What's wrong with globular clusters?

Globular clusters are amongst the oldest and most massive star clusters in the Universe. Their size and luminosity means that not only can we study the approximately 150 globular clusters in our own galaxy (the Milky Way) in quite a lot of detail, but we can also observe and study globular clusters in other galaxies. This is useful because globular clusters, like all types of star cluster, can provide unique insights into how galaxies form.

For many years astronomers have considered globular clusters to be examples of simple stellar populations, meaning that all the stars in them are thought to have formed at the same time and out of the same gas cloud, meaning that their initial chemical compositions were thought to be very similar. However, recent observations have shown that many globular clusters show evidence for multiple stellar populations with different chemical compositions (e.g., Gratton et al. 2012).

Colour magnitude diagram for the globular cluster NGC 2808.
Each dot represents a star in the cluster. The distribution of
dots into multiple but distinct lines suggests the presence of
multiple populations (Credit: Piotto et al. 2007).

How do astronomers know that there are multiple populations in these globular clusters? Well, if you measure the colour and brightness of all the stars in a cluster and plot their distribution then a single population of stars will form a single distribution in a narrow line, but astronomers have found that globular clusters appear to show multiple distributions.

The image on the right shows one of these plots, referred to as a colour-magnitude diagram (the magnitude of a star is a measure of its brightness), for the globular cluster NGC 2808. The stars are distributed in a narrow band, but closer inspection shows that this band is actually made up of multiple, narrower bands.

This means that the globular cluster is made up of multiple populations of stars, each with a distinct chemical signature that is different from the other populations. Astronomers can measure the chemical compositions in the different populations using spectroscopy, confirming that these discreet bands in the colour-magnitude diagram are caused by different chemical abundances.

The origin of these multiple populations aren't currently known. There are various possibilities that are being considered by astronomers, mostly involving multiple bursts of star formation within the clusters (e.g., D'Ercole et al. 2008), with the second generation of stars being chemically enriched by some process.

This then leads to the question of what could cause the chemical enrichment. There are various ideas that are being investigated, ranging from material being ejected by evolved stars, thrown off by rapidly-rotating stars, or even violent ejections by interacting massive binary stars. Astronomers are currently trying to work out which of these effects are responsible, though its a difficult task because most of this enrichment would have occurred many billions of years ago!

Understanding these massive star clusters is important because they represent some of the oldest star clusters that we can study and their formation appears to be closely related to the formation of their host galaxy.

How to make a star cluster

In my last two posts I've been discussing how stars are distributed at the time they're born. This is an important question because many of our theories for how stars form suggest that their environment can play a crucial role in how they accrete material. Our current observations suggest that stars form directly out of the dense gas that is found in filamentary structures across molecular clouds. But if stars form with an elongated and filamentary distribution, how do spherical star clusters form?

The answer to this question has eluded astronomers for many decades, and though there is still considerable debate in the community, a picture is emerging whereby star clusters arise when filaments of dense gas merge.

Star clusters forming in the Rosette Molecular Cloud
(Credit: Schneider et al. 2012)

The figure to the right shows an image of filaments and star clusters in a star forming region known as the Rosette Molecular Cloud (so-called because its very close to the famous Rosette Nebula). The background image shows the distribution of dense gas in the cloud, with the density of the gas ranging from low-density (black) to high-density (green and red).

On top of this are marked (in white) the positions of the filaments that make up the molecular cloud, and on top of that (the turquoise stars) are the positions of known star clusters.

If you inspect the image closely you'll see that the majority of the star clusters (which were known about well in advance of this study) sit at the intersections between the filaments. In fact out of the 14 star clusters in this molecular cloud, 13 of them are found at these intersections. This is unlikely to be a coincidence, so it appears that the formation of star clusters is closely linked to overlapping or merging filaments.

