Vampire star

By Phil Plait | September 12, 2007 10:05 am

On June 7, 2007, NASA’s Swift observatory detected a blast of X-rays coming from near the center of the Milky Way. Swift is designed to see high-energy light from gamma-ray bursts, vast explosions usually coming from incredibly far away.

Because this one was near the Galactic center on the sky, it made astronomers Craig Markwardt and Hans Krimm suspicious that it might actually be from some much closer object, one inside the Milky Way. They decided to follow up on the object using RXTE, another high-energy satellite. What they found is nothing short of extraordinary.

The object turned out to be a pulsar, the leftover remnant of a supernova explosion. When the massive star’s core collapsed in the explosion, the normal matter in it was crushed to unbelievable densities. Electrons combined with protons (and antineutrinos for those keeping score) to form neutrons, and the leftover ball had the mass of a star crushed into a sphere only a few miles across. It’s essentially an atomic nucleus the size of a city, where one cubic centimeter — the size of a sugar cube — would have about as much mass as all the cars in America combined.

This neutron star spun madly, and had a strong magnetic field. A star like that can sweep up material around it, which then follows along the magnetic field lines of the star and slams into the magnetic poles. This heats up the star, causing it to glow in two spots (the north and south magnetic poles). As the star spins, we see these glowing spots blip on and off like a lighthouse as they pass into and out of our field of view. The star pulses, and so we call this kind of neutron star a pulsar.

In the case of the pulsar discovered by Swift in June, though, things are a little different. The star had a companion, a normal star that was originally maybe not too different than the Sun. It survived the supernova explosion, and aged. Fast forward a few billion years. The second star started to become a red giant, and its outer layers puffed up. The pulsar greedily started eating that material, pulling it right off the other star. It got incredibly hot as it did so, and a fierce pulsar wind started which pummeled the other star, literally eroding away much of the other star. In the meantime, the material falling on the pulsar sped up its rotation.

Fast forward a few more million years. So much material has been drained off the second star that it is a shell of its former self. It may have started with thousands of times the mass of Jupiter, but it has now been whittled down to a mere 7 Jupiter masses or so. It is stretched into a teardrop shape by the incredible tidal forces from the pulsar. A thin stream of matter connects it to the pulsar, as the tiny but dense cinder continues to drain the star’s matter. Sometimes that stream becomes unstable, and a bigger blob of matter slams into the pulsar. When that happens, the pulsar erupts in a burst of X- and gamma rays, which is what Swift detected. The flare fades with time, and it was good planning that allowed the astronomers to detect this bizarre system before it faded back into obscurity.

The system is totally awe-inspiring. The pulsar spins 182 times per second; that’s faster than a kitchen blender’s blade. The second star was detected because as it orbits the pulsar the pulsar orbits it. This mutual gravitational dance causes the pulses to be slightly delayed or advanced as measured on Earth, and that allows astronomers to determine the size and orbit of the star. It turns out that the second star, the one being drained, orbits the pulsar at a distance of 230,000 miles — the same distance the Moon orbits the Earth. However, instead of taking a month to orbit as the Moon does, this star orbits the pulsar once every 55 minutes!

Imagine: an object that is far more massive than Jupiter, being tossed around at well over one million miles per hour! Neutron stars are scary. Here’s a scale diagram of the system:

In reality, the pulsar would be a microscopic dot on this scale, but you get the idea.

I used to work on the public outreach for Swift, and it’s great to see it doing such amazing work (and to see my friend Aurore Simonnet still doing artwork for it; she drew the diagrams above). I think it’s perhaps the single most successful NASA mission ever, and it’s still going strong. You tend not to hear the success stories, but Swift ranks very near the top.

Sometimes I have to sit back and chuckle: it seems fantastic (literally, like a fantasy) that systems like this vampire pulsar can even exist. But it seems even more fantastic that just by carefully examining a handful of photons from it we can deduce so much information about it. The Universe unfolds before us, and really, all we need is the will to see it.

CATEGORIZED UNDER: Astronomy, Cool stuff, NASA

Comments (52)

Links to this Post

  1. Telescope Fun » Blog Archive » Vampire star | September 12, 2007
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  1. Sam Platts

    I love what you are doing but why is it called “bad”? I want to teach an astronomy course and I am bouncing around the internet looking for resources and organization. And what are you skeptical about? Stars do exist, after all, at present anyway. Do you read astronomy magazine?

  2. Chip

    Sam Platts:
    Go to the homepage for this website:
    and scroll down a bit and read where it says:
    “Welcome to the homepage for Bad Astronomy!”
    You’ll find a good explanation of why the word “bad” is used.

