"Look around you." "Everything moves, everything orbits." "It is how we are born." "The Sun, the moon, the planets and the stars watch." "We are at the Centre." "Our place." "But, Nicolaus Copernicus came and said that the Sun stood at the Centre and that we moved around it." "Just like the other planets." "An Earth that moves?" "But can't we see or feel that?" "That was 1543." "Galileo made a telescope and pointed it at the sky in 1610." "He discovered moons around Jupiter and saw how these orbited around the planet in hours and days." "Later Christiaan Huygens and Dominique Cassini discovered moons around planet Saturn." "Everything seemed to orbit something." "Could it be true?" "Johannes Kepler described these movements Isaac Newton caught them." "From apple to planet: everything obeyed his laws." "What about us?" "We became usedto it and knew no better than the Sun stood at the Centre and we orbited around it." "And the stars that we see?" "What about that band of nebulous light, the Milky Way?" "Different cultures had given it different interpretations." "For the Egyptians it was Isis' wheat, the Incas saw it as golden star dust, for the Eskimos it was a band of snow and the Christians thought of it as the road to Rome." "It turned out that the Milky Way is made of stars:" "hundreds, thousands millions of stars." "And then We discovered the nebulae." "Some were spiral shaped, other oval or irregular." "Where did they belong to?" "How did everything fit together?" "We, the stars, the Milky Way, the nebulae?" "In the middle of the 19th century we knew that the stars were in fact like the sun." "That they must be very far away." "And that the Sun was very close by" "And that was the Universe, a collection of stars" "And our place in all that?" "Spiral Galaxy the Milky Way unravelled" "A film by Maarten Roos in collaboration with Pieter-Rim de Kroon" "In the middle of the 19th century the introduction of photography" "caused a big change in the field of astronomical observations." "Before, astronomers had used visual techniques to map the stars of the northern sky." "Looking through a telescope they had swept the northern sky and catalogued each star, their positions and brightness," "up to a certain limit." "This catalogue of northern stars was also called a 'Durchmusterung', a German word for survey." "With the introduction of astronomical photography an astronomer at the southern hemisphere said:" "'Let us complete the northern catalogue with all the southern stars, using photographic techniques'" "This astronomer was David Gill, the director of the Observatory Cape of Good Hope." "I am sitting next to a photographic telescope very similar to the one used by David Gill." "To take photos with the telescope you need to constantly check that it stays pointed at the same part of the sky" "You have to compensate for the daily apparent movement of the stars in the sky" "So, there is a place where the photographic plate is put, about here." "That is the focal plane of the telescope." "Parallel to it, there is another telescope, here below." "You look through this second telescope with your eye to check that the instrument follows the stars correctly and at the same time an image is taken with the other telescope," "Of that bit of the sky." "That could take several minutes or even longer." "When done, the exposed plate had to be changed for an unexposed one and the telescope pointed at a different part of the sky." "And this procedure continued until one had covered the entire sky with photographic plates" "em started this big project to photograph the entire southern sky." "But he did not immediately think about measuring all these plates" "Around the same time, the astronomer Kapteyn became professor of astronomy in Groningen (the Netherlands)." "It was expected that he would get a big telescope, but for unknown reasons this did not happen." "And so Kapteyn thought:" "'Well, there must be useful work to be done also without a telescope.'" "And the idea was born." "That he could collaborate With David Gill from the Cape Observatory to perform that huge task of measuring all the plates Gill had been collecting." "Both astronomers got into contact about this idea and it quickly led to an agreement:" "Kapteyn would measure the plates, while Gill continued ro collect new ones." "One can ask whether they realised the size of such an undertaking." "Kapteyn began enthusiastically and it took him several years to finish the work." "It was published in three thick volumes." "A copy can be found at the Kapteyn Institute in Groningen." "They contain nothing more than columns of numbers, indicating the positions and brightness of all stars on the plates." "This huge work became known as the Cape Photographic Durchmusterung." "It was this big achievement that lead Kapteyn to first distinguish himself at that time." "But in the meantime, he must surely