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# Planet Stellar

An abnormal Dyson Sphere
 (+2, -2) [vote for, against]

Start with a mostly-dead star. If you want to start with a live star, you need the technology to be able to cool the thing down enough to basically "turn it OFF" (mostly kill it, that is).

With the star mostly dead, it is now safe to approach it reasonably closely. There is a Special Distance that is critical for this Idea. In explaining it, let us assume the mass of this star equals the mass of the Sun. It happens that at the surface of the Sun, the force of gravitational attraction is something like 27 or 28 times the gravity of Earth. If there was a solid surface you could stand on, you would weigh that much more than you do on Earth (your bones probably aren't strong enough!).

However! The force of gravity diminishes with distance from the center of mass of the gravitating object. It happens that for the Sun, if you were about 6 million miles away from it (about 9.6 million Km), and had a surface upon which to stand, you would weigh just about the same as you do on Earth. (Note that the average mostly-dead star is a lot smaller than a live star like the Sun. For the variety known as a "white dwarf", its diameter can be some thousands of Km; for a "neutron star" (always somewhat more massive than a white dwarf), its diameter can be maybe 25 Km. If you could stand on the surface of a dead star you would be subjected to millions or even billions of Earth gravities. Nevertheless, at an appropriate Special Distance from either of those stars, the force of gravity will always diminish to One Gee.)

So the Idea here is to build a solid shell around a star, at the appropriate distance from the star where its gravitational force has diminished to One Gee (one Earth gravity). Meanwhile, the shell of a "Dyson Sphere" (see link) is typically described as being about the same distance from a star as would exist a planet in its "habitable zone" (see link)\ --a Dyson Sphere around our Sun would be about 93 million miles (almost 150 million Km) in radius. For a star less massive than the Sun, the diameter of this thing that I'm calling "Planet Stellar" would be smaller than 12 million miles, and of course for a mostly-dead star more massive than the Sun, the diameter of Planet Stellar would be greater than 12 million miles.

For a Dyson Sphere, colonies are described as being built on its inner surface, where the intensity of sunlight is normal. It is assumed that artificial gravity is used, to hold objects against that inner surface (they would naturally tend to fall toward the central star). For Planet Stellar, colonies would be built on its outer surface, where it is logical to take advantage of One Gee of perfectly natural gravity, from the surrounded mostly-dead star. Planet Stellar can have a nice thick atmosphere with no "roof" (just like a natural planet like Earth), and that same natural gravity will keep that air from escaping to Outer Space.

There is a lot of room on the surface of Planet Stellar: about 450 trillion square miles (1100 trillion sq Km). (Note to Britishers: I'm using the American counting system here: 1 Billion=1000 Million; 1 Trillion = 1000 Billion.) The Earth has a total surface area (including oceans) of about half-a-Billion square Km, so Planet Stellar would have a surface area rougly equal to 2 million Earths.

There are several Problems, of course, with this Idea:

First, it is likely that an unreasonably strong substance is required, to be able to build the shell. Can you imagine building a bridge from Alaska to Antarctica, across the whole Pacific Ocean, with NO support pillars underneath it, anywhere along its length? Now imagine building a bridge like that, only three thousand times as long! --and all of it under One Gee of force, trying to make it fall down to the star it surrounds.

On the other hand, perhaps there is a "dynamic" solution to that problem. See the "Earth-Space Web" link, and its description of a way to support non-rotating "solid" rings at various heights above the Earth's surface. If it could work for this Idea, we would need lots of those dynamic rings oriented in all directions, along the inner surface of the shell of Planet Stellar.

Second problem: It is necessary to keep the spherical shell positioned such that the star remains in the center of Planet Stellar. It may be possible to place various magnetic-field sources on the inner surface of the shell, to attract/repel/interact with the star's magnetic field. One reason we want a mostly-dead star, and not a completely-dead star, is because it will still HAVE a magnetic field!