Over the last decade astronomers have seen various strands of evidence pointing towards this picture (a good summary of the early evidence can be found here). However it wasn't until the launch of the far-infrared Herschel Space Observatory in 2009 that the filamentary structure of molecular clouds became so apparent, and soon after that the relation between clusters and filaments began to emerge.

So if stars clusters are found where filaments overlap, this suggests that the collision between the filaments might create the necessary conditions for a star cluster to form. The question this then poses is whether the filament collision occurs before, after, or even during the star formation process.

If the filament collision occurs before star formation then the collision is effectively bringing together large volumes of dense gas into a small space. This would allow star formation to proceed very rapidly in a very dense cluster of gas, leading to the formation of stars in a highly clustered distribution. This has sometimes been referred to as clustered star formation or in-situ cluster formation.

Alternatively, the filament collision might occur after star formation has begun, in which case the filament collision would be bringing together stars that have already formed, depositing them in a highly clustered distribution. This is usually referred to as conveyor-belt cluster formation.

Which of these two scenarios is right has big implications for how stars form and how the environment affects the star formation process. There are strands of evidence in favour of both scenarios, though neither has been conclusively shown to be true yet. Of course its possible that both scenarios might occur, perhaps in different environments, in which case it would be interesting to understand which process occurs more often, and whether the clusters that form from the two processes differ in some way. Hopefully that's a question we can answer soon!

What sort of environment do stars form in?

Last week we talked about the initial spatial distribution of young stars and how their distribution follows that of the dense gas in molecular clouds. But we also know that stars form in groups with a wide variety of sizes and densities, which astronomers think is really important for determining the type and sizes of the star clusters that form.

Distribution of young stars in the Perseus Molecular Cloud
(red, green and blue dots) projected against the gas
distribution (Credit: Evans et al. 2009)

The image on the right shows the distribution of young stars across the Perseus Molecular Cloud. These young stars were all detected by the Spitzer Space Telescope, an infrared telescope that was particularly effective in detecting young stars due to the copious amounts of infrared light they emit.

The molecular cloud is very elongated, as the image clearly shows, but even within that elongated structure the young stars are not evenly distributed, they're clumped into groups. Many of these groups represent the well-studied embedded star clusters typically found in molecular clouds, such as IC 348 and NGC 1333.

In addition to these dense and compact clusters there are also smaller groups, such as the clumps of young stars labelled B1 and B5, as well as numerous young stars that appear relatively isolated.

It appears that while young stars do like to form in groups, there are almost as many young stars that form alone - so is there a typical group size and density that stars form in? And if so, what is it?

One way that astronomers have attempted to tackle this problem is to study the distribution of densities that stars are forming at. To do this astronomers have measured the density of stars surrounding each young star. The distribution of densities is usually referred to as the surface density distribution of young stars.

The figure below shows such a distribution compiled from Spitzer Space Telescope observations of numerous nearby star forming regions. Mid-infrared observations from the Spitzer Space Telescope were chosen for this because it allows astronomers to peer deep within molecular clouds and hopefully identify all the young stars that are present. Hopefully this means no stars were missed!

The surface density distribution of young stars (both Class I and Class II young stars) identified from
Spitzer Space Telescope observations (Credit: Bressert et al. 2010)

The figure shows the fraction of stars born at various densities, from low densities on the left (surface densities of 1 star per square parsec) to high densities on the right (hundreds to thousands of stars per square parsec). The former represent stars that have formed in relative isolation, while the latter represent stars that have formed in dense groups or clusters.

Most notable in this figure is the fact that there is a smooth distribution from low to high densities, which suggests that stars don't just form at low and high densities (in isolation and in clusters), but at a wide range of densities, with groups and clusters existing over a variety of densities.

This is important for our understanding of star formation because it tells us about the conditions under which stars form, as well as the sort of environment where planetary systems form. A planet forming in a dense cluster faces very different conditions compared to one born around a relatively isolated star. In a dense cluster there could be multiple interactions or collisions between stars and planets, as well as a very powerful radiation field due to the close proximity of so many other stars, which could damage a forming planet's atmosphere.

