The Local Bubble, also called the Local Cavity, is a large empty space in the interstellar medium (the space between stars) in the Orion Arm of the Milky Way. It includes the closest stars and brown dwarfs, as well as the Local Interstellar Cloud (which contains the Solar System), the nearby G-Cloud, the Ursa Major moving group, and the Hyades (the nearest open star cluster). Scientists estimate that it is at least 1,000 light years across. It is defined by its low density of neutral hydrogen, about 0.05 atoms per cubic centimeter, which is one-tenth of the average for the Milky Way’s interstellar medium (0.5 atoms per cubic centimeter) and one-sixth of the density in the Local Interstellar Cloud (0.3 atoms per cubic centimeter).
The very thin gas in the Local Bubble is caused by supernovae (exploding stars) that occurred over the past 10 to 20 million years. At first, scientists thought a pulsar called Geminga in the constellation Gemini was the leftover material from a single supernova that created the Local Bubble. However, recent research suggests that multiple supernovae from subgroup B1 of the Pleiades moving group were responsible, forming a large remnant shell. Other studies propose that the subgroups Lower Centaurus–Crux (LCC) and Upper Centaurus–Lupus (UCL) of the Scorpius–Centaurus association caused both the Local Bubble and the Loop I Bubble. Research shows that 14 to 20 supernovae from LCC and UCL likely created these bubbles.
Description
The Solar System has been moving through the Local Bubble for about five to ten million years. It is now located in the Local Interstellar Cloud (LIC), a small area of denser material inside the Bubble. The LIC formed where the Local Bubble and the Loop I Bubble meet. The gas in the LIC has a density of about 0.3 atoms per cubic centimeter.
The Local Bubble is not perfectly round. It is narrower along the galactic plane and appears somewhat egg-shaped or oval. It may widen above and below the galactic plane, resembling an hourglass. The Local Bubble touches other regions of less dense interstellar medium (ISM), including the Loop I Bubble. The Loop I Bubble was cleared, heated, and maintained by supernovae and stellar winds from the Scorpius–Centaurus association, which is about 500 light years from the Sun. The Loop I Bubble contains the star Antares (also called α Sco or Alpha Scorpii), as shown in the diagram above right. Several tunnels connect the empty spaces of the Local Bubble with the Loop I Bubble. These tunnels are called the "Lupus Tunnel." Other bubbles near the Local Bubble include the Loop II Bubble and the Loop III Bubble. In 2019, scientists discovered interstellar iron in Antarctica, which they connected to the Local Interstellar Cloud. This discovery might be linked to the formation of the Local Bubble.
Observation
In February 2003, a small space observatory named Cosmic Hot Interstellar Plasma Spectrometer (CHIPSat) was launched and operated until April 2008. It studied the hot gas inside the Local Bubble. The Local Bubble was also the focus of the Extreme Ultraviolet Explorer mission (1992–2001), which studied hot sources of extreme ultraviolet light within the bubble. Objects outside the bubble’s edge were identified but weakened by the denser material in space. In 2019, scientists created the first 3D map of the Local Bubble using data about diffuse interstellar bands. In 2020, the shape of the dusty layer around the Local Bubble was studied and modeled using 3D maps of dust density from data about how starlight is blocked by dust.
Impact on star formation
In January 2022, a study published in the journal Nature showed that studies and computer models found the growing edge of a bubble gathered gas and dust, which helped create all young, nearby stars. These young stars are often found in molecular clouds such as the Taurus molecular cloud and the open star cluster Pleiades.
Connection to radioactive isotopes on Earth
Radioactive isotopes on Earth are linked to supernovae that occurred near the solar system. The most common source of these isotopes is deep-sea ferromanganese crusts, which continuously collect iron, manganese, and other elements. Scientists divide these crusts into layers and date them using methods like beryllium-10. Some layers contain higher amounts of radioactive isotopes. The isotope most often found in Earth's materials from supernovae is iron-60, which has been discovered in deep-sea sediments, Antarctic snow, and lunar soil. Other isotopes include manganese-53 and plutonium-244, also found in deep-sea materials. Supernova-related aluminum-26, predicted by studies of cosmic rays, has not been confirmed. Iron-60 and manganese-53 show peaks between 1.7–3.2 million years ago and between 6.5–8.7 million years ago. The older peak may have formed when the solar system passed through the Orion–Eridanus Superbubble, while the younger peak likely occurred when the solar system entered the Local Bubble 4.5 million years ago. One supernova that created the younger peak might have produced the pulsar PSR B1706-16 and turned Zeta Ophiuchi into a runaway star. Both objects originated from the UCL region and were released by a supernova 1.78 ± 0.21 million years ago. Another possibility for the older peak is that it came from a supernova in the Tucana–Horologium association 7–9 million years ago.