Third problem: We Want Daylight! You obviously can't expect to see much light from a dead star that is surrounded by a solid shell, after all. On the other hand, even a "mostly dead" star can emit quite a lot of light. Consider Sirius B, a white dwarf star that orbits the "Dog Star", also known as Sirius A. That white dwarf is emitting so much light that if a planet orbited it at Earth's distance from its Sun, the planet could well be decently illuminated, if not exactly warm. So, if we built a Planet Stellar shell for Sirius B, we could also build lots of "light pipes" (see link) to carry light from the inside of the sphere to the outside, where it could illuminate the landscape quite nicely. And whenever you want it to be night in your local area, just shutter the appropriate light-pipes (Note it can NEVER be night inside a Dyson Sphere, and you don't get to see the stars, either).

As an alternative, we could simply pick a mostly-dead star that was located in a "binary star system" (see link). Sirius B of course is associated with Sirius A, which is a very-much-alive star. Then all we need to do is build Planet Stellar such that it rotates around the star it surrounds, and the whole surface of the artificial world would have a day/night cycle, courtesy of the living star. (Note I am NOT saying we can rotate the shell fast enough to get a 24-hour day! There are significant limits to rotating a hollow spherical shell, after all --it will distort quite easily, for example. Not to mention major gyroscopic effects related to the rotating structure-support-rings....)

It is interesting that with respect to the Sirius system, the distance between the two stars is about the same distance as Planet Uranus is from the Sun. Sirius A is about 25 times as bright as the Sun, so its habitable zone is about 5 times the diameter of the Sun's (due to inverse/square law) --and Sirius B is well outside that zone. A Planet Stellar around Sirius B would get plenty of light for seeing, but not enough for warmth. (Not to worry; remember the surface of Planet Stellar is only a few million Km from a star that will take billions of years to become fully-dead/cold!)

Fourth problem: You Are Way Down A Deep Gravity Well! If you think it is difficult to put something into orbit around the Earth, where the minimum necessary velocity is nearly 18,000 miles per hour (nearly 29,000 Kph), well, the orbital velocity around a Sun-magnitude mass, at a distance of 6 million miles, is (if I did the calcs right) 267,000 mph (427,000 kph)! This also means that anything falling from Outer Space, like a comet (which are known to impact the Sun on occasion), is going to try to make a nice large hole when it reaches Planet Stellar's shell. You will need a MAJOR defence system to protect this thing!

That's enough for now; I'm sure other HalfBakers will come up with other problems (like cost). So? Enjoy!

 — Vernon, Mar 23 2010

Dyson Sphere http://en.wikipedia.org/wiki/Dyson_sphere
As mentioned in the main text [Vernon, Mar 23 2010]

Habitable zone http://en.wikipedia...wiki/Habitable_zone
As mentioned in the main text [Vernon, Mar 23 2010]

Earth-Space Web Earth-Space_20Web
As mentioned in the main text [Vernon, Mar 23 2010]

Light Pipes http://en.wikipedia.org/wiki/Light_tube
As mentioned in the main text [Vernon, Mar 23 2010]

Binary star system http://en.wikipedia.org/wiki/Binary_star
As mentioned in the main text; note these things are very common in the Universe. [Vernon, Mar 23 2010]

Kepler's Laws http://en.wikipedia...of_planetary_motion
The Third law is handy for computing an orbital period. (If you know the size and the period, you can compute orbital velocity, too.) [Vernon, Mar 23 2010]

Sirius http://en.wikipedia.org/wiki/Sirius
Some information about the Sirius binary star system. [Vernon, Mar 23 2010]

Wikipedia: Shell Theorem http://en.wikipedia.../wiki/Shell_theorem
An object enclosed by a spherical body will effectively be decoupled from the forces of gravity generated by that body - I guess that works both ways. [zen_tom, Mar 23 2010]

Inverse Square Law http://en.wikipedia.../Inverse-square_law
For anyone unfamiliar with it. [Vernon, Mar 23 2010]

