Neutron star question (again)

Factual Questions
1

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.

2

The most famous binary pulsar is the PSR 1913+16, 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, the shortest orbital period observed is 95 minutes.

3

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

This site 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?

4

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.

5

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.

6

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).

7

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).

8

???

Not sure I get that. A neutron star is the last stage of star before you get to a black hole (unless Quark Stars 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).

9

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.

10

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).

11

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.

12

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

13

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.

14

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.

15

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.

16

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.

17

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.

18

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?

19

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 ~

20

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?