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Exploring the Universe


Combining different disciplines, Leiden University researchers work together to formulate innovative solutions to societal problems. Below is an example from the field of fundamental sciences.

Overview research dossiers

From the Big Bang to life-bearing planets

Astronomers want to understand the Universe, from the Big Bang to the present day, and what the future will hold. In Leiden they focus on two key questions: ‘How did stars and planets originate’ and ‘How were  galaxies and black holes formed in the young Universe?’ A new generation of telescopes – just operational or still under construction -  will help them find the answers.  Maybe we will even detect signs of life on planets outside our solar system.

First there was the Big Bang, the point when the Universe and even space and time were created out of the void.  And after that there was darkness – because the Universe contained little more than hydrogen and helium gas. It was not until a  few hundred million years later, after the first stars were formed, that anything became visible. This thirteen-billion-year-old light is still en route to us and can be received by our telescopes.
The ‘early’ Universe is  an important research theme in Leiden astronomy. Hardly surprising, as the origin and evolution of stars, galaxies and black holes largely determine the history and the future of the Universe.

Leiden scientists study the physics and chemistry of star formation, both in nearby interstellar clouds and in distant  galaxies. They also develop computer models that simulate as closely as possible the growth of the Universe to date so that they can make predictions for the future.  The search for ‘exoplanets’ – planets outside our solar system –  and trying to discover whether they may contain life, is a particularly active research topic.

Progress in astronomy is often driven by  better observations, which means better telescopes. Leiden is a partner in the construction of the world’s most advanced instruments; one added advantage of this is that it entitles our researchers to guaranteed observation time once these much-coveted instruments are operational. Examples include the - largely Dutch - LOFAR radio telescope and the ALMA observatory in Chile. Both of these have recently been completed and have many productive years ahead of them. Leiden astronomers are also intensively involved in the construction of the European Extremely Large Telescope (ELT) and the James Webb Space Telescope, which will  further extend the boundaries of what can be observed in the coming decade.

Ultimately, these billion-euro projects and the simulations of the Universe on supercomputers all serve the same goal: they aim to complete the big story, from the Big Bang to the present day. But understanding the Universe is just one  benefit to come from astronomy: the technical demands placed on the telescopes and instruments drive engineers to extreme limits, and ensure innovation. Many of the technologies developed to make astronomy research possible are also applied to improving life on Earth. ISPEX, for example, the device for your mobile phone that allows you to measure particulates in the air, originated from the technology needed to detect exoplanets.

World-wide reputation
The astronomy institute at Leiden University has a strong international reputation, as does Dutch astronomy in general. MSc and PhD graduates and researchers  spread  out over the whole world, and hold prominent positions in the astronomy community. One of these is Leiden theoretician Tim de Zeeuw, who was Director-General of the European Southern Observatory (ESO) for ten years. Another is Ewine van Dishoeck, who played a crucial role in realising the major worldwide   observatory ALMA in Chile, and will become President of the International Astronomical Union (IAU) in 2018. But according to Van Dishoeck, that’s not the most important thing: ‘Here in Leiden it’s not about a few big names; research in Leiden is absolutely top quality and that applies from young to old, across a broad spectrum.’

For  Leiden Observatory,  communicating with the public is one of its core tasks, and Leiden astronomers are regularly to be found in the media. A deeper insight into the Universe has a cultural value that is recognised by broad layers of society. Director Huub Röttgering: ‘We want to understand our place in the Universe, and communicate that to the world.’

Leiden Observatory

To the edge of space and time

Large telescopes can look so deep into the Universe that they can also look back billions of years in time. From 2018, the successor of the Hubble Space Telescope, the James Webb Space Telescope, will be able to see the period just after the Big Bang, when the first stars and galaxies formed. Astronomers from Leiden are helping build instruments for the James Webb, and can’t wait to use it for observations.

