This zigzagging, orange-ish ribbon of dust and gas is one of the closest star-forming regions to Earth, just 450 light-years away. And yet in visible light, this looks like just a dark patch of nothing in empty space.
This gas forms part of filaments in the Taurus Molecular Cloud. This particular part of the Cloud is called Barnard 211 and Barnard 213, after early 20th astronomer Edward Emerson Barnard, who first cataloged these "dark markings of the sky." While Barnard's optical data only showed him darkness, he correctly guessed that something was hiding in the blackness, somehow obscured by the space around it. You can see the optical version of this cloud below on the left, which makes Barnard's intuition for the cloud's true contents look even more impressive.
The apparent blackness because of the extremely low temperatures of the gas and dust grains in the filament, which means it glows so faintly that you need to look in millimeter-long wavelengths - much, much longer than the wavelengths of visible light - in order to see them at all.
Despite their feebleness, the dust grains are still thick enough to block out the bright light of the stars forming inside of them. Because the Taurus Molecular Cloud doesn't have many massive stars, which sets it apart from most other star-forming regions, it remains pretty much entirely invisible in our familiar optical wavelengths. Astronomers at the European Southern Observatory explain what's going on inside the cloud:
These clouds of gas and dust are not merely an obstacle for astronomers wishing to observe the stars behind them. In fact, they are themselves the birthplaces of new stars. When the clouds collapse under their own gravity, they fragment into clumps. Within these clumps, dense cores may form, in which the hydrogen gas becomes dense and hot enough to start fusion reactions: a new star is born. The birth of the star is therefore surrounded by a cocoon of dense dust, blocking observations at visible wavelengths. This is why observations at longer wavelengths, such as the millimetre range, are essential for understanding the early stages of star formation.
This particular image overlays the millimeter-range observations of the filament from the ESO's APEX telescope with visible wavelength data for the rest of the field, showing all the other stars that share this patch of space. The big bright star on the left is known as Phi Tauri, which is actually much closer to us than the cloud itself.
Via the ESO.