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.