Thousands of millions years back in time

If we ‘only’ look 100 million light years around us, we see a peaceful part of the Universe, with neat old galaxies that look like the Milky Way, the galaxy that we are part of. The space in between is more or less empty. But if you use modern telescopes to look billions of light years away – and thus billions of years back in time – the ‘early’ universe looks very different indeed. Many galaxies are chaotic in shape and produce new stars at a rapid rate or have a gigantic black hole at their centre that sends out clusters of particles into space, causing shock waves there. And although fledgling galaxies can only grow

Since its launch in 1990, the Hubble telescope has been orbiting around Earth. Because the telescope is above our planet’s atmosphere, it can make extremely sharp images.

Since its launch in 1990, the Hubble telescope has been orbiting around Earth. Because the telescope is above our planet’s atmosphere, it can make extremely sharp images.

gradually according to computer simulations, by consuming their little brothers, they appear to be large and massive surprisingly soon after the Big Bang.

On to new long-distance records
Marijn Franx has been involved in the Hubble Space Telescope’s voyage of discovery into the early Universe from the outset: ‘Hubble radically changed this field of research at the time. In 1996, it proved easy for Hubble to see such extremely distant galaxies, because they were much brighter than expected.’ In 2016, Franx was on a team that discovered the current long-distance record holder, galaxy GN-z11, at 13.4 billion light years away, when the universe was only 400 million years old.

Franx: ‘Hubble has now done all that it can, but its

successor, the James Webb Space Telescope, will be able to see much further back in time. We have regular discussions with the people at NASA who are building it about the decisions that are made. In such projects, you never have enough money to do everything, but we as users tell them: ‘This is what we really need.’

It won’t be possible to perform maintenance on the James Webb, because it will be parked in space a long way from Earth. So decisions needed to be made 15 years ago about which observation instruments will be sent up with it for the next ten years, and this will greatly determine what the telescope will be able to see.

Galaxy GN-z11, to date the furthest galaxy to be observed, is part of the well-known Great Bear constellation. Source: NASA

Galaxy GN-z11, to date the furthest galaxy to be observed, is part of the well-known Great Bear constellation. Source: NASA

Turbulent childhood
Distance records in themselves are not that important. What astronomers want to understand is the turbulent childhood of the Universe. In galaxies, new stars constantly form from gas and dust clouds, but at present, in our corner of the Universe, stars form at a slow rate because there is hardly any gas and dust left between the stars. It you apply this logic to young galaxies, they must therefore have contained a lot of gas and dust. The mega-telescope on Earth, ALMA, was specially built to observe the millimetre radiation that this dust radiates. And ALMA is still making observations aplenty.

Franx: ‘We thought at the time that when the James Webb was launched ALMA would have made these observations, but ALMA sees very few extremely distant galaxies. It’s a real mystery. Perhaps they contain too little dust? We have therefore adjusted the James Webb program, to make it better at detecting those galaxies.’

A piece of the Universe in the computer

Simulations of galaxies help researchers understand astronomical observations better. The EAGLE simulation, a large project in which Leiden astronomers play a leading role, shows the evolution of the Universe, from just after the Big Bang to the present day.

Gas clumping together
Shortly after the Big Bang, 13.8 billion years ago, the Universe was made up of only hydrogen and helium gas. As an effect of gravity, this gas started to clump together, and formed large, rotating clouds of gas, from which  galaxies were formed, each consisting of billions of stars. The starting situation is known, so a computer can in principle use the laws of nature to back-calculate how the Universe developed to its present state.

According to Joop Schaye, Professor of the Formation of Stellar Galaxies, these  simulations made it possible for astronomers from Leiden and elsewhere to achieve a breakthrough: ‘All previous simulations created galaxies that were too small, too heavy and too spherical.’ The key was the improved modelling of galactic winds, enormous expulsions of gas from the centre of a young galaxy to the outside, which has a serious effect on the development of the galaxy.  The wind is produced by the black hole that is rapidly  formed in the core of a galaxy  and by the many exploding massive stars, the so-called supernovae.

