Pulsars are intrinsically cool.
Take a massive star. Let it cook for a few millions years. Then the core collapses, and the outer layers explode outward in a supernova. The inner part, the core, can become a black hole, or the almost-equally-bizarre neutron star, an object maybe 10 kilometers across, but packing the mass of an entire star.
The gravity of a neutron star is billions of times the Earth’s, and the magnetic field is even stronger. Not only that, but that sucker can spin: conservation of angular momentum means that it can spin dozens of times per second. The combination of the rapid spin and magnetic field means it sends out radiation like a lighthouse, two beams of radiation and matter that sweep around in a circle several times per second. When the beams sweep over the Earth, we see a little blip, a pulse, and that’s why these are called pulsars.
If the pulsar is in a close orbit with another, more normal star, its gravity can draw material off the star. This adds energy to the spin of the pulsar, speeding it up. Some, called millisecond pulsars, rotate hundreds of times per second. We’re pretty sure this idea is correct, because almost* every time we see a millisecond pulsar we see a companion star. Not only that, but the pulsar has to be so close to the companion star that its orbit is almost a perfect circle; the rules of gravity and tides ensure that.
But we’ve run into a problem: a millisecond pulsar has been found that has a wide, elliptical orbit. Worse, the companion star is the wrong kind.
The pulsar is dubbed J1903+0327, and it sits about 21,000 light years away, a fair chunk of distance. It’s orbiting a rather Sun-like star, and the orbit is odd: it’s too big, and too elliptical. All other millisecond pulsars have circular, small orbits.
The pulsar itself is more massive than usual for its type, too, which may be a clue to its origin… though astronomers really don’t know.
So what’s going on? Maybe we’re missing something… like a third star.
One idea is that there is another neutron star involved, one that isn’t pulsating so we don’t see it, and the two neutron stars (one pulsing, one not) orbit each other. That would make more sense to me. How would a system like this be created?
In this case, a few million years ago, it looked like this: we actually had three stars; two massive, and one not so massive. The two massive ones orbit each other close together, with the lighter one farther out. One of the big stars explodes as a supernova, leaving behind a pulsar. It draws material from the other massive star, speeds up its spin, and the orbit becomes a very tight circle. That first one is our millisecond pulsar.
Then, some time later, the second massive star explodes. The neutron star formed is a weak pulsar at best for some reason or another (it happens; not every neutron star is a pulsar). When the star exploded, it lost quite a bit of mass, and so its gravity decreased. When that happens, the orbits of the two stars becomes elliptical, a natural consequence of the mass loss. So now we have two neutron stars, one pulsing rapidly, the other one not, on an elliptical orbit. The third star doesn’t play a big role, except that it’s bright in visible light and it’s easy to see from Earth. The other neutron star is much fainter, and hard to see.
So now, today, when we point our telescopes at the system, we see a normal star apparently orbiting a millisecond pulsar, which doesn’t make sense. But that’s because we don’t see the second neutron star.
Is this the correct scenario? Beats me. It’s a guess, but a good one. Still and all, I love it when a good astronomical theory is tested by some individual weird object. It means there are more factors in play, and that means more fun.
*In the original version of this post, I left off the word "almost", but Ethan in the comments corrected me.
May 15th, 2008 at 12:43 pm
Missing a star? Man, I get upset when I can’t find my car keys.
May 15th, 2008 at 12:51 pm
Phil,
Sorry that I’ve got to chime in as a pulsar expert here, but all millisecond pulsars don’t have binary companions. While it’s true that about 80% of them do (as compared to about 4% for regular pulsars), there’s still a large number with periods below 25 ms (that’s the magic number to qualify as a millisecond pulsar) that have no binary companions.
A slightly older (but still mostly correct) writeup is available for free online here.
Ethan
May 15th, 2008 at 12:55 pm
I choose to think it’s a cosmic practical joke. Some very intelligent species in the past became capable of moving stars and realized that somewhere else in the galaxy, eventually, a species would arise that was capable of observing and understanding it’s surroundings. They realized that pulsars would be noticed (they do kinda stand out) and realized that their behaviour would be catalogued. Then, being a species of practical jokers, they messed with the orbit of just one neutron star, knowing that it would eventually be noticed and puzzle an entire species. Cheeky aliens.
Or maybe Phil’s idea is more plausible…. I’m not sure.
May 15th, 2008 at 12:59 pm
Ah, thanks Ethan. I corrected the post.
May 15th, 2008 at 1:20 pm
This post is awesome! I love reading about these things.
May 15th, 2008 at 1:27 pm
Oh, great. Another theory in crisis.
