The largest optical telescope in the world

When building astronomical telescopes bigger is often better. And this means that there's often a race to build the next big telescope that will push our view of the Universe further and deeper. But before we obsess about the next big thing, lets take a moment to consider the current holder of the World's Largest Optical Telescope crown, the Gran Telescopio Canarias, which has a mirror stretching 10.4 m from side to side!

The Gran Telescopio Canarias on the summit of La Palma
(Credit: GTC)

The GTC, as its usually known, is the largest optical telescope in the world. It's based at the Roque de Los Muchachos Observatory at the summit of the island of La Palma, an excellent site for astronomy as it is both high up (above the clouds) and also very dry. It's owned and operated by several astronomical institutions in Mexico, the United States, and primarily Spain.

The 10.4 m mirror of the Gran Telescopio Canarias,
made up of 36 hexagonal, smaller mirrors
(Credit: GTC)

The telescope's mirror isn't a single mirror but 36 hexagonal mirrors that fit together to produce a telescope with an equivalent diameter of 10.4 m. Thanks to their extremely precise alignment all the segments reflect light as if they were a single mirror. This is a common technique in astronomy for making really big telescopes, as it is a lot easier to make multiple medium-sized mirrors than one very large mirror. As you'd imagine for a telescope of this size, the GTC took over 6 years to build!

The telescope was completed in 2008 and saw its first scientific observations in 2009. Since then the telescope has done some amazing work in all areas of astrophysics using a selection of instruments, including both imaging cameras and spectrographs, that can operate throughout the optical, the near-infrared and the mid-infrared.

The telescope sits close to the summit of La Palma at a site well known for its dry weather and good observing conditions. There are many telescopes clustered together at the observatory here, owned and operated by astronomical institutions across Europe.

I've had the pleasure of visiting this observatory many times for work and it is a beautiful place, well worth visiting if you get a chance, and particularly if you can stay for sunset (or get up early enough for sunrise!).

Sunset view from the summit of La Palma showing the Gran Telescopio Canarias (right) and the
Italian Telescopio Nazionale Galileo (left) above the clouds (Credit: Nick Wright)

What's wrong with globular clusters?

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

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

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

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

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

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

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

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

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

Rehearsals aren't just for the theatre

We all know that actors rehearse, but did you know that telescopes rehearse as well? Large telescopes and surveys often rehearse the process of choosing targets, observing, and analysing the data so that they can be well prepared for the real thing.

Its a process that is particularly relevant with large surveys where efficient scheduling of observations is necessary to observe as many targets as possible in the time available. Modern telescopes also produce an incredibly large amount of data, sometimes many terabytes per night, and transferring this data from the telescope to different universities, and then processing and storing it all can be quite a mammoth task! Practising this process in advance allows potential issues to be identified and ensures that when the telescope is up and running the process proceeds smoothly.

The William Herschel Telescope
(Credit: Wikimedia Commons)

Most of the last month of my time has been spent preparing for an "Operational Rehearsal" for the WEAVE spectrograph being built for the William Herschel Telescope in the Canary Islands. This is quite an endeavour, as we're simulating over a week of observations on the telescope, which requires everyone involved to produce simulated target lists and spectra for them.

The goal of this work isn't just that the telescope operators and survey team will be able to experience the day-to-day running of the survey, its also so that the survey science teams become familiar with the process of supplying targets for the observations.

And so for the last month I've been busy simulating "fake" target lists, preparing "fake" spectra for those targets, and assembling everything into the format needed by the survey organisers and telescope operators. Its been quite a task, but hopefully it'll be useful for everyone involved in the survey to have gone through this rehearsal and learnt from it.

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.