A petabyte of data
A simulation such as EAGLE is a long-term project that will deliver a petabyte (a million gigabytes) of data.  The first tests can be done using a standard computer, but for the actual simulation, time has to be reserved on a super-computer, in this case, the French super-computer CÜRIE; even this computer needs tens of millions of hours calculating time (on thousands of computers at the same time).  EAGLE simulates in a ‘representative’ section of the Universe, around a hundred million lightyears in diameter, what happened from shortly after the Big Bang up to the present day.

Astronomers will be busy for many years analysing the

data. Schaye: Firstly, you have to recognise the galaxies in all those data, because the computer doesn’t know what a galaxy is.’ 

The computer only recognises pockets of space of a particular volume, mass and pressure and a couple of natural laws, and still the simulation reproduces the entire spectacular range of galaxies in the Universe:  elliptical galaxies, galaxies with spiral branches and barred spiral galaxies. Plus the sponge-like, large-scale structure of the Universe that becomes visible if you zoom so far out that individual galaxies are no larger than particles of dust.

Schaye and colleagues successfully simulated the different galaxies of the so-called Hubble series or ‘tuning fork’. This classification was made in 1926 by Edwin Hubble, and is still in use for classifying galaxies.

Schaye and colleagues successfully simulated the different galaxies of the so-called Hubble series or ‘tuning fork’. This classification was made in 1926 by Edwin Hubble, and is still in use for classifying galaxies.

Tuning the model
Simulations have two major advantages over observations using telescopes, according to Schaye: ‘We have the whole history of a phenomenon available, not only what you can see now in the Universe. And we repeat the process a number of times, each time with slightly different ingredients. Then we see how sensitive the outcome is to particular details.' The strength of the wind seems to be the most important 'tuning parameter' in the model to mirror a realistic Universe.

Simulations have become a branch of astronomy equal to theory and observations. Schaye: ‘We now have observers coming to us and saying, “Look at this, what is it?” That wasn’t possible before because the simulations were so unrealistic. Now they are very realistic and we can learn more from our observations. It’s important to continue doing that. Ultimately it all has to come together in one coherent story.’

The EAGLE simulations

Chemistry between stars and planets

In the large gas clouds between the stars, chemical reactions take place under extreme conditions, giving rise to both small molecules, such as water and common salt, as well as large complex molecules that can serve as the building blocks of life. This is known as astrochemistry and it is something we investigate in Leiden using unique telescopes, on Earth and in space, and in the Sackler Laboratory for Astrophysics.

Minus two hundred degrees Celsius
With its four astronomy professors - Ewine van Dishoeck, Xander Tielens, Harold Linnartz and Michiel Hogerheijde – Leiden University has one of the biggest astrochemistry groups in the world.

Astrochemistry takes place in the space between and around stars. In this space there is an almost-vacuum; the gas densities there are particularly low, radiation is intense and temperatures are well below minus two hundred degrees Celsius. This is not an environment  where one would expect chemistry to play an important role. However, the opposite is true. It may well be that individual chemical reactions in space take place very slowly, but the volumes involved are gigantic and processes often last millions of years. The net result is therefore an interesting mix of all kinds of different substances: small and large molecules, stable and highly reactive, and complex organic species. Prebiotic molecules have also been found that may have formed the first building blocks of life on Earth – and possibly elsewhere too.

Infrared and millimetre waves
Ewine van Dishoeck, Professor of  Molecular Astrophysics, is one of the founders of modern astrochemistry. In the course of time increasingly complex carbon-containing molecules have been discovered in space. These organic molecules – and water, too – can be identified at hundreds of lightyears’ distance because these substances emit radiation at infrared and millimetre wavelengths. Each type of molecule has its own spectrum, a kind of fingerprint comprising specific colours of light. To measure them, you need particularly sensitive telescopes such as ALMA, the new worldwide telescope that comprises 66 large dishes. This enormous telescope array in Chile can explore the Universe in much greater detail than ever before. Together with colleague Professor Michiel Hogerheijde, Van Dishoeck uses ALMA to look at chemical processes around young stars and to explore how new planets formed there.

