The Serpens South Cluster

Continuing our series on nearby star forming regions that are interesting and important to the history of astronomy I want to turn this week to one of the most recently-discovered regions, the Serpens South Cluster.

Serpens South is not as famous as some of the other nearby star regions such as Taurus and Ophiuchus, predominantly because it was only discovered in the last decade. The cluster is very faint in the optical part of the spectrum because it is still heavily embedded within its molecular cloud, so it wasn't seen by previous surveys that were predominantly performed in the optical part of the spectrum. Furthermore, because the Serpens and Aquila regions of the Galactic plane appear relatively unpopulated in young stars, astronomers hadn't studied the area in much detail.

The vicinity of the Serpens Molecular Cloud as
seen on inverted Deep Sky Survey plates.  The
presence of the molecular cloud is seen from its
obscuration of background stars (Credit: Eiroa et al. 2008).

The wider region first came to prominence in the 1970s when a dark cloud was discovered in the vicinity of the bright star VV Ser near the Aquila Rift (a dark cloud of gas and dust that extends along the Galactic plane in this area of the sky). The image to the left shows the night sky in the vicinity of the Serpens Molecular Cloud where its presence can be seen by the lower density of stars. The density of stars appears lower towards the molecular cloud because dust in the cloud obscures the starlight from stars behind the cloud.

A number of small bright nebulae were identified in the area at this time, including Sharpless 68 and the Serpens Reflection Nebulosity, both illuminated by nearby bright young stars.

South of this region lies the Westerhout 40 (W40) HII region, a modest cloud of ionised gas thought to be at a distance of 1500 light years (500pc). The HII region is the visible part of a larger star forming region where stars of all masses are currently forming including O and B-type stars, making this one of the nearest regions where O and B stars are in the process of forming.

Map of the dark clouds in Serpens that form part of the Aquila Rift. The main part of the cloud
is shown in the grey rectangular box, which includes the W40 region, the Serpens South Cluster
(the white star), and the HII region Sh2-62. The Galactic Plane can be seen across the bottom-left
corner of the image. The earlier image covers the region around Serpens Main and Serpens
NH3 at the top of the image (Credit: Bontemps et al. 2010).

When this area of the sky was observed by the Spitzer Space telescope in 2006 astronomers discovered a cluster of stars previously unknown, highly embedded within the molecular cloud and visible only to infrared telescopes such as Spitzer. The cluster is very close to the W40 HII region, as can be seen on the map of the region shown above. It was soon dubbed the Serpens South Cluster, and since then it has been the focus of considerable study.

The Serpens South Cluster as seen by the
Spitzer Space Telescope
(Credit: Spitzer/NASA)

The kinematics of the gas associated with the cluster are very similar to the gas surrounding W40, suggesting that the two structures are part of the same star forming complex and are likely at the same distance. A distance of 1500 light years is also in good agreement with that recently obtained from radio parallax measurements.

Early studies, primarily with the Spitzer Space Telescope (see image to the right) uncovered a cluster of about 50 stars, of which at least 35 were still in the process of forming, suggesting that the cluster is very young. Later studies in the far-infrared with ESA's Herschel Observatory detected even more highly embedded sources at even earlier evolutionary stages, providing evidence for even younger protostars still in the process of collapsing to form stars.

Given its youth, the density of the cluster is very high, with at least a few hundred stars per square parsec on the sky. This suggests that either the stars formed in a very dense and clustered state (as we see them now) or that clusters like this can form very quickly out of stars that form in a low density distribution. This is one of the key questions astronomers are trying to answer when they study young star clusters.

The infrared observations have also revealed an intricate network of filaments emanating from the cluster with a hub-like morphology. These filaments are thought to play an important role not just in how stars form and build up their masses, but also in how star clusters grow to their present sizes so quickly. Collisions between filamentary structures in molecular clouds may play a critical role in the formation of such dense clusters.

Studies of young and dense clusters such as Serpens South are important for understanding how young star clusters form and how this is related to the formation of stars within them. It seems that the two processes are critically connected and so to study one we must also study the other!

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!

Milky Way Astrophysics from Wide Field Surveys - Part I

The entrance to the Royal Astronomical Society's
headquarters at Burlington House in London
(Credit: Wikimedia Commons)

This week I'm at a conference in London at the headquarters of the Royal Astronomical Society where we're discussing scientific results from recent wide field surveys of the Milky Way. Wide field surveys is just another name for surveys that cover a large area of space, and there are many surveys these days that fit that category, including a few that I work on.

Because these surveys cover such a large area of space they allow many different types of astronomical objects to be studied, from young stars to old stars, individual objects to the entire galaxy. So a conference like this is a great opportunity to stay in touch with a wide array of scientific results.

Today's talks have mostly been given by the leaders of the surveys, who have been telling us about their surveys, how we can get the data from the surveys, and highlighting some of the scientific results. This is a good opportunity to learn about new survey data and to think about how this data might be useful to solve some of the problems I'm trying to address.

The Milky Way - home of many many surveys! (Credit: ESO)

I've been really impressed with the surveys presented today. They've covered (almost) every part of the electromagnetic spectrum, from radio waves through the infrared and up to the optical part of the spectrum, and they've offered up a huge range of possibilities for future work. They also have some amazing names, including such gems as e-MERLIN and UWISH - astronomers really love acronyms!

