We seem to be inside a "local bubble" in a network of cavities in the interstellar medium, probably carved by massive star explosions millions of years ago. The interstellar medium (or ISM) is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space.
The ISM plays a crucial role in astrophysics precisely because of its intermediate role between stellar and galactic scales, with stars forming within the densest regions of the ISM and molecular clouds, and replenishes the ISM with matter and energy through planetary nebulae, stellar winds, and supernovae.
This interplay between stars and the ISM helps determine the rate at which a galaxy depletes its gaseous content, and therefore its lifespan of active star formation.
NASA astronomer's best guess is depicted in the map (below) of the surrounding 1500 light years constructed from multiple observations and deductions. Currently, the Sun is passing through aLocal Interstellar Cloud (LIC), shown in violet, which is flowing away from the Scorpius-Centaurus Association of young stars (image above).
The LIC resides in a low-density hole in the interstellar medium (ISM) called the Local Bubble, shown in black. Nearby, high-density molecular clouds including the Aquila Rift surround star forming regions, each shown in orange.
The Gum Nebula, shown above and below in green, is a region of hot ionized hydrogen gas. This complex nebula is thought to be a supernova remnant over a million years old, sprawling across the southern constellations Vela and Puppis. Inside the Gum Nebula is the Vela Supernova Remnant, shown in pink, which is expanding to create fragmented shells of material like the LIC. Future observations will aid astronomers to learn more about the local Galactic Neighborhood and how it might have affected Earth's past climate.
Over 13 billion years ago at least one of the domains of life may have begun in nebular clouds. If restricted to the Milky Way, which is 13.6 billion years old, the first chemical combinations would have had billions of years to become a self-replicating organism with a DNA genome long before the existence of Earth.
Nebular clouds are thought to be most likely environment for synthesizing and promoting the evolution of molecules needed for the origin of life. The building blocks for DNA could have been generated or combined within interstellar clouds and DNA would become part of the molecular-protein-amino acid complex. Hydrogen, oxygen, carbon, calcium, sulfur, nitrogen and phosphorus for example are continually irradiated by ions, which can generate small organic molecules which evolve into larger complex organic molecules that result in the formation of amino acids and other compounds.
Phosphorus, for example, is rare in our solar system and may have been non-existent on the early Earth; phosphorus is essential for the manufacture of DNA.
Polarized radiation in the nebula cloud leads to the formation of proteins, nucleobases and then DNA. The combination of hydrogen, carbon, oxygen, nitrogen, cyanide and several other elements, could create adenine, which is a DNA base, whereas oxygen and phosphorus could ladder DNA base pairs. Glycine has also been identified in the interstellar clouds.
Fast forward 4.6 billion years, on Earth the steps leading from the random mixing of chemicals to the first nano-particle would likely require hundreds of millions and even billions of years before the first self-replicating molecular compound was fashioned. Even after billions of years, the first replicon may not have possessed DNA.
A map of the local ISM within 10 light-years based on recent observations (top of the page) shows 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.
Image credit top of page: Linda Huff (American Scientist), Priscilla Frisch (U. Chicago)