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!

Gaia's first data release is here!

Last wednesday the Gaia satellite made its first major data release, exactly 1000 days after the satellite was launched back in 2013.

Gaia's view of 1 billion stars in the Milky Way (the strange arcs across the image are due to the way Gaia
scans across the sky and are not real) (Credit: ESA)

This first data release contains over 2 million stars with positions, distances and proper motions, as well as positions for over 1 billion stars. The astronomical community has been pouring over this data for the last week, and there are already many exciting results and revelations that have been announced, including the discovery of over 3000 variable stars that can be used to independently calculate distances, and over 2000 background quasars.

There's sure to be many more exciting results from this mission and its future data releases, so stay tuned!

Back in business

It's been a long and busy summer, hence my absence from this blog. I've had quite a few conferences to attend, as well as the necessary presentations at those conferences that I've had to prepare.

However, that's all over now, I've got lots to tell you about from my busy summer, and it's also the first Gaia data release this week, so hopefully they'll be lots of interesting science coming out of that. Stay tuned!

The perils of star formation in the vicinity of massive stars

Some collaborators and I have recently had an interesting paper published in which we show that stars born in the vicinity of massive OB stars may be less likely to form planetary systems that stars born further away from such stars.

In our Galaxy stars form in a wide variety of environments, from small regions with only a handful of stars, up to massive star clusters and associations with millions of members. The larger regions are also home to tens to thousands of massive OB stars that emit immense amounts of energetic radiation that can affect nearby stars.

The OB association Cygnus OB2 (Credit: CXO).

One of the questions facing astronomers is whether stars born in the vicinity of multiple OB stars might grow up differently to stars born in regions devoid of such objects. In particular it has been suggested that the radiation from these massive OB stars could erode the disks of material that surround young stars.  These protoplanetary disks are thought to be in the process of forming planetary systems just like our Solar System, so its important we understand how they form and evolve.

The study, lead by my colleague Mario Guarcello, focusses on young stars in the OB association Cygnus OB2, a region home to thousands of massive and luminous OB stars, as well as many less massive stars like our Sun. In the paper they study the spatial distribution of stars with and without protoplanetary disks and find that stars nearer to OB stars are less likely to have a disk around them than stars further away from the OB stars (see figure to the right).

The fraction of stars with disks (y-axis) plotted
against the strength of ultraviolet radiation
from massive stars (x-axis). The fraction of stars
with disks is lower when the ultraviolet flux is
higher (Credit: Guarcello et al. 2016).

This suggests that the OB stars are somehow eroding or destroying the protoplanetary disks around stars in their vicinity, most likely due to the photo-evaporation of material in the disks by the harsh ultraviolet radiation that these stars emit.

This result is very important for our understanding of where planetary systems are forming in our galaxy and what factors are hindering this process. As we start to search for planetary systems in distant star clusters we may find that such systems are rarer, or perhaps have fewer planets in them, than those around stars that aren't in star clusters.

This may also tell us something about where our Sun and its Solar System formed. If protoplanetary disks are eroded in massive clusters and associations, then it is unlikely that our Sun formed in an environment such as this.

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!

The new generation of multi-object spectrographs

Last month I visited Barcelona for a short science meeting to discuss plans to use a new multi-object spectrograph currently being constructed. The instrument is called WEAVE, which stands for WHT Enhanced Area Velocity Explorer, and its going to be constructed on the William Herschel Telescope (WHT) in La Palma.

The William Herschel Telescope in
La Palma (Credit: ING)

The name of the instrument comes from the fact that one of its main goals will be to measure the velocities of stars over large areas, covering millions of stars over huge swathes of the sky. In doing so it will complement a lot of the work that Gaia is doing to measure the distances to stars and their proper motions.

The meeting was very productive and there's lots of interesting science that we're planning to do with this instrument. However, this isn't the only multi-object spectrograph (so-called because they can obtain spectra of multiple objects at the same time, thus making them very efficient instruments!) being constructed at the moment.

