Galactic Superbubbles
around the Solar Neighborhood & our Local Chimney

The first detailed map of space within about 1,000 light years of Earth places the solar system in the middle of a large hole that pierces the plane of the galaxy, perhaps left by an exploding star one or two million years ago.

The new map, produced by University of California, Berkeley, and French astronomers, alters the reigning view of the solar neighborhood. In that picture, the sun lies in the middle of a hot bubble - a region of million-degree hydrogen gas with 100-1,000 times fewer hydrogen atoms than the average gas density in the Milky Way - and is surrounded by a solid wall of colder, denser gas.

Instead, said astronomer Barry Welsh of UC Berkeley's Space Sciences Laboratory, the region around the sun is an irregular cavity of low-density gas that has tunnels branching off through the surrounding dense gas wall. Welsh and his French colleagues suspect that the interconnecting cavities and tunnels, analogous to the holes in a sponge, were created by supernovas or very strong stellar winds that swept out large regions and, when they encountered one another, merged into passageways.

"When we started mapping gas in the galaxy, we found a deficit of neutral gas within about 500 light years, suggesting that we are in a bubble-shaped cavity perhaps filled with hot, ionized gas," Welsh said. "But the Local Bubble is shaped more like a tube and should be called the Local Chimney."

If this system of interlocking, gaseous cavities is characteristic of the entire galaxy, it presents a dramatic confirmation of a 30-year-old theory of the Milky Way, Welsh said.

Welsh is presenting the findings on Thursday, May 29, at the American Astronomical Society meeting in Nashville, Tenn.

At the moment, the origin of the cavities is anybody's guess, Welsh said. The local cavity has been around for a few million years and could easily have been caused by a supernova punching through the top and bottom of the galactic disk, the intense stellar winds from 10 or so hot stars, a powerful gamma-ray burst, or even a large star moving through the area. Each of these could theoretically sweep dense gas out of the region, leaving only tenuous, ionized hydrogen.

Astronomers have known since the 1970s that the Solar Neighborhood lies in the middle of an enormous "Local Bubble" of million-degree, ionized hydrogen gas, surrounded by a wall of colder, denser neutral gas. Within this hot bubble, gas density is much sparser, with some 100 to 1,000 times fewer hydrogen atoms, than the average density of the rest of the Milky Way's spiral disk. The Local Bubble was thought, at first, to be an asymmetric cavity of 330 to 490 light-years (ly) -- 100 to 150 parsecs (pc) -- in diameter.

A Tubular Cavity Penetrating the Local Disk

On May 29, 2003, a team of astronomers (including Barry Welsh, Rosine Lallement, Francoise Crifo, Daphne Sfeir, and and Jean-Luc Vergely) presented the first detailed map of interstellar space within 1,000 ly (300 pc) of the Solar System. They also announced confirmation of the hypothesis that the so-called Local Bubble of the spiral disk's gas clouds actually pierces the disk of the galaxy, perhaps as the result of exploding stars around one or two million years ago (CNRS press release).

The tubular cavity of hot, low-density gas was found to be irregular with tunnels branching off through the surrounding dense gas wall, which is suspected of having interconnecting cavities and tunnels like an interstellar sponge, as first suggested by astronmers nearly 30 years ago (Cox and Smith, 1974). The astronomers hypothesize that energetic supernova explosions created fast-moving expanding bubbles of hot gas that collided with the surrounding cold gas of interstellar space, which in turn became compressed into thin shells. Eventually, these shells of cold gas met other expanding hot cavities and broke up to form small tunnels or pathways between the expanding voids (U.C. Berkeley press release; and Welsh et al, 1999). Such "hot chimneys" have been detected in other galaxies.

One portion of the Local Bubble's wall appears to have collided and merged with the shell of another enormous bubble of hot, ionized gas that's called (Radio) Loop I. Located far above the galactic plane, within 490 ly of the Local Bubble, Loop I's brightest feature is the North Polar Spur, which is thought to be created by supernovae and stellar winds from the 13-million-year-old, Scorpius-Centaurus Association of young and massive, OB-type stars (more discussion and illustrations). In addition to Loop I, astronomers have also detected also two other expanding bubbles nearby, called LOOP II and LOOP III (more discussion).

Over the last five to 10 million years, the Solar System has been moving through the lower density region of interstellar gas of the Local Bubble. As a result, Earth and its lifeforms have avoided dangerous flows of cosmic radiation and gas. Astronomers, however, have discovered a denser cloud of interstellar gas about 25 ly (7.7 pc) in diameter called the "Local Fluff" (or "Local Interstellar Cloud") that is moving towards the Solar System. Stretched out towards Constellation Cygnus, the stellar winds of young stars in a star-forming region of the Scorpius-Centaurus Association near the Aquila Rift (a high-density molecular cloud) have been blowing the Local Fluff so that its denser parts may reach Sol's heliosphere in around 50,000 years (Straizys et al, 2003).