Interstellar Gas

[Dekan]

I was reading Fred Hoyle's SF novel "The Inferno". Its scenario is, the centre of our Milky Way galaxy has exploded and become a quasar. High-energy particles from the quasar are rushing towards the Earth. Scientists are discussing the potentially catastrophic effects these particles will have on the Earth.

 

One scientist says, we shouldn't worry, because:

 

"There is gas, interstellar gas, all along the plane of the Galaxy. The gas will act as an effective shield."

 

Another scientist refutes this reassurance, by asking:

 

"How much gas? Along a unit-column, what? A hundredth of a gram for a column with a cross-section of one square centimetre....high-energy particles pass very easily through one hundredth of a gram of hydrogen."

 

Is the gas really that thin - so that a column of one square centimetre, with a length of about 30,000 LY, contains only 100th of a gram of hydrogen?

[swansont]

3 x 10 ^ 10 cm/s * 3.15 x 10^7 sec * 30,000 * 1 cm^2 is about 3 x 10^22 cm^3. The interstellar medium has a density of about a proton per cm^3 (or perhaps less), which would be 0.05 grams. So yeah, that's about right.

[insane_alien]

intergalactic space is even less dense

[Dekan]

3 x 10 ^ 10 cm/s * 3.15 x 10^7 sec * 30,000 * 1 cm^2 is about 3 x 10^22 cm^3. The interstellar medium has a density of about a proton per cm^3 (or perhaps less), which would be 0.05 grams. So yeah, that's about right.

 

 

Thanks swansont for the figures. I thought Hoyle couldn't be wrong on his own subject, even if he did make dubious claims about prehistoric birds.

 

It does seem to beggar belief though - a column of gas, with the not tiny cross-section of one square centimetre, and extending for 30,000 light-years, containing less than 0.05 grams of hydrogen!

 

Thanks also insane_alien. Your mention of the intergalactic medium reminds me of Poul Anderson's "Tau Zero", where the runaway starship had to go, not just into intergalactic space, or even inter-galactic-cluster space, but into what Anderson termed "inter-Clan" space (ie between clusters of clusters of galaxies), before the gas got thin enough for the crew to go outside the ship without being clobbered by relativistically mass-swollen particles.

Edited May 16, 2011 by Dekan

[Airbrush]

I believe I read the average density of the universe is about one atom per cubic meter. I also believe I read the average density of the middle of the great voids is also about one atom per cubic meter. My guess is that the average density of the universe is only a tiny bit more dense than the center of the great voids.