In addition to WEAVE there is also 4MOST for the VISTA telescope, MOONS for the VLT, and PFS for the Subaru telescope. There are also plans to build a multi-object spectrograph for the upcoming European Extremely Large Telescope (the E-ELT) called MOSAIC. This won't be constructed for almost a decade, but once operational it'll allow scientists to obtain spectra of hundreds of stars in other galaxies, allowing us to study how stars and star clusters evolve in other environments.

Each of these new instruments will be able to take spectra of hundreds of stars at the same time, allowing scientists to efficiently study the properties of lots of stars. This sort of approach is critical for building up large samples with which to do exciting science. I'm really excited to be part of one of the teams that hopes to do this, there's going to be lots of wonderful science to do!

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!

How do you form a massive black hole binary? (Part 2)

Following the recent discovery of gravitational waves from a merging black hole binary system, I've been dedicating a few posts to exploring how such a system could have formed in the first place. Last time I talked about how such massive black holes could form, and here I want to discuss how a binary black hole system could form.

Artist's impression of a binary system comprised of
two massive OB stars (Credit: Universe Today)

The answer to this question depends a lot on where the black holes formed. It's a lot easier to form a binary black hole system in a star cluster where there might be many black holes than it would be to form such a system in relative isolation.

Lets deal with the more difficult case of forming a binary black hole system in isolation first. Most massive stars (the precursors of black holes) are actually born in (and spend most of their lives in) binary systems composed of two massive stars orbiting each other. However, there are many events during a massive star's life that can disrupt the binary.

Schematic of a binary star system undergoing
common-envelope evolution
(Credit: Adrian Potter)

First, the star swells up and inflates to become a red supergiant. These are the largest types of star known, with diameters hundreds of times their original size and many thousands of times larger than our own Sun. If the red supergiant is in a binary system then it is possible that its outer atmosphere could spill over onto the secondary star, forming an envelope of material that encompasses both stars.

This process is known as common-envelope evolution, so-called because the two stars effectively share their outer envelopes (see the figure to the right). The stars in the common envelope experience a drag on their binary orbits, slowing them down and shrinking the binary system. The phase is typically quite short-lived, but can actually end with the two stars merging!

If the binary system survives the common-envelope phase it may be disrupted by the material the star has ejected during this phase. Red supergiants expel a considerable fraction of their mass through stellar winds. This weakens the binary system by taking away some of the mass holding it together, which causes the binary system to widen. If the system widens sufficiently the stars may actually separate and the binary will be no more!

Finally, the last act in the life of a massive star is a supernova explosion. Again, this expels considerable mass from the star and therefore from the binary system, which could disrupt the binary. In fact this is a commonly-considered mechanism for the disruption of binary systems composed of two massive stars.

This presents a difficult path for a massive binary system to negotiate if it is to become a black hole binary. The system must survive a potential common-envelope phase while one (or both) stars are red supergiants and it must survive the loss of considerable mass from both stars from stellar winds and supernova explosions that can weaken the binary. However, if the system negotiates these obstacles then it could form a binary black hole system just like that observed to merge by LIGO.

Next time I'll talk about a potentially much simpler way to form a massive black hole binary system, and that's in a star cluster!

How do you form a massive black hole binary? (Part 1)

The science news media has been buzzing over the last fortnight after the LIGO (Laser Interferometer Gravitational-wave Observatory) team announced the discovery of gravitational waves. The signal appears to have come from the merger of two black holes, each approximately 30 times the mass of our Sun, that were orbiting each other and have since coalesced. Many posts have been devoted to this amazing discovery and its implications, but I'd like to talk about how the binary black hole system might have formed and what it means for our understanding of black holes, starting in this post with how you might form such massive black holes.

Artists impression of the collision of two black holes, as detected by the Laser Interferometric
Gravitational-wave Observatory (Credit: Vox.com)

First, a little background. Black holes broadly come in two varieties: stellar-mass black holes, with a mass a few times that of our Sun, and supermassive black holes, with millions of times the mass of our Sun. The less massive variety are found throughout galaxies, such as the X-ray binary Cygnus X-1, which was the first black hole discovered (see image below), while their more massive cousins are thought to lie at the centres of most large galaxies like our own.

