The Crab is still crabby

By Phil Plait | October 4, 2010 10:00 am

A thousand years ago, and 6500 light years away from Earth, a high mass star exploded. An octillion tons of gas blasted outwards at speeds of thousands of kilometers per second, forming tendrils and wisps as it raced away. At the center of the conflagration, the core of the star had collapsed into an ultradense object called a neutron star. It has the mass of the Sun crammed into a ball only 20 – 30 km (12 – 18 miles) across, and is spinning at a rate of 30 times per second.

All this happened a long time ago. The debris is what we now call the Crab Nebula, and is one of the best-studied objects in the sky. And that’s a good thing, because even now the Crab is capable of throwing tantrums… and we can see it when it does!


This image is a brand spanking new shot of the heart of the Crab Nebula taken by the Hubble Space Telescope. And by new, I mean it was taken on Saturday, October 2! It’s a bit hard to see what’s going on, so I created an annotated version:


The pulsar is labeled. It’s sitting right at the center of the gas cloud, which extends way beyond the edges of this picture. As the pulsar spins, it emits a fast stream of particles that act like a wind, compressing the gas in the nebula and creating those circles of light. They look elliptical because the whole system is tilted, and you’re seeing it like a DVD held at an angle. From what I can tell, the bottom left is the side toward us, and the upper right is farther away, as if we’re looking down on it.

In mid-September, just a couple of weeks ago, several orbiting observatories noted that there was an increased amount of gamma rays coming from this part of the sky. Gamma rays are the highest energy form of light, and there aren’t many sources in the sky that can create them at all, let alone in quantities that can be seen. The Crab is the brightest continuous gamma-ray source we know, and so it was immediately put on the Most Wanted list.

hst_crab_dec1993Hubble was quickly pointed at the pulsar, and that image above was taken. Now note the rectangular bit to the left of the pulsar. Some of that stuff was seen in earlier images, but not this bright! You can compare it to the image on the right, taken in December 1993 by Hubble, which I’ve rotated and scaled to be close to what we see in the new image. The overall structure is very roughly the same, but clearly that gas to the left is brighter in the new image. I suspect that rectangular structure in the new image is actually a ring, the shape distorted by density variations in the gas.

What must have happened [note: I’m conjecturing here and may be way off, but the basic stuff is probably correct] is that something occurred on the pulsar: maybe it had a starquake, or a bit of material fell on it (the crushing gravity of a neutron star guarantees that anything hitting it will be doing so at a large fraction of the speed of light, generating a lot of explosive energy; even a marshmallow hitting at that speed explodes like an atomic bomb!). But something violent happened, and a blast of gamma-rays ensued.

Pulsars like this have incredibly strong magnetic fields, and this can affect how matter and energy flow from the star. Apparently, a lot of this energy was directed out the poles of the star. Focused in this way, a beam of high-energy particles and energy slammed into the gas surrounding the star, lighting it up, and generating an expanding wave of pressure which created that rectangular ring, like a snowplow piling up snow.

I suspect this is the case in part due to the ring of material on the other side of the pulsar, which I marked with a dotted line. That too must be due to energy flowing out of the star’s pole. How do I know? Because if that ring were in the plane of the other circles of gas we see, it would be centered on the pulsar; literally, the pulsar would appear to be right in the center of the ring. But it’s not, it’s off-center, indicating the ring is not in that same plane. From the geometry we see, it must be above that other material, so it must have been due to material coming from the star’s poles, not its equator. For the same reason, the material on the other side — the new stuff — must be due to something happening at the other pole of the star as well.

Almost certainly, that rectangle/loop was there before this recent flare of gamma-rays occurred; it’s way too big to have been created in the past two weeks! It’s probably been there for years, carved by steady emission from the star’s pole, and this new flare lit it up. Note too that if you start at the pulsar and go through that loop, you can see a very faint arc of material even farther out (labeled in that second image). That’s just what you’d expect if a beam of matter was pouring out of the star’s pole and plowing through the surrounding gas. That arc is a shock wave where that beam ends and is ramming that material in the nebula.

