Just Try to Imagine th​e Two Million Mile Per Hour Winds at the Milky Way’s Core

On Earth, wind starts knocking over old and weak trees at not much above 40 or so miles per hour. Double that and it's not safe to be outside at all. Double it again and we're fully in apocalyptic, civilization-erasing weather. The highest recorded wind speed on Earth is 253 mph, observed on Australia's Barrow Island during tropical storm Olivia. Tornadoes certainly offer wind still more brutal, but also brutal enough to destroy our instruments.

New observations collected by the Hubble Space Telescope, and presented at this week's meeting of the American Astronomical Society, show cosmic winds blasting outward from the heart of the Milky Way at over two million miles per hour. These winds, a gaseous assault of charged particles, are the result of a still mysterious eruption that occurred at least two million years ago. Mostly, they consist of silicon, carbon, and aluminum, the sorts of heavy elements produced in the guts of newborn stars.

The Milky Way's screaming core is a question as well as an answer. The winds help explain a pair of odd structures existent to this day at the galaxy's center, known as the Fermi bubbles. These are turbulent clouds of gas extending above and below the plane of our galaxy (which you might envision as a bumpy frisbee). The bubbles are enormous, extending 30,000 light-years above and below the mainstream Milky Way. They are also highly energetic; imagine blowing up a balloon with a jet exhaust, basically.

The cosmic wind description comes courtesy of Andrew Fox of the Space Telescope Science Institute in Baltimore, who used the Hubble telescope's spectrograph instrument to probe ultraviolet light emanating from a distant quasar located on the far side of one of these bubbles. With this light, it was possible to characterize the insides of the bubble itself based on how the quasar's light was affected by it.

What Fox and his team found is that gas on the far side of the bubble is racing away from Earth (and the galaxy's core), while gas on this side is racing toward Earth (still away from the core)—at two million miles per hour.

"This is exactly the signature we knew we would get if this was a bipolar outflow," explained Rongmon Bordoloi, also of the Space Telescope Science Institute, in a statement. "This is the closest sightline we have to the galaxy's center where we can see the bubble being blown outward and energized."

The winds feeding the Fermi bubbles are a scorching 17,500 degrees Fahrenheit, which is actually a bit cooler than the galaxy's deepest core. This suggests some amount of mixing with interstellar gases, a potential clue in the hunt for an explanation for the phenomenon. In addition, we know that the bubbles are relatively short-lived, suggesting that they're periodic events that occur throughout the Milky Way's lifetime.

Fox hopes that further quasar-based investigations (this is the first of 20 quasar probes) will lead to new information, particularly that relating to the total mass of ejected material making up the Fermi bubbles. Comparing this mass to the outward velocities of cosmic materials should reveal the energy needed to cause the phenomenon. This energy may reveal a cause—likely either a chorus of near-simultaneous supernova births, or a collection of stars falling into the galaxy's central black hole. Either set of events would release lots of energized gas very quickly, perhaps enough to form our bubbles/winds.

"It looks like there's a link between the amount of star formation and whether or not these outflows happen," Fox noted. "Although the Milky Way overall currently produces a moderate one to two stars a year, there is a high concentration of star formation close to the core of the galaxy."