The Earth's 'Cosmic Bubble' Was Blasted Out by 15 Enormous Supernovas

Scientists determined that no new stars had formed within the Local Bubble for 14 million years, and any stars currently within the Local Bubble either formed over 14 million years ago, or else drifted in as they orbited around the center of our Milky Way galaxy.

In the new study, which was published in the journal Nature on January 12, 2022, scientists at the Space Telescope Science Institute in Baltimore, Maryland, Harvard University, and the Smithsonian Institution used data collected by the European Space Agency's (ESA) Gaia Space Observatory.

Since its launch in 2013, Gaia's purpose has been to measure with unprecedented accuracy the positions, distances, and motions of stars, with the aim of constructing the largest and most precise 3D catalog of space. Additionally, Gaia is expected to detect near-Earth asteroids that may pose a threat to our planet, and to discover planets orbiting neighboring stars, new supernovas, and the distribution of dark matter around our galaxy.

The new study determined that beginning 14 million years ago, multiple supernova explosions blasted out the Local Bubble, sending hydrogen gas and dust, the components of star formation, to the bubble's edge. Along that edge, a frenzy of star formation then began.

By analyzing the seven largest star-forming regions along the Local Bubble's surface, the scientists were able to determine how fast the bubble is expanding, which is around 4 miles per second (6.4 km/s). This, in turn, allowed them to determine that it required 15 supernova explosions to have blasted out the Local Bubble.

Young stars on the surface of the Local Bubble are moving in a direction perpendicular to the bubble's surface, and by tracing their motions backwards, a picture of the Local Bubble 14 million years ago emerged in which there were multiple star births and deaths near the bubble’s center. 

The Local Bubble abuts other bubbles, in particular, the Loop I Bubble, which contains the well-known reddish star Antares. Several tunnels connect cavities within the Local Bubble with cavities within Loop I Bubble, one of which is known as the "Lupus Tunnel". Other bubbles that are adjacent to the Local Bubble are the Loop II Bubble and the Loop III Bubble. Questions remain as to how the bubbles interact with one another.

What's a supernova?

A supernova is nature's most powerful explosion and light show. It typically occurs when a massive star runs out of hydrogen, and nuclear fusion slows. The star can no longer counteract its own gravity and its core collapses, releasing vast quantities of gravitational potential energy, and blasting several solar masses worth of material out into space at high velocity.

A shock wave expands into the interstellar medium, spewing elements and weeping up gas and dust, triggering new star formation. As the star collapses, the shockwave can induce fusion reactions in the star’s outer shell, creating new atomic nuclei in a process called nucleosynthesis. In fact, supernovae are thought to be one of the original sources of the elements heavier than iron in the Universe. 

At its height, a supernova can shine brighter than the light of an entire galaxy, but supernovas don't last long, typically fading over the course of several weeks or months. The star that caused the supernova is either completely destroyed or it collapses, forming either a neutron star or a black hole. 

Another mechanism for the creation of a supernova is when a star, such as a white dwarf, accumulates new material and nuclear fusion is suddenly reignited. A star can acquire new material by sucking it from a companion star, or by colliding with another star.

The first supernova ever observed by humans, known as HB9, may have been recorded on a rock carving in the Kashmir region dating to 4500 BCE. In 1006 AD, an intensely bright supernova in the constellation Lupus was observed and recorded by astronomers in China, Japan, Iraq, Egypt, and Europe.

The famous Crab Nebula was formed by a supernova which was observed in 1054, while supernovae also occurred in 1572 and 1604. The former was observed by the famous astronomer Tycho Brahe, while the latter, which was the last supernova visible to the naked eye in the Milky Way galaxy, was closely observed by Johannes Kepler who kept meticulous notes on its brightness. 

Today, both professional and amateur astronomers locate hundreds of supernovae every year. On October 6, 2013, a supernova was detected in the galaxy NGC 7610, which is 160 million light-years away from Earth, just three hours after it detonated.

On September 20, 2016, an Argentinian amateur astronomer named Victor Buso was testing his telescope, which was trained on the galaxy NGC 613, when he noticed something odd. Buso contacted authorities, who confirmed that he had captured the initial moments of the "shock breakout" of a visible supernova. The odds of capturing such an event were put at between one in ten million and one in a hundred million.

The star that exploded was identified on Hubble Space Telescope images, and astronomers the world over lauded Buso's discovery. Alex Filippenko, an astronomer at the University of California at Berkeley, who carried out extensive follow-up observations, explained that, "Observations of stars in the first moments they begin exploding provide information that cannot be directly obtained in any other way."

Our fancy neighborhood

At the time the Local Bubble formed, our Sun was about 1,000 light-years outside the bubble, and it only entered around 5 million years ago. According to the scientists, the Sun should exit the Local Bubble in around eight million years, but at that time, the Local Bubble may no longer exist, since its expansion has been seen to be slowing.

In the meantime, our unique position at the center of the bubble gives us a ringside seat to the process of star formation, where dying stars give birth to new stars in a never-ending cycle.