This finely detailed picture exhibits the center of NGC 1097, a barred spiral galaxy that lies about 48 million light-years from Earth within the constellation Fornax. This image reveals the intricacy of the net of stars and mud at NGC 1097’s middle, with the lengthy tendrils of mud picked out in a darkish purple hue. The extent to which the galaxy’s construction is revealed is thanks to 2 devices on the NASA/ESA Hubble Space Telescope: the Wide Field Camera 3 (WFC3) and the Advanced Camera for Surveys (ACS).
The idea that a single image can be taken using two different cameras is not very intuitive. However, it makes far more sense after delving into how beautiful astronomical images like this one are composed. A helpful starting point is to consider what color is, exactly. Our eyes can detect light waves at optical wavelengths between roughly 380 and 750 nanometres, using three types of receptors, each of which is sensitive to just a slice of that range. Our brain interprets these specific wavelengths as colors. By contrast, a telescope camera like the WFC3 or ACS is sensitive to a single, broad range of wavelengths to maximize the amount of light collected. Raw images from telescopes are always in greyscale, only showing the amount of the light captured across all those wavelengths.
Colour images from telescopes are indirectly possible, however, with the help of filters. By sliding a filter over the aperture of an instrument like the WFC3 or ACS, only light from a very specific wavelength range is let through — one such filter used in this image is for green light around 555 nanometres. This yields a greyscale image showing only the amount of light with that wavelength. This multicolor image of NGC 1097 is composed of images using seven different filters in total.