Now, this all these descriptions may sound a little dry, but remember what you’re seeing here. The above Hubble image shows a region trillions of kilometers across, tens of thousands of times larger than our entire solar system. And the energies involved are vast and numbing: in gamma rays alone, the Crab emits thousands of times the Sun’s total energy! In other words, put the Crab pulsar where the Sun is now, and we’d be cooked in seconds just from gamma rays.

And this new flare of gamma rays from the Crab doubled the intensity for several days.


The Crab is a fantastically complicated place, with a lot going on at once. It’s hard sometimes to know what’s what. But we do get hints, and we’re getting a glimpse of its three-dimensional structure, providing essential clues to the events we’re witnessing.

Remember too: stars like these are what create nearly all the elements in the Universe besides hydrogen and helium. The iron in your blood and calcium in your bones came from stars like this that exploded billions of years ago. When we study objects like the Crab, we’re learning not just about astronomy, but literally about where we came from.

Image credits: NASA/ESA/Hubble. Tip o’ the aperture door to Evan Keane/Astronomyblog

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

Comments (45)

Links to this Post

  1. Novedades en el Cangrejo « Pablo Della Paolera | October 4, 2010
  2. Astronews Daily (2455475) | October 5, 2010
  1. Steven Vallarsa

    I think this is suppose to say “A thousand years ago, the first tendrils of light from a supernova 6500 light year away poured onto the Earth…”

  2. Gary Ansorge

    So, I guess this “star quake” could also be referred to as “Shake and Bake”?

    AH, nature, cooking up heavy elements right where we can observe. That’s one hell of a kitchen.

    Great images, Phil.

    Gary 7
    PS; the mental image of a marshmallow exploding over Hiroshima is just too cool.

  3. Shawn

    @Steven Vallarsa
    I read that statement a dozen times, trying to figure it out. I thought maybe the BA was turning creationist. :)

  4. I love it when actual observation produces an image that matches the “artist’s rendering” that so frequently must accompany these sorts of discussions. The close-up image looks just like some of the pulsar graphics used in Sagan’s Cosmos series.

    @ Gary 7:

    PS; the mental image of a marshmallow exploding over Hiroshima is just too cool.

    Particularly for the late residents of Hiroshima, I would think.

  5. Brian Utterback

    Well, something is wrong with the first line. Phil, don’t tell me you finally got that tachyon telescope working?

  6. Phil, you said a 1000 years ago and 6500 light years away”. . . if it’s 6500 light years away how would you know that that happened 1000 years ago? Tom

  7. Josh

    1000 years ago, the event was observed. The event was 6500 light years from us. The spacetime invariant between ourselves and the supernova is equal to -1.4 x 10^7 square lightyears. That’s the only measurement between the two events that is true for all observers.

  8. beanfeast

    Hope you don’t mind but I’ve got a few questions.

    Can you give some idea of the distances shown in this image. How far is it from the neutron star to the new glow? Is the image of the neutron star distorted or is it an inability to resolve two stars. If it is the later, how far apart are they, how long do they take to orbit and what are their masses?

  9. Ed H.

    Ah, Phil…

    Gotta love your geeky audience. Steven, Tom, and Josh all beat me to the punch.

  10. Wayne on the plains

    When dealing with galactic scale distances, it’s common to not explicitly talk about the look-back time since it’s all in the present epoch relative to the age of the Universe. Phil has stated on multiple occasions that when he talks about “when” he means our observation of it, regardless of the distance. Having said that, I must admit that this particular wording is just asking for this sort of trouble in the comments.

  11. Increased gama ray observations . . . hubble gotchu!


  12. DrivethruScientist

    I see something like a wheel with a bar or stick through it and the pulsar at the center. The “wheel” was there in the ’93 pic, but not the “bar” that runs through it. Am I looking at it correctly?