have thought about other questions in Astronomy." "And one of those questions caught his attention early on:" "the properties of the ensemble of stars we see in the sky, the Milky Way Galaxy." "At the beginning of the 20th century around 1904, he made an important discovery in this area." "He noticed that the stars did not move randomly through Space, but that their movements had preferential directions." "Within the astronomical community that was a very important discovery:" "the fact that there was regularity in something astronomers had always considered to be chaotic!" "Thanks to this discovery he established his name even more so, which helped him pursue his plans for further research on the Milky Way Galaxy in collaboration with institutions abroad." "At the end of his working life," "Kapteyn had constructed an idea about how the big ensemble of stars of which we are part, fits together." "He had found a model that best fitted all the data he had collected overtime:" "information about distances and movements of the stars." "We now call this the Kapteyn model." "The Kapteyn model was much like a thick disc." "In the centre part of this disc the stars were densely packed in space." "And towards the edges of the disc the stars were further and further apart." "In terms of size, from the centre towards the edge of the disc was about 30,000 light years," "meaning that light has to travel 30,000 years to cover that distance." "In the direction perpendicular to the disc the size was much smaller, of the order of 7,000 light years." "So, it's a model that is quite easy to understand." "And Kapteyn must have felt it as a great satisfaction that he had found a model of which he could say:" "'This is how things must be.'" "Jacobus Kapteyn had found a model for the Universe:" "a disc like structure with the Sun near the Centre." "It explained the Milky Way." "But there were other scientists who disagreed with this model." "New observations indicated that the Universe must be much bigger than what Kapteyn had proposed." "A big debate took place in the early 1920s." "It became clear that the spiral nebulae must be very tar away way beyond the borders of the Kapteyn Universe." "Slowly this new vision gained terrain." "And the idea that we were also part of such a flat spiral nebula began to grow" "And that the Sun was not at the Centre." "At the end of the 1920s the young astronomer Jan Oort derived the rotation of the Galaxy from movements of the stars in the Sun's neighbourhood." "It also explained the star streams" "Kapteyn had discovered at the beginning of the 20th century" "But how could we get an impression of the entire Galaxy?" "Large quantities of dust between the stars obscured a clear view to distant parts and the Centre." "For twenty years people searched for a solution." "I studied astronomy and mathematics and physics of course." "In April/March 1943 all students had to sign a declaration of loyalty towards the german occupiers" "It you did not want to do that, you had to work in Germany." "Most students did not want that either and they had to hide." "That is more easily said than done, because where to hide?" "At a farm?" "But I did not know any farmers." "Or in the North-East Polder, where a lot of students hid." "I was lucky to hide here at this Observatory, together with a fellow student, Hans Hubbenet." "We came here in May 1943." "We were put in a small room, door locked, curtains closed, and we sat silently all day long." "After 6 p.m., when the Observatory's personnel had left, we were allowed to leave the room, but at 8 a.m. we had to be back." "And so we did day after day, and it went fine." "We were still in a scientific environment, so I could not have imagined any better place to hide." "There were a couple of other students as well." "Hans and I were the only ones hidden, but there was another student, three years older than me," "Henk van de Hulst." "Henk hacl finished his studies, except for the final exam, which he could not do, because the University was closed." "And he had one big advantage: he was an assistant at the Observatory." "So on his ID card was written:" "Assistant." "Wehreas on mine it said:" "Student." "And the Germans were arresting the students." "But an assistant could be an accountant's assistant or a pharmacy's assistant or anything else." "So Henk could move around freely." "Early in 1944 something happened that we found very special." "Henk received an invitation from Prof. Oort, the director of the Leiden Observatory, to come and work with him for a few months." "Today this is quite normal." "But at the time, it was like going to a different continent!" "Contacts were rare." "And Henk went to Leiden, and from time to time we received news about his stay, the people at Leiden, the things that were going on there." "Henk came back after a few months with a very intriguing story." "What was