Local Bubble

• 1. Introduction

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  The Local Bubble is the vast, thinly filled cavity of space that the Solar System currently calls home. At least 1,000 light years across, it sits within the Orion Arm of the Milky Way, and its gas is so sparse it holds only about 0.05 atoms per cubic centimeter. That is roughly one tenth the average density of the interstellar medium across the galaxy. What hollowed out such an enormous region? And what does it have to do with the young stars forming right now in our cosmic neighborhood?

• 2. A Cavity in the Cosmos

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  The Local Bubble is not a perfect sphere. It narrows in the galactic plane, taking on an egg-shaped or elliptical outline, and widens above and below that plane into something closer to an hourglass. It abuts neighboring bubbles of similarly thin interstellar gas, most notably the Loop I Bubble, which lies some 500 light years from the Sun and was carved and maintained by supernovae and stellar winds in the Scorpius-Centaurus association. Several tunnels, collectively called the Lupus Tunnel, link the cavities of the two bubbles. Loop II and Loop III Bubbles also border the Local Bubble, making this region a patchwork of overlapping voids. Where the Local Bubble and the Loop I Bubble pressed against each other, a minor clump of denser gas formed: the Local Interstellar Cloud, with a density of roughly 0.3 atoms per cubic centimeter. The Solar System has been drifting through the Local Bubble for somewhere between five and ten million years, and right now it sits inside that Local Interstellar Cloud.

• 3. Supernovae That Made the Void

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  Multiple stellar explosions within the past ten to twenty million years blasted away the gas that once filled this region. For a time, the pulsar Geminga, found in the constellation Gemini, was the prime suspect: a single supernova remnant thought to have excavated the entire bubble. That picture eventually gave way to a more complicated one. Multiple supernovae in subgroup B1 of the Pleiades moving group are now thought to have been responsible, producing what scientists call a remnant supershell. A separate line of research points to two subgroups within the Scorpius-Centaurus association: Lower Centaurus-Crux, known as LCC, and Upper Centaurus-Lupus, known as UCL. In this account, between 14 and 20 supernovae from LCC and UCL together shaped both the Local Bubble and the adjacent Loop I Bubble, with LCC credited for the Local Bubble specifically and UCL for the Loop I Bubble. One supernova from UCL, dated to 1.78 plus or minus 0.21 million years ago, may have simultaneously created the pulsar PSR B1706-16 and sent Zeta Ophiuchi racing away as a runaway star.

• 4. Fingerprints in Deep-Sea Sediment

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  Iron-60, a radioactive isotope produced in supernova explosions, has turned up in deep-sea ferromanganese crusts, Antarctic snow, and lunar soil, leaving a record of nearby stellar deaths written into Earth's own geology. Scientists slice those crusts into dated layers, using isotopes such as beryllium-10 to fix the ages, and read the concentrations of radioactive material like tree rings of the cosmos. Iron-60 and manganese-53 both show a peak between 1.7 and 3.2 million years ago, likely tied to the Solar System's entry into the Local Bubble around 4.5 million years ago. A second, older iron-60 peak dating to 6.5-8.7 million years ago probably traces back to the Solar System's passage through the Orion-Eridanus Superbubble, or possibly a single supernova in the Tucana-Horologium association between 7 and 9 million years ago. Plutonium-244 also appears in deep-sea materials, while supernova-originated aluminium-26, which cosmic ray studies had predicted, was not confirmed. In 2019, researchers found interstellar iron in Antarctica and connected it to the Local Interstellar Cloud itself, adding another thread to this geologic and cosmic record.

• 5. Mapping an Invisible Void

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  Launched in February 2003, a small space observatory called the Cosmic Hot Interstellar Plasma Spectrometer, or CHIPSat, examined the hot gas filling the Local Bubble until the mission ended in April 2008. The Extreme Ultraviolet Explorer ran from 1992 to 2001 and catalogued hot sources of extreme ultraviolet radiation within the bubble; anything beyond the bubble's edge showed up dimmed and attenuated by the denser interstellar medium outside. Progress in mapping the bubble's structure accelerated in 2019, when astronomers published the first three-dimensional map of the Local Bubble using observations of diffuse interstellar bands. The following year, in 2020, researchers retrieved and modeled the shape of the dusty envelope surrounding the bubble by analyzing three-dimensional maps of dust density drawn from stellar extinction data. Together, these two observational advances turned what had been an educated sketch of the Local Bubble into a detailed, measurable shape.

• 6. A Nursery for Nearby Stars

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  A paper published in the journal Nature in January 2022 brought one of the most striking conclusions about the Local Bubble: the expanding surface of the bubble itself is responsible for the formation of every young, nearby star. As the bubble grew, its shell swept up gas and debris, compressing material until it collapsed into new stellar systems. Molecular clouds like the Taurus molecular cloud and the open star cluster Pleiades are among the structures where these new stars have taken shape. The bubble that supernovae cleared out millions of years ago has, in effect, been manufacturing stars along its own rim, turning destruction into construction across an enormous swath of our galactic neighborhood.