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!

What are OB associations and why should you care?

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

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

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

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

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

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

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

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

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

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

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

The Scorpius Centaurus association
(Credit: Akira Fujii)

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

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

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

The distance to the Moon

I've recently been watching the wonderful HBO series From the Earth to the Moon, a dramatisation of NASA's Apollo missions to the Moon, which I'd highly recommend. As I've been watching it I've been reminded of some of the amazing contributions to science that these missions made. Its certainly true that their original motivation wasn't scientific, but that doesn't mean they didn't achieve anything scientifically.

One of their most important achievements is helping to precisely measure the distance to the Moon. Knowing the distance to the Moon and how this changes is important for understanding the orbit of the Moon, which influences tides on Earth, and whether the Moon is spiralling towards or away from the Earth.

Calculations from the ancient Greek astronomer
Aristarchus used to estimate the distance to the Moon
(Credit: Wikipedia)

Prior to the Apollo missions there were various methods to measure the distance to the Moon, including using radar and simple trigonometry. The ancient Greeks were the first to try to measure the distance to the Moon using trigonometry.

The astronomers Aristarchus and Hipparchus both succeeded in using this method, with the latter measuring a distance of about 410,000 km, an estimate which is only off by about 25,000 km, or 7% of the total distance.

To improve the distance measurements that were available at the time, the Apollo programme decided to take special reflectors to the Moon and leave them on the lunar surface. These specially-designed reflectors allow the distance between the Earth and the Moon to be measured by aiming lasers on Earth at the positions of these reflectors and then timing the amount of time it takes for the laser to be reflected back to Earth. Since we know how fast light moves through the vacuum of space we can use the time this journey takes to calculate the distance travelled.

The lunar ranging equipment, as left by Apollo 11 (Credit: NASA)

Once the reflectors were installed by Neil Armstrong and Buzz Aldrin of Apollo 11, a number of telescopes around the world were able to use them to measure the distance to the Moon. Additional reflectors were also left on the lunar surface by the Apollo 14 and 15 missions, with the latter using a particularly large reflector array that was three times the size of the other two reflectors. The majority of distances measurements to the Moon since then have used the Apollo 15 reflector due to its size.

Thanks to the precise measurements that these reflectors have allowed we have learnt a considerable amount about how the distance between the Earth and the Moon is changing. The distance to the Moon, which is approximately 385,000 km, is now known with an accuracy of better than one part in 10 billion. The exact distance changes throughout the Moon's orbit around the Earth, as well as due to a number of smaller effects.

One of those small effects is that the Moon is very slowly moving away from the Earth, at a rate of about 3.8 cm per year. While this is only a tiny fraction of the total distance between the Earth and the Moon it is surprisingly high! Don't worry though, at that rate it would take millions of years for there to be any perceptible change in the Moon's appearance from Earth!

Artist's rendering of the lunar core (Credit: NASA)

Variations in lunar rotation and orbit, measured thanks to these reflectors, have also provided evidence that the Moon probably has a liquid core. This core is thought to be about 20% of the Moon's radius. Seismographic measurements since then have refined this picture, suggesting that the Moon may have a solid inner core surrounded by a fluid and partially-fluid outer core.

Perhaps the most important use of knowing the precise distance to the Moon is the impact of the Moon on the rotation of the Earth, due to tidal gravitational forces. We now know, for example, that the Moon has a small effect on the length of the Earth day, which is changing very slowly due to this effect.

Nick Wirght (2015), The distance to the Moon (Blogger).