If you look to the southern sky around this time of year - away from the glow of the city lights - you can see two faint clouds, sitting below the Milky Way.
These are the Large and Small Magellanic Clouds, they have been known since people first came to the southern hemisphere with many Aboriginal groups in south-eastern Australia depicting them as cranes, such as the people of the Lower Murray, who call them "Prolggi". Their name in English, however, comes from the Portuguese explorer Ferdinand Magellan, who recorded their existence on his circumnavigation of the world.
Their names bely their true nature, because they are more than just clouds but galaxies all of their own. In fact, they represent collisions on the largest scale that seem to be unfolding in slow motion. Drawn in by the gravity of our own Milky Way galaxy, they have a complicated history of motion astronomers are still working to unravel. We can see the two galaxies are seemingly bound together by a "bridge" of gas, aptly named the "Magellanic Bridge", and trailing behind them is the "Magellanic Stream" with the "Leading Arm" in front of their path; remnants of their historic and ongoing cosmic dance. All of this spans across most of the sky, yet much of it is invisible to the naked eye.
Studying the collisions of ... nearby galaxies may offer us a glimpse into our own future.
Currently, we are uncertain as to how exactly the two clouds have collided, but based on our current evidence, there are two potential scenarios.
In the first scenario, around 2 billion years ago, the Large and Small Magellanic Clouds, which were already orbiting one another, began to fall towards the Milky Way. As they fell, around 200 to 300 million years ago, the Small Magellanic Cloud punched through the Large Magellanic Cloud, expelling its gaseous contents to form the Magellanic Bridge and the Magellanic Stream. It may have already collided at this point in a similar way even earlier around 500 million years ago. In this scenario, these galaxies are not orbiting the Milky Way and have come as close to the Milky Way as they ever will. Their journey from here will only take them further and further from us.
In the other scenario the Large and Small Magellanic Clouds may have already orbited the Milky Way one to three times. Starting around 2 billion years ago, through their mutual gravitational attraction, they formed a pair and started orbiting one another, something we see stars do. As they orbited one another, the more massive Large Magellanic Cloud pulled gas from the Small Magellanic Cloud, forming the Bridge. This complex interaction also flung off gas from both galaxies, forming the Leading Arm and a braid in the Stream - one braid from the Small Magellanic Cloud and one from the Large Magellanic Cloud.
Studying the collisions of these nearby galaxies may offer us a glimpse into our own future, when the Milky Way collides with our larger neighbour, the Andromeda galaxy, in about 4.5 billion years.
We aren't sure whether the Magellanic Clouds will stay with us forever. But they will be with us for a long time still to come, both as clouds in the night sky; and one day in the future, the gas from their braided stream may feed into our own Milky Way to fuel the birth of new stars.
- Nicholas Killerby-Smith is an astrophysics PhD student at the Australian National University.
