According to the Density Wave Theory, the arms of the Milky Way are comparable to ________.?

Question

a. Light waves
b. Ocean waves
c. Sound waves
d. Electromagnetic waves

Answer 1

Probably d), but I'm going to take a stab here and say that the entire contents of the milky way (that we see) are comparable to the swirl of water around the bathtub drain. Looking at galaxies comparable to the milky way, I swear I'm staring straight at a big black hole.

Answer 2

b. ocean waves

The following is from the web site listed below:

-- begin quote
Density waves pass through the disk like water waves pass over the ocean.
- Stars move through the spiral arms.
- Gas clouds try to move through, but some are induced to form stars (collision or compression)
-- end quote

As an ocean wave passes by in the deep ocean, a blob of water doesn't move with the wave, but executes a roughly circular or elliptical motion and ends up in about the same place after the wave has passed (unless there is a current, in which case the motion of the current is superimposed on the action of the wave).

A small neighborhood of interstellar gas in the galaxy behaves much like this. There is a current of sorts (the rotation of the gas about the center of the galaxy). Separately from this, the gas passes through the density wave; the gas doesn't travel with the wave, but its motion is disturbed as it passes through.

The answer c (sound waves) has some merit because sound waves are a type of density wave. In a sound wave, however, the wave travels at about the same speed as the air molecules. In an oceanic wave, the wave travels more slowly than the atoms in the water. Similarly, a density wave in a galaxy travels more slowly than the atoms in the gas.

-- added a few minutes later:
Brant makes a very good point. Sound waves compress the air, and density waves compress interstellar gas; so from that point of view, c is the best answer. If you look at the speed of the wave versus the speed of the particles, then b is the best answer, as I mentioned above. The bottom line is that this is a badly worded question, and I really don't know what answer is expected by the teacher and/or textbook. (Brant -- I'm pretty sure you mean that A and D are the same thing.)

-- one further thought:
It is a relatively subtle point as to whether the gas travels more quickly than the wave or vice versa, whereas compression is a really important property of the density wave, so I think Brant deserves "best answer" here. "Sound waves" is probably the answer expected by the questioner.

Answer 3

I would pick sound waves. Dr. Bob gives a good argument for ocean waves, but sound really is a compression wave. It is one in which the density of the medium is the primary attribute of the wave, not the gravitational amplitude.

In any case, you have three different answerers so far, with three different answers. That means it is a good question. Deserves a star. (Even though it's not really *your* question.)

BTW, I think A and B eliminate each other since they are both the same thing.

Oh, gosh, yes! I did mean A and D! Sorry.

Answer 4

i usually think 'resonance waves' which are like standing sound waves.

picture a microphone moving around the milkyway at the distance of the Sun...

"ssssssssssSSSssssssssssSSSssssssssSSShhhhh"

every time it moves past a standing wave it gets louder.

(note: thats a metty-for.. we ain't talking 'sound waves')