Hopefully as we start to learn more about the various types of planetary system that exist, and especially once we start studying the atmospheres of these planets, we can hopefully address the question of what impact the birth environment has on a forming planetary system.

Star formation, filaments, and the initial distribution of young stars

I've talked before about molecular clouds, the birthplaces of stars, but only in very general terms. Today I want to go a little deeper to look at how stars are distributed within molecular clouds when they form, and how this can lead to the formation of star clusters.

Molecular clouds aren't just big fluffy clouds of gas that uniformly form stars, they have considerable structure. This is clear if you look at molecular clouds using an infrared telescope, particularly one that's tuned to the far-infrared, such as the Herschel Space Observatory (the further into the infrared you go, the cooler the material you can study because cooler things radiate light at longer wavelengths, i.e. the far infrared).

A far-infrared image of the Taurus molecular cloud, showing the filamentary structure of the gas
(Credit: Herschel Space Observatory)

This image from the Herschel Space Observatory shows the incredibly filamentary structure of the gas in the Taurus molecular cloud. This structure is thought to arise due to a combination of shock compression (due to collisions between material) and self-gravity (meaning the filaments can form gravitationally-stable structures by themselves). These filaments can be seen on all spatial scales, from the large filament spanning the entire image, through to small filaments emanating from the larger ones.

Many of these filaments are dense, containing many times the mass of our Sun in molecular gas. This high density means that they can become gravitationally unstable, which can lead them to collapse and potentially form stars. Simulations of filaments suggest that a single filament may actually fragment into multiple stars distributed along it's length.

The NGC 1333 molecular cloud. Contours
show the distribution of dense gas, while
 the red dots show the very young stars that
are still forming (Credit: Gutermuth et al. 2008)

One of the predictions of this model of filamentary star formation is that the densest gas in star forming regions should be distributed in filaments and that the youngest stars - those that are actually still forming - should be found within these filaments.

The image to the right shows that the distribution of very young stars (red dots) in the star forming molecular cloud NGC 1333 are almost entirely projected against (and therefore likely to be within) the filamentary dense gas shown by the contours.

This provides strong evidence that stars form directly out of the dense, filamentary gas in molecular clouds.

Not only do the young stars trace the spatial distribution of the dense gas, but they also appear to have very similar motions. Numerous studies have measured the radial velocities (the speed of an object along the line of sight - i.e. towards or away from us) of stars such as these and found that they are almost identical to the velocities of the gas (e.g., Walsh et al. 2004, Hacar et al. 2016).

Figure showing the Orion A molecular cloud. The
background greyscale image show the dense gas
while the red dots show very young stars
(Credit: Megeath et al. 2012)

This picture where star formation occurs in dense gas is seen in all the star forming regions we've studied, and not just on small scales, but also on the much larger scales of giant molecular clouds such as the Orion A cloud, shown on the left.

This is important for understanding star formation because it not only tells us where stars will form, but also provides clues as to how stars form (i.e. the process requires very dense gas) and what must occur to set up the necessary conditions (i.e. the gas in molecular clouds must be compressed in dense filamentary structures that become self-gravitating).

It is also important for understanding star clusters because it tells us not just the initial spatial distribution of groups of stars that may go on to form clusters, but also their kinematics, i.e. how those stars are moving.

The next question to ask is how do these elongated and substructured groups of stars evolve into the classic centrally-concentrated images of star clusters we see so often? But that's a question for another day!

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!

How do you form a massive black hole binary? (Part 3)

This post brings to an end a three-part series of posts about how massive black hole binary systems form, such as those recently observed to merge by the LIGO gravitational wave detector. The first two posts discussed how such massive black holes could form and how a binary system of black holes could form in isolation. Here I'm going to talk about a much more efficient way to form binary black holes, and that's in a star cluster!