Artist's impression of Cygnus X-1,
a black hole accreting material from its
companion star (Credit: Chandra X-ray Observatory)

How supermassive black holes form isn't really understood at the moment, but fortunately we do have a good idea how stellar mass black holes form. When very massive stars (O- and B-type stars) have exhausted all the nuclear fuel in their cores then they can no longer produce the necessary energy to support the weight of their heavy outer layers. Under the force of gravity the star collapses in on itself, a process known as core collapse. This collapse is incredibly violent and leads to two things, the first being the release of an immense amount of energy (a supernova) and the second is the creation of a very dense remnant such as a neutron star or a black hole.

This process is well understood and there is considerable evidence for it, as well as for the existence of black holes. However, the gravitational wave source that was observed in September wasn't just a black hole, it was two black holes orbiting each other in a binary system, and each black hole was thirty times the mass of our Sun, which is quite large for a typical black hole! So how do you make a black hole this big?

Wolf-Rayet star #124, as imaged by the Hubble
Space Telescope, showing all the material thrown
off by the star as it nears the end of its life
(Credit: HST)

The main factor determining the mass of a black hole is how much material the massive star that it formed from expelled during its life. This material might be expelled by stellar winds during the late stages of its life, such as while the star is a red supergiant or a Wolf-Rayet star (see image to the right), or much of the material might be expelled when the star explodes as a supernova.

The best way to prevent a star from losing most of its mass is to weaken its stellar winds so that they don't expel much material. The strength of a star's winds is usually proportional to the amount of elements like carbon, nitrogen or oxygen (known as metals to astronomers) that the star has. These elements accelerate the loss of material from stellar winds, so their absence would help to reduce the amount of mass lost and therefore increase the mass of the black hole produced.

Scientists have even gone as far as estimating that to produce a black hole as massive as those in the recent binary black hole merger they'd need to form from a massive star with as little as one hundredth the amount of metals that there are in our own Sun. Since the amount of these metals in a typical star has increased over the lifetime of the Universe, you'd need to form these stars very early in the Universe for them to have few enough metals to produce such massive black holes.

However, you could also make such massive black holes by other processes, possibly starting with a smaller black hole and slowly growing it. Material falling into black holes is absorbed by them, increasing their mass, though this is surprisingly hard to do efficiently, so it would be a slow process. Alternatively you could grow a black hole more rapidly by merging it with another black hole, in exactly the same way that the gravitational wave source that was detected was a merger of two black holes.

Whatever the process, the discovery of this merging black hole binary system suggests that black holes as massive as this must be relatively common in the Universe for scientists to have observed such a merging system so easily. Whether the black holes formed with such masses, or if they grew by accretion or mergers we may never know until we are able to detect and study more black holes and better understand the properties of these elusive but amazing objects.

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!

NASA's next big space telescope reaches a critical stage

The space telescope that will one day replace NASA's Hubble Space Telescope (HST) has reached a critical stage in its construction this month as work entered the final assembly phase. Engineers working on the James Webb Space Telescope (JWST) started installing its mirrors last fall and by December had fitted 9 of the 18 primary flight mirrors. This month they started on the 10th mirror and the final stage of the assembly process.

The team is using a precise robotic arm to carefully position the massive gold-coated mirrors onto the growing observatory. Inside the huge clean room at NASA's Goddard Space Flight Centre, the massive observatory is starting to take shape.

Engineers installing the 9th primary flight mirror onto JWST
(Credit: NASA)

The 18 hexagonal-shaped primary mirrors each weigh approximately 40kg and measure over a metre in diameter. They were built at the Ball Aerospace labs in Boulder, Colorado, and then transported to NASA's laboratory in Maryland. Together they will produce a single mirror 6.5 metres across, making JWST the largest space telescope ever constructed.

Construction work is on schedule for completion in time for a launch in 2018. Once constructed and launched JWST will have the light-gathering power to peer back in time to when the first stars and galaxies were forming in the Universe. By observing these objects astronomers hope to understand how the Universe that we see around us was constructed. JWST will also aid the ongoing search for habitable exoplanets, the study of nearby forming stars and star clusters, and the large-scale structure of the Universe.

Artist's impression of the completed James Webb Space Telescope with its 18
gold-coated mirrors and large sun shied at the bottom (Credit: NASA).

Look out for more news on JWST's construction, mirror installation, and testing!

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!