Ringworld Parameters

In article <XKudnacv3dgFuXvZnZ2dnUVZ_s2dnZ2d.DeleteThis@speakeasy.net>,
Erik Max Francis <max.DeleteThis@alcyone.com> wrote:
>
>Science fiction authors then came along and thought about making it
>solid and habitable. If you want to live on the outside, then you need
>to make it very small and around a very small red dwarf.

Or you live with a low surface gravity, as in Pohl and Williamson's
FARTHEST STAR.
--
http://www.cic.gc.ca/english/immigrate/
http://www.livejournal.com/users/james_nicoll
http://www.cafepress.com/jdnicoll (For all your "The problem with
defending the English language [...]" T-shirt, cup and tote-bag needs)

JRS: In article <1155902027.159320.244520 DeleteThis @m79g2000cwm.googlegroups.com>
, dated Fri, 18 Aug 2006 04:53:47 remote, seen in news:alt.books.larry-
niven, kuyper DeleteThis @wizard.net posted :
>
>A Dyson sphere has a surface gravity which is equal to the surface
>gravity of the enclosed star, divided by the square of the ratio of the
>radius of the sphere and and the radius of the star:
>
>Gdyson = Gstar*(Rstar/Rdyson)^2
>
>Therefore, if you make the sphere small enough, it could have a surface
>gravity equal to that of Earth; it could therefore sustain an
>atmosphere like earth's.

Not the right criterion.

To retain a long-term atmosphere, escape speed must exceed some modest
multiple of the mean molecular speed at the top of the atmosphere.
Earth has retained air, but AIUI has not retained hydrogen or helium.

But escape energy is proportional to surface gravity and to radius; in
fact, escape speed is what you would get by falling one radius in the
surface gravity field.

For any Dyson sphere around the Sun, the radius must be much greater
than that of Earth, so correspondingly less surface gravity is needed
for retention..



Consider a sphere around the Sun, radius R. Gravity goes as 1/R^2, so
escape energy goes as 1/R. Area goes as R^2, energy radiated is
constant and goes as R^2T^4 so T goes as 1/rootR; molecular energy is
proportional to T. As one increases R, escape energy falls faster than
molecular energy.

Remember, a little Solar Wind does no harm; but install a Dyson Sphere
at the photosphere, and it is *necessary* for the continued existence of
Sol that said sphere be able to retain an atmosphere at 6000K rather
well.

--
© John Stockton, Surrey, UK. ?@merlyn.demon.co.uk Turnpike v4.00 MIME. ©
Web <URL:http://www.merlyn.demon.co.uk/> - FAQqish topics, acronyms & links;
Astro stuff via astron-1.htm, gravity0.htm ; quotings.htm, pascal.htm, etc.
No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News.