The nearest site of dense star formation, the ρ Ophiuchi Molecular Cloud

A few months ago I talked about the Taurus Molecular Cloud, a prominent and well-studied site of active star formation, which also has the privilege of being the closest such region to us. It isn't the only nearby region of star formation though, and this week I want to talk about another well-studied nearby region called the ρ Ophiuchi (pronounced rho oh-fee-ook-ee) molecular cloud.

ρ Ophiuchi is not much further away than the Taurus Molecular Cloud, lying at a distance of 400 light years from us (120 parsecs), but one of its truly special features is that it is visible from both hemispheres. Due to the orbit and rotation of the Earth, astronomical objects visible from one hemisphere are often not easily seen from the other hemisphere, simply because the Earth does not spin into a position where they could be observed.

This presents a difficulty for astronomers because astronomical observatories can therefore only observe about half the sky. Telescopes and radio antennae in the Southern hemisphere can only observe the Southern sky, and vice versa (this isn't strictly true as it depends on the precise position of the observatory, but it serves as a good general rule). This can be annoying for astronomers because it means they can't always observe their targets with the ideal telescope. For example, my favourite region Cygnus OB2 is in the Northern sky, and thus I have to use telescopes in the Northern hemisphere to study it, a very disappointing situation when one considers the wonderful telescopes available in the Southern hemisphere!

ρ Ophiuchi is one of a small number of regions that straddles the Northern and Southern skies and can be observed from observatories in both hemispheres. It is therefore much easier to study ρ Ophiuchi than it is to study the northern hemisphere Taurus Molecular Cloud for example.

The ρ Ophiuchi star forming region is made up of quite a few different components, as the image below shows. The region is very close to one of the subgroups of the Scorpius-Centaurus OB association (the subgroup is known as Upper Scorpius), which leads to a number of bright stars in the area, many of which can be seen with the naked eye. Amongst these, Antares or Alpha Scorpius is one of the most impressive. The 'Alpha' designation means that it is the brightest star in the constellation of Scorpius, and is actually the 15th brightest star in the night sky. The star is a red supergiant, a very massive star coming towards the end of its life, which will one day explode as a supernova.

The Ophiuchus Clouds and surrounding area, covering 5 x 6 degrees. The image shows
both the dark clouds of Rho Ophiuchus (L1688 and L1689) and many of the nearby
naked eye stars (Antares, Alpha and Sigma Scorpius) and the nearby globular cluster M4.
(Credit: Robert Gendler / Nick Wright)

The triple star system ρ Ophiuchus, from which the star forming region gets its name, also contains a number of massive stars, slightly less massive than Antares, and not as far towards the ends of their lives, but still very bright and impressive. They'd be even brighter in the night sky if they weren't partly embedded within the molecular cloud. These stars are what astronomers refer to as B-type stars.

The young stars of ρ Ophiuchi are actually much fainter and less massive than those in the nearby OB association. They are younger as well, only about 1 million years old, making them stellar toddlers! Most of these young stars are still embedded in the molecular cloud that they formed in, so they are difficult to see in images like this, though the molecular clouds themselves can be seen as they appear as dark clouds obscuring the background starlight. The two main clouds in ρ Ophiuchi are known as L1688 and L1689, though they're both made up of many smaller clouds.

The first proper studies of ρ Ophiuchi came with the advent of infrared astronomy in the 1970s. Infrared radiation can penetrate into the dark and obscuring clouds, allowing astronomers to see the young stars forming within them. These early studies revealed hundreds of young stars deeply embedded with the molecular clouds, many still in the processes of forming and others at the end of the formation process.

The cluster of young stars that the infrared observations uncovered is larger and denser than those in the Taurus Molecular Cloud, though not as dense as some of the more massive star clusters such as the Orion Nebula Cluster. They therefore provide a nice contrast between these two other prominent regions.

The young stars of the L1688 cloud in Rho Ophiuchus, as seen in infrared light (Credit: Spitzer Space Telescope)

By studying this large population of forming stars astronomers were able to garner insight into the star formation process and study how stars appeared to change appearance as they formed. These infrared studies also revealed an important type of object known as a starless core. These are giant globules of gas, dense and massive enough to be held together by their own gravity, but without a star inside of them. Because they are gravitationally bound but not supported by any outward forces they must be in the process of collapsing to form new stars.

By studying these starless cores astronomers have been trying to understand how stars start to form, as well as how the properties of the core affects the properties of the star that forms. For example, astronomers have discovered that the distribution of masses of these cores is very similar (if slightly larger) than the distribution of masses of stars, suggesting that stars probably form directly out of these cores with a direct correlation between the mass of the core and the mass of the star that forms within it.

Rho Ophiuchus (right) and the Pipe Nebula (left) projected against
the Galactic Centre, with many of the bright stars of the Scorpius-Centaurus
OB association also visible (Credit: Maurice Toet)

Star formation in the region is thought to have started when a shock wave from the nearby Scorpius-Centaurus OB association triggered the collapse of the ambient gas clouds in the area. This may have been caused by winds that emanate from massive stars or even possibly a supernova explosion when one of the most massive stars in the OB association died. Many astronomers think that shock waves such as this are a common trigger of large star formation events in our galaxy, and there has been considerable work to trace back these triggering events to their source.

Recent far-infrared and sub-mm observations of ρ Ophiuchi have allowed astronomers to trace the molecular gas and dust that makes up the densest parts of the molecular cloud where star formation is most active. These molecular maps have revealed that the gas has a highly filamentary structure on large scales, with multiple dense clumps on the smaller scales where stars are beginning to form.

Rho Ophiuchus (right) and the Pipe Nebula (left) with a colour map
projected on top showing the density of molecular gas
(Credit: ESO, S. Guisard and J. Kainulainen)

It is thought that stellar winds and supernovae sculpt the gas in these molecular clouds into these massive filamentary structures, which then become gravitationally unstable and collapse to form the dense cores that are the precursors of forming stars. Trying to understand how these filaments of gas are created and how stars form from them is an area of very active research at the moment.

ρ Ophiuchi has proved to be not only an amazing location to study young and forming stars, but also to study all the processes that lead up to star formation: the sculpting of molecular gas, the collapse of long filaments into dense cores, and the formation of protostars within them. It has, and continues to be, a valuable resource for astronomers!

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!