As we discussed last week, forming a binary black hole in isolation is very hard. There are lots of processes that could disrupt the binary system before both stars form black holes, and once the binary has fallen apart it's hard to put it back together. But not so in star clusters! In star clusters there are lots of (gravitational) interactions between stars (or black holes) that, under the right circumstances, can lead to the formation of new binary systems. In fact there have been many simulations to explore exactly this sort of process!

The dense and massive star cluster R136 in the 30 Doradus nebula. The bright blue dots are predominantly massive
OB-type stars, many of which will end their lives as black holes (Credit: HST).

The basic idea is a simple 3-step process first put forward by Sigurdsson & Hernquist in 1993. The first step is that star clusters are full of massive stars, and these massive stars are heavier than normal stars and so they naturally sink to the centres of the clusters (we observe this process, called mass segregation, in many star clusters). The second step is that once these stars reach the ends of their lives they explode as supernovae and leave behind stellar remnants such as black holes. This we also know to regularly take place.

Simulation showing the development of mass segregation in a star cluster after only 2 Myrs (right)
compared to a lack of mass segregation at the beginning of the simulation (left). The massive stars (big circles)
are clearly more centrally concentrated in the right-hand panel (Credit: Marc Freitag).

The third step, which is where the clustered environment of the star cluster is critical, is that close encounters between stars (or black holes) in the centres of star clusters can quickly lead to the formation of binary systems, and therefore potentially black hole binary systems. Outside of star clusters the density of stars in space is very low and close encounters are very rare (which is fortunate for our Sun and solar system otherwise we'd always be bumping into other stars!). But star clusters are dense and the stars within them are moving very fast, meaning that close encounters between stars are common. If the stars come close enough to each other in just the right way then a binary system forms.

Many studies have confirmed this general picture and have estimated that it should take (on average) about 1 Giga-year (1000 million years) for enough interactions to occur between stars and black holes to form a binary black hole system in a dense cluster. This means that such binary black hole systems are most likely to be found in older clusters, such as globular clusters, though it shouldn't be impossible to find them in younger clusters.

Does the Omega Centauri globular cluster house
a central black hole? (Credit: ESO)

There is good evidence that this happens quite often. A recent study of the massive globular cluster Omega Centauri found that the dynamics of its stars suggest a particularly massive central black hole, while another study even found evidence that there might be a black hole within the nearby Orion Nebula Cluster. Confirming these theories is difficult because directly detecting black holes isn't always easy, but the evidence is there to suggest they might be occupying many of our favourite star clusters!

Further dynamical interactions within the star cluster can lead to two important effects for the binary black hole system. The first is that the binary will shrink over time due to dynamical interactions with individual stars - this brings the black holes in the binary closer together, making an eventual merger more likely. The second effect is that these black hole binary systems can be easily ejected from the star cluster, often at quite high velocities, through interactions with other binary systems.

The most likely clusters to produce binary black hole mergers are intermediate age clusters with ages of a few Giga-years. Younger clusters are unlikely to have had enough time to form the binary black hole systems, while older clusters will have formed them but they might have all merged by now.

This process implies that a typical star cluster, which contains many massive stars, will easily produce many black holes, and most likely a few black hole binary systems that will one day merge and release gravitation waves just like those observed in September.

Given how many star clusters there are in the Universe its hardly surprisingly that such a black hole merger was observed so easily!

Planet-forming discs around young stars truncated by close encounters in Orion

The protoplanetary disk of HL Tau, as observed by the
ALMA Observatory. Discs like those would be continually
eroded by close encounters in dense star clusters like
the Orion Nebula Cluste (Credit: APOD)

Astronomers have for a long time suspected that the environment in which stars form could very strongly influence the formation of planetary systems. A recent paper by Simon Portegies Zwart at the Leiden Observatory shows this to be case and reports results of simulations that explore this effect. The simulations show that close encounters between stars in a dense cluster can truncate the protoplanetary discs around stars where planetary systems are thought to form. His simulations are able to reproduce the properties of protoplanetary discs in the nearby Orion Nebula Cluster, suggesting the stars and discs in this cluster have already felt the impact of their crowded environment. These interactions will not only lead to an erosion of the disc (leaving less material available to form planets) but also reduces the orbital radii at which planets can form, impacting the types of planetary system that might form.

