Blue Hot, Red Not

With sayings like “red hot” and thoughts of cool blue swimming pools, it is no wonder that people associate red with hot and blue with cold. However, in reality colors associated with heat obey the opposite of the common consensus; blue indicates hotter temperatures than red. Perhaps one of the most deceiving examples, simply because of daily experience, is the Sun. A yellow star, the Sun is relatively cool compared to other stars, but it represents the hottest object within 50 trillion miles of Earth, suggesting that yellow (a close neighbor or orange and red) means hot.

Therefore, when a recent study published in the journal Science Communication took a sample of 8,866 individuals and gave them two photos of the same galaxy, one with more red than blue and the second more blue than red, and asked which photo was hotter 71.5 percent responded that the photo with more red was hotter. The other 28.5 percent answered correctly that the bluer photo indicated a hotter, more energetic region.

While the daily temperature ranges humans experience on Earth are too small to see red hot verses blue hot, space objects such as galaxies and stars are ideal examples for the inherent relation between color and temperature. With a temperature of 5,800 degrees Fahrenheit, a red star is cool compared to a 60,000 degrees Fahrenheit blue star. At a basic level, these colors not only correspond to temperature but energy, as well.


Understanding why a rainbow has multiple colors is similar to understanding why red stars are cooler than blue stars. When white light, which consists of the seven colors of the visible spectrum (red, orange, yellow, green, blue, indigo, violet), is dispersed into its individual colors, the colors separate according to their energy. Therefore, the more energetic colors of violet, indigo, and blue always appear at the bottom of a rainbow while the less energetic colors of red, orange, and yellow appear the top.

Similarly, the more energetic stars with hotter temperatures appear blue while the cooler, less energetic stars appear red. (Indigo and violet stars do not exist because no star has a hot enough surface temperature.) Due to daily experiences with red-hot stoves and red-hot fire, humans often see red and think hot, but in the grand scheme of science their conception is incorrect. However, with a simple understanding of the inherent relation between color, temperature, and energy, these misunderstandings can easily be set right.