Expert: Philip Stahl - 11/26/2009

Question
QUESTION: Forgive me for belaboring the subject of bars in spirals, but I was wondering that if, as you said in a previous reply, "The barred spiral shape persists because an essentially standing density wave exists in the galaxy over time.", then could the following explanation of bars also hold true?:
"Over time, the orbits of stars become unstable and instead of being circular, have grown more elongated and get locked into place creating a bar."
Thank you again for your time.

ANSWER: Hello,

The short answer here is "not really" because - as I noted in an earlier response, this again wrongly  assumes the bars are purely material distributions affected by mechanical forces.  Or to put it another way, your unstable stellar orbits agglomerate and *form the bars*. But this isn't likely since it would take more than half of the stars in a given galaxy (extending from near the center to the periphery) to assume homologous parabolic orbits at once and remain in them for a long enough time to be visible to astronomers on Earth. Generally, unstable stellar orbits are the exception, not the rule- meaning that they exhibit such large deviations that they don't follow the normal circular-elliptical paths traced out via galactic rotation.

Bear in mind also, stars (irrespective of their orbit) are discrete entities. The barred forms of barred spirals don't display a discrete composition. This is exactly what leads us to propose density waves as "shocking" the ambient material in barred spirals into a kind of standing wave pattern over time. (Much like when you sprinkle sawdust on the top of a drum and apply differing beats, the sawdust assumes distinct different "standing patterns")

To form the sort of "bars" that appear in barred spirals, tens of billions of stars would have to partake of almost synchronous elongated orbits and also be very close together to present a view to us that exhibits no discreteness.

It simply isn't a plausible hypothesis.

---------- FOLLOW-UP ----------

QUESTION: Thank you for your patience in helping me understand barred spirals. I trust your answer that the best explanation is density waves. I just want to ask you two more questions about them. Actually these are two statements I have seen on the web and wonder what your take is about them. I don't know if they contradict the density wave theory or not. (Maybe the first one does.)

1.- It has been said that the bars could be the result of a tidal interaction with another galaxy.
And..
2.- The bar structure acts as a type of stellar nursery, fueling star birth at their centers. The bar is thought to act as a mechanism that channels gas inwards from the spiral arms through orbital resonance, in effect funneling the flow to create new stars.

I am probably done asking about bars, much to your relief, I'm sure!
Thanks a million.

Answer
Hello,

Actually, I don't mind answering any questions on astronomy ....well, ok...I hate the absurd ones concerning how close we are to "gamma bursters" or supernovae that may wipe us out - when global warming (albeit some on this site don't accept it, alas) is a much more immediate threat. Barred spirals are kind of in peculiar class wherein the wherewithal to really understand their basis isn't available to most laypersons. Which is a shame because they *are* intriguing critters.

Anyway, to your current questions:

1) Yes, bars *could* manifest as a result of interaction with another galaxy. The problem here is to set up, establish the parameters (including boundary conditions to initiate such interaction and dynamics) that would allow it. For example, the interacting galaxies NGC 2207 and IC 2163 disclose no bars in either galaxy. Why is this? How come interaction leads to bars in some dynamical pairs, and not in others? Proximity alone isn't an adequate response, since the pair referenced are extremely close together to the extent their spiral arms appear to be inter-penetrating each other's central disks. So, again, if interaction is the cause- why no bars here? Or is this one of the (many) exceptions that disproves the "rule"? If so, what exactly is the "rule"?

2) The bar as "stellar nursery" puts the cart before the horse. Ok, the bar can engender stellar bight, but this still avoids answering how the bars got there in the first place! Appealing to "orbital resonance" is fine, but if that be the case what are the specific resonance classes or manifestations (e.g. 3:2, 5:2, etc) and how do they pertain to barred origin, not just funneling flow to create new stars. The funneling of flow to bring about new stars is a hypothesis to explain new stars, but not a hypothesis to account for the bars in the first place.

So, it seems to me those who propose either (1) or (2) still have lots of work to do if they are insisting these can account for the *origin* of barred spirals.