Monday, 24 August 2015

Why is there a giant ring of young stars surrounding us?

A few weeks ago I talked about one of the nearest (and most important) regions of star formation, the Taurus Molecular Cloud. This region however isn't alone in being a relatively nearby region of star formation, in fact it forms part of a group of such regions that circles us in the night sky and which is known as the Gould Belt.

The Gould Belt is a ring of nearby star forming regions and young star clusters that surrounds our Sun. It was first observed by John Herschel (son of the famous William Herschel who discovered infrared radiation), who noted it as a band of bright stars that circled the sky, tilted away from the plane of the Milky Way, which also circles the night sky.

The bright stars of the Scorpius-Centaurus OB association,
part of the Gould Belt of young stars and star forming regions
(Credit: Akira Fujii)
The belt was named after an American astronomer called Benjamin Gould who performed the first detailed study of the structure in the 1870s. Gould was a pioneering and prominent astronomer, the first American to earn a doctorate in astronomy, and would go on to found the Astronomical Journal, one of the most prominent astrophysical journals in the world.

In the early 20th century astronomers were able to use spectroscopy to measure the speeds of the stars in the belt and found that they were moving with similar motions. This meant that all the stars in the Gould Belt were part of a single coherent structure. The distinctive pattern of the stellar motions has also revealed that the belt is both expanding and rotating. The rotation of the belt is thought to be caused by the fact that it sits embedded within the Milky Way galaxy, which is itself rotating as well.

The system appears to be broadly flat and pancake-shaped, approximately 2000-3000 light years across, but only about 400-500 light years thick. There is also evidence that the belt isn't perfectly circular and is more oval-shaped, a distortion that is also thought to be due to the rotation of the Milky Way as it stretches and twists the belt.

Illustration of the Gould Belt across the night sky relative to the distribution of molecular clouds in our galaxy. The Gould Belt is shown in red and the Galactic Plane is shown in blue. (Credit: Nick Wright / Thomas Dame)

The discovery of molecular clouds of hydrogen gas in the second half of the 20th century revealed that the Gould Belt was made up of many such clouds. When it was later realised that such clouds were where stars form it was quickly recognised that the Gould Belt represented a major site of star formation. It is now known that these star-forming molecular clouds make up most of the mass of the Gould Belt. This includes many well known structures such as the Taurus Molecular Cloud, the Orion Nebula, and the Rho Ophiuchi cloud complex.

While there are still many stars forming in the Gould Belt there is also a considerable history of star formation dating back almost 60 million years. This has lead to a huge collection of young star clusters (such as the Pleiades and Alpha Persei clusters), many OB associations (the famous Scorpius-Centuaurus association amongst others), and a number of luminous supergiant stars (such as Antares, the bright red star in the constellation of Scorpius). Supergiants are massive stars that are coming to the ends of their short lives and these objects were most likely born in one of the star forming regions in the Gould Belt. In fact the Gould Belt includes the majority of massive stars in the solar neighbourhood.

The young star clusters and OB associations that make up the Gould Belt,
shown in 3D relative to the plane of the Milky Way.
(Credit: New Scientist)

Since the discovery of the Gould Belt astronomers have been trying to understand how such a large and coherent structure formed within our galaxy. It was first thought that it formed when a massive star exploded as a supernova. The shock wave from the supernova would have swept up huge clouds of gas, compressing them and triggering the formation of new stars within them. However, if this were the case the ring should be aligned with the Galactic Plane and not pointing out of the plane (as the above image shows), so this theory appears to be ruled out.

A recent suggestion is that the Gould Belt was produced when a massive cloud of gas collided with our galaxy, in the same way that dwarf galaxies are known to collide with our Milky Way galaxy. This collision would have lead to a giant ring of expanding gas in the Milky Way that would be inclined at the same angle to the Milky Way of the collision itself. The shock wave from this would lead to the triggering of star formation and the creation of young star clusters, just as in the structure we see today.

This theory represents the best explanation astronomers have for the Gould Belt at this present time. Recently weight was added to this theory when astronomers found evidence for similar structures to the Gould Belt in other galaxies, suggesting that this phenomena may not be that rare.

The Gould Belt is just one of many structures in our galaxy that we can observe, from local star-forming clouds up to massive spiral arms. Understanding how these structures relate to the continual process of star birth and death and the evolution of our galaxy is one of the major tasks for astronomers today. Next time you look up at the night sky and see the bright stars and star clusters that are part of the Gould Belt think of how these objects are a part of the continual evolution of our galaxy!

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