Planet Stellar

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!