Why are we interested in star clusters?

This blog is all about star clusters, which are pretty stunning and amazing astronomical objects, but some of you might be wondering why astronomers are so interested in them. So in this post I thought I'd give my Top 5 Reasons to Study Star Clusters! I hope you enjoy it!

The Orion Nebula Cluster, seen in X-rays and
optical light. By estimating the masses of the
young stars formed here we can study the
distribution of stellar masses that form
(Credit: Chandra X-ray Observatory)

1. Star formation. Perhaps the most important reason to study star clusters is because we believe the majority of stars form in groups or clusters and so by studying young star clusters we hope to learn something about how stars form. For example, by measuring the masses of all the stars in a cluster we can study the mass distribution of recently-formed stars (known as the initial mass function), which is one of the most important products of the star formation process.

Hubble Space Telescope image of an evaporating
protoplanetary disk (known as a proplyd) in the
Orion Nebula. The disk (dark silhouette in the
centre) is being eroded by radiation from a
nearby bright O-type star leading to the tail of
material stripped off(Credit: NASA/HST)

2. The impact of environment on star formation. Stars form in many different environments, from small groups of only a handful of stars, up to dense clusters with millions of stars and many thousands of luminous and massive OB stars. In dense clusters young stars are very close to each other, which can lead to close encounters that might disrupt binary systems or planetary systems. The bright OB stars that are present in the most massive clusters can also erode the disks around stars in which planets form, potentially hindering the creation of a full solar system like our own. Understanding how the environment that stars form in affects their final properties is therefore very important!

3. Stellar evolution. Just like humans, stars change as they age in many different ways, from subtle changes in their luminosity and slowing their rotation, to dramatic changes as they switch their source of nuclear fuel. To study these changes we need to know how old the stars are, but unfortunately its very difficult to measure how old individual stars are (we can't ask stars how old they are like we do with humans!). If you have a group of stars, however, you can often work out how old the group is by studying which stars have come to the ends of their lives and which haven't. This allows you to estimate the age of the cluster, and therefore all the stars in it. Once you know their ages you can study how the stars have evolved over time, an area of research known as stellar evolution.

The life cycle of a star like our Sun (upper row) and a more massive OB-type star (lower row).
Both types of star form in star forming regions and star clusters, but evolve through different
phases. By studying this process in star clusters with known ages, astronomers can
calculate how long this evolution takes (Image credit: SciOly.org)

4. Star clusters can be used to study star formation in distant galaxies. Its easy to study star formation in our own galaxy, the Milky Way, because we can observe stars forming deep within molecular clouds and the young stars that have recently formed. But in distant galaxies these things are too small and too faint to observe, so our understanding of star formation in other galaxies, whether it is different in any way, and how much star formation has been occurring, is limited. Star clusters however are bright and we can easily observe them in distant galaxies. Its also relatively simple to get a good estimate of their mass, the type of stars in them, and how old they are. This is really useful for astronomers because it means we can study not just the current star formation in these galaxies, but also star formation that occurred in the past. Astronomers call this the star formation history of a galaxy and its useful for understanding how galaxies evolve over cosmic time.

