The Taurus Molecular Cloud in the constellation Taurus the Bull lies about 450 light-years from Earth. This image shows two parts of a long filamentary structure in this cloud, which are known as Barnard 211 and Barnard 213. Their names come from Edward Emerson Barnard’s photographic atlas of the “dark markings of the sky,” compiled in the early 20th century. In visible light, these regions appear as dark lanes, lacking in stars. Barnard correctly argued that this appearance was due to “obscuring matter in space.”
We know today that these dark markings are actually clouds of interstellar gas and dust grains. The dust grains — tiny particles similar to fine soot and sand — absorb visible light, blocking our view of the rich star field behind the clouds. The Taurus Molecular Cloud is particularly dark at visible wavelengths as it lacks the massive stars that illuminate the nebulae in other star-formation regions such as Orion. The dust grains themselves also emit a faint heat glow but, as they are extremely cold at around –436° Fahrenheit (–260° Celsius), their light can only be seen at wavelengths much longer than visible light, around 0.04 inch (1 millimeter).
These clouds of gas and dust are not merely obstacles for astronomers wishing to observe the stars behind them. In fact, they are 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 millimeter range, are essential for understanding the early stages of star formation.
The upper-right part of the filament shown here is Barnard 211 while the lower-left part is Barnard 213. The millimeter-range observations from the LABOCA camera on APEX, which reveal the heat glow of the cosmic dust grains, are shown here in orange tones and are superimposed on a visible-light image of the region, which shows the rich background of stars. The bright star above the filament is Phi (Φ) Tauri while the one partially visible at the left-hand edge of the image is HD 27482. Both stars are closer to us than the filament and are not associated with it.
Observations show that Barnard 213 has already fragmented and formed dense cores — as illustrated by the bright knots of glowing dust — and star formation has already happened. However, Barnard 211 is in an earlier stage of its evolution; the collapse and fragmentation is still taking place and will lead to star formation in the future. This region is, therefore, an excellent place for astronomers to study how Barnard’s “dark markings of the sky” play a crucial part in the lifecycle of stars.