Well, it turns out that our Solar System’s specific corner of the Milky Way is basically a hollow shell. Our Sun sits inside a vast, thin chamber of superheated gas that astronomers call the Local Hot Bubble.
Nobody is sure why they didn’t give it a catchier name like the Local Hot Pocket, but since this area is such an oddity, researchers are determined to figure out how it got here.
Recently, a group of scientists created a detailed map of this space, showing that the bubble is lopsided in both shape and temperature, while also uncovering a puzzling tunnel that stretches out toward the Centaurus constellation.
The latest findings on the bubble’s layout and temperature confirm the idea that it was carved out by supernova explosions, which blew the structure open and heated it up. Meanwhile, the existence of that tunnel hints that our pocket connects to a neighboring bubble of thin gas.
An environment that is a million degrees hot
The defining feature of the LHB is its heat. This zone is estimated to span over 1,000 light-years, with temperatures sitting right around a million Kelvin. Since the atoms are so far apart, that intense heat doesn’t actually burn up the objects inside, which is definitely a good thing for us. It does, however, create an X-ray glow, which is the clue that allowed scientists to discover it years ago.
Mapping out a space while you are stuck inside it is way harder than it sounds. Think of a fish—assuming it was smart—trying to explain the shape of its tank while floating in the center. It’s a tough task, but the right tools make it a lot more doable.
Trying to explain the shape of the very vessel that holds us
This is where eROSITA, a high-tech space telescope from the Max Planck Institute, comes into play. Led by astrophysicist Michael Yeung, a team used this gear to get the clearest look at the LHB yet.
We know from past research that the LHB likely formed when a series of stars blew up about 14.4 million years ago. The fact that our Solar System is dead center is just a lucky coincidence. Before this, though, the bubble’s shape was pretty vague, looking like a weird, lumpy blob.
Trying to escape Earth’s halo
A huge benefit of eROSITA is its location. Traces of Earth’s atmosphere reach surprisingly far into space, forming a massive hydrogen cloud called the geocorona that extends over 600,000 kilometers from the surface. When particles from the Sun hit this outer layer, they produce a fuzzy X-ray glow that looks a lot like the LHB itself.
The eROSITA instrument is on a spacecraft about 1.5 million kilometers away. It sits in a steady gravitational spot between the Earth and Sun, making it the first X-ray telescope to look at the universe from totally outside our planet’s glowing halo.
The team broke down the telescope’s data into about 2,000 pieces and carefully examined the light in each one to create a map of the bubble. They discovered that the structure is growing taller rather than wider compared to the galactic disk. This isn’t surprising, because there is less resistance pushing back from the vertical directions.
The uneven heat patterns they found align with the theory that supernovae created this bubble, hinting that stars might have been exploding in our vicinity as recently as a few million years ago.
This map also sharpened our view of the LHB’s outline, enabling the team to build a 3D model. The final shape looks a lot like the expanding gas of a bipolar nebula, though it is somewhat more jagged and uneven. It also held a surprise.
The Centaurus Tunnel
“We were unaware of an interstellar tunnel heading toward Centaurus, which cuts a clear gap through the cooler gas between stars,” says astrophysicist Michael Freyberg from the Max Planck Institute for Extraterrestrial Physics. “This area shows up extremely clearly.”
We still aren’t sure where this tunnel actually leads. It heads toward a mix of cosmic objects, including the Gum nebula, another bubble right next door, and some molecular clouds.
This discovery might also be evidence that the galaxy is actually a vast web of connected hot bubbles and tunnels, a theory proposed in 1974 that lacked proof until recently. We could be close to uncovering that entire system now, which would help us better understand the recent history of our galaxy. The full study appears in the journal Astronomy & Astrophysics.