The highlight of the day for me was probably a presentation about a sub-mm survey called ATLASGAL. The sub-mm part of the electromagnetic spectrum is between the infrared and the microwave parts of the spectrum. One of the advantages of observing in this part of the electromagnetic spectrum is that it is not absorbed by dust and so can be used to study objects across our entire galaxy, even on the far side of our galaxy that would normally be obscured and inaccessible to us.

Part of the Galactic Plane of our galaxy seen by the ATLASGAL survey showing a number of prominent
star forming regions, including Messier 20, The Triffid Nebula (Credit: ESO/ATLASGAL)

The survey data has been used by a team of astronomers to survey the majority of our galaxy in the sub-mm part of the spectrum and identify hundreds of dense clumps of molecular gas where massive stars are forming. Sub-mm emission is one of the most reliable and efficient methods to identify dense star forming regions. It's an exciting project and I'm looking forward to seeing more results from the survey in the future.

Where the wild things are: Cygnus OB2

This week I want to tell you about one of my favourite astronomical objects, and one which I have spent many years studying, the Cygnus OB2 association.

Cygnus OB2 is a loose group of young stars known as an association, a sort of less-dense version of a star cluster. As the name would suggest, the Cygnus OB2 association is the 2nd OB association catalogued in the constellation of Cygnus. This constellation lies very prominently in the plane of our galaxy where the majority of stars and nearly all young stars in our galaxy are found.

The Milky Way, showing the constellation of Cygnus and the position of the Cygnus OB2 association
(Original image credit: Axel Mellinger)

From the image above you wouldn't believe anything particularly exciting was happening in Cygnus, as the region appears mostly dark, particularly around Cygnus OB2 itself. The reason for this is that between us and Cygnus OB2 there is a vast cloud of obscuring dust that absorbs the majority of starlight emitted by the association, hiding it from us. This cloud of dust is sometimes referred to as the Cygnus Rift or the Great Rift, and can be seen extending across much of the Galactic Plane, as the above image shows.

Optical photons are readily absorbed by dust particles, but infrared photons are less susceptible to this problem and can penetrate these obscuring dust clouds. An infrared view of the Galactic Plane, such as that shown below thanks to Japan's Akari satellite, reveals the prominent sites of star-formation activity in our galaxy, radiating brightly thanks to the many young stars forming within them.

The Milky Way in infrared light, as seen by the Akari satellite, with major infrared-bright regions labelled
(Credit: ESA)

The brightest and most prominent of all these star forming regions is probably the Cygnus region, despite it being more distant than many of the other star forming regions shown on this image. This brightness is testament to the intense amount of star formation that has, and still is occurring in this region. In fact when this region was first studied at radio wavelengths the bright structure was so prominent as to be marked with an 'X', hence leading to the name of Cygnus X for the whole star forming complex.

Detailed infrared view of the Cygnus X region, with
Cygnus OB2 in the centre (Credit: NASA)

Zooming into this image we can begin to see some of the amazing structures present within the Cygnus X giant molecular cloud. Vast clouds of gas and dust can be seen collapsing to form young stars, while huge pillars are constantly being sculpted and eroded by the recently-formed stars. This is a place where star formation takes place at the extremes!

Right in the centre of the Cygnus X giant molecular cloud is the Cygnus OB2 association, a massive group of young stars, as populous as some of the most massive young star clusters in our galaxy, yet nowhere near as compact.

The diffuseness of Cygnus OB2, coupled with the obscuring dust clouds, led to it being maligned for many decades. Only in the last 15 years, thanks to the revolutions in infrared and X-ray astronomy, have researchers been able to penetrate the extinction and uncover the thousands of massive, young stars in this huge OB association, including some of the most massive and luminous stars known!

As I said at the beginning of this post, Cygnus OB2 is a region I have studied for many years, hoping to better understand its origins and its content, and therefore to appreciate its role in the continual evolution of our galaxy. Next week I hope to share some exciting news and discoveries about Cygnus OB2 that we will be publishing very soon, so stay tuned!

Where stars form

Yesterday we were treated to a stunning image on the Astronomy Picture of the Day website, which showed off one of the regions in our galaxy where stars are in the process of forming. The image is shown below in infrared light, using data from NASA's Spitzer Space Telescope.

The W33 star forming region as seen in infrared light (Credit: NASA / Spitzer Space Telescope)

This region is called W33, so called because it was the 33rd object catalogued by Gart Westerhout in his survey of radio sources in our Galaxy. Many of the sources catalogued by Westerhout are regions in which stars are forming, such as this one.

Astronomers refer to these regions as massive star forming regions, not just because they are massive (this image is about 100 light years wide!), but also because they are the sites where massive stars are forming. Massive stars are the hottest and most luminous stars that exist, and they play an important role in how a galaxy evolves thanks to their luminosity, the strong winds that emanate from their surfaces, and the supernova explosions in which they end their lives.

For these reasons, and because of their short lives and inherent rarity, massive stars are important objects to study. Furthermore astronomers aren't entirely sure how they form, so regions such as this where massive stars are known to be forming, are important to study.