this story?" "Oort had told Henk that in the years before the war," "American physicists and engineers had detected radio emission from the Universe." "That was something completely new and unheard of." "Where did that radio emission come from?" "Oort wanted to detect and analyse that emission too, as soon as the war was over." "And he said: 'It would be so interesting, scientifically speaking, if there was a specific wavelength where the radio emission were stronger than at other wavelengths," "So that you could concentrate on that.'" "And so he asked Henk:" "'Since you are going back to Utrecht now, could you look into this hypothesis:" "is there a wavelength where the radio emission is stronger than at other wavelengths?" "'" "And so as we were hidden here, we heard this story." "We asked Henk how he would proceed and he said that he would check it With Prof. Rosenfeld." "Rosenfeld was a Belgium professor of theoretical physics, a real master of quantum mechanics, etc." "So, he went to see this professor." "And Rosenfeld asked him:" "'What is the most abundant gas or element in the Universe?" "'" "Answer: 'Hydrogen, Professor.'" "'Well', he said, 'then we should start by looking at hydrogen." "And that's not so difficult, because hydrogen is the most simple atom that we know'" "It consists of one electrically charged proton at the centre and one electrically charged electron that orbits the centre, just like a planet around the Sun." "And both proton and electron also spin around their own axis." "They can do that in the same direction" "Or in the opposite direction." "And in the first case the system has more energy than in the second case." "When the electron spin ﬂips, it loses energy," "Which is emitted as radio emission." "And Rosenfeld said: 'Henk, now you calculate at what wavelength that emission is radiated.'" "Well, that was not too difficult, a quick, simple, calculation." "The answer was: 21 cm!" "At a wavelength of 21 cm we could expect more radiation than at all the other wavelengths around it." "But the real question was: 'Is that radiation detectable?" "'" "Because Rosenfeld quickly calculated that the probability of a spin flip occurrence, is very low." "Per hydrogen atom it happens once every ten million years!" "Imagine, ten million years." "And when you hear that you think:" "'There's no chance one can detect that radiation.'" "But there is so much hydrogen in the Universe that adding all those tiny chances, a detectable amount of radiation should be produced." "Henk did some further work to determine how much radiation could be expected and he concluded that it should be possible to detect it." "In April 1944 there was a meeting of the Dutch Astronomy Club, the association of professional Dutch astronomers" "They met three times a year, still today." "But back then there was only a small number of participants, because students could not attend and also because travel was difficult during the war." "At this small meeting Henk presented his prediction:" "'We expect radio emission from the Universe at a wavelength of 21 cm and we, in the Netherlands, must make an effort to detect it, to discover it.'" "The prediction by Henk van de Hulst was but a prediction." "It was impossible to do scientific experiments in those times, especially when this had to do with radio techniques." "But if it were true, if that emission really existed, it would be a very powerful tool to study the Galaxy." "Because emission from that wavelength went practically unhindered by the clouds of dust, so emission from all parts oi the Galaxy would be visible!" "After the war, Oort immediately wanted to build a large telescope, with a diameter of 25 meter." "Yet, industry and technique were only slowly building again." "Financial support for such a project was also not available." "But Oort became impatient." "Fortunately the Dutch Telecommunication Service (PTT) had moved several german radar dishes from the dunes to the radio station at Kootwijk." "They were the so-called Würzburg dishes, that had been part of the Atlantic Wall." "Engineer De Voogt from the PTT made one of these dishes available for research on the Galaxy in 1948." "We hope this is the right spot." "No traces were left of the old radar mirror," "Which was removed in the late 50s." "We do see the old main building of the radio station and a few other buildings." "Looking at them we feel that this must be the right spot." "We are now in Hoeven, next to a german radar telescope, a Würzburg Riese." "I am a bit confused by the colour, because this white colour does not fit." "In fact, all those hundreds of radar telescopes, that formed the Atlantic Wall, had dazzle-painting green." "But for the rest it is a real Würzburg Riese telescope, 7.5 meters diameter, a small stair, everything." "In addition to a Würzburg mirror, a good receiver was also needed." "But 21 cm receivers did not