A great cavity in the interstellar medium (ISM),
at least 300 light-years across, within which the Sun and many nearby stars
resides. The Local Bubble has a neutral hydrogen density of only about 0.07
atoms/cm3 – at least 10 times lower than the average ISM
in the Galaxy – and also contains a thin gruel of million-degree X-ray-emitting
plasma. These observations have led astronomers
to conclude that the Bubble was formed between a few hundred thousand and
a few million years ago by several relatively nearby supernova
explosions that pushed aside gas and dust in the ISM leaving the current
depleted expanse of hot, low density material. A prime suspect for an object
left behind by this supernova activity is Geminga.
The "Bubble" may be a misnomer since it appears to have an
hourglass shape that is narrowest in the galactic plane and that widens
above and below the plane. In fact, in directions away from the galactic
plane the Bubble appears to be opened-ended, bursting into the galactic
halo, so that "Local Tube" describes it better. Inside it are numerous
cloudlets, oriented in sheet-like structures near the Bubble's boundary.
The Sun, along with several neighboring stars, is presently embedded in
a group of such cloudlets, known as the "Local Fluff Complex" or, more prosaically,
as the local interstellar medium (LISM), that is passing through the Local
Bubble. These floating islands of neutral hydrogen atoms have resulted from
the expansion of an even larger bubble, the Loop I superbubble, up against
our own cavity. The Loop I superbubble was created by supernovae and stellar
winds in the Scorpius-Centaurus
Association, which lies about 500 light-years away. When the Local Bubble
and the Loop I collided, the Local Fluff Complex formed at the boundary
between the two. This boundary lies 50 to 130 light-years away and through
it the cloudlets are invading our Local Bubble. The Sun lies very close
to the edge of a cloudlet named the Local
Interstellar Cloud and is moving roughly perpendicular to it.
| Map of the cold, dense interstellar gas surrounding
the Local Bubble in the plane of the Galaxy. White areas represent
regions of extremely low gas density (which are probably filled with
plasma); dark areas reveal where large condensations of cold, dense
gas occur. Notice that the local cavity is surrounded by many of these
condensations, but this "wall" is broken in several places by low
density interstellar tunnels that link the local cavity with other
nearby bubble cavities such as the Pleiades
and GSH 238+00+09.
Branching off from the Local Bubble, through the surrounding dense gas,
appear to be a number of tunnels that open out into other cavities. 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.
If this system of interlocking, gaseous cavities is characteristic of the
entire Galaxy, it represents a dramatic confirmation of a 30-year-old theory
of the Milky Way.
AND INTERPLANETARY MATTER