On a clear night in the Southern Hemisphere the sky is filled by the brightest parts of the Milky Way and the prominent dark nebula named the Great Rift, a veil of interstellar cloud drawn across the Milky Way itself for almost a third of the sky from Cygnus to Centaurus. The Great Rift is most visible where it contrasts with the bright hub of our galaxy near the constellation Sagittarius.
The Great Rift is formed by overlapping clouds of cosmic dust about 300 light years distant between our own Orion Arm of the galaxy and the Sagittarius Arm, the next spiral arm inwards towards the galactic centre. The visible gap between the bright spiral arms isn’t empty – the darker dust clouds are some of the densest parts of the galaxy and make up around half of its mass.
As the galaxy rotates around its centre about once every 200 million years, the bright spiral arms rotate more slowly than the galaxy’s ring of dark cosmic dust. It’s thought that as cosmic dust passes through the slower spiral arms it is compressed by gravity into stellar nurseries where molecular hydrogen and traces of heavier starstuff become dense enough to form hot proto-stars. But the process of star formation is hidden by the surrounding dust clouds, and although the Great Rift is filled with new stars, when viewed from the earth it is strongly back-lit by some of the brightest parts of our galaxy, including the galactic core itself.
For Inca astronomers the patterns of the Great Rift provided a celestial shadow theatre and a practical innovation. The shapes of the intersteller clouds were given ancient names from ancient legends: Yacana the llama, her baby and her shepherd; the serpent Mach’acuay and Atoq the fox who pursue Hanp’atu the toad and Yutu-Tinamou the bird across the sky. A painting at the Qoricancha Museum in Cuzco, Peru, housed in a former Inca Temple of the Sun, shows the legendary figures in the clouds of the Great Rift:
Inca astronomers also used the dark constellations of the Great Rift and other parts of the sky along with bright stellar constellations to mark the astronomical calendar and time their planting, harvests, herding and religious ceremonies. Yutu-Tinamou, the round dark nebula beside the Southern Cross known as the Coalsack, marked the southernmost point of the celestial sphere while the passage of the major dark constellations may have been used to mark the passing seasons.
The Coalsack dark nebula also appears in Australian Aboriginal astronomy as the head of the Dinewan the emu, the king of the birds in Dreamtime legend, with the broadest parts of the Great Rift and Milky Way forming his neck and body. A rock carving of an emu at a national park in New South Wales is thought to represent the “Emu in the Sky” at a particular time of year. Dinewan is shown flying, with his legs stretching back, but in legend he forever lost the ability of emus to fly.
In other Australian legends the Coalsack is the watchful head of a tribal lawgiver, or a giant fish speared by two brothers, depicted by the stars of the Southern Cross. In New Zealand Maori it is “Te Paniwi a Taewa”, the Black Fish. The first European to describe it was the Spanish navigator Vicente Yanez Pinzon during a voyage to South America in 1499. The most prominent dark nebula in the skies except for the Great Rift itself, it was named and renamed by other explorers of the southern oceans: “il Canopo fosco” (the dark Canopus, a bright southern star), or the “Macula Magellani” (Magellan’s Spot), or the “Black Magellanic Cloud”.
Space telescopes and new astronomy techniques mean it’s now possible to look through even some of the densest dark clouds of interstellar dust in very low and high wavelengths, at least partially. Among the most interesting regions is the galactic core itself, otherwise obscured by the clouds of the Great Rift but visible in hard x-rays, gamma rays, infrared and radio wavelengths. The central few light years of our galaxy are crammed with thousands of stars orbiting the centre of mass of our galaxy, which happens to also be the location of a supermassive black hole, the radio source known to astronomers as Sagittarius A*, or SgrA*. The black hole can’t be seen directly, but when SgrA* swallows a star the hot accretion disk of matter just outside the black hole gives off a burp of x-rays.
SgrA* is more than 4 billion times the mass of our sun and getting bigger. Fortunately it is a very long way off and, after devouring most of the nearby stars, it is going through a period of relatively slow growth – so slow that it probably won’t grow large enough to consume the rest of the galaxy. We of the Solar System can at least count ourselves lucky that we’re not on a world around a star like S2, which orbits SgrA* at an average of four times the distance from the Sun to Neptune, unenviably close in astronomical terms: the speed required for S2’s stable orbit around such a large black hole is around 5000 kilometres a second, or around 1.6 percent the speed of light. By peering through the clouds of Great Rift, astronomers are now using observations of S2 and other stars to map the very centre of our galaxy.