Galactic Arms Point Forward or Backward?

Hello group, I have a question about galactic arms.

In Grade 8, my science teacher told the class that as a galaxy spins, it
throws the stars out and forward, so that the ends of the arms point in the
direction of the galactic rotation. On the blackboard she drew a diagram of
the arms being thrown "out and forward" by the spinning galaxy.

I told her that this is impossible, that the galactic arms start out
straight. As the galaxy starts to rotate, the outer stars would move more
slowly than the stars near the center, and gravity would pull them in.
Therefore, the ends of the arms should point in the direction opposite the
galactic rotation.

After a minute of debate, she started to get angry, so I backed down,
letting the class think she was right and I was wrong. The old junior high
has since closed down, so I don't know where she is now. AFAIK she still
teaches students her version. To this day I wonder if any other student has
bothered to question this view. IMO, reacting so strongly against having
your view questioned does not encourage scientific curiosity.

Now that the memory of this debate has resurfaced, I was wondering if an
astronomer could finally confirm for me whose view of galactic rotation is
the correct one? Thanks in advance.

Eric J. Roberts wrote:
Hello group, I have a question about galactic arms.

In Grade 8, my science teacher told the class that as a galaxy spins, it
throws the stars out and forward, so that the ends of the arms point in the
direction of the galactic rotation. On the blackboard she drew a diagram of
the arms being thrown "out and forward" by the spinning galaxy.

I told her that this is impossible, that the galactic arms start out
straight. As the galaxy starts to rotate, the outer stars would move more
slowly than the stars near the center, and gravity would pull them in.
Therefore, the ends of the arms should point in the direction opposite the
galactic rotation.

After a minute of debate, she started to get angry, so I backed down,
letting the class think she was right and I was wrong. The old junior high
has since closed down, so I don't know where she is now. AFAIK she still
teaches students her version. To this day I wonder if any other student has
bothered to question this view. IMO, reacting so strongly against having
your view questioned does not encourage scientific curiosity.

Now that the memory of this debate has resurfaced, I was wondering if an
astronomer could finally confirm for me whose view of galactic rotation is
the correct one? Thanks in advance.

Trying not to fall into the kind of language that probably encouraged
the teacher to argue -

The most important fact is that the orbital period increases with
distance from the center of a spiral (even when the velocity remains
constant). One mechanism for making spiral arms, of the disorganized
or "flocculent" sort, is what you were talking about at the time.
Any collection of stars or whatever that doesn't hold itself
together gravitationally will be progressively sheared toward
a spiral section by this differential rotation. As star-forming
events age they fade, so there is a clear favoritism to which
features we see. There are some classic numerical simulations -
look up Seiden and Gerola's stochastic self-propagating star formation
or SPSF models.

Furthermore, in a differentially rotating and self-gravitating
disks, there exist spiral-shaped wave modes which, once excited,
can propagate around the disk for very long times. These so-called
density waves can be excited by bars or close gravitational
encounters with other galaxies, and are most important in grand-design
spirals - those in which one can trace a few arms for 180 degrees
or more around the center. The math is very similar for leading and
trailing waves, so in principle leading arms could exist. In nature,
only a few leading arms are known (either one or two of the arms
in NGC 4622, seen in a Hubble Heritage image, must be leading,
because they wind in different directions), and there are large
numbers of spirals in which we have enough data to show that the
arms are trailing.

But "flinging stars outward"? I have no idea where that might have come from.


Bill Keel

"Eric J. Roberts" wrote in message news:oOE8c.594$li5.450@pd7tw3no...
Hello group, I have a question about galactic arms.

In Grade 8, my science teacher told the class that as a galaxy spins, it
throws the stars out and forward, so that the ends of the arms point in the
direction of the galactic rotation. On the blackboard she drew a diagram of
the arms being thrown "out and forward" by the spinning galaxy.

I told her that this is impossible, that the galactic arms start out
straight. As the galaxy starts to rotate, the outer stars would move more
slowly than the stars near the center, and gravity would pull them in.
Therefore, the ends of the arms should point in the direction opposite the
galactic rotation.

After a minute of debate, she started to get angry, so I backed down,
letting the class think she was right and I was wrong. The old junior high
has since closed down, so I don't know where she is now. AFAIK she still
teaches students her version. To this day I wonder if any other student has
bothered to question this view. IMO, reacting so strongly against having
your view questioned does not encourage scientific curiosity.

Now that the memory of this debate has resurfaced, I was wondering if an
astronomer could finally confirm for me whose view of galactic rotation is
the correct one? Thanks in advance.

You are right, she is wrong. The angular directions of galactic
rotations have been measured directly by way of Doppler shift. They
spin with the spiral arms trailing, just like a stirred cup of coffee.
Never, ever trust an elementary school teacher to know a damn thing
about natural science.

-Mark Martin

Mark Martin wrote:
"Eric J. Roberts" wrote in message
news:oOE8c.594$li5.450@pd7tw3no...
Hello group, I have a question about galactic arms.

In Grade 8, my science teacher told the class that as a galaxy
spins, it throws the stars out and forward, so that the ends of the
arms point in the direction of the galactic rotation. On the
blackboard she drew a diagram of the arms being thrown "out and
forward" by the spinning galaxy.

I told her that this is impossible, that the galactic arms start out
straight. As the galaxy starts to rotate, the outer stars would move
more slowly than the stars near the center, and gravity would pull
them in. Therefore, the ends of the arms should point in the
direction opposite the galactic rotation.

After a minute of debate, she started to get angry, so I backed down,
letting the class think she was right and I was wrong. The old
junior high has since closed down, so I don't know where she is now.
AFAIK she still teaches students her version. To this day I wonder
if any other student has bothered to question this view. IMO,
reacting so strongly against having your view questioned does not
encourage scientific curiosity.

Now that the memory of this debate has resurfaced, I was wondering
if an astronomer could finally confirm for me whose view of galactic
rotation is the correct one? Thanks in advance.

You are right, she is wrong. The angular directions of galactic
rotations have been measured directly by way of Doppler shift. They
spin with the spiral arms trailing, just like a stirred cup of coffee.
Never, ever trust an elementary school teacher to know a damn thing
about natural science.

-Mark Martin

I remember seeing a TV program in which a bunch of newly qualified primary
school teachers were asked basic science questions, and the answers were
pretty terrifying. The one that sticks in my mind was "what are trees
primarily made out of?" Most of those interviewed explained that trees suck
up soil from the ground, and are thus made from that.

When the interviewer responded with "suppose I suggested that they are made
from carbon?" one of the teachers replied "I'd say you're mad".

Ian

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I remember seeing a TV program in which a bunch of newly qualified primary
school teachers were asked basic science questions, and the answers were
pretty terrifying. The one that sticks in my mind was "what are trees
primarily made out of?" Most of those interviewed explained that trees suck
up soil from the ground, and are thus made from that.

When the interviewer responded with "suppose I suggested that they are made
from carbon?" one of the teachers replied "I'd say you're mad".

Sarcasm and being offensive are still part of the teacher's arsenal,
then. As opposed to knowledge, of course.
The experiment in which someone grew a tree in a pot, weighed the soil
before and after the tree had grown, and found the difference is
negligible is a couple of hundred years old at least.
--
Those who can't do, teach.
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