The stars are not alone. In the disk of our Milky Way Galaxy about 10 percent of visible matter is in the form of gas, called the interstellar medium (ISM). The ISM is not uniform, and shows patchiness even near our Sun. It can be quite difficult to detect the local ISM because it is so tenuous and emits so little light. This mostly hydrogen gas, however, absorbs some very specific colors that can be detected in the light of the nearest stars. A working map of the local ISM within 10 light-years based on recent observations is shown above. These observations show that our Sun is moving through a Local Interstellar Cloud as this cloud flows outwards from the Scorpius-Centaurus Association star forming region. Our Sun may exit the Local Interstellar Cloud during the next 10,000 years. Much remains unknown about the local ISM, including details of its distribution, its origin, and how it affects the Sun and the Earth.

 

 

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 is 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. In addition to Loop I, astronomers have also detected also two other expanding bubbles nearby, called LOOP II and LOOP III.

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 life forms 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).

 

The Local Bubble is composed of very sparse, hot gas from supernovae that exploded within the past two to four million years. The "Local Bubble" of low-density, hot and ionized gas, is actually part of a tube-like chimney that extends through the local region of the spiral disk into the surrounding galactic halo, and so may can act as a vent for the energetic hot gas produced by supernovae. 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.