The spectral fingerprints needed are measured in

the Leiden Sackler Laboratory for Astrophysics. Researchers here also study how molecules can be formed in space. This is the field of work of Harold Linnartz, Professor of Laboratory Astrophysics. This makes it possible to understand the data from large telescopes: what are we seeing in space and why can we see it there?  Xander Tielens, Professor of the Interstellar Medium, focuses his research on a special kind of molecule: the PAH (polycyclic aromatic  hydrocarbons). These large molecules are found as particulates in car exhausts, but they are present everywhere in space. They are also present as the remnants of exploding stars, where they play an important chemical role.

'The Herschel space telescope, that orbits around the Earth at 1.5 million km distance, has measured long wavelength infrared and submillimetre radiation in space, including the fingerprint of water. Leiden astrochemists have shown that large amounts of water are present around almost all young stars, enough to fill thousands of oceans.

'The Herschel space telescope, that orbits around the Earth at 1.5 million km distance, has measured long wavelength infrared and submillimetre radiation in space, including the fingerprint of water. Leiden astrochemists have shown that large amounts of water are present around almost all young stars, enough to fill thousands of oceans.

Pieces of a big puzzle
The data from the Laboratory for Astrophysics, the observations made with ALMA, and also those that will soon be made with the James Webb Space Telescope provide detailed pieces of a large puzzle that astrochemists in Leiden want to resolve. What does the chemical evolution of the Universe look like? Does the chemistry in interstellar clouds determine the atmosphere of a planet? Is the existence of life linked to the processes that take place on minuscule, icy dust particles? Can PAHs teach us more about how large molecules behave in space?

To us, Earth is very special, but it may in reality not be that unique, if every planetary system in our Milky Way galaxy and far beyond has undergone the same chemical evolution.


High up on the Chajnantor plateau in the Chilean Andes is ALMA, the Atacama Large Millimeter/submillimeter Array – an advanced telescope that can measure the radiation of several of the coldest objects in the Universe. ALMA is a collaboration of Europe, North America and East Asia. Source:  ESO

High up on the Chajnantor plateau in the Chilean Andes is ALMA, the Atacama Large Millimeter/submillimeter Array – an advanced telescope that can measure the radiation of several of the coldest objects in the Universe. ALMA is a collaboration of Europe, North America and East Asia. Source: ESO

Searching for life in the Universe

Is there extra-terrestrial life out there? It now looks as though we can sketch out an answer to this enduring question. Leiden Observatory is helping to build new instruments to find the most promising exoplanets.

Lifeless ‘gas giants‘
The first exoplanet – a planet that orbits another star than our Sun – was only discovered in 1995, but over 3,000 have since been found. Their existence is usually detected indirectly, and they are almost all ‘gas giants’ like Jupiter, where no known life can exist.

If we want to directly observe exoplanets that are like the Earth, we will need much more powerful telescopes and instruments. Then it will even be possible to study their atmosphere. If chemically active gases such as oxygen (O2), ozone (O3) and methane (CH4) are present in its atmosphere, this is a strong indication that an exoplanet is teeming with life.

Mini solar eclipse
It is always difficult to obtain a good image of an exoplanet, because the star that it orbits emits much more light than the exoplanet itself reflects. This ‘outshining’ of a weaker source of light by a much stronger one is also the reason why we cannot see stars in the daytime. Only during a full solar eclipse, when the Moon passes before the Sun, do we see stars in the sky near the Sun. The solution for detecting an exoplanet appears simple: shield off the image of a star in a telescope with a black disc, a kind of mini solar eclipse that will reveal the exoplanet

Unfortunately, what works on the scale of the Earth and moon does not automatically work in a telescope that is millions of times smaller. Essentially, this is because light has wave-like properties and some of the starlight travels around the edges of the black disc and thus mixes with the light from the exoplanet in the telescope.

