Any iconic image of the universe you can think of was probably taken by the Hubble Space Telescope. Take the Eagle Nebula, for example. What you might not realise is that this spectacular, coloured photograph was originally black and white.
The photo originally taken in 1995, was black and white, on which experts then use coloured filters to tease out the final product.
How they turn an uninspiring black and white image to a coloured masterpiece?
Light from astronomical objects comes in a wide range of colors, each corresponding to a particular kind of electromagnetic wave. Hubble can detect all the visible wavelengths of light plus many more that are invisible to human eyes, such as ultraviolet and infrared light.
Astronomical objects often look different in these different wavelengths of light. To record what an object looks like at a certain wavelength, Hubble uses special filters that allow only a certain range of light wavelengths through. Once the unwanted light has been filtered out, the remaining light is recorded.
Hubble's many filters allow it to record images in a variety of wavelengths of light. Since the cameras can detect light outside the visible light spectrum, the use of filters allows scientists to study "invisible" features of objects — those only visible in ultraviolet and infrared wavelengths.
Many full-color Hubble images are combinations of three separate exposures — one each taken in red, green, and blue light. When mixed together, these three colors of light can simulate almost any color of light that is visible to human eyes.
The red, green, and blue filters represent different elements that are part of the nebula’s chemical make-up. Here,in case of Eagle nebula, red indicates the presence of sulfur, green is for oxygen, and blue represents hydrogen — the most abundant element in the universe.
After layering one filtered image atop the other, the final colourful image gives astronomers an idea of where and how much of each of these elements are in the nebula.
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