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!

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!

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!

Star birth and the Sea Serpent

What do sea serpents have to do with star formation I hear you ask! Well normally the two may not be connected, but this sea serpent is no ordinary snake, it is the giant constellation of Hydra, otherwise known as the Sea Serpent! Within this constellation lies a very important young star known as TW Hydrae, which has the honour of being the closest T-Tauri star to the Sun.

T-Tauri stars, as I've discussed before, are young stars, named after the prototypical example T Tauri. They are stars which have almost completely formed but are still accreting some material from their surroundings, which can cause them to glow brightly. This accretion process is not fully understood and so astronomers try to study as many different accreting stars as possible so that they can better understand how this is happening and how the accretion is affecting the star.

Artists impression of the TW Hydrae star
and its protoplanetary disk (Credit: NASA)

TW Hydrae is particularly important because it is the closest T-Tauri star at a distance of only 176 light years, and therefore it is the easiest such object to study. It was first recognised as a young star by American astronomer (and later astronaut) Karl Gorden Henize, who noted extreme H-alpha emission, typical of strongly accreting objects, in 1976.

The isolation of the star, not near any other T-Tauri stars or known star forming regions, suggested to some astronomers that it might not be a truly young star. However thanks to further observations of the star a few years later it was shown to be a bona fide and fully accreting young T-Tauri star, despite its apparent isolation.

The star itself is similar to our Sun, a little less massive and therefore a little redder, but much younger. Current estimates put its age at about 5-10 million years old, considerably younger than the 4.6 billion year old Sun that we orbit.

Fortunately for us of course the Sun is not a young star, because young stars like TW Hydrae can be very volatile objects. Young stars are surrounded by a disk of gas and dust known as a protoplanetary disk. These disks are created as material falls towards the star and gathers in a disk around it, due to a process known as the conservation of angular momentum (which basically means that things that spiral in towards a star like to keep spiralling, or at least orbiting the star and this can sometimes stop the material from falling onto the star).

Material from this disk is occasionally accreted onto the star, channeled along magnetic field lines before pummelling onto the star's surface. When this material hits the stellar surface it is heated to incredibly high temperatures, approximately 3 million degrees, which causes the star to briefly shine brightly. You don't want to be near such stars when they're accreting!

The stars and motions of the TW Hydrae association (shown in red and labelled "TW Hya" and with new members shown in black) relative to other nearby OB associations (Credit: Murphy et al. 2015).

Recently astronomers have uncovered other low-mass stars in the vicinity of TW Hydrae, creating a small group of about 20-30 stars known as the TW Hydrae association. These stars are all very young and share a common motion through our Galaxy, suggesting that they probably formed together. This discovery will be useful for astronomers looking for other young stars to study, as well as for studying how stars form in groups such as these. And importantly, TW Hydrae isn't alone any more!

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!

Tracing the spiral arms of the Milky Way

Our galaxy, the Milky Way, is thought to be a huge spiral galaxy like many such galaxies we see across the Universe. One of the tasks that modern-day astronomers are trying to achieve is to map the size and structure of our galaxy so we can better understand how it formed and how it will evolve in the future.

The spiral galaxy Messier 100 - similar to our own Milky Way? (Credit: ESO)

One of the key tasks in such work is to map the spiral arms of our galaxy. This is important because spiral arms are thought to be where the majority of dense gas is found in galaxies, and therefore where the majority of star formation takes place. Spiral arms aren't fixed objects though, the stars in our galaxy actually move in and out of the spiral arms as they orbit within our galaxy. Spiral arms are actually thought to be density waves that rotate around our galaxy, independently of the stars in our galaxy, just like waves in the ocean move independently of the water in them.

Spiral arm model of the Milky Way with four arms.
The Sun is located towards the top of this image.
(Credit: Georgelin & Georgelin 1976)

Identifying spiral arms is easy when you're outside of a galaxy and looking at it face on, but its much harder when you're embedded within the galaxy and all you can see is the plane of our galaxy. We can't directly see the spiral arms of our galaxy, but we can trace their presence by looking for signposts that identify them. Signposts such as giant molecular clouds, star forming regions, and bright young stars are all indicators of where spiral arms are found.

The Milky Way was first identified as a spiral galaxy thanks to the work of William Morgan from Yerkes Observatory who showed that the distribution of bright and hot OB stars, which are known to be very young objects, appear to be distributed in spiral arms. Morgan identified three spiral arms, which he labelled the Perseus, Orion and Sagittarius arms.

Later studies that attempted to discern the spiral structure of the Milky Way used the radio emission from hydrogen gas to trace its structure, but it can be tricky to determine the distance to such gas, making it hard to reveal the 3-dimensional structure.

A major breakthrough came in the 1970s when scientists combined radio measurements of hydrogen gas with optical measurements of the distances to the young stars associated with the gas. This work lead to a model made up of four spiral arms called the Norma, Scutum-Centaurus, Sagittarius and Perseus arms. While many researchers debated the distances to the various star forming regions used for this model (and therefore the exact structure and number of spiral arms the model predicted), this picture was for over 30 years the standard model of the spiral structure of the Milky Way.