  13. Deepsix

    Neat. Looks like this (:0)

  14. Question: If the event occurred 6500 light years from earth, wouldn’t the light from the event take 6500 years to get here?

  15. Navneeth

    14. Brian Lang, in short: yes.

  16. Thomas Anderson

    #2 Gary:

    I think at high relativistic speeds that Marshmallow would explode below Hiroshima. Think the anime Akira. BIG CRATER!


    This sort of reminds me of my dad blowing smoke rings through smoke rings.

    Captn Tommy

  17. Chris Winter

    “…the crushing gravity of a neutron star guarantees that anything hitting it will be doing so at a large fraction of the speed of light, generating a lot of explosive energy; even a marshmallow hitting at that speed explodes like an atomic bomb!”

    Quite literally. If the marshmallow weighs 4 grams, by my calculations the kinetic energy it possesses when moving at the speed of light is 1.8E+14 j, or 180 Terajoules.

    The Hiroshima bomb had an estimated yield of 63 TJ.

    Looks like John Campbell was not so far off when imagining that lightspeed-muzzle-velocity cannon in The Black Star Passes.

  18. 14. Brian –

    If your question has to do with Phil’s statement that has been hashed and rehashed here, then the event Phil is talking about happened 7500 of our years ago, reaching Earth in approximately 1010 CE (common era), or 1000 years ago.

  19. Woof

    #18 Chris Winter:

    By my calculations the kinetic energy of a 4g marshmallow moving at c is… (carry the 2)… INFINITE.

  20. @Woof: But is it infinitely delicious? Answer that, science!

  21. Woof


    Only if you drop it into an infinite cup of hot chocolate.

  22. Messier Tidy Upper

    Marvellous images & write up there. Thanks BA. :-)

    There is just one thing though ..

    The Crab Supernova – which produced that first Messier object (M1) the Crab Nebula and the Crab Pulsar there – took place in 1054 didn’t it?

    So 1054 + 1000 = 2054 not 2010 so it has not quite been 1,000 years yet – even my maths can work that much out! 😉

    The energies involved and the bizarre nature of it though are just staggering to contemplate. I don’t know if anyone can fully grok how extreme these objects are but the BA sure does a nice job of trying to describe them for us. :-)

  23. Messier Tidy Upper

    @ Wof & Naked Bunny with a Whip :

    Here’s a thought – a marshmellow of neutron star matter may or may not be infinitely delicious but given the amount of material it sure would be filling! 😉

    As explosively filling as that final after-dinner mint in the Monty Python sketch .. :-)

  24. Messier Tidy Upper

    Nb. Yes, it was indeed SN 1054 that produced the Crab Nebula – see :


    BTW. Proxima Centauri is Wiki’s featured article today too I see. :-)

  25. @Woof: How much funding will NASA need to run this experiment?

  26. Messier Tidy Upper

    @ ^ : I’m guessing an INFINITE amount! 😉

    (Sorry I’m not Woof but I’ll still answer that ‘un, after all we haven’t got an infinite amount of time to wait. Meow. 😉 )

  27. Phil

    Here at Jodrell Bank Observatory we have been (almost) continuously monitoring the Crab Pulsar for over forty years. I have just been talking to one of the observers and she assures me that there has been no unusual activity – no flux change, profile change, timing chaneg etc – recently.

  28. Messier Tidy Upper

    I think I read in an old textbook once that the study of the Crab Nebula is half of all astronomy. :-)

    I may, of course, be mistaken..

    I’ve just noted Wikipedia suggests that the Crab Pulsar (under its entry) may have an orbiting planet :


    “In 1970, astronomer Curtis Michel proposed the presence of a planetary companion to explain certain variations observed in pulsar timing[16]. The hypothesized object would have a mass of 0.00001 Solar masses (i.e 0.01 Jupiter masses or 3.3 Earth masses) and be located at 0.3 Astronomical Units from the pulsar.”