  3. Jalbietz

    These vignettes are great, please keep them coming.

    Sam, welcome! This site is a great resource, explore it a bit more, and I think the answers to your questions will become clear :)

  4. Rowdower

    Okay, this is just freaky. I know that pulsars can spin fast, but that’s not so amazing. (Well, it is, but with what we’re talking about here, it’s something else entirely.) 55 minutes to traverse nearly 1.5 million miles is incredible. I wish I could see this in action. The universe is an amazing place. We live in such a mundane location. lol

  5. Dan

    Kind of reminds me of a hyperkinetic girl I dated once. She had a tendency to spin wildly and drain the mass of money from my bank account from very great distances.

    Aside from that, pulsars are probably my favorite things out there. They’re just so freaky wild.

  6. andy

    One of the interesting questions to ask here is whether this accretion process could create a protoplanetary disc around the pulsar. This system may vaguely resemble the process that generated the planetary system around the millisecond pulsar PSR B1257+12.

  7. PK

    This is very cool! Does the red giant have enough food left for the pulsar to form a black hole eventually? Or is the pulsar too messy an eater for this?

  8. I have a question that some here might be able to answer – Common sense tells us that it would be impossible to stand on the surface of a neutron star, right? The gravity would be so strong it would crush an astronaut into a one-atom thick puddle in a fraction of a second.

    But… what if the star was spinning so fast that the centrifugal force would cancel out the gravitational pull at the equator? Is such a scenario possible then, and if so how fast would the star have to spin? Tidal forces would still be a problem I would think, but I have the feeling that there could be a certain set of circumstances (star mass, star diameter, spin rate etc.) that would make it possible (if not particularly comfortable) to stand on a neutron star.

    Comments anyone?

  9. DrFlimmer

    @ Elwood:

    Well, I don’t know how fast it would need to spin, but I guess it would rip apart the neutron star.

    I wonder if a neutron star could catch up so much mass from a companion that it “needs” to form a black hole, like SN Typ 1a’s when a white dwarf reaches the Chandrasekhar border and becomes a neutron star.

  10. John Powell

    No, long before the centrifugal force counteracted the gravitational pull at the equator to the extent that it allowed an astronaut to stand on the surface you would have passed the point where the neutron star-stuff would have ‘inflated’ to ordinary matter.

  11. Doc

    @Elwood Herring

    Tides would very definately be a problem – a human would likely be ripped in half.

    You might want to check out a couple of works of science fiction on this subject. They describe the environment around neutron stars pretty well, and I believe they’re still reasonably accurate (Phil or someone else with more astronomy background than I’ve got may have different opinions).

    “Dragon’s Egg” by Robert L. Forward, ISBN: 034543529X


    “Neutron Star” by Larry Niven, ISBN: 0345336941

  12. This is by far the most interesting thing I’ve read in a long while. Thanks again Phil. Great stuff.

  13. bjswift

    Classically, at “breakup velocity,” the gravitational acceleration (g = GM/R^2) equals the cetri[fug/pet]al acceleration (a = v^2/R = (2pi*R/P)^2/R = 4pi^2R/P^2).

    Using some standard numbers of R=5 km, M=1.4 Msun: g ~ 7×10^12 m/sec^2.

    The rotational accelaration is MUCH less than g for P=1/(55 Hz): a ~ 6×10^8 m/sec^2.

    Of course, you should really use GR to do this properly, but the point is that neutron stars are usually nowhere near breakup velocity and are very round.

    In fact, the classical breakup velocity is around a period of 0.1 millisec (or 10,000 rotations/second)!

  14. bjswift

    Here’s an abstract from AAS a few years ago that will be of interest:…206.1702C

  15. Thanks – I have a copy of Niven’s Neutron Star but I haven’t read it for years. I think I’ll read it again now.

    I think Doc’s right about the tidal forces – they would be far too strong to withstand. I don’t know about neutron star-stuff “inflating” though – back to what we call “normal” matter – would that really happen? I thought that once matter had been compressed to neutronium it would stay compressed.

    Anyway – it’s all food for thought. Thanks.

  16. bjswift

    @above: and 20% c –> ~0.5 millisec –> 2000 rotations/sec (at R=5 km; sorry for multi-posting!)

  17. Matt

    I read stuff like this and it’s just amazing. Such an awe-inspiring universe we live in, with astonishing, beautiful events like this happening all around us–why do so many feel the need to add an invisible friendly giant to the mix?

  18. Bunsen

    Allow me to put this simply…

    Science is SO DAMNED SEXY.

  19. Bunsen

    Oh, and Phil – do you have any higher resolution versions of those images that could be a little more background-friendly?

