The sun is a distant star. The sun is at the center of Earth's solar system. Which revision uses an appositive to combine these two sentences correctly?

A distant star is located at the center of Earth's solar system.
A distant star, the sun, is at the center of Earth's solar system.
The sun is a star, which is at the center, of Earth's solar system.
The sun is at the center of Earth's solar system because it is a star.