    (Brackets original.)


    Which raises a few questions like :

    1. Has anybody confirmed or rebutted this idea since?

    2. If true then wouldn’t this be the first exoplanet ever discovered?

    3. How come I’ve never heard of this before – why has it seemingly been kept so quiet? Has anybody else here heard of that before and can anybody here enlighten us on this Crab pulsar possible planet (!) further?

  29. Messier Tidy Upper

    ^ NB. My source link for the quoted info. & comment above is here :

    & I’ve also asked this as a question on the Bad Astronomy Universe Today (BAUT) forum here :!

    Which I hope is okay netiquette~wise. My apologies if not. I’ve mentioned & linked this thread there too btw. I’ll check back on both threads there & here for more.


    PS. I post as ‘StevoR’ there & ‘Messier Tidy Upper’ here, sorry if that causes any confusion or problems. I guess I should make it consistent but.. long story, anyhow.

  30. #29 MTU:
    “I think I read in an old textbook once that the study of the Crab Nebula is half of all astronomy.”

    Pretty much so. One of my astrophysics lecturers was fond of saying, “Astrophysics can be divided into two parts – the Crab Nebula and everything else.” That was nearly 30 years ago, but I’m told the saying still applies today.

  31. @ dave from manchester

    So much for my thought that whatever the event was it may cause a glitch in the pulsar…

  32. Messier Tidy Upper

    @ ^ Neil Haggath : Thanks – that’s exactly the saying I was thinking of that I read somewhere. Perhaps your lecturer wrote the book I read? 😉 :-)

  33. #29 MTU

    My observer friend at Jodrell has never heard any one speak of a possible planet in the twenty years she has been working on the crab. Pulsar timing wasn’t so exact in 1970

  34. Chris Winter

    Woof is right, of course; I neglected the relativistic effects. Shall we say the marshmallow moves at near lightspeed, and its K.E. is much more than 180 TJ?

  35. VJBinCT

    Back in 1967, I was one of the first to see the crab pulsar pulsing with the naked eye (on a TV monitor). As a grad student working at a NASA facility, I happened to be at Lick Observatory for observations completely unrelated. The night before we started, a U of Cal group hooked up a TV camera at the coude focus of the 120 inch telescope and strobed the frame capture off of a variable frequency oscillator. With a small beat frequency you could see the pulsar modulation happening at a human eye compatible rate. Very cool. I’m glad I got word of what they planned to do at dinner.

  36. t-storm

    If it was moving at light speed wouldn’t it’s KE be 0.5 * m * c^2?

  37. Joel

    @t-storm: No, because of the relativistic effects it would experience at that velocity. As the velocity of an object approaches the speed of light, so its mass increases, becoming infinite at c. 0.5*m*v^2 works fine for most things in everyday experience, because the relativistic effect is so small as to be negligable, but at c the kinetic energy would be 0.5*infinity*c^2, which works out as infinite.


  38. Yeebok

    I love the way you explain stuff Phil, it’s illuminating and makes you think and re-examine the pics, it’s a neat ability you have to promote excitement at science.
    My take on what you’ve described is an ‘hourglass nebula’ pair of rings, with a jet shooting through it’s centre to the left (and an associated disturbance past it) as well as an outer equatorial ring.. ?
    It’s an amazing picture and I’m glad you took the time to explain it.

  39. r0blar

    ” In other words, put the Crab pulsar where the Sun is now, and we’d be cooked in seconds just from gamma rays.”

    I think in about 8 minutes :)

  40. Anchor

    Phil, I have to amplify #28 dave from manchester England, who notes that “there has been no unusual activity – no flux change, profile change, timing change etc – recently.”

