Why we miss the dark

The dark is dust

The dark patches in the Milky Way are not holes. They are clouds of dust and gas that block the light of the stars behind them. A dust grain is tiny, roughly 10 to 100 nanometres across, with a rocky core and, in the coldest clouds, a mantle of ice (Fraknoi et al. 2022).

Such grains dim starlight by absorbing and scattering it; astronomers call this extinction. Blue and violet light is lost faster than red, which is why stars behind dust look redder than they are (Fraknoi et al. 2022).

In 1919 the astronomer E. E. Barnard published a catalogue of 182 dark markings in the sky (The Astrophysical Journal 49, 1-23).

Not dark matter

The word "dark" is used for two very different things. A dark nebula is ordinary matter, dust and gas, that blocks light. Dark matter is something else: according to NASA it makes up about 27 percent of the universe, against about 5 percent for ordinary matter, and it can be detected only through its gravity. It does not absorb light, reflect it or emit it.

In infrared you see through it

A dark cloud is dark only in visible light. At an infrared wavelength of 2 micrometres the dust blocks about one sixth of what it blocks in visible light (Fraknoi et al. 2022), which is why infrared telescopes can see the stars behind the cloud. What we read as a shadow or as an animal is a window to such a telescope.

Figure and ground

Why do we see the stars as the figure and the dark as the background? Our perception keeps dividing what we see into two: a figure that stands out and a ground behind it. The Danish psychologist Edgar Rubin described this in 1915 in Synsoplevede figurer (roughly: perceived figures), with the famous vase that can also be two faces in profile. You see one or the other, never both at once.

The space around and between things is called negative space in the visual arts, and in Gestalt psychology it belongs to the perception of figure and ground. Whoever reads that space as the figure sees something different from whoever reads the dots as the figure.

This is how the Western sky is built: stars are the dots, lines connect them, and the dark is what is left over. The Emu and Yacana turn this around: the dark shapes are the animals. It is not a different piece of sky, but a different way of looking. See it for yourself on the overview page, where you can slide between the two.

• To the overview page with the slider

See for yourself

From Europe the finest example is the Great Rift: the dark lane that splits the Milky Way in summer under very dark skies, from the Summer Triangle to Sagittarius (NASA Night Sky Network). You need a truly dark place for it.

The Emu and Yacana are best seen from the Southern Hemisphere. Read their stories on the pages about the Emu in the Sky and about Yacana.

• The Emu in the Sky
• Yacana, the sky llama