May 15th, 2008 at 1:40 pm
Wouldn’t the second star wreak havoc on the first pulsar when it goes nova so close to it?
May 15th, 2008 at 1:41 pm
This should be easy to test. Doppler shift should be evident in the milissecond pulsar, caused by the shift in speed due to the fact that it is orbiting the “quiet” neutron star.
If doppler shift is detected, we have a second neutron star.
If no doppler shift is detected, we are in trouble. Either there is a second neutron star, but we are seeing the system face-on, so we’ll never know, or we need a new theory!
May 15th, 2008 at 1:42 pm
Is it possible that this theoretical pulsar pair somehow captured the third, sun like star after the supernova blast? How far does a sunlike star have to be from a supernova not to be effected? What is the distance between the pulsar and sunlike star in this case?
As a further question, we know the damage a nearby supernova can do to our planetary ozone layer (I’m sure we’ll know a lot more when your book comes out BA), but what kind of damage should we expect on star of the system effected?
May 15th, 2008 at 1:43 pm
kx,
Don’t worry. Pulsars are very tough.
May 15th, 2008 at 1:55 pm
Andre,
but the pulsar would probably not stay where it is, wouldn’t it?
Unfortunately I don’t know enough physics to work out all the maths involved.
May 15th, 2008 at 2:15 pm
kx and Andre,
Sometimes supernova explosions are fairly symmetric (spherical) and sometimes they’re pretty asymmetric. Pulsars, when they’re created, can either get very small kicks (like the Crab Pulsar), or very very large ones (up to ~1000 km/s is not rare). They can get shot out of star systems or they can barely change their orbit.
While an explosion can impart a tremendous amount of energy, if there’s a companion not all that close, it may not be disturbed so much. Why? Because even though a lot of energy is released, the amount that reaches the companion star decreases with the distance between the supernova and the companion. Since not all supernovae are created equal (more massive=bigger explosion), it won’t necessarily eject anything.
There’s no universal rule for these things; there’s a lot of variety!
Ethan
May 15th, 2008 at 2:23 pm
Boggles the mind. This is what make science so great.
Interesting that it is us who admit that we don’t know everything.
May 15th, 2008 at 2:55 pm
Hmmm…”The pulsar itself is more massive than usual for its type, too, which may be a clue to its origin… though astronomers really don’t know.”
Maybe the pulsar and neutron star were orbiting so close that their orbits decayed and they merged into a single object (giving off lots of gravitational radiation in the meantime). That could possibly be a reason for the mass of the pulsar being larger than usual.
May 15th, 2008 at 3:10 pm
Quick Question- What is the difference between a ‘cold’ and ‘hot’ neutron star?
May 15th, 2008 at 3:13 pm
So, when the companion star dumps material onto the pulsar, wouldn’t the pulsar slow down, due to conservation of momentum? The extra mass should constitute an external force, shouldn’t it? Ah, but then if the material is rotating, it will transfer its angular momentum to the pulsar, yes? So the pulsar will speed up. Right, got it. Thanks for your help.
May 15th, 2008 at 3:13 pm
And since angular momentum is proportional to r^2, it will cancel out any decrease in speed. Does that even apply in a rotating system?
May 15th, 2008 at 3:23 pm
What if we’re just not in the plane of fire for the second pulsar’s “lighthouse beams”?
May 15th, 2008 at 3:27 pm
From http://en.wikipedia.org/wiki/Pulsar
“Pulsars are highly magnetized rotating neutron stars which emit a beam of detectable electromagnetic radiation in the form of radio waves. …The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name.”
May 15th, 2008 at 4:23 pm
Is there any chance that the third object could be a black hole?
May 15th, 2008 at 4:49 pm
Maybe the system is quite young, and the companion hasn’t had time to circularize its orbit? Also, couldn’t infidel have a point, about Earth not being in the line of the pulsar’s beam?
May 15th, 2008 at 5:27 pm
Very weird, indeed. I can’t even begin to imagine what’s responsible for this weirdness. Well, maybe I could.
BTW, given the spin rate of these objects Phil, would it be too much to make a new amusement park ride called “The Pulsar”?
May 15th, 2008 at 6:57 pm
I am rather skeptical of this explanation given for pulsars. You want me to believe that there are objects:
1.) made out of some theoretical type of matter
2.) has the mass of a star
3.) and is rotating faster than a dentist drill
even though one out of every five examples of this type of pulsar does not have the hypothesized mechanism for speeding up its rotation?
There has to be a better explanation for what we are observing!
