How do astronomers differentiate between a star and a planet?

Astronomers differentiate between stars and planets primarily based on their motion relative to the background of fixed stars. Planets exhibit apparent motion against the backdrop of stars, which is often referred to as retrograde motion or their changing position over days or weeks. This movement is due to planets orbiting the Sun at different distances and speeds than the stars, which are so far away that their positions appear constant in the night sky.

In contrast, stars are fixed points of light that maintain their relative positions over time, leading to the impression that they do not move when observed over short periods. This characteristic of planets is one of the most straightforward ways for astronomers to distinguish them from stars, making option B the correct choice.

The other options present common misconceptions. While it is true that stars often appear to twinkle more than planets due to their distances and the effects of the Earth's atmosphere, this is not a definitive factor for differentiating between the two celestial bodies. The notion that planets are always larger than stars is inaccurate, as many stars vastly outsize planets. Finally, while stars do not emit their own light in a traditional sense, they actually generate light through nuclear fusion, whereas planets reflect light from stars, making this statement misleading.