exist at that time." "And at the Technical School at Delft, no engineers had been trained during the war." "Oort had to manage with a student of physics from Leiden, who started to try to put together a receiver." "Yet, after a year and a halt he had not made much progress and then he did something wrong and the entire receiver caught fire." "On March 10, 1950 a year and a half worth of work was lost." "Then Oort found a good engineer, someone from the new generation after the war:" "Lex Muller." "In addition Muller was also an active radio amateur." "In the end of 1950 he came to work at the" "Dutch Foundation for Radio-emission from the Sun and the Milky Way Galaxy." "He came to Kootwijk and five months later he managed to deted the 21 cm line emission from the Milky Way Galaxy" "May 11,1951." "One and a half month earlier a student at the Harvard University (USA)," "Harold Ewen, had detected the emission for the first time." "No surprise, because the Americans had a huge advance because they had a lot of experience with microwave techniques, which they had developed during the war." "Henk van de Hulst was at Harvard at that time and he organised the communication between Harvard, Leiden and Kootwijk." "The results of those first detections were published together." "For several weeks Lex Mulder did test observation." "And concluded that he needed a much better receiver in order to do a large survey" "So, he took everything apart and spent a year building a better receiver." "In March 1952, just before the new receiver was finished," "I went to Kootwijk for a month and a half to calibrate our radio telescope." "We had to know exactly in which direction the instrument would be pointing." "For this we needed new scale-indicators in azimuth and height, corresponding to the horizontal and vertical movements of the telescope." "Together with a technician from radio station Kootwijk we improved the existing scale-indicators." "Because they were inaccurate and had a wrong reference point." "We also determined the small errors that still remained," "So that we could correct for them." "Once this was finished, in June 1952, the first large survey along the Milky Way equator started." "After my study of electronics, I came here in 1954." "There was a small cabin next to the telescope where I worked." "We mapped the Milky Way." "The observations were done day and night by students from Leiden, who came to Kootwijk." "There were always three students, one observed during the day, one during the evening and another during the night." "I supported them during the day and evening." "In the telescope there was a clock that needed to be set right." "In order to move the dish there were two handles, one tor the azimuth, one for the elevation." "Because the Earth rotates, one had to correct the position every three minutes." "Lists for these positions were prepared in Leiden." "It was a complete manual process." "The first series of measurements lasted one year from June 1952 through June 1953." "In the meantime, Henk van de Huist received an invitation from Oxford to give the famous Halley Lecture in May 1953." "In that lecture he wanted to show the first map of the distribution of hydrogen clouds in the Galaxy." "So, we had to make that map before his talk." "In order to put together the first map of the Galaxy," "Oort, the director of the Leiden Observatory, organised a crash-action." "All the personnel of the Observatory participated." "I did too, together With my fellow students" "Maarten Schmidt and Gart Westerhout." "We sat there, I'd almost say, locked up in the Observatory's lecture room for a week." "Our task was to process the observations from Kootwijk, do some calculations and give the results to Van de Hulst so that he could put together a map." "Everybody had a pile of observations from the same position in the sky." "Each observation was a registration oi the radio emission as a function of wavelength or frequency I have an example here." "This registration starts as a wiggling line near zero where there is no radio emission." "It rises to a maximum, a lot of radio emission." "It then drops a bit, rises again to an even higher maximum and finally drops back to zero." "What happened?" "The receiver changed frequency very slowly over a one or two hour period." "It recorded the hydrogen radio emission at different frequencies." "Why was this done?" "Well, I remind you of the Doppler effect that everybody knows from a passing police car or ambulance." "Such a car produces a sound of a certain tone." "And you know that as the car approaches, the tone sounds higher than when it moves away" "The higher the tone, the higher the frequency of the sound wave (shorter wavelength)." "And that depends on the speed of the car relative to you." "The same principle was applied at the radio receiver in Kootwijk." "It observed the radio waves emitted by hydrogen