May 15th, 2008 at 7:13 pm
It isn’t exactly a ‘hypothesized’ mechanism, I believe. It simply matches our understanding of rotational dynamics, notably conservation of momentum. If a rotating object gains mass, it must rotate faster, or else shed momentum some other way. If a pulsar is accelerating, (angularly) and is close enough to a companion star to steal mass, that matches our understanding of rotational motion.
I would guess that the 1 in 5 pulsars that spin fast but have no companion were quite heavy to begin with, leading to high angular velocity. They should not, however, accelerate.
May 15th, 2008 at 7:37 pm
Jigsaw man,
The hypothesis i was referring to is: millisecond pulsars are rotating objects gaining mass. (not that adding mass to a rotating object speeds them up)
May 15th, 2008 at 7:39 pm
Grand Lunar,
I think the ride would very quickly be renamed “The Vomitator” or “The Regurgitron”.
May 15th, 2008 at 7:46 pm
I am aware. I just feel that rotating pulsars is closer to ‘meets expectations’ than ‘experimental hypothesis.’ Possibly I’m just thinking about it sideways; I y=tend to do that.
May 15th, 2008 at 7:53 pm
@occam: there’s a lot of evidence for the “recycled pulsar” theory, but it’s not the only possible explanation.
The best evidence is that we actually *see* pulsars being spun up. Binary systems called high-mass X-ray binaries consist of a high-mass star and (in some cases) a pulsar, in which matter from the companion is falling on the pulsar. We can often observe the rotation of the pulsar, and what we see is that the rotation rate of the pulsar is controlled by the infalling material – as the rate of infall varies, the torque on the pulsar increases or decreases in just the way predicted by theory. Now, high-mass X-ray binaries lead to very slowly spinning pulsars, so they are not directly applicable, but they give us confidence that low-mass X-ray binaries (in which it is much more difficult to see the pulsar’s rotation) exhibit the same phenomenon. In such a case the theory predicts that the neutron star should be spun up.
As for isolated millisecond pulsars, many millisecond pulsars (and in particular many of the ones we have found) occur in globular clusters. Globular clusters are very dense, so over the (very long) lifespan of a millisecond pulsar, there is plenty of time for a third star to come along and break up a binary pair.
There are also plenty of stars available in a globular cluster to kick 1903 into an elliptical orbit – or rather, there would be, if it were in a globular cluster. But it’s not, it’s floating around loose in the galaxy. Which is one reason it’s so weird.
May 15th, 2008 at 8:01 pm
I kind of like the globular cluster idea they mention in the NRAO article…
May 15th, 2008 at 8:12 pm
What about the passage of a passing star, through the system?
May 15th, 2008 at 8:25 pm
Wow, amazing. Things like this drive home just how big, strange and wonderful the universe is.
May 16th, 2008 at 1:05 am
I’m guessing time & / or temperature.
Like white dwarfs, neutron stars cannot fuse elements into energy and thus shine only by radiating away their massively hot temperatures they were born with.
A young – and hence still very* hot – neutron star would therefore be a bit different from one that has cooled down over the aeons.
The young hot neutron star or pulsar would have – usually – a far higher spin rate, immensely strong magnetic field, a searingly blue hot surface etc …
Vs
the old cold neutron star which would’ve spun down to a slower roatation or spin rate, had its magnetic field fade away a fair bit in intensity – and hence perhaps stopped pulsing, cooled down to a mere white or yellow hot surface etc .. (Ultimately, neutron stars and white dwarfs cool all the way to red heat and finally cold black dwarfs – but that will take longer than the cosmos has been around for yet!
)
That’s just my guess anyway I’m not really 100% sure what you’re referring to here.
—————-
* “very” is utterly inadequate at describing the extremes in play with neutron stars, pulsars & magnetars – but then no words really come close to describing them.
May 16th, 2008 at 1:10 am
Hmm .. couple of other quick thoughts :
Could this be a case similar to the “black widow pulsar” which actaully evapourated its companion star away entirely?
Or could this pulsar have formed through the merger of two white dwarf stars?
In either case, this scenario has taken place in a triple star syetem with only the sun-like star & milli-second pulsar remaining ..
Either of those options plausible &/or testable?
May 16th, 2008 at 8:55 am
To StevoR:
Thanks for the info…
Okay, I’m still looking through my back issues of ScienceNews for an article about neuton stars that mentioned the temperature of such objects – that I found a little odd. When/if I find the article I’ll rephrase my question a little more clearly…
May 18th, 2008 at 12:47 am
At 465 revs per sec, the surface at it’s equator would be traveling at the speed of light if the diameter were greater than 205 km (127 miles), and that is ignoring the speed of the body in its orbit. Taking the latter into account, the size of the body would have to be even smaller.