The Gould Belt

The Gould Belt is a vast ring of active stellar nurseries, young stars and molecuar clouds encircling our Solar System. I am part of the JCMT Gould Belt Survey and the Spitzer Gould Belt Survey who study this collection of star-forming regions in submillimetre and infrared wavelengths respectively.

The Belt (called Gould’s Belt in North America, and named for Benjamin Gould, who identified it in 1879.) is actually a fragmented ring of of star-forming clouds, young stars and nebulae that comes in at around 3000 light years in diameter. Containing lots of bright, young O- and B-type stars, in sits neatly inside the local spiral arm of our own galaxy the Milky Way. The Sun appears to be situated roughly in the middle of it – but it is not know where it came from or what it really is. To study the star-forming regions does not necessitate any understanding of the belt itself, but it is curious to wonder how it came to be.

The image below (from galaxymap.org) shows approximately where in our Milky Way galaxy the Gould Belt lies. You have to image the entire Solar System as an invisible speck near the middle of the white ring, denoting the Gould Belt.

One common theory is that there was some ancient supernova that exploded and sent radiation and material outward. Like a ripple from a stone dropped in a pond, this caused an ever-growing ring of activity in the surrounding interstellar medium. This model works fairly well, since the local bubble – a region of low density in which the Sun and a few other stars sit – fits reasonably well inside the Gould Belt. Perhaps both of these structural features could have resulted from an ancient explosion.

It is also possible that some much larger scale interaction has taken place. A recent paper on arXiv suggests that the belt originated 30 million years ago when a giant dark matter clump collided with a giant molecular cloud in the Milky Way’s spiral arm.

Maybe there is no Gould Belt! It could be that we only think we can see a ring-shaped pattern in the layout of our local region. After all when it comes to detailed structure in the galaxy, we cannot see much further out than our own spiral arm. Maybe these kinds of shapes are merely coincidental from our viewpoint.

The image at the top of this post was created by my office-mate Jason Kirk and it shows the star-forming clouds within the Gould Belt. The belt itself is marked as a blue ring and the local bubble is shown as a shaded area. The size of the star-forming regions is proportional to their mass, assuming a uniform density. This image was created for the Spitzer Gould Belt Survey and I have always found it handy when thinking about the Gould Belt.