Gary 7

I don’t know enough about SNR, so you could be right….

I would note that the Planetary Nebulae that I was talking about are also optically thin to optical light, but it’s optical depth at the ionization edges that matter.

-Rob

It’s not that the balloon wants to move in the opposite direction of gravity, it’s that it moves along the gradient of air pressure (high to low), and that is due to gravity. In an accelerating car, the acceleration is the substitute for gravity, creating a horizontal pressure gradient. The balloon therefore moves along that gradient, again from high to low.

I could have been more clear and written “acceleration” and not “gravity” but what I wrote is still essentially correct.

Rob Knop: the gas in the nebula is optically thin to optical light, so any visible photon generated will just pop right out of the gas. I expect that given the age and low density of the nebula it’s a pressure-driven ionization and not photoionization, and therefore the segregation of the light emitted is due to a real mass distribution, and is not an ionization effect.

“Like the bubbles in a soda bottle, the helium in the balloon is lighter than air and wants to move opposite the direction of gravity.”

Not exactly.

I’ve been reading the helium balloon explanations and everyone seems to be all around it with derivatives but here’s what they teach in pressure instrument calibration school -

The reason a helium balloon rises is because of the pressure differential around the balloon (Bernoulli). To simplify, air presses down on the top, up on the bottom, to the left from the right and to the right from the left (you can add front an rear if you wish). In calm air the force of the air pushing from the left and right are equal, so the balloon will not move in those directions. There is a pressure differential between the air pushing on the bottom of the balloon and the air pushing on the top of the balloon. Because air is more dense closer to the earth there will be a net force upward. If this force is greater than the force of gravity acting on the mass of the helium and its container the balloon will rise.

Likewise with the balloon in the car. When you hit the brakes the air in the passenger compartment tries to maintain its initial velocity. This results in a high pressure area at the front and a low pressure area (relatively) at the rear. It’s the force caused by this difference in pressure that pushes the balloon to the rear. The lower inertia of the helium filled balloon allows the very small force of the pressure differential to cause the rearward movement. If I remember correctly, the inertia of an air filled balloon is greater than the pressure differential force so it would continue to move forward, albeit somewhat slower.

The difference in pressure is demonstrated in pressure instrument calibration school using a high accuracy, high resolution barometer. Each student, in turn, takes the barometer places it on the floor and records the reading. He/she then moves the barometer to the top of another student’s head and records the reading. The subject’s height can then be calculated by the difference in air pressure between the two levels. Accuracy is typically within 2 – 3%. Pretty good considering the uncontrolled classroom atmosphere.

If you had a pressurized air vessel (closed system) with a helium balloon in it the balloon would maintain its initial position. It would neither rise nor fall no matter what its initial position. If a helium balloon rose due to being lighter than air, a helium balloon in a highly pressurized closed vessel should rocket to the top as the air is very dense (heavy). This is not the case. It does not move because the air pressure is equal all around the balloon (no pressure differential to move the balloon).

Sorry to all who knew this but the ‘lighter than air’ thing doesn’t really explain the particulars of the effect.

As an unrelated aside -

When dealing with a high quality pressure calibration device the biggest source of error is ……

GRAVITY

The equipment is so accurate, the gravity at the use site can cause an error in measurement as much 10x that of the equipment itself. This is corrected by getting a gravity report and using software to apply a correction factor.

It’s a pretty interesting field.

Greg

Refering to the ‘bulge’, I’m guessing it bulges out, as do our schooner glasses, and I think, so does any other large glass.

Ivan.