Lets say an alien space ship visted

[muriel_volestrangler]

9. "a power source of this ship was a white dwarf star of .5 miles diameter"

What does this mean?

Astronomers may have discovered the smallest and heaviest white dwarf star ever seen, a smoldering ember about the size of our moon but 450,000 times more massive than Earth, a new study finds.

White dwarfs are usually about the size of Earth and are the cool, dim cores of dead stars that are left behind after average-size stars have exhausted their fuel and shed their outer layers. Our sun will one day become a white dwarf, as will about 97% of all stars.

https://www.space.com/moon-size-white-dwarf-smallest-densest-ever-discovered

Even if you could carve off a bit of a white dwarf to be 0.5 miles in diameter, I don't think it would stay a white dwarf - the interaction between gravity and other forces would fundamentally change its structure, density and temperature.