Stars are hard to study because, apart from the Sun, they are so far away that they appear as nothing more than geometric points of light. All that we can measure about this light (without getting complicated and breaking out the spectrometer) is its brightness and its colour, but fortunately this is enough for us to work out quite a lot about the star. It’s colour, for example, tell us how hot the star is at its surface.
If you’ve ever held a poker in a hot fire for a while, you would have noticed when you pulled it out that the hottest part of the metal glows with a dim red light. The hotter the fire, the brighter the glow, but you’ll also notice the colour changing. As the temperature goes up, the light shining from the hot metal changes from red to orange to yellow to white and on beyond the visual spectrum. When my jeweller cast the titanium for my wedding ring (titanium melts at almost 1700°C), he had to wear welder’s goggles to protect his eyes as the molten metal was hot enough to emit bright ultraviolet light!
Our sun has a surface temperature of almost 6500°C, which is why it shines with such a pure white light (and why we need to wear sunscreen). Cooler stars (such as Betelgeuse, at one of the corners of Orion) are more distinctly yellow or red, while hotter stars (like Rigel, also in Orion) appear visibly blue.
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