Special Orbit #2 Special_20Orbit_20_232
As mentioned in an annotation [Vernon, Mar 23 2010]

Planetary Shell 1 http://www.newmars....pic.php?f=57&t=7872
[MechE, Mar 23 2010]

Planetary Shell 2 http://forums.xkcd....ic.php?f=18&t=55610
[MechE, Mar 23 2010]

Planetary Shell 3 http://en.wikipedia.org/wiki/Dyson_sphere
Read under "Other Types" [MechE, Mar 23 2010]

Read the publication "A Visit to Suprajupiter" [MechE, Mar 23 2010]

As mentioned in an annotation [Vernon, Mar 23 2010]

Comets impacting the Sun http://solar-center...du/news/comets.html
As mentioned in the main text [Vernon, Mar 24 2010]

Not an original idea, more commonly done with a large gas giant or brown dwarf. It could theoretically also be done with an active star, with careful application of heat pumps to create hot spots to increase radiation away from the livable areas.
 — MechE, Mar 23 2010

 Hold on a minute.

 Let's examine a small portion of the shell. Take a patch, say 1km by 1km and consider that in isolation. If you stand on this platform, you are being stuck to it by the star's gravity, rather than by the gravity generated by the mass of the platform itself. Can we consider the platform to be in orbit? I mean, if the star rotates by itself, or if the platform rotates about the star, in terms of this system alone, the two are identical right? So if the platform is in orbit then so are the people standing on it. But if the platform is in orbit (i.e. freefall) then so are the people on it too, at the same rate, and wont that mean they lose the benefit of the gravity - in the same way a person in a spaceship orbiting the earth doesn't "feel" earth's gravity because they and their spaceship are moving at the same rate of accelaration - they still experience close to 1g, it's just that the spaceman and his vehicle are both experiencing it at the same time.

 If the platform can be considered to be in orbit, maybe we have to think differently about the shell - maybe it's not really "in orbit" if it entirely circles the star. But, if it rotated about the star, even just a little bit, wouldn't that cause sticking to the ground issues, the same as an orbit would? Then, structurally too, you'd need to make the thicknesses different at different angles from the axis of the spin, since the merest rotation, multiplied by the massive diameter is going to impart very different accelarations on the structure at the poles than at the equator.

 So you'd probably have to avoid spinning.

Then, how do you keep the sphere centrally focussed on the star, any minor perturbation from perfect alignment would be magnified in a feedback loop that would draw one side of the sphere into contact with the star (actually, I'm not 100% sure about that one, maths may be required, but run with me here) - is that where the magnetic fields come in? These would either (depending on the aforementioned maths) either be redundant, or have to be actively controlled based on the positioning of the whole system. It would be tricky to keep on track - like keeping up a hula-hoop.
 — zen_tom, Mar 23 2010

 [MechE], I claim this Idea is at least independently original with myself. If you know of other places where it has been discussed, please provide a link.

[zen_tom], the main text mostly assumes the shell of Planet Stellar is not rotating with respect to the star it surrounds. Building it with some rotation, to get a day/night cycle with respect to a second star, is discussed. Keeping the star centered is also discussed. Please read more carefully.
 — Vernon, Mar 23 2010

 I did read those bits, just expanded on them a bit - it's the standing up bit I'm trying to visualise.

 It's interesting, since in terms of gravitation, it should be analogous to us standing on earth and not flying off into space as earth rotates. Of course, we are trapped by gravity - but then, so is the spaceman in the international space-station, and so is, for that matter, the moon. All are feeling the effects of what is still, at least in the space-station's case, essentially 1g (gravity itself doesn't tail off that much up in orbit, it's just that the orbit of the space-station doesn't intersect with the matter generating the gravitational field i.e. earth)

There must be a value, linked to density, mass, diameter and distance that describes whether you can stand up on a surface and be considered, to all intents and purposes in terms of movement to be a part of that surface, or whether you will scate off as it and you go about your business independently and be considered to be "in orbit". At what point does that change, from one to the other?
 — zen_tom, Mar 23 2010

