The “void” between the stars is anything but empty. Interstellar space is filled with the gaseous remains of supernova explosions, wispy tendrils of plasma from stellar winds and star-forming nebulae. Some of this gaseous matter forms magnetized cloud-like structures, the closest of which is known as the “Local Fluff.”
However, our solar system sits inside a volume of space that contains 10 times less gas than the galactic average. The sun — and many nearby stars — is located in a “cavity” (known, unsurprisingly, as the “Local Cavity”) that measures approximately 300 light years across.
The presence of this region has been known for some time, but an international team of astronomers have now created a 3-dimensional map of this fascinating region of space in our cosmic backyard.
The team, led by Barry Y. Welsh from the University of California, Berkeley, managed to produce this map by analyzing the spectrum of light from over 1,800 stars up to 800 parsecs (2,600 light years) away from the sun.
As starlight travels through the interstellar medium, it passes through the thin gases between the stars. When the light is received by telescopes on Earth, an instrument called a spectrometer is used to analyze this light.
As far back as 1904, astronomers have known about this interstellar gas as particular wavelengths in the received starlight appeared to be missing from the spectrum. Certain gases absorb certain wavelengths, producing dark “bands” in the starlight spectrum. Astronomers use this method (known as spectroscopy) to detect what elements are present in the interstellar gases and what density it is.
The team used the European Southern Observatory (ESO) to collect the spectral data and combined it with previously published observations to create a comprehensive survey of the densities of the interstellar medium surrounding us.
What is obvious in the maps is a white region in the center (where the sun is located), highlighting the Local Cavity. On the edges of the cavity is a “wall” of very dense, neutral gas interspersed with “interstellar tunnels.” These tunnels connect our cavity with other surrounding cavities.
Interestingly, by analyzing one particular absorption line (caused by doubly ionized calcium — CaII), it appears that the Local Cavity contains filamentary structures of partially ionized gas, creating a honeycomb-like pattern of smaller interstellar cells.
Although the cause of this cell-like structuring is unknown at present, it is thought that the Local Cavity was formed after a series of supernovae detonated nearby, blowing the cool, dense interstellar medium away and replacing it with a very tenuous volume of hot plasma, creating a ‘bubble.’