Polaroid sunglasses

So what is the solution? Astronomer Matt Kenworthy: ‘This problem has been at an impasse for years, but suddenly people are coming up with new ideas.’ Kenworthy shows us a piece of plastic the size of a euro coin. ‘This was designed here.’ In the middle of the transparent ‘coin’ is a rosette in the colours of the rainbow – a microscopic polarisation pattern that has been etched into it with a UV laser beam. You could call the coin a very advanced pair of polarised sunglasses. The custom polarisation pattern manipulates light waves that

This coin, which was developed in Leiden, is called a ‘grating vector Apodizing Phase Plate’. It makes it possible to look into the light of a distant  star and yet be able to see the planets around the star.

This coin, which was developed in Leiden, is called a ‘grating vector Apodizing Phase Plate’. It makes it possible to look into the light of a distant star and yet be able to see the planets around the star.

pass through in such a way that the light from a distant star is separated from the light of the exoplanet.

All the light from space that enters one of the four Very Large Telescopes (VLTs, mirror diameter eight metres) in Chile, or the future European Extremely Large Telescope (ELT, mirror diameter 39 metres) will ultimately pass through these glass coins as the hunt for exoplanets continues. It will therefore be possible for the first time to use telescopes on Earth to view the atmosphere of large numbers of exoplanets.

Conditions for life
We will then also be able to see if water vapour, a condition of life, is present in the atmosphere of exoplanets and gases such as O2, O3 and CH4, which are generally produced by life processes. Exoplanet hunter Ignas Snellen: ‘If we can analyse whole collections of exoplanets, we will find out, for instance, if oxygen occurs on one type of exoplanet only.’ This would be another indication that this is the type of exoplanet where life arises. Snellen: ‘But it’s not a smoking gun. It’s about the combination and amount of such gases.’

They won’t venture a guess as to how long it will take before we find extra-terrestrial life: ‘We really don’t know. But we do know that life arose on Earth relatively soon after the circumstances here had become favourable. It may prove really easy to create life, and extra-terrestrial life may prove common.’



  • Huub Rottgering
  • Ewine van Dishoeck
  • Marijn Franx
  • Harold Linnartz
  • Michiel Hogerheijde
  • Rychard Bouwens
  • Bernhard Brandl
  • Jarle Brinchmann
  • Anthony Brown
  • Dirk van Delft
  • Marc van Hemert
  • Henk Hoekstra
  • Vincent Icke
  • Christoph Keller
  • Matthew Kenworthy
  • Koen Kuijken
  • Frans van Lunteren
  • George Miley
  • Simon Portegies Zwart
  • Tim de Zeeuw
  • Pedro Rodrigues Dos Santos Russo
  • Elena Maria Rossi
  • Paul van der Werf
  • Joop Schaye
  • Jacqueline Hodge
  • Xander Tielens
  • Ignas Snellen

Huub RottgeringScientific director / professor of Observational cosmology


+31 71 527 5851

Ewine van DishoeckProfessor of Molecular astrophysic


+31 71 527 5814

Marijn FranxProfessor of Extragalactic astronomy


+31 71 527 5870

Harold LinnartzProfessor Laboratory Astrophysics

Topics: Interstellar matter, Laboratory astrophysics, Astrochemistry, Interstellar ices, astronomical spectroscopy, PAHs, planet formation

+31 71 527 5804

Michiel HogerheijdeAssociate professor

Topics: Astronomical Interferometry, (Sub) millimeter astronomy, Interstellar Matter, Astrochemistry, Star and Planet Formation, ALMA, MATISSE

+31 71 527 5590

Rychard Bouwens Associate professor

Topics: Galaxies, HST, High redshift galaxies, Galaxy formation, Euclid, JWST, MUSE, Galaxy formation, Starburst galaxies

+31 71 527 8456

Bernhard BrandlProfessor of Infrared astronomy


+31 71 527 5830

Jarle BrinchmannAssociate professor

Topics: Stars, Galaxies, Interstellar matter, Euclid, HST, Integral Field Units, MUSE, Stellar population synthesis, Wolf-Rayet stars

