We've all seen stunning images showcasing the diversity and beauty of objects in space - from majestic spiral galaxies, colourful nebulae, or even the still mysterious planets in the Solar System. Such pictures have taught us much, but photography isn't the only tool used by the astronomers of the world.
Spectroscopy is one such tool, and is simply the splitting of light into a rainbow, or perhaps more appropriately, a space rainbow. As we're most familiar with them on Earth, rainbows occur when light from the sun, which appears white to our eyes, passes through a material and is refracted into the array of colours making it up - a spectrum. For rainbows in the sky, this material is water droplets in the atmosphere, but a prism of glass can serve equally well and indeed is the item of choice for many telescopes.
The true power of the technique comes when you realise that not all colours in a rainbow are equally bright, and that some colours are actually missing.
In addition to this, anything emitting light, from campfires and fluorescent lighting, all the way to stars and galaxies, can have their light studied in this way.
Think of a campfire and the orange light that flickers about, illuminating tents and faces engaged in conversation. Compare that to the blue flame from a gas stove that you might use to prepare a hearty meal, and you might ask yourself why the flames are different colours. The answer is that each is actually a different temperature - things glowing orange or red are cooler than things glowing blue or white. The same is true of the stars. Betelgeuse is a red giant star in the constellation of Orion, and its atmosphere is a little over 3500 degrees - much hotter than your oven, but relatively cool for a star. Sirius on the other hand, the brightest star in the sky, is closer to 10,000 degrees!
Consider a rainbow from the sun in the image pictured here, chopped up and stacked to fit. You'll notice lots of vertical black lines where colours appear to be missing. These correspond to atoms or molecules, mostly in the form of gases, in the atmosphere of the sun. Each kind of atom, from common elements like oxygen to those more rare like uranium, absorbs light at certain different colours - like a fingerprint. The more of the element, the darker the black line, and if we know where to look a rainbow has the power to tell us what the stars are made of - for every star has a unique spectrum.
Astronomers can also use it to study the clouds of gas and dust where stars are born, what the atmospheres of the planets in the Solar System are like, or to hunt for heavy elements made in the death throes of the very largest stars in supernovae explosions. So next time you happen to look at the night sky, give a moment to consider what secrets of the universe are yet to be found - hidden in literal rainbows.
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