass limit before neutronium will collapse into something else entirely (black hole or some other exotic entity)
4) we would also know the maximum diameter of a neutron star.

[Ring]

This site has a cross section of neutron star. The iron/electon crust is about 16km thick, and the neutrons are degenerate and superconducting.

http://zebu.uoregon.edu/~js/ast122/lectures/lec16.html

[scotth]

Now that is an intersting page, Ring. Might need to see if I can find some more stuff along that line.

[Ring]

As far as degeneracy is concerned neutrons and protons inside the nucleus are arranged in orbitals just as electrons are arranged in orbitals in an atom.

When a degenerate electron gas is formed in a white dwarf these orbitals are destroyed and the electrons only separation is due to the Pauli principle. As the pressure is increased the DeBroglia wavelengths of the electrons is forced to decrease and therefore their energy increases, which is what keeps the white dwarf from collapsing to a neutron star. However when the pressure increases sufficiently the electron motion approaches c and at that point the degeneracy pressure can no longer withstand the compression and the star collapses to a NS.

The same thing occurs with degenerate neutrons which BTW are also fermions

[SPOOFE]

Degenerate matter and neutronium are not the same thing. Degenerate is just highly compressed matter, atom still maintain their own identities.
Well, the site mentions how degenerate matter is any instance where particles are actually mashed together, and refers to neutron stars as containing degenerate matter.

Essentially, degenerate matter is in a state where the only force keeping it from collapsing to a singularity is the Pauli Exclusion Principle, I believe.

[JS Princeton]

Note that protons and electrons are also in neutron stars, though not as abundant as neutrons.

[FriendRob]

This link (http://zebu.uoregon.edu/~js/ast122/lectures/lec16.html) says

As with white dwarfs, neutron stars have an inverse relationship between mass and radius. As a neutron increases in mass, its radius gets smaller.

I think the reason is that with more mass, there are more energy levels available, so the neutrons can pack in more tightly.

The cited link also has a diagram of the structure of a neutron star.

I would think that in real life, most of the visible light coming from a neutron star would be radiated by the accretion disk of matter falling onto the star. This would probably be quite hot and have a black-body spectrum.

[Chronos]

For computing the orbital period, we can use Kepler's second law. In solar masses, AU, and years, the general form is (M1 + M2)P2 = a3, where the Ms are the masses of the two objects, P is the period, and a is the orbital radius (strictly speaking, the semimajor axis, which is a sort of average radius). Now, suppose we've got two neutron stars, just about to merge. Then the distance apart will be just over twice the radius of the stars, or about one ten millionth of an AU. Each one will have about two and a half times the mass of the Sun, for a total mass of 5 solar masses. So this gives us a period of 1.4*10-11 years, or a half a millisecond. of course, by this time, there isn't long to go before final merger.

What you'll get after the merger will definitely be a black hole. There's a very narrow range of masses where you can have a neutron star, and probably a narrower range yet for a quark star. Twice the mass of the lighest possible neutron star is easily more than the mass of the heaviest possible quark star.

[Polycarp]

Degeneracy is used in both a wide and strict sense.

The individual-nuclei-in-an-electron-"soup" degenerate matter of a typical white dwarf core is the strict usage.

Any compression of matter that results in the suppression of electron orbitals is the broad sense, and includes both strict degenerate matter and neutronium, and whatever you want to call a black hole as made of as well.