  20. “These millisecond pulsars are spinnning near their centrifugal breakup limit, with surface velocities nearly 20 percent the speed of light.”

    (From bjswift’s article)

    Wow! Just imagine what the sky would look like from the surface of a star spinning at that speed! Someone should try doing an artists impression of that – any takers?

    (btw my comments are still being caught in the spam filter, and take a while to appear on the page.)

  21. Click the images for higher-res versions.

  22. Arthur Maruyama

    Considering that Dr. Robert Forward got his doctorate for gravitational radiation (assuming that his entry in Wikipedia is accurate), I would guess that the most of the gravity science in “Dragon’s Egg” was probably correct for its day.

    On the short story “Neutron Star” Larry Niven himself admitted that the macguffin of the a space-faring race not understanding gravity tidal forces was weak in hind(most)sight, but that story and the rest of that collection are still enjoyable reads.

  23. Another impressive description of the awesomeness that is going on out there, Phil. Thank you very much.

    In the previous entry you asked the question: “But where are we ever to find such a person?”

    Well, every time I read an entry like this in your blog, the answer is clear in my mind…

  24. You honestly think SWIFT is more successful than, say, Voyager? I guess it would depend on what you consider successful, but you said “single most successful”. Both missions accomplish different goals. I would consider Hubble to also be “the most successful” …at getting everyday people interested in astronomy (among many other things). I would consider Spirit and Opportunity to also be “the most successful” …at roving (and getting people interested; I’ve rarely seen such complete coverage by mundane media). I would consider SOHO to also be “the most successful” …at solar observation.

    I don’t think any of these missions could even remotely be considered a failure or (goodness gracious me) a waste of money. They are all MONUMENTALLY successful missions.

    So, yeah. I don’t think it’s fair to call SWIFT “the single most successful”. :)

  25. Jarno
  26. Navneeth

    Electrons combined with protons (and antineutrinos for those keeping score) to form neutrons…

    Well, I was keeping count. 😉 Wouldn’t electrons need to combine with protons to produce a neutrons and neutrinos? (Isn’t it just inverse Beta decay?) Correct me if I’m wrong.

  27. Guy Who Doesn't Know Much About the Stars

    Yes… we do live in a mundane location of the galaxy. Still I’d rather live here than next door to a neutron star. Dang, that’s amazing… and a little scary. (Though I would like to live closer to a Nebula)

  28. DrFlimmer

    @ Navneeth:
    I’m not sure, just guessing: Neurinos are nearly weightless, so for the mass they do not really matter. But you are right, a “reversed beta decay” would need an electron, a proton and a neutrino. But it’s very unlikely that we get so many neutrinos to interact in such a short time (supernova explosion) when the neutron star is created. So there must be something else that is like a neutrino but in some way the other way around.
    I mean: We have only an elctron and a proton to form a neutron. The missing neutrino is replaced by something coming out of the new formed neutron which must interact in a way a neutrino would… but the other way around, because it comes out and does not get in. This is the mentioned ANTI-neutrino.
    I try to sum this up: Instead of putting a neutrino in we are getting an anti-neutrino out.
    I hope that this helped a bit, but if someone has a better idea: come along with it!

  29. andy

    Electrons+protons going to neutrons+antineutrinos does not conserve lepton number. So it can’t be right. You’d get electron neutrinos out instead.

  30. Gads, that’s incredible!

    I’ll give Markwardt and Krimm a gold star if they document rotation of the two stars around one another.

  31. BlondeReb3

    Truly amazing to believe that anything as strange and cool as that is in our Universe.

    That being said, I don’t want to be anywhere near one of those things!

  32. KaiYeves

    Space is cooooooool. “Vampire Star” sounds like a title for a manga, though.

  33. Willo the Wisp

    Ooh, great post. It’s all a bit scary, all those huge masses and high speeds. But this sort of thing is what I love about astronomy – the reality and wonder of it is way outside anything we could make up ourselves!

  34. Regner Trampedach

    Thanks for a great post Phil. What I find must amazing is how this little city-sized
    dot, whips around a 7 Jupiter-mass ball of gas at 112 km/s = 0.02% of the speed of light in vacuum, c. A rifle bullet travels at less than 1 km/s and Voyager 1 travelled at 17.2 km/s on November 2005…
    Using the numbers from bjswifton’s post of 12 Sep 2007 at 11:53 am, the break-up frequency would be 0.16 mHz, resulting in an equatorial velocity of 64% of c. This assumes classical mechanics and a spherical neutron star (both assumptions are obviously violated, but it gives a good feeling for what is happening). The actual 182 Hz spinning of the neutron star, results in an equatorial velocity of about 1.9% of c.
    Cheers, Regner

