The subtlety of cosmic violence

By Phil Plait | September 14, 2012 6:30 am

One of the most amazing things I can think of can be stated simply: some stars explode.

That’s incredible. An entire star, millions of kilometers across and massing octillions of tons, can go supernova, tearing itself to shreds. The explosion is so huge that it releases more energy in a few months than our Sun will over its entire lifetime.

And yet, when seen from a distance, supernovae can produce structures of astonishing subtlety and beauty. About 11,000 years ago, a star over 800 light years away exploded in the constellation Vela, producing an expanding gas cloud. Here is one small part of that nebula:

[Click to chandrasekharenate, or grab the ginormous 8300 x 8300 pixel version.]

That gorgeous structure is NGC 2736, also called the Pencil Nebula, part of the much larger Vela supernova remnant. This picture was taken using the 2.2 meter MPG/ESO telescope in Chile. As the debris from the titanic explosion expands, it rams into the interstellar gas surrounding it. That compresses the gas, and drives a shock wave through it. A shock wave occurs when an object moves at supersonic speed through some other material – although the gas in space is thin, in some places it’s thick enough that atoms and molecules in it do collide. It’s still a thin vacuum by our standards, but physics will not be denied.

Think of it this way: imagine two people standing a few meters apart, holding a rope between them. One of them snaps their end up and down sharply. A wave is created which moves down the rope, and a second or so later the other person feels the tug. The information that the first person moved the rope took some amount of time to travel down the rope in the form of that wave.

Sound is similar, in that it’s a compression wave. When something happens to make a sound – like a tree falling in a forest – it compresses the air, and that compression moves outward at (duh) the speed of sound. It’s a way of transmitting information from one spot to another.

But now imagine something moving faster than sound. Instead of hearing the sound first, the supersonic object would actually travel past you before its sound would. Because you didn’t hear anything first, that event would surprise you, right? You might even be… shocked.

Hence the term shock wave.

And that’s what’s happening in NGC 2736. The gas from the supernova is expanding far faster than sound in the surrounding gas, so the gas is shocked. It gets hugely compressed, and forms those thin filaments and ribbons. You see this a lot in space where one thing is slamming into another (see Related Posts below). The energy of the shock wave heats up the gas, which then glows, and from a safe distance we see it as a thread of light, finely detailed and structured.

They say that in space, no one can hear you scream… but in reality, if you pick the right place and scream hard enough, you can make yourself heard across thousands of years in time, and trillions of kilometers in space.

Image credit: ESO

Related Posts:

Revealing the Veil
The beating heart of W5
A dying star with the wind in its hair
The cold, thin, glorious line of star birth

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

Comments (14)

  1. Edd

    NGC 2736 is in Vela – it’s not part of the Veil Nebula. The Veil Nebula is in Cygnus.

  2. Chris

    So is the entomology of the word shock in “shock wave” really because the person was shocked to hear it?

  3. carbonUnit

    Well, they were going to call it as “SURPRISE!!” wave, but thought better… ūüėČ

  4. Scott P.

    Can’t fool me; clearly we’re looking at the Nexus.

  5. ozprof

    @1. Yes Edd, I think he meant the Vela supernova remnant.

    NGC 2736 is more commonly known as “Herschel’s Ray”. An interesting object to observe.


  6. Edd (1): You’re right, of course. I fixed the text. Funny; I wrote this last night with sinus issues and lack of sleep from all my travel, so the two nebulae got conflated in my soggy brain. Thanks!

  7. @Chris – nit-pickery alert: I think you mean “etymology” :)

  8. Ian

    Phil, were you being humorous about the shock wave thing? I mean, obviously gas doesn’t have any ears to hear the sound first. Is that actually the nature of the word?

  9. Regner Trampedach

    v0idation @ 7: I think there was a bug in his writing… Aah, – I kill my self. :-)
    Cheers, Regner

  10. Arek W.

    Right-Upper part of picture – do I see FSM?

  11. Chris

    @7 and 9

    Argh, wrong superhero coming to the rescue
    This is who I wanted

  12. Phil: great explanation.

    Arek: I think you do. RAmen!

  13. TheVirginian

    This is probably the result of living in Louisiana for most of my adult life, but the thick red figure near the bottom looks like a crawfish (moving to the left); the red streaks around it could be parts of other crawfish. The bluer part below it could be the water it’s swimming in, but seems more likely to be the steam rising from a kettle of boiling crawfish. Boiled crawfish are considered a great delicacy down here. If the person who named it had been a Cajun from south La., it probably would be known as the Crawfish Nebula. :)

  14. RaginKagin

    Someone beat me to the punch, but I’m going to say it again…that’s The Nexus.


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