The model changed again in 2008 thanks to data from NASA's infrared Spitzer Space Telescope, which allowed astronomers to count the number of stars all the way across our galaxy. The number of stars they counted suggested that there weren't four spiral arms, but only two, with a number of smaller spiral arms lying in between them.

Artist's conception of our new view of the Milky Way's structure thanks to results from the Spitzer Space Telescope.
The Sun's position is marked towards the bottom of this image.
(Credit: NASA)

This new model suggests that the Perseus and Scutum-Centaurus arms are the two major arms, while the Norma and Sagittarius arms are actually relatively minor arms. The two major arms connect up with the inner Galactic Bar, which dominates the central part of our Milky Way and may also play a role in the origin of the spiral arms.

Recently a flurry of results have taken this work even further with suggestions of a new and distant spiral arm that wraps completely around one side of the galactic centre, while other researchers have started using the distribution of star clusters to trace the structure of the Milky Way. Further improvements in the model of our galaxy's structure have come thanks to improved distance estimates for many of the stars and clusters in our galaxy, allowing the exact size and extent of the galaxy to be better determined.

Upcoming missions such as the Gaia observatory that will determine the distances to a billion stars across our galaxy will dramatically improve our understanding of our galaxy's size and shape. The motions that the Gaia spacecraft will measure will allow astronomers to study the orbits of these stars as well, improving our understanding of our galaxy from a purely structural model to a more advanced dynamical model.

The nearest site of star formation, the Taurus Molecular Cloud

Following my recent post about how stars form I wanted to take some time to talk about a few of the most prominent star forming regions that astronomers have studied and what they have taught us about the star formation process. The first of these regions I want to talk about is the Taurus Molecular Cloud.

The Taurus Molecular Cloud is, unsurprisingly, in the constellation of Taurus. It is the nearest star forming region to our Sun and so it is the region that astronomers have been able to study in the most detail. The proximity of the Taurus Molecular Cloud means that it spans a relatively large area on the sky, almost 10 degrees across, but the lack of a bright emission nebula means that all that can be seen even with a good backyard telescope is the obscuration of the background stars due to the gas and dust in the cloud.

The image below shows where the Taurus Molecular Cloud can be found in the night sky, approximately halfway between Elnath (the second brightest star in the constellation of Taurus, to the upper left) and the Pleiades open star cluster.

Location of the Taurus Molecular Cloud within the constellation of Taurus, and near the Pleiades star cluster. North is up in this image, East to the left (Image adapted from one by David Malin)

The Taurus Molecular Cloud was discovered in 1852 by J.R. Hind, which he noted as a faint nebulous object on the sky. Astronomers were soon able to take a spectrum of the light from the nebula and were able to confirm that it was indeed a giant cloud of gas. However, its importance as a site of nearby star formation wasn't immediately recognised.

The archetypal young star, T Tauri, visible in the centre
of this image and surrounded by a small dusty cloud

(Credit: University of Alaska)

At the beginning of the 20th Century many astronomers became interested in a number of variable stars identified in the vicinity of the Taurus Molecular Cloud, the most prominent of which was named T Tauri after the constellation in which it was found.

Searches for other variable stars in the vicinity of dark nebulae produced many candidates in the 1940s and 1950s. These stars were often found in the vicinity of young OB stars (massive stars with particularly short lives, hence they must be young), leading many astronomers to believe that they were particularly young stars themselves. This discovery led to the realisation that the dark nebulae that these stars were found near was likely where these stars had formed. This was how the modern theory of star formation began!

The star T Tauri is now firmly recognised as the prototypical young star, and its name has been given to the class of young stars that share its properties, T-Tauri stars. It is thought to be less than a few million years old, already formed but still accreting material and growing in mass. The star itself is actually in a binary system with a fainter star, and is thought to be surrounded by a disk of material that is in the process of accreting onto the star, which partly explains the variability of the star that initially brought it attention.

An infrared view of the Taurus Molecular Cloud (Credit: FCRAO)

With the invention of infrared detectors in the second half of the 20th century astronomers were able to start peering into these dark nebulous clouds to study the star formation process within them, and as the nearest such cloud Taurus was a major target for early infrared astronomy.

The infrared image shown here penetrates the dusty molecular cloud and allows astronomers to see the giant gas clouds that are in the process of collapsing into stars. This image shows the cloud in immense detail, with filamentary tendrils of gas and dense cores where stars will one day form.

Because the Taurus Molecular Cloud is not large enough to be forming any really massive stars the region is spared the destructive powers that these stars can inflict on their surroundings. This means that there is still considerable molecular material in the cloud, including many molecular ices, despite the fact that many stars have already formed. This means that star formation is still ongoing and may be able to continue for a while, all thanks to the tranquil nature of the stars formed here.

The rise of infrared astronomy has also led to an increase in the number of young stars discovered in the cloud. Prior to this only the bright and optically visible young stars, such as T Tauri itself, had been identified. But infrared observations allowed astronomers to peer into the dark and obscuring clouds and identify many more young stars, and several hundred are now known.

