Space isn't all empty - we have seen spectacular images from the Hubble Space Telescope
showing the "pillars of creation" in the Orion Nebula
- giant dust and gas clouds and the magnificent graves of giant stars. Most of these things are quite some ways away and it is rare to see anything about our own neighborhood
Things out there aren't all static, our sun has moved from its birthplace and those clouds have moved about too. Over the course of 5 billion years, our star has moved quite some distance (around the galaxy 20 times or so) and through a number of galactic environments. Looking around we can get some idea of where we have been, and where we are going.
Compared to the stuff around us, the sun is moving at about 16.5 km per second or about 50 light years every million years. This motion is inclined to the galactic plane by about 25 degrees. Thus, if one was to look at the sun side on, we appear to be bobbing up and down through the galactic disk every 33 million years.
Starting with the sun and moving away the first milestone of interest is the heliosphere. From the Sun blowing out is a stream of hot plasma that consists mostly of protons, the nuclei of helium atoms and electrons. This 'wind' goes far beyond the orbit of Pluto before it hits the interstellar gas. When it hits the interstellar gas at about 80 - 100 AU away it slows down and forms a termination shock and appears to stop at about 130 to 150 AU from the sun. The charged nature of the solar wind carries with it a magnetic field that causes the interstellar plasma to be diverted around the heliosphere.
At the edge of the heliopause is a structure known as the hydrogen wall. Here, as particles get bunched up protons and neutral hydrogen atoms exchange electrons. The exchange of charge causes the atom to decelerate and bunch up more forming a 'wall' of protons and neutral hydrogen atoms.
We appear to be on the edge of what is known as the Local Bubble - a roughly spherical area about 600 light years across. The Local Bubble is one of the most extreme vacuums ever found - the best laboratory vacuum is on the order of 10,000 times denser than an interstellar cloud, which itself is a few thousand times denser than the Local Bubble, having a density of less than 0.001 atoms per cubic centimeter (a good lab vacuum is about 100,000 molecules per cubic centimeter). The Local Bubble is also very hot (this refers to the velocity of the particles) with a temperature of 1,000,000 Kelvin. Compare this to the solar system which is 7,000 Kelvin and 0.3 atoms per cubic centimeter.
The cloud that we are near or passing through is known as the Local Fluff. This appears to be created by the winds from the stars in the star forming region known as the Scorpius-Centaurus association where giant molecular clouds sit about at about 100 Kelvin and 1,000 atoms per cubic centimeter.
Around the Local Bubble is a ring of stars known as Gould's Belt that shows up in the sky as a band of bright stars that form a circlea across the sky. Gould's belt was discovered in 1879 by the astronomer Benjamin Gould of Argentina. The discovery was made during a survey of the distribution of the nearby bright stars. This is a relatively young structure as cosmology goes - only a few tens of millions of years old and made up of O and B stars. This belt is tilted 20 degrees to the plane of the Milky Way. Our position within the belt is pure chance - we've been around the galaxy about 20 times during the life of the sun so far - and the belt has only existed for about 1/8th of a galactic rotation.
The Lockman Hole is in the sky as part of Ursa Major between the two pointer stars. The significance of this area is that it has the least gas between us and extragalactic space allowing for sensitive searches of distant objects.
The sun has been cursing through Gould's Belt on the Bubble side for the past several million years. While we are heading out of the Bubble, it is not likely that we will hit a dense cloud (denser than the Local Fluff) for several million more years. While scientists are unsure what will happen if the solar system happens to pass through a cloud, they consider it fortunate that we have been in a rather empty region of space for some time. Some simulations show that if the sun was to move into a moderately dense (10 atoms per cubic centimeter) interstellar cloud the heliosphere would become unstable and the solar wind would not be able to protect the Jovian planets. If it was to run into a dense cloud (1000 atoms per cubic centimeter), the solar wind would not push out to or beyond the orbit of earth, possibly causing the atmosphere of our planet to change in composition.
The emptiness of the Local Bubble does provide a clear path for shock waves from supernova to propagate easily. About 250,000 years ago we encountered some of the flow from the Scorpius-Centaurus association (this is the main component of the Local Fluff). Some of this material flows through the solar system and has been named the "Local Interstellar Wind". This wind is focused by the sun and produces a tail of helium that the earth passes through around the end of November each year.
The Local Bubble was created by a shock wave (or more than one). The question that is still open for debate is what created it? Some astronomers hold that it was created by shock waves from star formation in the near by nebulae. These shock waves would be heating the gas and causing it to blow up like a balloon. Other astronomers believe that the bubble was carved out with a supernova explosion in the Scorpios-Centaurus association or other nearby cluster (this was the first theory which gave the Local Bubble its name - the idea that we were sitting inside of a supernova remnant). If the supernova theory is correct, it happened about 100,000 years or so. The pulsar named 'Geminga' is speculated to be the remnant of the supernova.