Okay, I'll admit that the prosaic science explanation - that this is a solar prominence, matter ejected from the sun that appears dark because it is relatively cooler than the sun's surface - is probably the right explanation here.

Update: Take a look at this new video by a NASA solar science expert describing the event that took place.

It turns out that this sighting is of a transitory cavity in the solar plasma field, known as a coronal prominence cavity, or polar crown cavity.

We propose to define a polar crown cavity as a density depletion sitting above denser polar crown filament plasma drained down the cavity by gravity. As part of the polar crown filament, plasma at different temperatures (ranging from 50 000 K to 0.6 MK) is observed at the same location on the cavity dips and sustained by a competition between the gravity and the curvature of magnetic field lines.
(From A new look at a polar crown cavity as observed by SDO/AIA)

So, all of the Death Star fans will have to wait a bit longer to see an enormous circular ship in our solar system. Although, considering what happened to Alderaan, I don't think we want to see the Death Star in our solar system! End update.

However, science fiction fans can be excused for thinking of enormous ships that take energy from stars.

Fans of Stargate may be thinking of the ship Destiny, which refuels by taking plasma directly from the surface of stars. See a dramatic refueling, which starts at about 2'30" in the following video.

The Destiny is fueled by plasma from stars. The ship skims over a star's photosphere and uses an array of retractable ram-scoops on the underside of the hull (located towards the rear of the ship) to scoop in plasma to recharge its power reserves. Destiny normally refuels itself from cool K- and M-spectral class stars. Surface temperatures of these stars are low enough (less than 5200 K) for the ship's energy shielding to protect the crew and the ship's vital systems.

Fans of Golden Age science fiction might be thinking of the enormous alien ship - 12 million miles long! - that was mistaken for a comet in Jack Williamson's 1936 classic The Cometeers:

"Perhaps it's a comet." Still frowning, Bob Star swung back toward the observatory. "It looked like one - it was a short streak of that queer, misty green, instead of the point a star would show..."

Inside the chilly gloom of the observatory, Bob sat down at the telescope. Its mechanisms whirred softly, in swift response to his touch. The great barrel swung to search space with its photoelectric eyes, and the pale beam of the projector flashed across to the concave screen.

...He stepped up the electronic magnification. Vindemiatrix and the fainter stars slipped out of the field. The comet hung alone, and swiftly grew. Its shape was puzzling - a strangely perfect ellipsoid. A greenish football, he thought, kicked at the System out of the night of space - by what?

...Using ray filters and spectroscope, with the full power of the circuits, he strove to pierce that dull green veil, and failed."