The Antennae Galaxies, two interacting galaxies with a rich and vibrant star formation history.
Astronomers have been able to study its star formation history by observing the many
star clusters (bright blue dots surrounded by red clouds) (Credit: Hubble Space Telescope)

The Jewel box Cluster (NGC 4755), one of many
clusters that are important for measuring distances
in astronomy! (Credit: APOD)

5. Star clusters are an important step on the cosmic distance ladder. In actual fact, they're two steps! The first step is a 'local' step on the cosmic distance ladder, and it comes from a process known as the moving cluster method, which is essentially a perspective effect whereby if you know the direction that all the stars in a cluster are moving you can estimate how far away it is. The second step is a much more 'distant' step on the ladder, which uses the luminosity of globular clusters to estimate the distance of the galaxy that they're in. The method stems from the assumption that the brightness globular cluster in a galaxy usually has the same luminosity as the brightest globular cluster in another galaxy. Based on this, if you can measure how bright the globular clusters in a galaxy are, you can estimate how far away the galaxy is.

There are many other reasons to study star clusters, but these are some of the most important and wide-ranging, spanning the formation of planetary systems to the size of the Universe!

So next time you hear about star clusters or new research into our understanding of these amazing objects, think about all the different scientific topics that might be influenced by those new results!

Improving our census of star clusters in the Large Magellanic Cloud

A new paper published last week by Romita et al. presents results of a search for new star clusters in the nearest galaxy to the Milky Way, the Large Magellanic Cloud (LMC).

The LMC is a dwarf satellite galaxy currently orbiting our galaxy that has a number of key differences to the Milky Way. For a start the LMC is much smaller than the Milky Way, but critically it is also less chemically evolved, meaning that it has fewer 'metals', which is the name astronomers give to anything other than hydrogen or helium. By studying the distribution of star clusters in this galaxy we can try to understand whether star formation and the evolution of star clusters has proceeded any differently in this environment compared to in our own Galaxy.

The authors have targeted a 1.65 square degree area of the LMC that includes the massive star forming region 30 Doradus, the largest region of star formation in the LMC, and larger than anything in our own galaxy. It's a rich field of star formation as the images below show, and a good place to be hunting for new star clusters.

Images of the area of the LMC studied in this paper. On the left is a colour image compiled from the infrared observations used in the study, while on the right a black and white image is marked with the positions of the newly-discovered star clusters (red dots) relative to the positions of known molecular clouds (black ellipses). (Credit: Romita et al. 2016)

Using infrared images of the LMC the authors identify 65 embedded star clusters, 45 of which are new discoveries. Using their observations the authors are able to estimate the sizes, masses and luminosities of these clusters, all key properties of star clusters.

The authors compare the distribution of these star cluster properties with their distribution in our own Galaxy, and find that the LMC clusters are generally larger, more massive, and more luminous. Since these three quantities are often well-correlated with each other, it's not a surprise that all three properties are bigger in the LMC, but this does clearly show that LMC star clusters are typically more massive than those in our galaxy.

The authors also find the density of clusters in the LMC is 3 times higher than in the Milky Way, and that the mass of clusters in this area of the LMC is 40 times higher than an equivalent area in out galaxy. Both these results suggest that the LMC is producing star clusters at a much higher rate than in our own Galaxy.

These two results are actually linked. If a galaxy is forming more stars and producing more star clusters then it is likely that it will, on average, produce larger and more massive clusters than a galaxy that it is forming fewer star clusters. It is clear that the LMC is very actively forming stars and clusters at the moment.

However the authors note that this shouldn't surprise us because the LMC contains many more molecular clouds than the Milky Way, and since stars form in molecular clouds then more molecular clouds should mean more star clusters! They find that both galaxies display the same relationship (known as the star formation rate scaling law) between the amount of dense gas and the amount of stars (and star clusters) that are forming.

This means that while the environments of the two galaxies may be different, the star formation process that takes place within them isn't. We can therefore take what we've learnt about star formation in the Milky Way and apply it to other galaxies. This is an important step forward for understanding star formation across the Universe!

What are OB associations and why should you care?

Over the last year I've talked a lot about different types of star cluster, but I've also mentioned another type of stellar group known as an OB association, and you may be wondering what are these OB associations? In this post I'm going to talk about OB associations, what they are, and why they're so important for understanding star clusters.

