The Gould Belt: an Explosive Bubble at the Heart of the Milky Way

Scientific Summary

This article presents the Gould Belt, a local structure of the Milky Way, approximately 60 million years old, composed of gas and young stars, tilted by 18° relative to the galactic plane. With a diameter of approximately 3,000 light-years, it was likely born from a hypernova (equivalent to 10 supernovae) which created a circular shock wave containing 240,000 solar masses. Data from the Hipparcos satellite (1989–1993) allowed this structure to be mapped by distinguishing young, massive stars (O and B types) from older stars. The Gould Belt is an active region of star formation, where approximately 300 to 400 supernovae are expected in the next few million years.

What is the Gould Belt and what is its origin?

The Gould Belt is a local structure of the Milky Way, identified in 1879 by astronomer Benjamin Gould. It is a ring of gas and young stars, approximately 60 million years old, with a diameter of 3,000 light-years, tilted by 18° relative to the galactic plane. This inclination explains why, from Earth, the Milky Way appears curved: the Gould Belt, rich in bright, nearby stars, superimposes on the background galaxy. Its formation would be due to a giant hypernova (equivalent to 10 supernovae) which generated a circular shock wave containing 240,000 solar masses, deformed into a tilted elliptical ring by the variable density of the interstellar gas. Data from the Hipparcos satellite (1989–1993) allowed this structure to be precisely mapped by distinguishing two populations of stars: the young, massive stars (types O and B, younger than 40 million years) which form the ring, and the older stars (types A and F) which are distributed homogeneously. Currently, the Sun is crossing this belt, a region particularly rich in supernovae: it is estimated that approximately 300 to 400 supernovae still need to explode on its edges in the next few million years, at a rate of one every 40,000 years on average.

The Large Gas Bubbles of the Milky Way

What is the Gould Belt?

The Gould Belt, named in honor of Benjamin Gould (1824-1896), who identified it in 1879, is a young structure of our Milky Way, approximately 60 million years old. In reality, it is a "small bubble" 3,000 light-years in diameter, composed of gas and stars, located in our very close galactic environment, through which the solar system travels.

A Tilted Structure in a Galaxy Difficult to Map

The Gould Belt is tilted about 18 degrees relative to the plane of the Milky Way. We live in a spiral galaxy approximately 100,000 light-years in diameter, but it is very difficult to map its structure because we are inside it, which explains the uncertainty regarding the number of existing arms and even their naming.

The Spiral Arms of the Milky Way and Our Position

What appears clearly from Earth is the continuous white band of the Milky Way, but this galaxy is actually structured in spiral arms: infrared observations (Spitzer survey) now suggest two major stellar arms, the Scutum-Centaurus arm and the Perseus arm, complemented by minor arms or gas "spurs", including the Sagittarius arm and the Orion arm. Its total mass remains debated, with recent estimates ranging from approximately 200 billion to 1.5 trillion solar masses depending on the methods used. Our Sun orbits at approximately 220-230 km/s, at a distance of approximately 27,200 light-years from the galactic center, in the Orion arm, this local spur where the Solar System resides. At this speed, it takes about 225 to 250 million years to complete one orbit.
The Gould Belt contains many young, hot stars; it could constitute part of the embryo of the local arm to which the Sun belongs. This small local structure of the Galaxy is our "playground": it is this ring of molecular clouds and bright young stars that obscures the background of the Milky Way. This region forms a bright bar in front of the Milky Way, visible especially from the southern hemisphere; this bar is tilted relative to the Milky Way, giving the Milky Way a curved shape.

Why Do We See a Curved Band in the Night Sky?

When we look at the Milky Way, we mostly see a band tilted relative to the galactic plane because the stars near the Sun are very bright. The Gould Belt thus traces a large curve across the Milky Way, where many very active stars are forming.

Origin of the Gould Belt

The Hipparcos Satellite and the Revelation of the Belt

The Hipparcos satellite (HIgh Precision PARallax COllecting Satellite) of the European Space Agency operated from 1989 to 1993. For 4 years, it measured the position, parallax, and proper motion of stars in our Galaxy. The Hipparcos data enabled this simulation (image on the right), highlighting the Gould Belt.

The Gould Belt appears distinctly if we photograph only the stars of the Milky Way located within 1500 light-years of the Sun, while fading out the small and old stars.

Two Populations of Stars, Two Faces of the Belt

In the top image, we see a homogeneous distribution of all nearby stars within 500 parsecs (<1500 ly), whose mass is roughly equal to that of our Sun (if we photographed all stars, near and far, we would see the Milky Way drawn over them). We kept A and F type stars, of almost identical mass to our Sun, rather old; and in the bottom image, we kept O and B type stars, much more massive than the Sun (7 to 20 solar masses) and, of course, younger, less than 40 million years old. The brightest stars of the Gould Belt are distributed along a curved band relative to the Milky Way, which is what Benjamin Gould had spotted in the southern hemisphere skies in 1879. The Gould Belt is therefore a ring of gas tilted relative to the galactic plane.

