PDA

View Full Version : Gas in a vacuum

jplumey
2009-Jan-15, 03:27 PM
I was chatting with my five year old son who loves space and all things astronomy. Unfortunately, though I have a strong interest and I am learning all the time, I cannot always answer his questions. Kids have a way of asking simply profound questions, don't they?

Anyway, we were looking at pictures of space on my iPhone when we came across the Crab nebula. He asked what a nebula was and I tried to explain but then he asked a question for which I had no coherent answer, being a trilobite in the ways of astronomy. He said, quote, "Daddy, how can there be gas in space when it's a vacuum? You know, without air?"

I stumbled on the answer and told him I would tell him tomorrow, which is actually today. So, I'm new to the forums and I'm here to learn about astronomy and to make more astronomy buddies, but also to figure out how to explain this to my five year old son, who is obviously going to turn out much smarter than me.

Sp1ke
2009-Jan-15, 04:15 PM
Well, the places where the gas is - that's not a vacuum.
Where there's no gas - that's a vacuum.

Nick Theodorakis
2009-Jan-15, 04:25 PM
First of all, vacuum is a relative thing; no vacuum is perfect. According to the wikipedia article on vacuum (http://en.wikipedia.org/wiki/Vacuum), ultrahigh vacuums created on earth range from 10-9 to 10-12 torr (which is a much greater vacuum than that creatred by oridinary laboratory instruments), whereas space can vary from 10-6 to 10-17 torr.

Diffuse nebulae range in density from a few atoms per cubic centimeter to 10,000 or more atoms per cubic centimeter (as in the Orion Nebula).

Hope this helps.

Nick

jplumey
2009-Jan-15, 05:06 PM
Well, the places where the gas is - that's not a vacuum.
Where there's no gas - that's a vacuum.

hah! That's a simple answer. Nick's a more robust answer, but yes I get the point. I guess I never thought about what the word 'vacuum' really meant, in terms of space, not the thing that sucks up dirt around the house.

Thank you both. I'll be sure to look at the links provided!

trinitree88
2009-Jan-15, 05:13 PM
Even if you get all the gas, all the liquids, all the solids, out of a volume of space...it is still not perfectly empty. There are still photons from the zero point radiation, and the neutrino sea. pete

swansont
2009-Jan-15, 05:28 PM
hah! That's a simple answer. Nick's a more robust answer, but yes I get the point. I guess I never thought about what the word 'vacuum' really meant, in terms of space, not the thing that sucks up dirt around the house.

Thank you both. I'll be sure to look at the links provided!

In practical terms it's usually the "significantly lower number of atoms" sense, rather than "no atoms" (or "nothing at all") since as trinitree88 points out, even if you could somehow remove the atoms, you would still be left with a lot of things.

grant hutchison
2009-Jan-15, 05:37 PM
It's maybe also worth pointing out that those lovely colourful pictures are all taken with long exposures, because the light from the very diffuse nebular gas is pretty dim, as you might expect.
If you were actually floating next to an emission nebula and observing it with the naked eye, you'd see a dim, colourless light, or nothing at all.

Grant Hutchison