What is a light-year primarily a measurement of?

Which planet in our Solar System has the shortest day, completing one rotation in about 10 hours 33 minutes?

Which object is located at the center of our Solar System?

What is a pulsar?

Which planet is famous for having a Great Red Spot, a gigantic storm system observed for centuries?

What is the name of the process by which a star like the Sun produces energy by combining hydrogen nuclei into helium?

What is a black hole's event horizon?

Which galaxy is expected to merge with the Milky Way in the distant future?