Crabby Hubble

By Phil Plait | December 6, 2005 11:32 pm

When you go outside and look at the night sky, it seems changeless. But there are tremendous energies and motions out there, squashed by our lousy human perception of distance, perspective, and proportion.

Case in point: the Crab Nebula.

You may have already seen this image; it’s all over the blogosphere, as cool Hubble images tend to propagate quickly. Truly, it is amazing (it’s actually a composite of Hubble images with those from the Very Large Telescope). I’ve looked at the Crab Nebula my whole life and I’ve never seen it look so three-dimensional. But there’s more to this than just a pretty picture.

The Crab is an expanding supernova remnant; literally the gaseous remains of a star that blew up nearly a thousand years ago. The gas is really moving at a huge clip: 1400 kilometers per second. Given the amount of gas involved (a star blew up), the energy it took to get this stuff moving at that speed is mind-numbing– more energy than the Sun puts out in millions of years.

Yikes. All that energy was dumped into the gas all at once when the progenitor star exploded. And the Crab is considered to be relatively low-energy compared to other supernova remnants!

At a distance of 6500 light years or so, even that incredible expansion rate is apparently slowed to a crawl by perspective. Just like a distant car or airplane appears to hardly move at all, even the Crab Nebula’s ferocious expansion seems more reminiscent of a turtle than a crustacean. But it can still be seen.

If you take an old image, and compare it to a more recent one, the expansion is pretty easy to detect. Here’s a great example of such an animation. The two images, taken about 28 years apart, were carefully aligned, and you can see the movement of the gas as it screams out from the center of the nebula. Another good animation is here.

There’s more to it than a cool picture, too. By measuring the speed of the expansion, you can backtrack the gas to calculate how long it’s been expanding. That gives you the age of the nebula! When this is done, the number is about 1000 years– in fact, the date of the explosion is probably 1054 AD, in July. I’ve even heard an exact date of July 4, appropriately enough.

For my day job, developing educational activities for students, I rewrote and totally reworked an old lab exercise where you calculate this number (a preliminary draft version is online here; it’s the second activity listed). It was very cool to play with images of the Crab and see for myself how old it is. That exercise should be completed and ready to go in a couple of months. It can be performed online, so anyone who reads this blog will be able to do it for themselves. Imagine! With the tools literally at your fingertips right now, you can grasp some of the most incredible forces nature has to offer, and using some simple math determine when a star died, and when an iconic nebula was born.

I love this stuff.

Image credit: NASA, ESA, J. Hester and A. Loll (Arizona State University)

CATEGORIZED UNDER: Cool stuff, Pretty pictures

Comments (26)

Links to this Post

  1. Rural Rambles: Larry Ayers’ Weblog » Tangled Bank # 43 | December 14, 2005
  1. Whoa, the old/new images animation just blew my mind. Ouch.

  2. I love this stuff too. =D It’s the really practical elements of astronomy (one might say the really superficial ones) that affect me the strongest.

  3. Andy

    1400km/s ?

    the space shuttle goes at 7600+ km/s

    or is that a typo?

  4. Andy

    oh, KM/s.

    doh. nevermind

  5. moonflake

    Surely the calculated date of the explosion is not actually the date of the explosion, only the date at which the light from the explosion reached us? The actual explosion would have happened 6,500 years earlier than that. Or am I confused?

  6. Blake Stacey

    The actual explosion happened 6,500 years before we Earthers saw it, but today, we see it as it was 6,500 years ago. We still see the nebula as it was when it was about a thousand years old.

    To the best of my knowledge, the exact date (4 July 1054) comes from the date the Chinese astronomers recorded a “guest star” appearing in the sky. Not being an expert in calendrical conversions, I don’t know how tough it is to change a Chinese date into a Gregorian one. Given the whole Gregorian/Julian mess and a Chinese calendar about as complicated as the Hebrew one, conversion-wise, I’d expect a probable error of a day or two in either direction. (It’s like figuring out on what day Passover fell, 950 years ago.) I read this in L. A. Marschall’s **The Supernova Story** (1988), but I don’t have the book with me to check the details.

  7. antipodean

    hey, thats my background

    on another note about hubble, i found this in a DK guide to the universe, and i quote:
    “The French comet hunter Charles Messier (1730-1817) compiled a catalogue of 110 nebulous-looking objects in the sky that could be mistaken for comets. Not all were discovered by himself – many were spotted by another Frenchman, Pierre Mechain, and yet others had been found YEARS EARLIER BY ASTRONOMERS SUCH AS EDWIN HUBBLE.”

    I’m pretty sure Hubble was born a good 40+ years after Messier died, so how is this possible? You’d think when writing a science book they’d at least check their facts.

  8. antipodean

    oops, my bad, i misread the book, it actually says Edmund Halley, sorry about any confusion

  9. Jon Niehof

    That before/after image is neat, except what I presume is the after (more expanded) image pretty clearly has a different exposure time or filter–the dimmer stars appear brighter in it. So the dimmer nebulosity would also appear brighter and make the nebular appear to expand, which unfortunately confuses the issue.

