Description of the image: A look at the Gould Belt, our 100 million year old bubble. This homogeneous image shows all the stars very close to the Sun, within a distance of less than 1500 light-years (data from the Hipparcos satellite from 1989 to 1993). At the top, stars of type A and F with masses almost identical to our Sun, are relatively old, and at the bottom, stars of type O and B, much more massive than the Sun (7 to 20 solar masses) and younger, less than 40 million years old.
The Gould Belt, named in honor of Benjamin Gould (1824-1896), who identified it in 1879, is a young structure of our Milky Way, aged 100 million years. In reality, it is a "small bubble" 3000 light-years in diameter, composed of gas and stars, located in our very close galactic environment, in which the solar system travels.
The Gould Belt is tilted by about 18 degrees relative to the plane of the Milky Way. We live in a spiral galaxy with a diameter of ≈100,000 light-years, but it is very difficult to map its structure because we are inside it, which explains the uncertainty about the number of existing arms and even their naming. However, what clearly appears from Earth is the continuous white band of the Milky Way, but this galaxy is actually composed of at least four major spiral arms (Sagittarius arm, Orion arm, Perseus arm, Cygnus arm). Its mass is ≈400 billion solar masses, and our Sun orbits at ≈217 km/s, at a distance of ≈27,200 light-years from the galactic center, in the Orion arm (orange arm in the image). At this speed, it takes ≈240 million years to complete one orbit.
The Gould Belt contains many young and hot stars; it could be 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 for us. This region forms a bright bar in front of the Milky Way, visible mainly from the southern hemisphere, and this bar is tilted relative to the Milky Way, giving the Milky Way a curved shape.
Indeed, when we look at the Milky Way, we mainly see a band inclined relative to the galactic plane because the stars close to the Sun are very bright. The Gould Belt thus draws a large curve on the Milky Way, where many very active stars are forming.
The Hipparcos satellite (HIgh Precision PARallax COllecting Satellite) of the European Space Agency operated from 1989 to 1993. For four 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 when we photograph only the stars of the Milky Way located within 1500 light-years of the Sun while erasing the small and old stars (bottom image).
This is what is seen in the image on the right.
At the top, we see a homogeneous distribution of all nearby stars within 500 parsecs (<1500 ly), with masses roughly equal to that of our Sun (if we photographed all stars, near and far, we would see the Milky Way drawn over it). In the top image, we kept the stars of type A and F, with masses almost identical to our Sun, relatively old, and in the bottom image, we kept the stars of type O and B, 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 in a curved band relative to the Milky Way, which Benjamin Gould observed in the southern hemisphere skies in 1879.
The Gould Belt is therefore a ring of gas. But what gigantic explosion generated this ring inclined relative to the galactic plane? Several scenarios are under study, but it is likely that it was a gigantic hypernova equivalent to 10 supernovae, which generated this shock wave that now sweeps our nearby environment. This circular shock wave of 240,000 solar masses probably deformed into an elliptical ring and tilted upwards of the Milky Way, with the gas density being lower upwards than on the galactic plane.
The entire ring has tilted despite the gravitational force of the stars in the galactic plane that tends to bring it back to the plane. This shock wave of ≈1000 light-years has been jostling the interstellar medium of our nearby environment for a few tens of millions of years. The Sun is currently crossing this ring, where at the periphery, star-forming bubbles of the Gould Belt are exploding. The first generations of massive stars have already disappeared into supernovae and have spread their material into our stellar environment. On the edges of the Gould Belt, about 300 to 400 supernovae will explode in the next few million years. Statistically, one explosion occurs every 40,000 years, and we are in the front row to witness this "stellar fireworks" because we are currently crossing a particularly supernova-rich region.
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