Barred for life

By Phil Plait | April 3, 2007 6:16 pm

It’s been a while since I posted a pretty picture just to post it. Click it for a bigger version (or go here to get a choice of images, including a monster image at 48 meg!).

Of course, I can’t just leave it at that! Astronomy is so much more than just pretty pictures.

There’s a lot to see in this image, the latest release form Hubble. It’s NGC 1672, a barred spiral galaxy (NGC stands for New General Catalog, as my last real episode of Q and BA notes). All by themselves, normal spirals are weird enough. We know that stars closer in to the center revolve around the core of the galaxy faster than stars farther out, so that tells you right away that arms are not just bands of stars. If they were, they’d wind up tighter and tighter with time, and they wouldn’t last very long (I’m surprised I haven’t seen any creationists use that as an argument against an old Universe).

So what are they? Turns out, the best hypothesis so far is that they are standing waves, like cosmic traffic jams. If you were in a helicopter over a traffic jam on the freeway, it would look like the jam is a permanent fixture of the traffic. But in reality, cars leave the jam at the same rate as cars entering it. So while the jam itself stays put, the cars making it up always change. So it is with spiral arms: they are places where the matter in the galaxy is compressed, but stars enter the jam and stars leave. The arm looks permanent, but over time its resident stars, gas, and dust change.

Weird, eh?

But of course, things always get stranger. NGC 1672 is a barred spiral. The big arms don’t go all the way to the center; they seem to emanate from the ends of a rectangular structure. The bar here isn’t as obvious as in some galaxies (like, for example, our own Milky Way), but if you look at the high res version of the image you’ll see it better. Bars are really weird, and are a result of the complicated gravitational forces due to an extended mass like a galaxy. The gravity from individual objects is easy to understand — the farther you are from them, the weaker you feel their gravity, and so on — but when you take a few hundred billion masses and spread them out into a disk, things get messy.

In such a situation, I’d go to a bar too. Haha. Heh.

Perusing the very high res images, you can see just what a mess things are. Gas and dust are strewn everywhere, stars scattered like dandelion seeds in the wind… people who study such things have their work cut out for them. But for you and me, we can simply scan the image and look for interesting things. Foreground stars — ones in our own Galaxy — blaze out. Fainter, more distant galaxies can be seen right through NGC 1672 (I love that). Pinkish clouds denote regions where stars are forming; stellar nurseries. These really define the arms of the galaxy; massive stars are born, and outshine all the other stars. These stars are blue, giving the arms their bluish hue. They don’t live long enough to pass out of the arm; that takes millions of years; time the stars don’t have. They explode before then, becoming supernovae, and scattering their elements back into the galaxy from which they came.

It’s the cycle of life, but on a somewhat larger scale than here on Earth.

I said above that astronomy is much more than pretty pictures, but actually, there’s a whole lot you can learn from simply looking over images like these. I’ve barely scratched the surface here. Still, it is pretty. Sometimes, as I’ve said many times in the past, that’s enough too.

Comments (36)

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  1. old amateurastronomer

    Drat, I miss being able to go out observing. My favorite objects were galaxies. I know this galaxy is out of the range of my 8-inch F/5 Newtonian, but still…
    I’m glad they provide a wallpaper image of this galaxy. I have several others and am always looking for others.

    What a beautiful galaxy, makes you wonder if there’s anyone looking back at us!!! Hope so!

  2. One technicality: a traffic jam wouldn’t look like it was standing still. It would be slowly moving backwards along the highway — a sound wave propagating through the viscous flow of traffic.

    In fact, I’m willing to bet that the spiral arms do rotate around the galactic center, but against the angular momentum. It seems weird, but as you say the arms aren’t “real”, so they don’t carry momentum.

  3. Cameron

    Astronomy is so much more than pretty pictures, but the pretty pictures get people interested. I wonder how many backyard and professional astronomers got their start after looking at Hubble images.

  4. John Armstrong Says: “One technicality: a traffic jam wouldn’t look like it was standing still. It would be slowly moving backwards along the highway — a sound wave propagating through the viscous flow of traffic.”

    Usually, but not always. In my daily commute (for those of you in the Bay Area it’s I-680 northbound through Fremont) there’s a sizable grade which, after you crest the top, you can see the freeway for about three miles (5 Km) ahead of you. In heavy commute traffic it’s always stop-and-go, and you can see the “brakelight waves” propagating backwards towards you, as you said.

    But at the bottom of the grade the road takes a gentle left-right bend and everyone hits their brakes. I don’t understand why, especially if they’re only going about 20 MPH to start with, but they do. This creates a true standing wave where the cluster of brakelights stays in one place, but is made up of an ever-changing collection of cars. This is especially evident in the winter when it’s dark at commute time.

