The Sun isn’t the only star in this galactic neighbourhood. Other stars also call this neighbourhood home. But what’s the neighbourhood’s history? What triggered the birth of all those stars?

A team of astronomers say they’ve pieced the history together and identified the trigger: a series of supernovae explosions that began about 14 million years ago.

A series of stars in our neighbourhood exploded as supernovae starting about 14 million years ago. They created a vast bubble of gas about 1,000 light-years across called the Local Bubble. In the middle of that void sits the Sun. The Sun’s neighbours formed on the edge of that bubble, and the preceding supernova explosions were the catalyst for their formation.

A new study published in the journal Nature presents the findings. The title is “Star formation near the Sun is driven by expansion of the Local Bubble.” Catherine Zucker is the lead author and she’s an astronomer and data visualization expert.

“We’ve calculated that about 15 supernovae have gone off over millions of years to form the Local Bubble that we see today.”

Catherine Zucker, lead author, NASA Hubble Fellow at STScI.

Stars form from clouds of hydrogen gas called Giant Molecular Clouds (GMC.) For a star to form, enough of the gas has to gather in one spot. This happens when the density of the gas varies. Due to gravity, density begets more density, and if enough time passes and if conditions are right, sufficient gas clumps together to trigger fusion, and a star is born.

But stars can also form when supernovae explode. Supernova explosions release a tremendous amount of energy, and that energy travels outward in a shockwave. The shockwave pushes gas together into clouds and creates greater density. That can lead to new stars.

That’s what happened in our neighbourhood, and it formed stars on the edge of the Local Bubble, which is also the edge of supernova shockwaves. Inside the Local Bubble, the density of the Interstellar Medium (ISM) is much lower than the density of the ISM throughout the Milky Way. The series of supernovae explosions pushed the gas aside, forming the edge of the bubble of dense ISM and driving star formation there.

“This is an incredible detective story, driven by both data and theory.”

Alyssa Goodman, study co-author, CfA Professor.

The edge of the bubble has fragmented and collapsed over time into star-forming clouds. Its once-smooth edge is gone. The team reports that there are seven star-forming regions on the bubble’s surface in the form of molecular clouds. These include Orion A and Orion B, both prominent parts of the Orion Molecular Cloud Complex. “Remarkably, we find that every well-known molecular cloud within ~200 pc of the Sun lies on the surface of the Local Bubble,” the paper says. The exception might be the Perseus Molecular Cloud.

The star formation didn’t happen all at once. In their paper, the authors point out that it happened in four distinct epochs: 10 Myr ago, 6 Myr ago, 4 Myr ago, and in the present age. They don’t know exactly how many supernovae exploded to create the bubble, but they’ve constrained it between 8 and 26, settling on 15 as the most likely number. “We’ve calculated that about 15 supernovae have gone off over millions of years to form the Local Bubble that we see today,” said Zucker, who is now a NASA Hubble Fellow at STScI.

This figure shows the evolution of the Local Bubble and sequential star formation at the surface of its expanding shell. Selected time snapshots (seen from a top-down projection) are shown here. The central figure shows the present day. Stellar cluster tracebacks are shown with the coloured paths. Prior to the cluster birth, the tracebacks are shown as unfilled circles meant to guide the eye, since the modelling is insensitive to the dynamics of the gas before its conversion into stars. After the cluster birth, the tracebacks are shown with filled circles and terminate in a large dot, which marks the cluster’s current position. For time snapshots ? 14 Myr ago, we overlay a model for the evolution of the Local Bubble (purple sphere). The solar orbit is shown in yellow dots and indicates that the Sun entered the Local Bubble approximately 5 Myr ago. Image Credit: Zucker et al 2022.