Most new stars in the Milky Way form along its spiral arms, the diffuse white star clouds we see encircling the celestial sphere. Strange, then, that some of the brightest stars in our sky seem slightly offset from the band of the Milky Way. In the constellation Scorpius, for example, you see bright stars well north of the plane of the galaxy. The bright stars of Orion, at the opposite end of the sky, are found to the south. A coincidence? No, it turns out. These stars of are part the Gould Belt, a ring of bright young stars around the sky that formed under mysterious circumstances quite recently in the history of our galaxy.
John Herschel first noticed a profusion of bright stars in an arc through Orion, Canis Major, and what was then called Argo Navis (now Puppis, Carina, and Vela) while observing from South Africa. He published his work in 1847. This arc was eventually named after Benjamin Gould, the first American to earn a doctorate in astronomy, who mapped the band of stars all the way around the sky during observations made from Argentina in 1874. Both Gould and Herschel noticed the band of stars was tilted by nearly 20º from the plane of the Milky Way.
Even casual stargazers can see the brightest stars of the Gould Belt. Many of the bright blue-white stars of Orion and Canis Major are members, as are many stars in the southern constellations Vela, Carina, and Centaurus. The belt crosses the plane of the Milky Way in Crux, then continues into Lupus, Scorpius, and north through Ophiuchus and past Lyra into the area of Perseus and Cassiopeia.
Gould Belt stars are formally distinguished by their brightness, age, and location. Most of the hot and bright O and B-type stars in our skies of apparent magnitude 5.3 or brighter are members of the belt. In real terms, the starry ring extends in an elliptical disk about 2,400 x 1,500 light years across. The center of the Gould Belt lies about 325 light years away from the Sun in the direction of Perseus.
Astronomers believe the first stars of the Gould Belt formed about 30 million to 50 million years ago. Many of the first stars produced in the belt have already detonated as supernovae, triggering further star formation which still proceeds apace, most vigorously in the famous Orion Nebula in the sword of the constellation Orion. So brilliant Vega, for example, which lies not far from other bright stars of the Gould Belt, cannot be a member because it’s more than 500 million years old. But Rigel, just 8 million years old, is a likely one of a later generation of stars formed from the residual gas and dust that formed the first Gould Belt stars. The red supergiant star Antares and its bright blue-white neighboring stars are also associated with the belt.
How did the Gould Belt form? No one knows for sure. A supernovae can trigger star formation by compressing local gas and dust clouds, but astronomers suspect a single supernova would not pack enough force to create the Gould Belt. One long-standing hypothesis suggests a large cloud of intergalactic dust smacked into the plane of the Milky Way at a shallow angle. The collision provided material and mechanical compression to create the first Gould Belt stars. A recent study even suggests a cloud of dark matter passed through the Milky Way, and its gravitational influenced triggered a wave of star formation.
The Gould Belt has a strong influence on the appearance of our night sky. There are likely many other such “belts” in our galaxy, and astronomers have observed similar rings of stars in other nearby galaxies as well.
Publisher’s Note: Image at top of page is used with permission of David Kingham. This image, called “Oxbow Bend Reflections”, shows Gould Belt stars in Scorpius north (right) of the Milky Way. See more of David’s images at www.davidkinghamphotography.com