atoms." "When the atoms move towards us, we perceive a higher frequency, when they move away a lower frequency" "Higher frequencies mean shorter wavelengths." "Lower frequencies mean longer wavelengths." "That principle has been applied here." "This registration shows the intensity of the radio emission at different frequencies." "That corresponds to the number oi hydrogen atoms moving at different velocities, in that one direction in the Galaxy." "Our task was to measure the radio intensities at different frequencies." "From these we determined the number of hydrogen atoms and we calculated their velocities." "In addition, several measurements had been done at each position, because the radio emission is weak and the receiver has quite some noise." "We needed to reduce the noise and so we had to repeat the measurements several times" "We averaged three, five or ten of these in order to reduce the noise and amplify the signal." "And that was the product we delivered to Henk van de Hulst." "He then determined where in the Galaxy the hydrogen atoms with different velocities were located." "Here you see the result and you see that the hydrogen clouds form a spiral shaped band around the Galactic Centre." "Here is the Sun, here the Galactic Centre and the hydrogen clouds form an arm around it." "Very similar to what we knew from extragalactic spiral nebulae." "That was a first indication that our Galaxy has indeed spiral arms of hydrogen gas." "That was something completely new." "And that was the big surprise" "Van de Hulst unveiled during his lecture in Oxford:" "based on the observations from Kootwijk, a spiral structure had been discovered." "After Van de Hulst's Oxford Lecture a second survey started in November 1953." "In this survey we not only observed along the Milky Way equator, but also 10 degrees above and 10 degrees below." "We observed several hundreds of positions." "We measured the radio emission as a function of velocity and determined from those the positions of hydrogen clouds in the Galaxy." "Not only in the equatorial plane, but also above and below." "We made a three dimensional map of the distribution of hydrogen gas in the Galaxy." "And so we mapped the entire Galaxy, as it was visible from the Netherlands, because from the Netherlands one can not observe the entire Galaxy." "That resulted in this beautiful colour map." "It was so nice that it proudly hang on a wall in the Observatory for months and went to several international conferences." "You can see four spiral arms." "The outer arm, that was also observed in the previous survey, and the dark coloured areas in the inner parts of the Galaxy." "But, as you can see, a part is missing." "That is the part that cannot be observed from the Netherlands." "However, colleague astronomers in Sydney, Australia had also started to do radio observations." "The leader of that group, Frank Kerr, tame to Leiden to collaborate." "He processed his data in a similar way as had been done by us in our surveys." "And this map shows the combination of the Dutch results, that we saw in colour just before, and the Australian results." "There is a difference between both parts." "The Dutch part of the map looks sharper than the Australian part, because of the slightly different methods of calculation used." "But the spiral arms from both parts do connect!" "The triumphal point of the diagram is that you can clearly see that the hydrogen gas in the Galaxy is distributed in spiral arms." "Just like the paper's title says:" "'The Galactic System as a Spiral Nebula.'" "The Galaxy is a spiral galaxy." "Comparable to the spiral nebulae we have known for long from photographic observation, like this one." "That was the great result from the survey done at Kootwijk in collaboration with our colleagues from Sydney." "Our place had now been clearly shown:" "a planet around a regular star at the edge of a regular galaxy with stars, gas and dust in spiral arms" "A galaxy as there are millions and millions." "Dutch Astronomy had made a significant and valuable contribution to this new insight." "The Netherlands continue to play an important role in an area where new discoveries are still being made, new instruments are being developed and the frontiers of our know-how and knowledge are constantly being pushed out." "A film by Maarten Roos in collaboration with Pieter-Rim de Kroon with" "But do not think we have finished." "The change of vision that happened in the past one hundred years, take my word for it, will happen again in the coming one hundred years." "And we do not have the faintest idea how where that will lead." "We always have to be prepared that new perspectives will come that are now beyond our imagination." "One has never finished." "Music by" "with support from initiative  produced by" "Translation:" "Maarten Roos, with thanks to Margarida Serote Roos and Maria Suzete Serote Nunes"