Astronomy Picture of the Day

Explanation: What if you could see, separately, all the colours of the Ring? And of the surrounding stars? There's technology for that. The featured image shows the Ring Nebula (M57) and nearby stars through such technology: in this case, a prism-like diffraction grating. The Ring Nebula is seen only a few times because it emits light, primarily, in only a few colours. The two brightest emitted colours are hydrogen (red) and oxygen (blue), appearing as nearly overlapping images to the left of the image centre. The image just to the right of centre is the colour-combined icon normally seen. Stars, on the other hand, emit most of their light in colours all across the visible spectrum. These colours, combined, make a nearly continuous streak -- which is why stars appear accompanied by multicoloured bars. Breaking object light up into colours is scientifically useful because it can reveal the elements that compose that object, how fast that object is moving, and how distant that object is.