What Was the Gigantic Explosion That Generated the Gould Belt?

Several scenarios are under study, but it is likely that a gigantic hypernova equivalent to 10 supernovae generated this shock wave that currently sweeps through our local environment. This circular shock wave, containing 240,000 solar masses, likely deformed into an elliptical ring and tilted upward from the Milky Way, as the gas density is weaker upward than in the galactic plane.

A Tilted Ring Despite Galactic Gravity

The entire ring has tilted despite the gravitational force of the stars in the galactic plane, which tends to pull it back toward the plane. This shock wave, approximately 1000 light-years across, has been buffeting the interstellar medium in our local environment for a few tens of millions of years.

The Sun at the Heart of a Region Rich in Future Supernovae

The Sun is currently crossing this ring, where star formation bubbles of the Gould Belt explode at the periphery. The first generations of massive stars have already disappeared as supernovae and spread their matter into our stellar environment. On the edges of the Gould Belt, there remain about 300 to 400 supernovae that will explode within the next few million years. Statistically, 1 explosion occurs every 40,000 years, and we have front-row seats to witness this "stellar fireworks," as we are currently crossing a region particularly rich in supernovae.

References

FAQ: Everything You Need to Know About the Gould Belt

What exactly is the Gould Belt?

The Gould Belt is a local structure of the Milky Way, a ring of gas and young stars approximately 60 million years old, with a diameter of 3,000 light-years. It is tilted about 18 degrees relative to the plane of the Milky Way. This structure, identified by Benjamin Gould in 1879, contains many young, hot stars (O and B types) and is located in our nearby galactic environment, within 1,500 light-years of the Sun.

Why is the Gould Belt tilted relative to the Milky Way?

The 18-degree tilt of the Gould Belt is likely due to its explosive origin. A giant hypernova (equivalent to 10 supernovae) created a circular shock wave of 240,000 solar masses. This wave deformed into an elliptical ring and tilted upward from the galactic plane, because the gas density is weaker above the plane than within it. Galactic gravity tends to pull the ring back toward the plane, but it has persisted for 60 million years.

How was the Gould Belt discovered and mapped?

The Gould Belt was visually identified in 1879 by astronomer Benjamin Gould, who observed a concentration of bright stars forming a curved band in the southern hemisphere sky. Precise mapping of this structure was achieved using the Hipparcos satellite (ESA, 1989–1993), which measured the positions, parallaxes, and proper motions of millions of stars. By keeping only stars within 1,500 light-years and distinguishing young, massive stars (O and B types) from older stars, the Gould Belt clearly appears as a tilted ring.

What is the difference between O/B type stars and A/F type stars in the Gould Belt?

O and B type stars are young (less than 40 million years), very massive (7 to 20 solar masses), and very luminous. They are the ones that form the visible ring of the Gould Belt. A and F type stars are older and of comparable mass to the Sun; they are distributed homogeneously in nearby space and do not form any particular structure. The distinction between these two populations revealed the Gould Belt as a young, dynamic structure.

What is the likely origin of the Gould Belt?

The most likely origin of the Gould Belt is a giant hypernova, an explosion equivalent to 10 supernovae combined, which occurred approximately 60 million years ago. This explosion generated a circular shock wave containing 240,000 solar masses, which swept through the local interstellar medium. The shock wave deformed into a tilted elliptical ring, triggering the formation of new stars along its path. This region is still active today, with molecular clouds and forming stars on the edges of the ring.

Why is the Gould Belt an important region for studying supernovae?

The Gould Belt is an active star-forming region where many massive stars (O and B types) were born. These stars have very short lives (a few million years) and end as supernovae. The first generations of massive stars have already exploded, and there remain about 300 to 400 supernovae to come on the edges of the belt, within the next few million years. Statistically, one explosion occurs every 40,000 years. The Sun, currently crossing the Gould Belt, has a front-row seat to observe these phenomena, making this region a unique laboratory for studying supernovae and their effects on the interstellar medium.

What is the link between the Gould Belt and the spiral arms of the Milky Way?

The Gould Belt is a local structure, located in the Orion Arm, the small spiral spur where the Solar System resides. It could be an embryo of a spiral arm or a remnant of a galactic shock wave. It is not one of the major spiral arms of the Milky Way (like the Scutum-Centaurus arm or the Perseus arm), but a local bubble of gas and young stars, which influences our immediate galactic environment. Its tilt and dynamics make it a prime subject for studying the formation of small-scale structures in galaxies.