    That’s the sort of thing I do to pass the time stuck in traffic!

    – Jack

  5. absolutely

    something wrong about that image. the outer large spiral arms seem to me to lie in a different plane to the core inner arms (or bar).

  6. I often put astronomical images as my Windows Wallpaper, but couldn’t use this one.

    I don’t go to bars anymore.

    (hiss,boo)

    J/P=?

  7. MattFunke

    Phil: “We know that stars closer in to the center revolve around the core of the galaxy faster than stars farther out, so that tells you right away that arms are not just bands of stars. If they were, they’d wind up tighter and tighter with time, and they wouldn’t last very long (I’m surprised I haven’t seen any creationists use that as an argument against an old Universe).”

    Unfortunately, they do:
    http://www.creationscience.com/onlinebook/FAQ19.html

    Scroll down to the “Spiral Galaxies” entry. This isn’t the only place I’ve seen this argument.

  8. Kristin

    The BA said:

    “If they were, they’d wind up tighter and tighter with time, and they wouldn’t last very long (I’m surprised I haven’t seen any creationists use that as an argument against an old Universe).”

    They do – I’ve seen them use that very argument! I had a “discussion” with one who said that any other explanation of the spiral structure (spiral density waves, etc) was wrong and every astronomer knows it! *sigh*

  9. John Oliver

    From my graduate student days (long ago) at UCLA I recall noticing that traffic jams caused by an accident moving slowly forward along the highway eventually showing up behind our student apartment on Sawtelle. Spiral arms also rotate slowly direct around the galactic center.

  10. I’m confused about the traffic analogies in the comments. The traffic jam would appear to move backward to a helicopter? Really?

    Oh, and the picture is gorgeous… thanks for sharing!

  11. YinYang0564

    I’ve been reading about standing waves for ages, but never really understood them. Whether they move backwards or forwards over time is irrelevant, the concept holds; I finally get it (I will admit to ignorance, but not stupidity). The analogy was a good one for me.

  12. Gary Ansorge

    Ooh! Pretty picture.

    Tanks.

    GAry 7

  13. Josie

    I fully acknowledge that what hooked me on Astronomy was Hubble, but the past few weeks the BA sort of opened up floodgates. I can’t get enough!

    I’m actually looking about going back to add a degree now. Not so much because I want to work in the field, but because I want to know. Anyone else get hungry like this, reading the BA posts?

    There’s something elegant about the idea of something mundane like traffic jams on Earth applying itself in the same way to something as amazing as this picture.

  14. Hmmm. Yes, traffic jams can move, and I considered writing about that, but figured the post was long enough. I didn’t want to go into details which might be distracting. Yeah, spiral arms themselves probably do move.

    The creationscience.com site is full of lies. Wow. I mean, wow. That astronomy stuff is blatantly wrong, and almost all of it is explainable easily enough. That kind of willful ignorance/lying makes me pretty angry.

    Josie: well, wow (but this time in a good way!). Funny- writing these posts makes me want to learn more. Nothing probes your own blank gaps in knowledge like trying to explain something. :-)

  15. Josie

    BA: I actually live in Seattle, and attend the UW! I want to hunt down Greenburg and buy him a latte or five for his fight against Hoglund. I’d buy you one too, but you’re farther away and it’d get all cold on the way. See, around here, buying coffee for someone is the sign of the highest regard!

    *goes to dink around Space.com more*

  16. Viggen

    Given that such stars and celestial bodies are connected only by light and gravity, is this phenomenon considered an indirect evidence of gravity waves? The traffic jam analogy relies on people who visually observe conditions ahead of them and then react by pressing a foot on an accelerator or brake pedal. There is no “force” between the cars. These are stars following geodesics in spacetime, which would liken the turbulence here to the contours of a spacetime ocean. Given, stars are not test particles, but why would it organize in this way? The galaxy looks to me more like the Charybdis Whirlpool sampled at 100 billion loci by stochastically placed stars… and a whirlpool would be considered related to transverse waves in water if not direct evidence. I apologize, this analogy is thin and stretching.

  17. Josie

    *yes, I’m very bored at work, forgive me*

    I’m reading through the movie reviews, and I got to Superman Returns. BA, please say you’ve seen superdickery.com! The movie just drove the point home further! Superman is a Dick!

  18. Okay, since I started the traffic jam analogy I should explain a little further. What you’re really looking at in a traffic jam or in a spiral galaxy is a sound wave! Particles (air molecules, cars, stars…) move through a medium with varying levels of density, and the propagation of the dense region through the medium has (almost) nothing to do with the motions of the particles themselves.

