An astronomer looks at the light coming from two stars: star A and star B. Star B is emitting bluer light than star A. Which star is hotter?
What we need to know to answer this question is that hotter stars emit more blue light. Whereas, cooler stars tend to emit light that’s more red. This might be a bit surprising and counterintuitive at first. After all, normally we think of red as a warm color and blue as a cold color. However, this has links to something else we study in physics, the electromagnetic spectrum.
After all, the stars that we’re talking about are emitting light. And light is a form of electromagnetic radiation. For now, we only need to consider this sort of middle bit of the electromagnetic spectrum. Over on the left, we can sort of ignore the radio waves and microwaves for now. And on the right, we can ignore the x-rays and 𝛾-rays. In the middle, we’ve got the infrared, IR; the visual spectrum; and ultraviolet, UV. We know that the electromagnetic spectrum, as we’ve drawn it here, as we go from left to right, we’re increasing in energy. In other words, ultraviolet radiation has a higher energy than visual light. And visual light has higher energy than infrared radiation.
Now the stars that emit higher energy radiation are also hotter. Hence, higher energy stars tend to emit from this part of the spectrum which happens to be overwhelmingly bluey purply. Whereas, lower energy stars tend to emit from this part of the spectrum which happens to be more red. Therefore, hotter stars tend to be more blue and cooler stars tend to be more red. And our final answer is that star B is hotter than star A.