Audio
Fred Watson on The Coalsack
The Coalsack facts

Distanceabout 600 light-years from Earth

Size20 - 30 light years across

Type of nebuladark

The Coalsack forms the head of our best known Indigenous constellation, the Emu in the Sky.

Close to the star at the foot of the Southern Cross is a black patch in the Milky Way – the stars in that part of the sky are blocked out by a dark nebula called the Coalsack.

A nebula is a massive cloud of gas and dust left over from the explosion of long-dead stars that eventually coalesces under incredible gravity to form new stars. Some of these clouds absorb heat from nearby stars and glow (emission nebulae), some reflect light from stars around them (reflection nebulae), and some block out light from stars or bright nebulae behind them (absorption or dark nebulae) and appear as dark patches against the sky. 

While the Coalsack blocks out light from background stars, another famous dark nebula – the Horsehead nebula – appears in stark silhouette because it blocks out the light from a bright emission nebula.

At some 600 light-years away, the Coalsack is one of the closest and darkest nebulae seen from Earth. 

Did you know?

Stars that have recently exploded can also form nebulae. When a massive star becomes a supernova, the ejected material can form a nebula around the neutron star, like the Crab Nebula. Smaller stars (like the sun) don’t become supernovas, but the shell of gas they expel as they die is called a planetary nebula. This kind of nebula has nothing to do with planets –  being round and light they looked like a planet to William Herschel who named them around 200 years ago.

Where is the Coalsack?

Finding the Coalsack should be easy – but you won’t see it unless you’re well away from bright lights.

The Coalsack sits near the foot of Crux (the Southern Cross), tucked between the brightest star (α Crucis) and Mimosa.  If you’re in a dark area you should see the roundish (coal sack-shaped in fact!) black patch between the two stars.
 

How to find the Coalsack during National Science Week

During National Science Week the Southern Cross is lying on its side with the head of the cross facing west, so the Coalsack will appear as a dark patch in the top left hand corner between the brightest star at the foot of the constellation and Mimosa.

To find the Southern Cross, face south, look about two hand-spans to the right (west). If you are in a dark area you’ll see the Milky Way rising up from the ground. Glance up a couple of hand-spans and you’ll see the cross on its side, with the pointers almost directly above it. The Coalsack is a black patch of sky above and to the right of the brightest star at the foot of the cross.

Indigenous stories of the Coalsack


The Coalsack forms the head of the best known Indigenous constellation the Emu in the Sky. If you can see the Coalsack, you might also be able to see the rest of the Emu – the shadow of dark nebulae running virtually the length of the Milky Way.

From its head near the Southern Cross, the Emu’s neck passes between the Pointers, and its body lies across Scorpius. The Emu’s orientation in the sky indicates the availability of different seasonal foods. For example, a carving of the Emu in Ku-Ring-Gai Chase National Park north of Sydney lines up with the Emu in the Sky just when real-life emus are laying their eggs.

While the Emu constellation is known to Indigenous groups from around Australia, there are some alternative interpretations of the Coalsack itself. In the central desert it’s the neck of a wedge-tailed eagle whose footprint is the Southern Cross. In parts of Victoria, the Wotjobaluk see the Coalsack as the top of a giant pine tree that can be climbed to the sky, while the Mara see the dark patch as a deep hole in a river represented by the Milky Way.