    Traffic jams often propagate against the flow of traffic, as evidenced on I495 around DC: often you’ll come upon the scene of an earlier accident five miles after the jam you were in breaks up. Sometimes (as other commenters evidence) they actually do stand still and sometimes they move forwards. I’ll revise my earlier statement a little to form a new conjecture: the motion of the density wave is related to the cause — congestion behaves differently from accident behaves differently from fixed road conditions.

    You can also see the same phenomenon in water waves. Individual water molecules a ways offshore don’t move much. Try putting a beach ball beyond the breakpoint and you’ll see it just bob up and down, even as waves roll past it. The water itself stays put, and the wave is made up of different molecules every moment until it finally breaks.

    So we come back to spirals. Okay I was wrong before, but now there’s the question: how do the arms move, if at all? And what can that tell us about how the spiral formed?

    Not to steal Dr. Plait’s line, but I think it’s incredibly cool that we can even know enough to ask that question, and that we might have a shot at answering it.

  19. Crux Australis

    Man, I would *love love love love* to see Phil go through the points on that site one by one. I know you’re busy Phil, but your legions of fans would be forever grateful!

  20. George

    There is also AIG [http://www.answersingenesis.org/docs/4005.asp]. Sad to see such little sense offered, yet done in a manner that would mask the truth to someone seeking it. They may really believe this stuff, so they may not be really offering lies, but, at best, it is very disingenuous.

    Simply a gorgious image. I have printed the 8 meg. version and I’m framing it. :)

  21. slang

    A lot of those lies are covered in the Talk Origins archive, http://talkorigins.org , a very useful resource when dealing with creationist nonsense. Check the Astronomy and Cosmology section at http://talkorigins.org/indexcc/ This is just a part of the archives, there are many, much more in-depth articles. Have a browse :)

  22. Beautiful, it really is. But I was under the impression that a Galaxy spins at the same speed around it’s center, like a vinyl record. The outer stars seem to spin faster because there further out. Am I wrong?

  23. George

    Angelo, think of the wind speeds of a hurricane or the speed of our planets revolving around the sun. The further out you go in either case, the less speed you will see. What is odd for galaxies is that the outer regions do move faster than expected which helped introduce dark matter, and other models, to help explain this strangeness. The vinyl record you mentioned, or disk, is a popular analogy of this.

  24. Viggen

    Okay, since I started the traffic jam analogy I should explain a little further. What you’re really looking at in a traffic jam or in a spiral galaxy is a sound wave! Particles (air molecules, cars, stars…) move through a medium with varying levels of density, and the propagation of the dense region through the medium has (almost) nothing to do with the motions of the particles themselves.

    Well, I would agree in principle about this being a form of “soundwave.”

    My major question is “varying density of what?” In the analogy about cars, the wave propagates without any direct help from Newton’s laws (which are valid in this regime) because people are judging and mediating the motion of the particles (cars) without a direct mechanical interaction with one another. If a traffic jam wave were the same as a soundwave or water wave, every car would need to be crashed into by the car behind it and in turn crash into the car ahead with no net change in the average position of the car. As such, traffic jam waves proceed with no contact force between the cars. Stars are entering and leaving these structures (mostly) without colliding with one another, which means that this wave is not dependent on mechanical contact in the manner of a soundwave or water wave much the way a traffic jam wave is not dependent on mechanical contact. If these stars were colliding with something, I think we would know it. As such, this is not a transverse wave in water, nor a longitudinal wave in air. The wave is not itself a medium of stars, much like a traffic jam is a wave of human judgement rather than of colliding cars. What is the “varying density” these objects are traveling through? Can is be spacetime? Is there any way to account for this sort of structure without directly invoking general relativity?

    I’m not disagreeing with you:-) I am simply focusing my question: can we consider such a structure to be evidence of gravity waves? Is my understanding shortsighted in some manner and requiring a revision of logic?

  25. Gary Ansorge

    Viggen: Gas and dust in intellarstellar space are the means by which compression waves propagate. Even in our Solar system, we refer to “sound” waves propagating thru the very thin solar media. Is that gas thin? Yes! But thin doesn’t mean non-existant,,,

    Gary 7

  26. Thomas C. Mapother IV

    It’s nice to see something from time to time the way a galaxy looked say only 5 million years after the dinosaurs died out which btw were planted there to make us think the earth is somehow older then 4000 years old, that the light from NGC 1672, or as I like to call it “Rufus” traveled for 60 million years to reach us only proves how truly sneaky Satan is, no wonder the Nazis faked the moon landing for the Americans who later destroyed tower 7 so we could later get cheap oil made from the so called dinosaurs bones or the so called plants they ate, the real answer god made the oil knowing some day we would drive cars, thanx for the pic btw, hail xenu!

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