We all, by now, can easily recognise the Horsehead Nebula, a cloud of gas and dust shaped, roughly, like the head of a horse. Most of those images, though, show the formation as it would appear -- albeit enhanced -- in visible light. In infrared, as seen in a new image by NASA's Spitzer Space Telescope, the curling wisp of material becomes all but unrecognisable.
The Horsehead Nebula is what is known as a dark nebula. This means that the material therein is so dense that it blocks the light. In the visible light spectrum, this means that we cannot see what is behind the nebula; stars that look like they're in the dark space are actually in front of the nebula. However, infrared imaging is based on temperature, which allows astronomers to see details and phenomena invisible to the naked eye. In this spectrum, the Horsehead Nebula almost disappears into a thin wisp of material.
It's also a very small part of a much larger group of nebulae in the constellation of Orion -- the Orion Molecular Cloud Complex. This group is absolutely enormous, hundreds of light years across in the region between Orion's Belt and sword. The Orion Complex is important not just for its size, but for its function: it's one of the most active stellar nurseries in the night sky.
As such, it hosts many protostars and very young stars, all of which burn very hot due to the intense temperatures involved in stellar formation. In the infrared spectrum, this means the complex appears very bright. At the cooler edges of the Complex, the Horsehead Nebula almost vanishes into the background: in this image, bright blue represents very high heat, while green is cooler, and red is cooler still.
You can see the Flame Nebula (emission nebula NGC 2024) on the left glowing with high intensity. This is because it is very close to the westernmost star in Orion's Belt, Alnitak -- seen in bright blue at the top of the nebula -- which shines ultraviolet light into it.
To the right of the Flame Nebula, a second, smaller bright region shines -- reflection nebula NGC 2023, glowing with the light of fluorescent molecular hydrogen emission and home to a very hot B-type star. Both nebulae were caused by newly formed massive stars.