  35. Jan

    Can anybody shed more light on how the companion ends up with this tear-drop shape? At a scale small relative to the radius of the orbit I would have expected an elipsoid. Does the pointy end of the tear drop point towards the neutron star?
    Cheers, Jan

  36. Anne

    Not to be a party pooper, but this is the eighth accreting millisecond pulsar to be found. The fastest one, 4U1820-30, has an orbital period of *11 minutes*. Now that’s a compact binary…

    If matter is going to get pulled from the companion to the neutron star, it needs to first escape from the companion – close to the companion, the combined gravity pulls toward the lighter but much nearer companion. As you move away from the companion, at some point the pull from the neutron star becomes stronger, and matter falls towards it instead. If you draw the outline of the region where that happens, you get a teardrop shape. It’s a teardrop roughly because on the side towards the neutron star, as soon as matter crosses the center of mass of the system it falls towards the neutron star. That is the point of the teardrop (leaving aside things like stellar and pulsar winds and radiation pressure). The teardrop shape is called the Roche lobe, and in any binary system it exists as an invisible dividing line. Roughly, stellar matter inside a star’s Roche lobe may stay put, but any that overflows is doomed. So if the companion is large enough to overflow its Roche lobe, any extra matter gets gobbled by the neutron star, leaving only a teardrop.

  37. wright

    Beautiful piece, Phil. The mind boggles (well, this layman’s mind, anyway) just contemplating the incredible forces romping around in that system.

  38. Jan

    Thanks, Anne, makes sense. Wikipedia has a diagram:
    A bit of googling brought me to , which has some nice numerical simulations of the accretion process.

  39. spicoli
  40. Can anybody shed more light on how the companion ends up with this tear-drop shape? At a scale small relative to the radius of the orbit I would have expected an elipsoid. Does the pointy end of the tear drop point towards the neutron star?

    nevresim takimi modelleri ceyiz havlu deseni kenari
    bilgisayar tamiri teknik yetkili servis sorun arıza veri kurtarma sirket sozlesmesi

  41. Karnbeln

    ..I think I’ll skip mentioning the pareidolia I see in these photos this time.

  42. KaiYeves

    Pareidolia? You mean I’m not the only one who thinks the red giant looks like a human heart?

  43. Aaron

    Amazing post – thanks Phil!

    Is it a fair assumption that if we go back far enough in time what we originally had was a binary star pair orbiting one another?

    I barely know enough of the science to comprehened it all (so I keep reading) but I would think that a supernova event for the now neutron star would have all but obliterated the second star, in fact wouldnt the pre-supernova expansion of the first star been enough to create a real issue for the pair? Is it assumed that the 2nd star formed post-neutron formation, and if so how could it do so in such close proximity to such a gravational magnent? So many questions. Love Science!

  44. That’s not pareidolia. They’re pieces of artwork. Anything you see in them was probably put there intentionally.

  45. Buzz Parsec

    Andy and Dr. Flimmer…

    Imagine a virtual neutrino-antineutrino pair forming in the vacinity of a proton. Along comes an electron. The neutrino, electron and proton combine to form a neutron and the antineutrino wanders off on its own.

    I think this is how you get the electron+proton -> neutron+antineutrino interaction without relying on the extremely improbably 3-way electron+proton+neutrino all arriving at the same place at the same time. I’ve never really understood what is meant by “an antiparticle is just a normal particle travelling backwards in time”, but in this case, the produced antineutron if regarded as a neutrino travelling backward in time is a neutring arriving rather than an antineutrino departing, so making that substitution, we have electron+proton+neutrino->neutron, which is just reverse beta decay.

  46. andy

    Buzz Parsec: electron+proton+neutrino->neutron doesn’t work!

    You have a lepton number of 2 on the left hand side (2 leptons, 0 antileptons), and a lepton number of 0 on the right hand side (0 leptons, 0 antileptons). Thus you’ve managed to propose a process that is an even worse violation of lepton number conservation!

    If you try to draw the Feynman diagram for what you suggest, you find it doesn’t work. Plus, even if it did, it’s a 4-particle interaction (electron+proton+neutrino+antineutrino).

    Furthermore, reverse beta decay is proton+electron–>neutron+neutrino.

    Whether BA is a bad astronomer I don’t know, but the reaction he put in this post is definitely bad particle physics.

  47. sandswipe

    SPOILER WARNING- didn’t the end of 2001 (book version) have a pair pretty close to this at the end, right before the mysterious imaginary hotel room?

    End spoilers

    The only thing more awesome then science fiction is science. Well, except for the warp drives. :)

  48. Sarah

    do these effect the science community and the genrel public? How?


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