Key takeaways

  • The Milky Way is not static; it moves and changes over time, including its internal structures.
  • A massive wave-shaped structure called the Radcliffe Wave spans 9,000 light-years and is composed of star-forming gas.
  • Recent research shows that the Radcliffe Wave oscillates like a traveling wave, not just orbiting the galactic center.
  • The Gaia spacecraft has provided the most precise map of the Milky Way, helping scientists understand the Radcliffe Wave’s motion.
  • Scientists are still exploring why the Radcliffe Wave formed and how many such structures might exist in the Milky Way.

Looking out at the seemingly constant sea of stars surrounding us, it’s easy to conceive of the Milky Way galaxy as static, with everything inside it fixed and immovable.

While the timeframes on which our galaxy travels sometimes defy our understanding, it does actually move.

Not all of these dynamic processes are easily visible. Just a few years ago, astronomers found a massive, wave-shaped structure reaching 9,000 light-years along a spiral arm of the Milky Way, about 500 light-years from the Solar System at its closest point.

This zigzag of star-forming gas, known as the Radcliffe Wave, is a fascinating finding in its own right, yet we still don’t know very much about it. However, a team of scientists led by astronomer Ralf Konietzka of Harvard University has just discovered that the Radcliffe Wave, like many other objects in the Milky Way, is in motion.

And not merely in an orbit around the galactic center. The Radcliffe Wave oscillates as a periodic traveling wave.

“By using the motion of baby stars born in the gaseous clouds along the Radcliffe Wave,” says Konietzka, “we can trace the motion of their natal gas to show that the Radcliffe Wave is actually waving.”

Our understanding of the Milky Way’s three-dimensional features has grown substantially in recent years, due mostly to the Gaia mission.

Gaia is a spacecraft that shares Earth’s orbit around the Sun and has been meticulously surveying the Milky Way for years. It employs parallax to accurately determine the locations of stars in three dimensions. But that’s not all: it also measures qualities like appropriate motion and velocity.

This means we now have the most precise map of the Milky Way to date, complete with star locations, directions of motion, and speeds of movement. Scientists utilized this data to uncover the Radcliffe Wave in 2018, and will publish their results in 2020 after creating a 3D map of the structure.

There was insufficient information at the time to fully comprehend the structure, but later releases of further Gaia data have revealed critical information. This allowed the researchers to ascribe locations and movements to clusters of young stars trapped in the Radcliffe Wave’s star-forming material.

Extrapolating from this information, the researchers discovered that the structure is actually undulating, resembling a massive cosmic snake flowing across Milky Way. The team’s simulations show that this motion may be controlled by the gravity created by conventional matter in the galaxy, so we don’t need to start mapping out dark matter to explain it.

The team’s data even indicate that the supernovae that nearly erased the bubble of space in which the Milky Way sits were formed in a cluster of stars within the Radcliffe Wave.

However, there are many more questions to be answered. Why did the wave form? And why is it moving the way it does? And how many of them are there? Is the Milky Way intertwined with yet-to-be found sinusoidal configurations of undulating gas?

“The question is, what caused the displacement giving rise to the waving we see?” explains Harvard University astronomer Alyssa Goodman. “And does it happen throughout the galaxy? In every galaxy? Does this happen on occasion? “Does this happen all the time?”

The researchers’ theories include supernova explosions, gravitational interactions with satellite galaxies, and collisions with other big galaxies.

The Milky Way has already collided with several other galaxies, and it looks to be colliding again right now. Last year, researchers discovered that dark matter may have a significant impact on the overall structure of a galaxy. There are several elements that might be in play.

“Upcoming deep and wide surveys of stars, dust and gas will likely uncover more wave-like structures,” the study’s authors write, “and measurements of their motions should provide insights into the star formation histories and gravitational potentials of galaxies.”

The team’s findings have been published in Nature.

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