The diagram shows six stars. Each star is colored approximately according to the light that it emits. Which star is the hottest? Which star is the coolest? Which star emits light that is most similar to the Sun?
Taking a look at this diagram, we see the six stars labeled a, b, c, d, e, and f. We see that these six stars have different sizes as well as different colors. Now, the sizes are not important in terms of the temperature of the star. But the color is. So as we answer these three questions about these six stars, we’ll only pay attention to the color that each star has. And speaking of color, it’s the color of an object that’s giving off energy as heat that can indicate to us roughly what the temperature of that object is. This is one of the reasons why when we look, for example, at the flames produced by a campfire, those flames have different colors. The color depends on the temperature of the air.
There is an outer portion of the flame. This often has a red tint or color to it. And then, there’s an inner part, a hot core of the flame, which typically has more of a yellow or a gold or even a white color. Along these same lines, if we look at the flame produced by a propane torch, then typically at the core of that flame, where the combustion is happening very efficiently, the temperature is hotter. And we see a blue or a white flame. Then, outside that hotter core, we may see flecks or even a solid flame of gold or orange or yellow. This tells us that that air is at a cooler temperature than the blue and white flame core. So the color of a combusting or a burning object does indeed roughly indicate the temperature of that object. And the same rule applies to celestial bodies, to these six stars.
We see that our first two questions talk about the hottest and coolest star. To answer these questions, it will be helpful to develop a color temperature scale. To create this scale, we’ll write down colors of combusting, that is, burning objects. And we’ll write them down in a particular order, starting with the colors that indicate the coolest temperatures of combustion. In other words, as we write down colors of stars, we’ll start out with those indicating low star temperature. And we’ll move up to those indicating high star temperatures.
Arranged this way, here are the colors we’ll write down. Starting with red, we then go to orange to yellow, white, then to blue-white or a bluish white, and finally to blue. Notice that this progression of colors going from low to high temperature matches up with the examples that we saw from our everyday life of a campfire flame or a propane torch. Combusting stars then work the same way as those more familiar burning objects.
Based on this color temperature chart, let’s now look at this first question, which of the six stars in the diagram is the hottest. We see that the star indicated with c is a blue star. And that, according to our scale, this color indicates the highest temperature. So that’s our answer to our first question. It’s the blue star that must be burning the hottest. Then, having looked at the high end of the color temperature scale, we now consider the other end. Which star on our diagram has the lowest temperature? That lowest temperature star will be indicated by a red color. And we see that star e is that color. So option e will be our answer to the second question of which star is the coolest. That’s based on the fact that its color, according to our color temperature scale, indicates the lowest burning temperature.
Then, finally, we have this last question. “Which star emits light that is most similar to the Sun?” Now, at this point, we have to be careful. The reason is, in many pictures that we see of the Sun and especially in drawings, the color of the sun is either yellow or an orange, like choices a or choice d. But actually, if we were to look at a photograph of the Sun, taken at the middle of the day on a clear day, we would see that the core color of the Sun doesn’t look yellow or orange. We would notice that while there’s some fringing of a yellowish type color, the core of the Sun looks white.
And in fact, if we were to look up, the particular temperature range corresponding to a white burning object, we would see that that temperature is approximately 6000 kelvin, which is the temperature of the surface of our Sun. Based on this fact, we won’t choose option a or option d as the star that emits light most similar to our Sun. Instead, a star which is actually white, but to our eye may appear to be fringed with yellow or gold, is a star with a temperature in the range of the surface temperature of our Sun. And therefore, it’s a star that emits light most similar to our Sun.
Based on all this, our answer for this final question is choice f. It’s the apparently white star, which emits light which is most similar to the light emitted by our Sun.