[zen_tom], there has to be a rotation factor, before you can devise such a formula. Remember that for Earth, "geosynchronous orbit" is the place where orbital speed can keep an object directly above an equatorial location on the rotating Earth. And there also is "Special Orbit #2" (see link), where the orbital speed equals the speed of the planet's rotation.
 — Vernon, Mar 23 2010

What stops the air seeping through the shell into the vacuum within?
 — pocmloc, Mar 23 2010

[pocmloc], "air tight construction", such as is used in building any space ship or submarine. A minor matter, compared to the other Problems listed. Even if there was some seepage, remember that when building Planet Stellar in the first place, any air it was given came from "outside" the construction locality. Replacement air can be likewise imported as needed (but the better the air-tightness of the shell, the less that will be needed).
 — Vernon, Mar 23 2010

[+] and I want to see the religions that pop up. You *would* have to choose the star rather carefully I imagine, for a juxtaposition of both radiant output and gravity.
 — FlyingToaster, Mar 23 2010

Since the shell is not gravitationally affected by the star, either solar sails arrayed in and adjustable around some openings in the shell, or relatively weak ion type thrusters could be used for station keeping.
 — MechE, Mar 23 2010

seems silly to ignore the fact that if the sphere was spinning the equatorial parts would be "in orbit" around the sun and thus only the polar parts would be forced to act structurally to any great extent. This property of motion could also be used to induce a gravity like sensation at the equator. then you would have a huge ring world which is a pretty baked idea. At the heart this is either a really poorly thought out idea (for all the reasons listed) or a pretty obvious "if you had a ring world and expanded it to a solar size then rounded it into a sphere it would be really big. I would presume that it would be easier to produce two ring worlds, both in motion but different in diameter and spinning around perpendicular axi to capture the rest of the solar energy.
 — WcW, Mar 23 2010

[MechE], thanks for the links! That earliest one is about the same time I had the Idea myself. I recall reading a Larry Niven article... (see link). The article didn't happen to mention this, but it sufficed to get me thinking.
 — Vernon, Mar 23 2010

Speaking of Larry Niven, I'm actually currently reading (and thoroughly enjoying) "Ringworld". What a coincidence.
 — DrWorm, Mar 24 2010

wonder how small you could make one of these with a black-hole at the center, assuming you want a 14psi atmosphere. [edit: the words "not likely" come to mind]
 — FlyingToaster, Mar 24 2010

I would assume (without doing any calculations, naturally) that if a megastructure was close enough to be gravitationally held by a black hole, it would already be within the event horizon.
 — DrWorm, Mar 24 2010

Nope, depending on the mass of the black hole it is entirely feasible to build one of these around it. The critical bit is tidal effects on the surface.
 — MechE, Mar 24 2010

The "special distance" described in the main text is completely distinct from the type of mass that gravitates, at the center of the shell. All that matters is that there be enough mass to provide at least 1 Gee of gravitation. The more mass, the farther from its center will be the Special Distance. If there is not enough mass for 1 Gee, then humans won't be squished trying to traverse the surface of the object, and the only thing you might want a shell for is to hold atmosphere INSIDE the shell, near the gravitating body (like the Moon, for example).
 — Vernon, Mar 24 2010

 [ZT], the gravityless situation you describe would only occur if the sphere were rotating at an angular velocity such that the centripetal force due to rotation was equal and opposite to the gravitational pull of the star. Despite the fact that this is the definition of a stable orbit, the shell would not need to rotate to maintain orbit due to the fact that the gravitational pull of the star would be acting on all parts of the shell with the same intensity, ergo the structural integrity of the shell would be holding it up.

Also, you would only be weightless at the equator, and gravity would increase as you moved towards the poles (if the shell were spinning).
 — MikeD, Mar 24 2010

 [annotate]

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