They say a picture is worth 1000 words. But sometimes you need 1000 words to describe a picture. Astronomical images often beg more explanation and so today I'd like to do them justice by taking us on a journey through a famous nebula - the Veil:
Wow, this image is gorgeous.
Before I dive into the picture above I'd like to take a step back and put this image in context. This nebula is part of a much larger structure called the Cygnus Loop. The Cygnus Loop is a supernova remnant from a star that exploded probably somewhere between 5000-8000 years ago. It takes up an area on the sky equivalent to 36 full moons! If you happened to be alive at the time, this supernova would definitely have been visible from Earth.
Whoa, wait a minute. If this thing is 36 times the area of the full moon, why don't we get a lovely view of it at night? The short answer is that these images are composite images that are captured by telescopes that put the human eye to shame.
The blue image with the labels above is an ultraviolet images. Humans can only see visible light, so this telescope is capturing very faint emission with a long exposure time that humans could never hope to see because our eyeballs don't have access to this part of the electromagnetic spectrum.
The first image up top with the lovely pink color is a color-composite RBG image that has been processed to highlight some very interesting features of the nebula. The pink color is representative of a very narrow filter type of exposure that tracks a specific emission feature known as H alpha, or Hydrogen alpha. The teal tracks my favorite emission feature, Oxygen III, or OIII for short. While these emission features are very interesting to astronomers because of the processes they track in nebulae, they also make for a beautiful image.
What color are these transitions? OIII is a green color and H alpha is red (look to the left). So in astronomical images, astronomers usually attempt to keep these transitions closely related to their true colors. But while these transitions are actually visible to the human eye, we wouldn't be able to see them looking up at the sky.
This image is created by combining a cumulative 36 hours of exposures. This telescope also has superior light collecting ability. The amount of light a telescope can capture is related to the area of the mirror. Put this way, you can see why the human eye falls short. But this is why we build telescopes, right?