How Dense Is A Nebula?

[L. Marcus]

Hiya, folks. Just had a little question: What is the average density of a nebula? Or the interstellar medium? Or the solar wind? I got a reading on the density of a planetary nebula, at 1,000 particles per cubic centimeter - and those clouds are pretty dense compared to the others (at least I think so . . .)

 

Anyone?

 

Bueller?

[Foxiekins]

Re: How Dense Is A Nebula?

 

I believe the Solar Wind is around 1 proton per cubic centimeter...

[L. Marcus]

Re: How Dense Is A Nebula?

 

I seem to recall that the interstellar medium has something like ten hydrogen atoms per cubic meter . . . ?

[Basil]

Re: How Dense Is A Nebula?

 

The solar wind at about 1 AU out is 2 - 10 atoms/sq. cm, travelling about 400 km/sec.

 

The average interstellar dust is about 1 grain in a cube 100 meters on a side.

 

Throughout the galaxy, the average density is 10^-15 grams/cubic km.

 

Hmm...

Sorry, I thought I had more-useful information.

[L. Marcus]

Re: How Dense Is A Nebula?

 

The solar wind at about 1 AU out is 2 - 10 atoms/sq. cm, travelling about 400 km/sec.

 

The average interstellar dust is about 1 grain in a cube 100 meters on a side.

 

Throughout the galaxy, the average density is 10^-15 grams/cubic km.

 

Hmm...

Sorry, I thought I had more-useful information.

Any idea where I can get some more info?

 

By the by, the data's for a warp drive in a new Hard SF setting. Does Not Work In Dense Nebulae (-1/4), or some such . . .

[Captain Obvious]

Re: How Dense Is A Nebula?

 

I have no data on nebular densities (although I'm interested in seeing what others have), but for a C+ drive like those used in Saberhagen's Berserker series, I'd let ships travel above the maximum safe speed with the side effect of taking hull damage from abrasion by all the gas molecule impacts.

 

Although, that may not be what you're after, I realize....just thought I'd chime in.

[L. Marcus]

Re: How Dense Is A Nebula?

 

I have no data on nebular densities (although I'm interested in seeing what others have), but for a C+ drive like those used in Saberhagen's Berserker series, I'd let ships travel above the maximum safe speed with the side effect of taking hull damage from abrasion by all the gas molecule impacts.

 

Although, that may not be what you're after, I realize....just thought I'd chime in.

You're welcome to chime whenever you feel like it, Cap.

 

The idea with the drive and nebulae is that the warpfield gets 'shortcirquited' when gas and plasma of a high enough density interacts with it.

[shadowcat1313]

Re: How Dense Is A Nebula?

 

Fire Fusion and Steel for Traveller had a section on odd science drives, and one thats discussed is the Bussard Ram Scoop. an idea involving using a scoop to suck up hydrogen molecules while on the move, but with the density of such molecules the scoop would have to be huge, or use electromagnetic technology

 

it quotes 2 hydrogen atoms per cubic centimeter

[Nyrath]

Re: How Dense Is A Nebula?

 

An average area of space contains about one or two atoms of hydrogen per cubic centimeter.

A nebula has a density of 10 to 10^5 atoms per cubic centimeter (10 to 100,000).

A molecular cloud has a density of 10^7 to 10^9 molecules per cubic centimeter.

Earth's atmosphere at sea level is about 10^19 atoms per cubic centimeter.

[Curufea]

Re: How Dense Is A Nebula?

 

Although I think the space between the galaxies would be much less dense. So wouldn't be "average".

[Nyrath]

Re: How Dense Is A Nebula?

 

Although I think the space between the galaxies would be much less dense. So wouldn't be "average".

True. So two atoms per cubic centimeter in a galaxy.

This was a plot point in Poul Anderson's classic Tau Zero. A Bussard Ramjet starship suffers an accident, damaging the deceleration system. If they turn off the drive, the interstellar hydrogen atoms will impact the hull, causing enough radiation to kill the crew.

 

So they have to go where the interstellar density is exceedingly low. They have to go, ahem a little further than they planned...

[Cancer]

Re: How Dense Is A Nebula?

 

How complex do you want to get?

 

When you introduce the interstellar medium ("ISM") to students, one of the phrases you introduce is the "three phase ISM". Because of how gas cools by radiation at various temperatures, there are only limited temperature regimes which are stable (that is, will persist for times longer than the collapse time of the cloud ... though it has to be noted that those collapse times are rather long on a human timescale!). These are characterized as "hot" (millions Kelvin), "warm" (thousands Kelvin), and "cold" (tens Kelvin). Very roughly, these states can be considered to be in pressure equilibrium ... which means that the product of (particle density) * (temperature) is approximately the same in all of them. This isn't exactly the same as saying the density of atoms varies in the same way, because the ionization fraction changes drastically among the three phases: the hot state is about 100% ionized, and the cold state is <1% ionized. (A decent helpful page is here.) It also doesn't tell you about dust, which is strongly more concentrated in the cold phase.

