Which type of stars did astronomers classify as "of the first magnitude"?

The classification of stars as "of the first magnitude" refers to their brightness as seen from Earth. Historically, this system, known as the apparent magnitude scale, categorizes stars based on how bright they appear in the sky. Stars of the first magnitude are indeed among the brightest, not necessarily the biggest in size. The brightest stars that can be seen without a telescope fall into this category.

While this classification may seem to imply a relationship with size, it is actually tied to luminosity, distance, and the observer's perspective. Many first-magnitude stars can be relatively small or, conversely, exceedingly massive; their classification reflects their brightness rather than their physical dimensions.

The other choices do not align with the first magnitude classification. The faintest stars would fall at the opposite end of the magnitude scale, categorized as higher magnitude numbers. Stars closest to Earth can vary widely in brightness; proximity does not inherently mean they belong to the first magnitude. The term "first discovered stars" is not a recognized category in the context of brightness, making it irrelevant to the classification based on magnitude.