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Bear with me. I want to explain this to my 6 and half year old.

If, all things being equal, we could reach out to the Sun with a suitable implement, a gigantic space knife, could we cut a piece off the Sun? Take a slice from it? Do we end up with a body of Hydrogen and Helium, or is it impossible to extract anything from it?

I know that the Sun is a massive ball of plasma, the sheer mass compressing the gas through gravity until fusion is achieved. What I have been asked is, effectively, what happens if you try to take a piece of that plasma? Of our sun?

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    $\begingroup$ I'm voting to close this question as off-topic because this is not really astronomy but a rather odd "what if" question. $\endgroup$
    – StephenG
    Feb 26 '18 at 19:17
  • $\begingroup$ Fair enough. I was hoping for an actual, genuine answer - there isn't any other group of experts on this topic! $\endgroup$ Feb 26 '18 at 19:19
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    $\begingroup$ It is a physics question, but the physics of stars is pretty much ignored by all but the astronomy and astrophysics communities, so I'd like to keep it open here. $\endgroup$ Feb 27 '18 at 15:45
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The idea of removing material from the sun for either fuel or to lengthen its lifespan is called "star lifting". Various hypothetical methods have been suggested, including heating parts of the atmosphere or squeezing it using magnetic fields. My own tentative suggestion is to combine these with chaos control to manipulate turbulence into making coronal mass ejections in desired directions.

The problem is not just that the sun is hot (5777 K at the surface), but that the stuff that is easy to scoop up with a huge magnetic field is a fairly sparse plasma - you don't get that much material despite the vast scales. Of course, you may want to compress the plasma into a smaller and more convenient ball, but now the ideal gas law tells you that it will heat up proportionally - to get a cool hydrogen ball you will need to bleed off all that heat slowly.

Which is why I think Dyson spheres are probably the easiest way to start.

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    $\begingroup$ I'm going to wait until tomorrow but this is a not unreasonable answer on the practicalities while ignoring the fantastical aspect of my question. Good stuff. $\endgroup$ Feb 26 '18 at 21:41
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    $\begingroup$ I'm going to accept this as the most reasonable answer that I can work with. Thanks - now to try and format this for my daughter! This isn't exactly the Stack site which is most amenable to humour, is it? $\endgroup$ Feb 27 '18 at 9:42
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    $\begingroup$ @KevinTeljeur - Thanks. Reformulating physics for kids is a fascinating topic on its own. That is where you actually get to see if you have understood it yourself. Good luck! $\endgroup$ Feb 27 '18 at 15:44
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We do not have, so far, have the technology to gather the Suns resources, besides Solar Panels here on Earth. A Dyson's Sphere is a theorized technology that could be able to harness the Suns energy. I am sure NASA is working on a solution, but at 5600 Deg. Celsius / 10,000 Fahrenheit, not even titanium can withstand a Close Encounter with our closest star, much less grabbing a piece of it. The Parker Solar Probe is a planned mission to the Sun, in the Summer of 2018. The probe is supposed to approach the Sun and orbit 3.67 million miles from the surface. We will have to see if it lasts very long. We will have to see!

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    $\begingroup$ Really has little to do with the question asked. $\endgroup$
    – ProfRob
    Feb 27 '18 at 12:41
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A "space knife" is a non-starter: You can't cut a plasma with a knife because it behaves like a gas, not like a solid. You can't put anything into the sun or go anywhere near it because it is hot enough to vaporise anything. Any concept that involves "flying over the surface and scooping up hydrogen" is not going to work.

Getting close to the sun is hard. You are orbiting the sun at 30km/s If you want to get close to the sun, you will need to slow down a lot that requires a lot of fuel. Then to get away from the sun you will need to speed up again. This takes a lot more fuel, far more than you could hope to gather from the sun.

It is possible to collect some hydrogen without going anywhere near the sun. You can collect hydrogen and helium from the solar wind. The sun blows plasma off its surface and this could be collected using magnetic fields. This "wind" is not very dense: about 3-6 atoms per cm³. So you need to scoop a very large region for a long time to get a reasonable amount of gas. This gas is hot (36000 degrees) charged (its a plasma) and diffuse.

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    $\begingroup$ Your orbital mechanics are off -- a parabolic or hyperbolic flyby, with the "slingshot" effect to gain speed, will let you get nice and close. $\endgroup$ Feb 28 '18 at 12:12
  • $\begingroup$ The delta v to to from a circular orbit to parabolic one are substantial. $\endgroup$
    – James K
    Feb 28 '18 at 18:20
  • $\begingroup$ OK, -- I wasn't requiring that we start out from Earth. $\endgroup$ Feb 28 '18 at 20:22

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