Local Bubble

The Local Bubble is a vast, low-density cavity within the interstellar medium (ISM) of the Milky Way. The interstellar medium refers to the gas and dust that exists in the spaces between stars. Located in the Orion Arm, this bubble contains many of our closest neighbors, including stars and brown dwarfs. It also houses the Local Interstellar Cloud, which contains our own Solar System.

This region is significant because it defines our immediate galactic neighborhood. By studying it, astronomers can understand how massive stellar events shape the structure of our galaxy.

This enormous cavity was created by the energy of multiple supernovae. A supernova is the powerful explosion of a star. These explosions occurred within the last ten to twenty million years. As these stars exploded, they pushed the surrounding gas outward. This process created a remnant supershell of material. Some researchers believe the Pleiades moving group caused these explosions. Other studies suggest the Scorpius–Centaurus association was the source. Specifically, the subgroups Lower Centaurus–Crux (LCC) and Upper Centaurus–Lupus (UCL) may have been responsible. It is estimated that between 14 and 20 supernovae originated from these groups to form the bubble.

The Local Bubble is not a perfect sphere. Its shape is more complex and varies depending on its location. It appears narrower within the galactic plane. In other areas, it may look like an egg or an ellipse. Above and below the galactic plane, it might widen into an hourglass shape. The bubble also interacts with other nearby cavities. One major neighbor is the Loop I Bubble, which was created by the Scorpius–Centaurus association about 500 light years away. A specific connection called the "Lupus Tunnel" links the Local Bubble to the Loop I Bubble.

Scientists have used several advanced missions to map this region. The Extreme Ultraviolet Explorer operated from 1992 to 2001 to study hot EUV sources. Later, the Cosmic Hot Interstellar Plasma Spectrometer (CHIPSat) examined hot gas from 2003 to 2008. In 2019, researchers achieved a major milestone by creating the first 3D map of the Local Bubble. They did this using observations of diffuse interstellar bands. By 2020, scientists used 3D dust density maps to model the dusty envelope surrounding the bubble. These tools allow us to visualize a structure that is otherwise difficult to see.

The scale of the Local Bubble is immense. It is estimated to be at least 1,000 light years in size. The density of gas inside the bubble is very low. It measures about 0.05 atoms per cubic centimeter. This is only about one tenth of the average density found in the Milky Way's interstellar medium. For comparison, the Local Interstellar Cloud has a density of 0.3 atoms per cubic centimeter. This makes the cloud about six times denser than the bubble itself.

The expansion of the bubble has a direct impact on the birth of stars. As the bubble expands, its surface sweeps up interstellar gas and dust. This material collects and collapses on the outer edge of the bubble. This process is responsible for the formation of all young, nearby stars. Examples of these stars can be found in molecular clouds like the Taurus molecular cloud. The Pleiades open cluster is another example of stars formed this way.

We can find physical evidence of these ancient explosions right here on Earth. Radioactive isotopes provide a historical record of nearby supernovae. Scientists study these in deep-sea ferromanganese crusts, Antarctic snow, and lunar soil. Iron-60 is a common isotope used to track these events. There is a notable peak of iron-60 from 4.5 million years ago. This likely occurred when our Solar System entered the Local Bubble. Another peak occurred between 1.7 and 3.2 million years ago. In 2019, researchers even found interstellar iron in Antarctica that may relate to the Local Interstellar Cloud. These tiny particles allow us to trace the history of our galaxy through time.