 

The relative proportions of the three phases is something we wish we knew. I think it is fair to say that BY VOLUME IN THE GALAXY -- that is, sort of how likely you are to encounter a phase if you travelling in a starship -- the "cold" state is the least common and the "hot" state is the most. The numbers I recall dimly (I did stars, not ISM) are sort of hot:warm:cold 60:30:10. Those numbers are "ancient" ... that is, from my circa-1980 pre-Hubble Space Telescope grad school days ... and could be hideously antiquated now.

[L. Marcus]

Re: How Dense Is A Nebula?

 

Nyrath and Cancer have both now deserved a spot each in my little house shrine . . . Thanks for the info, guys!

 

Now for the tough one: Do you know where to get any detailed info on the density of matter within, say, 100 lightyears or so of Earth?

 

Anyway, is there any great differences?

[Cancer]

Re: How Dense Is A Nebula?

 

Nyrath and Cancer have both now deserved a spot each in my little house shrine . . . Thanks for the info, guys!

 

Now for the tough one: Do you know where to get any detailed info on the density of matter within, say, 100 lightyears or so of Earth?

 

Anyway, is there any great differences?

 

Yes, I know where to look.

 

Oh, wait ... you want the details in a convenient format? Hmm. That's harder. Lemme do a bit of googling and see what I can find. It's quite possible that this isn't going to be something that has been satisfactorily translated out of Tech-ese, though.

 

That you limit it to 30 parsecs helps, sort of. That's a pretty piddly small part of the Galaxy. Yeah, to first order, that volume is mostly hot phase: we're mostly in the "Local Bubble".

 

Turns out that phrase "Local Bubble" is one where Google feeds you lots of good stuff. Other links are here and here ... those have graphics, the second one may be more technical than you want. A simpler site (with less info) is here.

[L. Marcus]

Re: How Dense Is A Nebula?

 

Yes, I know where to look.

 

Oh, wait ... you want the details in a convenient format? Hmm. That's harder. Lemme do a bit of googling and see what I can find. It's quite possible that this isn't going to be something that has been satisfactorily translated out of Tech-ese, though.

 

That you limit it to 30 parsecs helps, sort of. That's a pretty piddly small part of the Galaxy.

I'm researching -

 

Or rater, you're researching () for a Hard Tech SciFi campaign, taking place three hundred years from now.

 

I call it 2300 CE.

[Nyrath]

Re: How Dense Is A Nebula?

 

Many astronomers have a suspicion that the local bubble was created when the star Geminga went supernova about 100,000 years ago.

http://en.wikipedia.org/wiki/Geminga

http://www.scienceblog.com/community/older/archives/D/archnas1799.html

 

These maps may prove useful:

You can see Geminga in the first map, a little below and left of center. It's off center now, but it was approximately in the center of what is now the local bubble 100,000 years ago.

[L. Marcus]

Re: How Dense Is A Nebula?

 

Many astronomers have a suspicion that the local bubble was created when the star Geminga went supernova about 100,000 years ago.

http://en.wikipedia.org/wiki/Geminga

http://www.scienceblog.com/community/older/archives/D/archnas1799.html

 

You can see Geminga in the first map, a little below and left of center. It's off center now, but it was approximately in the center of what is now the local bubble 100,000 years ago.

Well, they most certainly probably will! Thanks many bunches!

 

Looking at that second map feels like looking at some kind of cosmic mine field . . .

[Nyrath]

Re: How Dense Is A Nebula?

 

Before I forget, those images are from The Guide to the Galaxy by Henbest & Couper, Cambridge 1994, ISBN 0-521-45882-X. Out of print, but you might be able to see it through an inter-library loan.

 

Used copies go for about $45 on http://www.bookfinder.com

 

A most useful book, jammed packed with useful astronomical information. The larger maps trace out the spiral arms.

 

And another note: in the map, the galactic core is in the direction of the top of the map, the left side is "spinward", the right side is "trailing", and the bottom is "rimward"

[Nyrath]

Re: How Dense Is A Nebula?

 

Ah, the paperback version of Guide to the Galaxy is available for $28 from Amazon.

http://www.amazon.com/exec/obidos/ASIN/052145882X/qid%3D952114822/sr%3D1-16/103-2315286-8680647

[Cancer]

Re: How Dense Is A Nebula?

 

I blundered across this today, looking for something else. It is terse in the extreme, and skims a lot of detail, but it's self-documenting (that is, it cites the technical literature directly) and covers a very large scale of information.

 

(This is part of the justification documents for an ESA space mission.)

[L. Marcus]

Re: How Dense Is A Nebula?

 

Ye gads, that's a monster of a document!

[Cancer]

Re: How Dense Is A Nebula?

 

Ye gads' date=' that's a monster of a document! [/quote']

 

Now you know why so few proposals for space missions get submitted!

[L. Marcus]

Re: How Dense Is A Nebula?

 

Yeah! The teams all die of old age before the specs are ready!

[Cancer]

Re: How Dense Is A Nebula?

 

I'll let you in on a secret: you never throw away old documents. You just recycle the source files, updating now and then.

 

A hard drive crash is your worst nightmare.