Scientists have discovered a huge wave-shaped structure stretching about 9,000 light-years in length snaking along a spiral arm of the Milky Way, just 500 light-years from the Solar System at its closest point.
Called the Radcliffe Wave, this wave of star-forming gas is an extraordinary discovery in itself, and there’s still a lot we don’t know about it. But a team of scientists led by astrophysicist Ralf Konietzka of Harvard University has just learned one thing: Like many objects in the Milky Way, the Radcliffe wave is moving. And not just an orbital movement around the galactic center. The Radcliffe wave oscillates as a traveling periodic wave. “Using the movement of small stars born in gaseous clouds along the Radcliffe wave”explains Konietzka, “we can trace the movement of their natal gas to show that the Radcliffe wave is indeed waving“. Our understanding of the three-dimensional properties of the Milky Way has improved dramatically in recent years, largely thanks to a project known as Gaia. Gaia is a spacecraft that shares Earth’s orbit around the Sun and has been carefully mapping the Milky Way for several years. It uses parallax to measure the positions of stars in three dimensions with high precision. But that’s not all: it also measures properties such as self-motion and speed. This means we now have the most accurate map of the Milky Way to date, including the position of the stars, their direction of travel and how fast they are moving. Scientists used this data to discover the Radcliffe wave in 2018, publishing their findings in 2020 after putting together a 3D map of the structure.
There wasn’t enough information at the time to understand the structure in more detail, but a subsequent release of additional Gaia data provided vital information. In this way, the researchers were able to assign positions and movements to the clusters of baby stars embedded in the star-forming material that makes up the Radcliffe Wave. Extrapolating from this information, the researchers discovered that the structure is, in fact, undulating, like a giant cosmic serpent winding through the Milky Way. The team’s calculations reveal that this motion may be influenced by the gravity produced by normal matter in the galaxy; we don’t need to start mapping dark matter to explain it. The team’s measurements even suggest that the supernova that virtually liberated the bubble of space in which the Milky Way resides was born in a cluster of stars within the Radcliffe Wave. But, of course, there are many more questions to answer. Why did the wave form? And why does he move like that? And how many of them are out there? Is the Milky Way laced with sinusoidal arrangements of undulating gas that have yet to be discovered? “The question is: what caused the shift that gave rise to the waves we see?” says astronomer Alyssa Goodman of Harvard University. “And does it happen all over the galaxy? In all galaxies? Does this happen occasionally? It always happens?” Theories, the researchers say, range from supernova explosions, to gravitational interactions with satellite galaxies and encounters with other large galaxies. We know that the Milky Way has merged with numerous other galaxies in the past and that it currently appears to be undergoing another collision. Research last year found that dark matter can have a rather dramatic effect on the overall structure of the galaxy. There are many factors that could be at play. “Next deep and broad surveys of stars, dust and gas will likely discover more wave structures,” the researchers write, “e measurements of their movements should provide information on the history of star formation and the gravitational potentials of galaxies.”
The team’s findings were published in Nature.