The distribution of young stars (red stars and triangles) in the Taurus Molecular Cloud, show against a map of the molecular hydrogen in the cloud. Yellow diamonds, blue squares and green circles show young stars with known outflows. (Credit: Narayanan et al. 2012) 

This large sample of young stars has been vital in helping astronomers learn about star formation. For example, the distribution of these stars, as can be seen in the image above, coincides strongly with the distribution of the dense molecular gas, suggesting that stars form in regions of particularly dense gas. Furthermore with so many young stars all roughly of a similar age, astronomers have been able to produce models for how young stars of a given age would appear as they finished forming, and were then able to compare these models with the stars discovered in Taurus.

All in all the Taurus Molecular Cloud has been vital for how astronomers have learnt about the star formation process. It has provided a rich, nearby laboratory to study the dark nebulous clouds in which stars form and also to observe the final stages of the star formation process itself. Next time you look up and see the constellation of Taurus, see if you can spot the dark clouds of the Taurus Molecular Cloud and think about how important this region has been for astronomy!

How do stars form?

Stars, like our Sun, are forming all the time across out Galaxy and in other distant galaxies. Understanding how stars form and what causes different types of star to form is one of the most important areas of research in astrophysics. Today I'd like to discuss the star formation process, what we know about it, and what we are still trying to understand.

Stars form out of dense clouds of gas (mostly made of hydrogen and helium) known as molecular clouds, so-called because many of the atoms in them have cooled and formed molecules. These molecular clouds are huge and are mostly found in the spiral arms of galaxies such as our own. The clouds are very cold, with temperatures of only 10 to 20 Kelvin (about -253 Celsius) and made of molecular gases such as H2 and CO.

The Whirlpool galaxy imaged in visible light (left) showing young stars and star-forming regions delineating the spiral
arms and a radio image (right) showing emission from the CO molecule tracing the molecular clouds in which stars form (Credit: NASA / PAWS)

These molecular clouds are thought to be held in balance between the inward force of gravity (which tries to make them collapse) and the outward pressures of magnetic fields and the motions of the molecules in the cloud (which are trying to make the cloud expand and disperse).

Eventually though something has to give and some part of the molecular cloud will begin to collapse. As it does so it will also cool as the molecules in the cloud release energy through as process known as radiative cooling, which helps the cloud collapse further. If the molecules weren't able to cool down while the molecular cloud contracted then the increase in density would cause them to heat up and the molecular cloud would expand and disperse, so this cooling is critical for star formation.

The dark cloud Barnard 68 (Credit: Marco Lombardi)

As this happens the molecular cloud will begin to fragment into smaller and smaller clumps of gas, each becoming denser and denser as they contract in towards their centres. In fact the density can reach so high that no light can penetrate to the centres of these clumps, making them so dark that they even block the light from background stars. We call these objects dark clouds, because they appear as dark patches on the night sky!

Once these dark clouds are dense enough that they can block out starlight then they cool even faster because they are no longer being heated by the light from nearby stars. Once these clouds have cooled even further then they can even block infrared radiation and become so cool as to not even emit infrared radiation. Only the coldest objects in the Universe are so cold as to not emit infrared radiation!

Once the centre of the clump has collapsed considerably a dense, gravitationally stable core forms in the centre, known as a protostar, which begins to heat up as it continues to contract. The protostar continues to grow in size by accreting more material from the surrounding molecular cloud, its core getting denser and hotter as it does so, and after a while the protostar begins to radiate energy into the surrounding molecular cloud.

A forming protostar surrounded by a disk of material accreting onto it
(Credit: ESO)

At this point the protostar is massive enough that it attracts considerably more material from the surrounding molecular cloud, which falls towards the star. Due to the conservation of angular momentum this material spirals in towards the star and forms a disk of material that orbits the star, slowly accreting onto the star in bright bursts that illuminate the surrounding cloud. With each burst of accretion the star becomes hotter and more massive.

Eventually the core of the protostar becomes so dense and hot that the temperature is high enough for nuclear fusion to take place. At first the star can only burn deuterium, but as it gets hotter it will eventually burn hydrogen just like our own Sun. The star is now beginning to shine quite brightly and the radiation from the star prevents further material accreting onto the star and may even begin to disperse the remaining material in the disk that still surrounds the star.

Once the star has started fusing hydrogen into helium we say that it has fully formed. Hydrogen fusion is the process by which the vast majority of stars create their energy, and the star can usually maintain this for billions of years before it runs out of hydrogen in its core.

This is the rough process by which we think stars form, and there is a lot of evidence to support this picture, including observations of forming stars and computer simulations that try to model the entire process. There are however a number of outstanding questions that scientists are still trying to answer, such as: How are stars clustered when they form (for example in clusters and OB associations) and what causes this? What causes stars to form with different masses? And what brings the star formation process within a molecular cloud to a halt? These are questions that astronomers such as myself are actively trying to answer!