    I’m afraid that your explanation of the changes in the structures you point out between December 1993 and last Saturday doesn’t quite square with what’s already been seen. In 1996, for example, a Crab pulsar MOVIE sequence composed of eight images taken by the Hubble WFPC2 every few days OVER A PERIOD OF ONLY A FEW WEEKS shows the structures near the pulsar changing very dramatically. The researchers on that observation attributed the changes to an ionized pulsar wind interacting with the intense magnetic field.

    That sequence can be found here:

    As the team leader Jeff Hester observed, “Watching the wisps move outward through the nebula is a lot like watching waves crashing on the beach — except that in the Crab the waves are a light-year long and are moving through space at half the speed of light…You don’t learn about ocean waves by staring at a snapshot. By their nature waves on the ocean are ever-changing. You learn about ocean waves by sitting on the beach and watching as they roll ashore. This Hubble ‘movie’ of the Crab is so significant because for the first time we are watching as these ‘waves’ from the Crab come rolling in.”

    The release story also notes: “The most dynamical feature in the inner part of the Crab is the point where one of the polar jets runs into the surrounding material forming a shock front. The shape and position of this feature shifts about so rapidly that the astronomers describe it as a “dancing sprite,” or “a cat on a hot plate.” The equatorial wind appears as a series of wisp-like features that steepen, brighten, then fade as they move away from the pulsar to well out into the main body of the nebula.”

    Comparable changes were also observed in x-rays by the Chandra Observatory in a sequence of 7 exposures taken between November of 2000 and April 2001.

    In other words, one doesn’t need to invoke some particular energetic event to explain what we’ve been seeing since 1993 – you don’t need 17 years: these changes have already been noted on a timescale of DAYS.

    A more plausible scenario is to assume that the pulsar suffers from a more or less constant rain of gas and dust from the supernova remnant falling onto its surface, with charged particles strongly funneled onto the magnetic poles. The more or less steady release of energy “rolls outward” in the form of dynamic shockwaves in an otherwise constant pulsar wind, mediated by the rapidly-rotating magnetic field rooted in the pulsar.

    Elsewhere, a number of theorists have suggested that much of the pulsed emission observed from the Crab and other young pulsars comes not only from the canonical ‘rotating lighthouse-beam effect’ as the dipolar magnetic field sweeps across our line of sight, but is also generated at a region some distance from the pulsar where charged particles are accelerated to near the speed of light inducing synchrotronic emission. They suggest that the ring-like structures that appear to be off-set from the position of the pulsar are the regions where the magnetic field beams pile up packets of charged particles and accelerates them to near-light speeds as the magnetic axis whips around like a natural particle accelerator.

    The “dancing sprites” observed above the ROTATIONAL (as opposed to the magnetic) axis of the pulsar are interpreted as a plasma confined within the relatively narrow compass of the magnetic dipole region (i.e., the ring you indicate) where the outflowing pulsar wind is also preferentially confined to produce polar jets that interacts with the SN remnant that would otherwise be compelled to fall into the pulsar. The pulsar “jets” blowing outwards prevents them from falling in, while the jets inject them with energy and dynamic shocks that make them ‘dance’ in place, somewhat analogous to a ping pong ball that bobbles about on a vertical stream of air.

  41. Messier Tidy Upper

    @35. dave from manchester England Says:

    #29 MTU – My observer friend at Jodrell has never heard any one speak of a possible planet in the twenty years she has been working on the crab. Pulsar timing wasn’t so exact in 1970.

    So no planet then & wiki got it wrong? Oh well, thanks for that anyhow. :-)

    @41. r0blar Says:

    ” In other words, put the Crab pulsar where the Sun is now, and we’d be cooked in seconds just from gamma rays.”
    I think in about 8 minutes.

    LOL. Good point, gotta allow for the travel time light minutes betwixt Earth & Sun. 😉

    @42. Anchor :

    Thanks for that informative comment & links. :-)

  42. S. Koter

    The human race will require loads of iron, calcium and the works, in time ahead. Nuetron star, the egg that makes chickens. Go crab

  43. Lost

    Phil, what is your response to Anchor and Dave?


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