Here's why:

* Brightness is influenced by both temperature and size: A star's brightness (apparent magnitude) is determined by its temperature and its size. A large, cool star can be just as bright as a small, hot star.

* Stars have different spectral types: Stars are classified based on their temperature and spectral characteristics. The hottest stars are classified as O-type stars, while the coolest stars are classified as M-type stars.

* Examples:

* Betelgeuse is a red supergiant, meaning it is very large and relatively cool. It appears bright because of its enormous size, but its temperature is relatively low compared to hotter stars.

* Sirius is a white main-sequence star, which means it is smaller but much hotter than Betelgeuse. It appears bright due to its high temperature.

In summary: A star's brightness is not a reliable indicator of its temperature. You need to consider its spectral type and size to understand its true temperature.