![]() ![]() ![]() Some very hot stars emit light primarily at ultraviolet (UV) wavelengths, while some very cool stars emit mostly in the infrared. Astronomers can learn a lot from studying the details of the spectrum of light from a star. Like our Sun, every star emits light at a wide range of wavelengths, all across the visible spectrum and even outside the visible spectrum. In the bottom, a layer of thick clouds appears in the infrared wavelengths. In the top, only the wavelengths of visible light show. (a) Visible light is part of the electromagnetic spectrum, which ranges from gamma rays with very short wavelengths, to radio waves with very long wavelengths. The full range of electromagnetic radiation, or the electromagnetic spectrum, is shown in Figure below. EM radiation with wavelengths shorter than 400 nm or longer than 700 nm exists all around you - you just can’t see it. Visible light ranges from wavelengths of 400 nm to 700 nm, corresponding to the colors violet through red. The color of visible light is determined by its wavelength. Visible light - the light that human eyes can see - comes in a variety of colors. The smallest red-shifted galaxies are among the most distant galaxies we have detected so far at 13 billion light-years away. How long does it take light to travel from one side of the galaxy to the other? 100,000 years! If an astronomer looks through a telescope at a star that is 1,000 light years away, is she seeing the star as it is now? The galaxy we live in, the Milky Way Galaxy, is about 100,000 light-years across. That means the light from Proxima Centauri takes 4.22 years to reach us. Proxima Centauri, the closest star to us after the Sun, is 4.22 light-years away. That’s a long way! But by astronomical standards, it’s actually a pretty short distance. A light-year is a unit of distance that is defined as the distance that light travels in one year. Since astronomical distances are so large, it helps to have a unit of measurement that is good for expressing those large distances. ![]()
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