+31 71 527 8470

Anthony BrownResearcher

Topics: Stars, Milky Way galaxy, Telescopes and instrumentation, Solar System Gaia, Astrometry, Astronomical distance scale, OB associations, Galaxy structure and dynamics

+31 71 527 5884

Dirk van DelftExtraordinary professor of Material heritage of the natural sciences

+31 71 527 8496

Marc van HemertProfessor emeritus of Theoretical chemistry

+31 71 527 4244

Henk HoekstraProfessor Observational cosmology


+31 71 527 5594

Vincent IckeProfessor emeritus of Theoretical Astronomy

Topics: Stars, Interstellar matter, Computational astrophsyics

+31 71 527 5843

Christoph KellerProfessor of Experimental astrophysics

Topics: Telescopes and instrumentation, Exoplanets, Planets, Stars, E-ELT, VLT, ING, iSPEX, SPHERE, High contrast imaging, Adaptive Optics, Aerosols, Atmospheric research, Planet forming disks, Polarimetry, Sun/ Solar physics

+31 71 527 8427

Matthew KenworthyAssociate professor


+31 71 527 8455

Koen KuijkenProfessor of Galactic astronomy

Topics: Cosmology, Dark matter, Gravitational lensing, Galaxy structure and dynamics

+31 71 527 5848

Frans van LunterenProfessor of History of the natural sciences

Topics: History of science

+31 71 527 8412

George MileyProfessor emeritus of Astronomy


+31 71 527 5849

Simon Portegies ZwartProfessor of Numerical star dynamics


+31 71 527 8429

Tim de ZeeuwProfessor of Theoretical astronomy

Topics: Galaxies

+31 71 527 5879

Pedro Rodrigues Dos Santos RussoAssistant professor

Topics: Science communication

+31 71 527 8419

Elena Maria RossiAssociate professor

Topics: Hypervelocity stars as probe for near field Cosmology, White Dwarfs as electromagnetic and gravitational wave sources, Supermassive black hole formation, Tidal disruption events

+31 71 527 5877

Paul van der WerfDirector of education / professor of Extragalactic astrophysics

Topics: Galaxies, Interstellar matter, (Sub)millimeter astronomy, Infrared astronomy, Radio astronomy

+31 71 527 5883

Joop SchayeProfessor of The formation of galaxies

Topics: Galaxies, Cosmology, Interstellar matter, Dark matter, Galaxy formation, Intergalactic medium, High-performance computing, Gravitational and gas dynamics, Large scale structure of the Universe, MUSE

+31 71 527 8443

Jacqueline HodgeAssistant professor

Topics: Galaxies, Astronomical interferometry, (Sub)millimeter astronomy, Radio astronomy, ALMA, Gravitational and gas dynamics

+31 71 527 8450

Xander TielensProfessor of Physics and chemistry of the interstellar medium

Topics: Interstellar matter

+31 71 527 8465

Ignas SnellenProfessor of Observational astrophysics


+31 71 527 5838


In small groups and in English

The number of first-year students of Astronomy has grown spectacularly in recent years, from around twenty-five to more than a hundred. But the teaching has remained small-scale: at the Leiden Observatory, researchers, lecturers and students still know each other personally. 
The courses are taught in English from the second year of the bachelor’s phase and throughout the full master’s programme. At the moment around half of the Master students and lecturers are from abroad. 

Astronomy research is highly internationally oriented – there are no fewer than 43 different nationalities at the Leiden Observatory. Even as a student you will carry out research using large telescopes on La Palma, and you will learn how to play a part in international projects.      
Apart from astronomy, physics and maths, programming and handling big data are important components of the programme. An astronomer always has to be an experienced data analyst, a skill that is very much in demand in today’s society. Director Huub Röttgering: 'You can give an astronomy graduate a computer and he or she will be able to do some very useful work.’

Leiden astronomers also work on teaching activities outside the University. Some of our researchers are involved in the NOVA mobile planetarium, which introduces astronomy to school pupils.  And the international outreach network Universe Awareness, specifically aimed at disadvantaged children up to ten years old, originated in Leiden, and this is the main basis of the network.

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