OB associations are very similar to young star clusters in that they are a group of young stars. They're not as compact as star clusters though, and the stars in them are spread out over a larger area of space than for star clusters.

OB associations were first identified just over a century ago by Arthur Eddington who noted the presence of groups of stars distributed across large areas of the sky but sharing a common motion. The actual term OB association was first penned by Victor Ambartsumian in 1947, who noted that these associations appeared to consist mostly of bright and blue stars known as O- and B-type stars. The image below shows the distribution of these stars near the Sun, showing how they appear to fall into a number of rough groups.

The distribution of nearby O-type (filled circles) and B-type
(empty circles) massive stars from Hipparcos. The boxes show the
positions of known OB associations (Credit: Preibisch & Mamajek 2008)

Ambartsumian noted that the low density of the OB associations meant that they weren't gravitationally bound (meaning that they weren't held together by their own gravity - in the same way that dense star clusters are). This means that they must be in the process of expanding and dispersing, and also implies that they must be relatively young structures if we are able to observe them before they have dispersed.

This revelation came at the same time as other scientists were beginning to understand how long stars of different types lived for. This revelation came from understanding the nuclear fusion that gives stars their energy, and then comparing the source of energy that each star has with the rate at which it is radiating that energy away. The O- and B-type stars are amongst the most luminous of all stars (as well as being the most massive), meaning that they are radiating away their energy the fastest and therefore have the shortest lives of all stars. This discovery confirmed the youthfulness of the OB associations that Ambartsumian had hypothesised.

If OB associations are not gravitationally bound then this means that they're probably in the process of expanding and dispersing into the Galactic field. And if OB associations are expanding then this means they were probably smaller and more compact in the past. This has led some astronomers to suggest that OB associations are the expanded remnants of compact star clusters that have been disrupted by some process.

The most common explanation for this disruption is that young star clusters are thought to be held together by the giant gas clouds that they formed in, and when star formation finishes this gas is blown away by powerful winds that come from massive stars. Without this gas the star cluster doesn't have enough mass to hold themselves together by gravity, and so the cluster begins to expand and disintegrate. The schematic below illustrates this sequence of events.

The sequence of events leading to the disruption of a star cluster: (1) The star is born embedded within a cloud, (2) the winds from the young stars disperse the cloud, and (3) without the gravitational potential of the cloud holding the cluster together the star cluster disperses and is briefly visible as an OB association. (Credit: Nick Wright)

There are other suggestions for how star clusters might be disrupted, such as tidal heating of the cluster, or possibly that some of the young star clusters that appear so ubiquitous may not actually be gravitationally bound in the first place and would therefore naturally expand and disperse.

This all means that OB associations are quite important objects to study, because by studying them we can effectively observe the process of star clusters being destroyed. Unfortunately they're not easy objects to study because the stars in an OB association are often spread over a large area of the sky and they can sometimes be difficult to distinguish from the older stars that make up the Milky Way Galaxy. For this reason our knowledge of OB associations, and our census of those that exist in our galaxy is rather slim.

The Scorpius Centaurus association
(Credit: Akira Fujii)

The most well studied OB associations are those nearest to us (this is often the case in astronomy). Associations such as the Scorpius-Centaurus association (see image on the left), the Perseus OB association, and the Lacerta I association were all discovered thanks to the bright and blue O- and B-type stars in them.

Other, slightly more distant OB associations include a number of slightly larger and more populous OB associations in the constellation of Cygnus (some of which I've discussed before here and here), as well as the Orion I association that surround the bright clusters in that constellation. Sometimes OB associations include a number of smaller clumps or clusters of stars within them, for example the double clusters h and chi Persei are part of the larger Perseus OB associations discovered in 1943.

