Local Bubble
The Local Bubble is a vast cavity within the Orion Arm of our galaxy. It stretches at least 1000 light years across the Milky Way. This region contains the nearest stars and brown dwarfs known to astronomers. Its defining feature is an exceptionally low density of neutral hydrogen atoms. Measurements show about 0.05 atoms per cubic centimeter inside this space. That figure represents roughly one tenth of the average density found in the rest of the interstellar medium. The surrounding environment holds approximately 0.5 atoms per cubic centimeter. Even denser pockets exist nearby, such as the Local Interstellar Cloud which measures 0.3 atoms per cubic centimeter. This creates a stark contrast between the empty void and the crowded regions outside it.
Multiple stellar explosions carved out this massive cavity over millions of years. Scientists now believe that supernovae from the Scorpius, Centaurus association created the structure. Subgroups Lower Centaurus, Crux and Upper Centaurus, Lupus contributed significantly to its formation. Research indicates that 14 to 20 supernovae originated from these groups. These events occurred within the past ten to twenty million years. Earlier theories suggested a single pulsar named Geminga was responsible for the entire bubble. Modern studies point instead to multiple explosions creating a remnant supershell. The Loop I Bubble formed alongside the Local Bubble through similar violent processes. The star Antares resides within the neighboring Loop I Bubble today. Several tunnels connect the cavities of the Local Bubble with the Loop I Bubble. One specific tunnel is known as the Lupus Tunnel.
Our solar system has traveled through this region for five to ten million years. We currently reside inside the Local Interstellar Cloud, a denser pocket within the larger bubble. This cloud formed where the Local Bubble met the Loop I Bubble. Gas density in our immediate vicinity measures approximately 0.3 atoms per cubic centimeter. The shape of the Local Bubble resembles an hourglass rather than a perfect sphere. It appears narrower along the galactic plane while widening above and below it. This elliptical form abuts other bubbles of less dense interstellar medium. The Sun entered the bubble around five million years ago according to recent models. Researchers found interstellar iron in Antarctica that relates to the Local Interstellar Cloud. That discovery might be connected to how the Local Bubble originally formed.
Space missions have mapped the hot gas and dust boundaries of this cavity. A small observatory called CHIPSat launched in February 2003 and operated until April 2008. It examined the hot gas within the Local Bubble directly. The Extreme Ultraviolet Explorer mission studied EUV sources between 1992 and 2001. Sources beyond the edge of the bubble were identified but attenuated by denser matter. In 2019 researchers reported the first three-dimensional map using diffuse interstellar bands. Another study from 2020 retrieved the shape of the dusty envelope surrounding the region. They modeled the structure from three-dimensional maps of stellar extinction data. These observations revealed tunnels connecting different cavities in the local neighborhood. The Lupus Tunnel remains one of the key structural features discovered through these efforts.
Expanding shockwaves collected debris to trigger the birth of nearby young stars. A paper published in Nature in January 2022 confirmed this mechanism. Observations showed that the expanding surface of the bubble swept up gas and dust. This material collapsed to form new stars on the outer shell rather than inside. New stars typically appear in molecular clouds like the Taurus molecular cloud. The open star cluster Pleiades also formed under similar conditions. The Sun entered the bubble around five million years ago during this process. Research indicates that all young nearby stars owe their existence to this expansion. The action of the expanding surface collected enough material to spark stellar births. This process continues to shape the immediate galactic environment today.
Scientists analyze iron-60 found in deep-sea sediments to trace specific supernova events. Samples are divided into layers dated using beryllium-10 isotopes. Some layers show higher concentrations of radioactive elements from ancient explosions. Iron-60 appears in Antarctic snow and lunar soil as well as ocean crusts. Manganese-53 and plutonium-244 have been detected in deep-sea materials too. These isotopes peaked between 1.7 and 3.2 million years ago according to data. A second peak occurred 6.5 to 8.7 million years ago in older samples. The younger peak likely originated when the solar system entered the Local Bubble 4.5 million years ago. One supernova creating this peak might have produced pulsar PSR B1706-16. That event released Zeta Ophiuchi as a runaway star about 1.78 million years ago. Another explanation suggests one supernova in the Tucana, Horologium association created the older peak 7 to 9 million years ago.
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Common questions
What is the Local Bubble and how large is it?
The Local Bubble is a vast cavity within the Orion Arm of our galaxy that stretches at least 1000 light years across the Milky Way. This region contains the nearest stars and brown dwarfs known to astronomers while maintaining an exceptionally low density of neutral hydrogen atoms.
How did scientists determine the origin of the Local Bubble structure?
Scientists now believe that supernovae from the Scorpius, Centaurus association created the structure through multiple explosions rather than a single source. Research indicates that 14 to 20 supernovae originated from subgroups Lower Centaurus, Crux and Upper Centaurus, Lupus within the past ten to twenty million years.
When did the solar system enter the Local Bubble region?
Our solar system has traveled through this region for five to ten million years and entered the bubble around five million years ago according to recent models. The Sun currently resides inside the Local Interstellar Cloud which formed where the Local Bubble met the Loop I Bubble.
Which space missions mapped the boundaries of the Local Bubble?
A small observatory called CHIPSat launched in February 2003 and operated until April 2008 to examine the hot gas within the Local Bubble directly. The Extreme Ultraviolet Explorer mission studied EUV sources between 1992 and 2001 while researchers reported the first three-dimensional map using diffuse interstellar bands in 2019.
How does the Local Bubble influence star formation nearby?
Expanding shockwaves collected debris to trigger the birth of nearby young stars as confirmed by a paper published in Nature in January 2022. New stars typically appear in molecular clouds like the Taurus molecular cloud or open star clusters such as the Pleiades under similar conditions created by the expanding surface.