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| 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) |
Friday, 19 February 2016
Planet-forming discs around young stars truncated by close encounters in Orion
Tuesday, 16 February 2016
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!
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.
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.
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.
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!
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| 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) |
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| 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.
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| 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.
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| 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) |
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| The Jewel box Cluster (NGC 4755), one of many clusters that are important for measuring distances in astronomy! (Credit: APOD) |
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!
Monday, 1 February 2016
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.
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!
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.
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| 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) |
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!
Tuesday, 26 January 2016
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.
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.
Look out for more news on JWST's construction, mirror installation, and testing!
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.
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| Engineers installing the 9th primary flight mirror onto JWST (Credit: NASA) |
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.
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| 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!
Thursday, 21 January 2016
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!
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.
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.
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!
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 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.
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| 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 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.
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| 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.
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| 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) |
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.
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| 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) |
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!
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