There are a few OB associations that you can see without the aid of a telescope, but not many unfortunately because they are often very diffuse. The Alpha Persei cluster in the constellation Perseus is part of the larger Perseus OB association and is easy to observe. And while you may not be able to see most OB associations there are a number of young clusters you can observe that may one day become OB associations!

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 III

The final day of the Wide Field Surveys meeting started by focussing on the later stages of a star's life, including red giant stars (big, bright and red), asymptotic giant branch stars (bigger, brighter, and redder!), planetary nebulae (a type of dying star), and white dwarfs (the final remnant of a dying star).

The focus then shifted to star clusters, which is my area of interest, and in fact this was when I gave my own presentation to the audience, and then finally there were presentations and discussions about future surveys, which can be useful to think about what research might be possible in the future.

All in all its been a very interesting meeting, with a wide array of science topics covered, which has been useful to stay up to date in current research results in different areas. I'm now a little exhausted though, so I'm looking forward to the long weekend to relax!

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!

Globular Clusters: the grandaddies of all clusters

This week I heard a fascinating talk about globular clusters, so I wanted to take this opportunity to tell you about globular clusters, and some of the science being done to study and better understand these amazing astronomical objects.

Globular clusters are a type of star cluster, but they're a very special type of star cluster, partly because they're very massive, but also because they're very old, almost as old as the Universe itself. For this reason they provide a glimpse of how stars, and star clusters, formed a long time ago.

The Omega Centauri globular cluster, the largest in
our galaxy (Credit: ESO)

Globular clusters are huge and contain millions of stars, considerably more than other types of star cluster. Although there are no fixed boundaries for the size of a globular cluster, they typically contain between 100 thousand and a few million stars, though there are some even larger than this.

The largest globular cluster in our galaxy is Omega Centauri, which is thought to contain approximately 10 million stars. Though it is quite distant it is relatively bright and can be seen with a pair of binoculars, as can many of the other globular clusters in our galaxy.

In addition to being massive, globular clusters are also very old. The ages of globular clusters can be determined by studying how the stars in the cluster have evolved based on our understanding of stellar evolution (the study of how stars evolve). The globular clusters in our galaxy seem to be about 11-13 Gyr old (1 Gyr = 1000 million years). This is just younger than the approximate age of the Universe, 13.7 Gyr, suggesting globular clusters were some of the first objects to form in the Universe.

The M80 'Scorpio' globular cluster (Credit: Baran Observatory)

It is thanks to their old age that globular clusters are so round and spherical. Globular clusters are held together by the mutual gravitational attraction of all the stars in the cluster, and over time this gravitational attraction has smoothed out any small structural differences in the cluster and created an almost perfect ball of stars. In fact globular clusters are really useful for studying how gravity affects clusters of stars, simply because they've been around for so long and have been relatively undisturbed by the galaxy.

The reason globular clusters haven't been disturbed by the galaxy they reside within is because of where within the galaxy globular clusters are found. The oldest stars in our galaxy are found not in the disk of the Milky Way galaxy, but in its halo, which is a spherical and diffuse cloud of stars within which the disk of our galaxy sits. Globular clusters are as old as the oldest stars in our galaxy and are found distributed across the Galactic halo in relative isolation compared to the spiral-shaped disk of the galaxy.

The structure of our galaxy, showing the globular clusters in the Galactic halo (Credit: University of Oregon)

Our galaxy contains approximately 150 known globular clusters, with experts suggesting that there may be 10 to 20 still to be discovered. Because of their brightness we can identify globular clusters in other galaxies and some large galaxies have many more globular clusters than the Milky Way, for example the Andromeda Galaxy may have as many as 500 globular clusters!

One of the great mysteries of globular clusters is how they formed, whether they formed in a single burst of star formation or whether they are made up of multiple generations of stars. This also raises the question of whether globular clusters are significantly different from some of the smallest type of galaxy we know of, for example the dwarf spheroidal galaxies. All of these unanswered questions are very important for understanding how stars and galaxies form, which makes globular clusters an important area of astronomical research.