  10. Rodolfo Granados


    The space shuttle does NOT travel at 7600 Km/sec. At that speed it would complete an orbit in less than 6 seconds! Its speed is around 7.7 Km/sec

    Absolutely stunning image of the Crab Nebula!

  11. Chip

    Animations spanning 28 years are cool, but has anyone every gone back to the earliest photographed images of the Crab Nebula (1890s?) and carefully put together images collected from every year of observation? A 100+ years of smoothly expanding gases, though still a short animation from that distance, would be interesting.

  12. more energy than the Sun puts out in millions of years

    Okay, now my brain hurts. Humans just aren’t built to grok anything that large.

  13. Evolving Squid

    >1400 km/s

    The speed of light is approximately 300 Mm/s. Despite that speed, it still takes 214 years to travese a light year.

    This would seem to indicate that in about 1.4 million years, the nebula bits will pass our solar system. Better get a net and put on some coffee :)

    On a more serious note, though, it also implies that planetary nebulae have a relatively limited life-span. Presumably the Crab nebula will dissipate, from our point of view, long before 1.4 million years. Is there any research/thoughts about how long planetary nebulae last?

  14. Marlayna

    I read on one of those links that they calculate the speed of the gas based on the spectrum of the light it gives off.

    Now I’m confused. :( What does that have to do with anything?

  15. Marlayna– when you compare the two pictures, you are only measuring the gas speed in terms of degrees/year or something like that. The spectrum gives you the speed in km/sec. Those can then be combined to get a size of the nebula and its distance. If a blob is moving at, say 500 km/sec, and you know how far it traveled in a given amount of time, you get a size of the nebula. Once you know the size of the nebula in km, you can measure its apparent size and get the distance to it.

  16. Irishman

    I think Marlayna’s question may have more to do with how to calculate the speed from the spectrum.

  17. Evolving Squid

    The speed of the gas would produce a doppler shift in its spectrum, the same way a car racing by you seems to raise in pitch as it approaches and lower in pitch as it passes.

    One can measure the doppler shift, and from that calculate the speed of moving stuff.

  18. Andy

    also, couldn’t you measure how far it’s gone from where the star used to be, since we know when the nova occured? it’d be extremely rough, but that should be good enough for back of the envelope calculations. you could probably even do with just a picture that had a scale on it, but you’d have to make sure you meausured the gas farthest from the star, where it’s path is perpendicular to your line of sight.

  19. Marlayna

    Thanks, I think I got it :)

  20. moonflake

    The idea that when we look up at the sky we look back in time, to the universe as it was thousands or millions or billions of years ago… we already have built time machines, they’re called ‘telescopes’. Love it.

  21. Irishman

    Here’s what bugs me about the Doppler shift in spectra – how is it that we conclude that the spacing of gaps and lines is shifted and not from a different combination of elements? For instance, I saw a TV show discussing this. The show came up with a reenactment of some 19th century astronomer making discoveries, and showed him and his lab assistant operating the telescope, showed him taking the image and then going to develop it. They spent five minutes on this silly reenactment, and then squeezed in one example of comparing the spectra to show the shift. To me it would have been much more informative to skip the silly acting and show a dozen spectral comparisons and demonstrate how the comparisons are made and why scientists interpret the changed patterns as shifts rather than just different combinations.

  22. JusANuttaBackYahdah

    Telescopes are a time machine, moonflake, but did you know they work in both directions? I’m just returning from a small star party I held for a local kids group. I always enjoy showing something like the Andromeda galaxy; telling the folks that the light took 2.5 million years to reach us which is always followed by a WHOA!!! It’s funny though, I always see the future when I get a kid to react like that ūüėČ
    One WHOA-fact about the Crab not mentioned is that various records of the sky event that created this nebula show that the supernova was naked eye visible in daytime skies for some twenty-odd days. Now that’s a WHOA!
    Clear skies ūüėČ

  23. Jon Niehof

    Irishman–good question! In fact, there have been times when composition and doppler were confused, resulted in the supposed “discovery” of substances like “nebulum.” We know it’s a Doppler shift because the line spacing is quite unique and there are a *lot* of them. If every single one of dozens of lines is shifted by an amount that indicates the same velocity, and dozens of lines of some element are shifted by an amount that *also* indicates the same velocity, Occam’s Razor (as well as general probability) suggests that it’s more likely a doppler-shifted spectrum of elements we already know, rather than some exotic material that *just happens* to be like that.

  24. Pittsburghmuggle

    I like the double star in the lower left corner of both animations – you can see the smaller star moving!

  25. Lorena

    TO COMMENT 13: “The speed of light is approximately 300 Mm/s.”

    is Mm/s a new way or scientific way of saying hundred thousand meters per second? because all my life mm, has been millimeter, and I thought the speed of light near was 300.000 m/s


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