How come there aren stars of different colours like purple or green stars?
The colors our eyes see are fine-tuned by evolution to detect only a narrow part of the spectrum where most of the Sun’s energy is concentrated, between 400 and 700 nm, centered at 550 nm. All the colors you and I see are in this narrow range: from violet at 400 nm to red at 700 nm. Each perceived color in this range is quite narrow, amounting to typically 50 to 80 nm only. But stars, like the Sun, do not put out light in narrow ranges. They radiate light in very broad ranges — which is why the Sun itself looks white to the eye: it’s radiating strongly across all the colors we can see. If the Sun were radiating ONLY at 550 nm, it would look green. Now stars DO have different colors, caused mostly by their different temperatures. But those are colors only in the very broad sense, that the center of their outputs are shifted away from the Sun’s 550 nm center, more to the red end, or more to the blue end. But almost all stars still radiate strongly across the entire human-visible spectru
