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TEDxLakeComo 2009 - Alessandro Boselli

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We should first define what are galaxies. Galaxies are objects that populate the Universe. They are hundreds of billions, we are dealing with big numbers. They are objects that evolve with time, they are not static objects. They form, they live, they die, they transform themselves. A galaxy is a system composed of stars gravitationally linked, where stars are kept together by gravitational forces, the same forces that keep the Moon linked to the Earth and the Earth linked to the Sun. Galaxies are of different type, shape and size. They are very large objects, they can contain between about 10 millions and several hundred billions of stars, and have dimensions that span between 3000 and 100000 light years. To measure the dimension of these objects or distances in the Universe we use special units such as the light year. I remind you that light has the special property of having a finite speed, light covers 300000 km per second, thus if you know that one year is composed of about 30 millions of seconds, you can easily calculate by multiplying these two entities the dimension of galaxies in kilometres: zeros would not fall into the screen, it is for that reason that we use special units. Galaxies are not only formed by stars, but they includes also other components such as gas, dust, magnetic fields and charged particles. They also include what astronomers call “dark matter”, of which we do not know the very real nature. Galaxies are fascinating because of their beauty, I indeed edicated my life to their study, (I have been working on them for the past 20 years). It is always for the same reason, the property of light of having a finite speed, that we can observe objects at such large distances that their images we see today are those of galaxies as they were 13 billions years ago. I remind you that the Universe has been formed from the Big Bang which astronomers dated at about 13,7 billions years ago, with a precision on this measurement of the order of 10%. Observing galaxies at different distances allows us to observe the Universe at different epochs, galaxies are thus unique tracers to study cosmology, which is the science studying the formation and evolution of the Universe, which is to my opinion in absolute one of the most interesting topics. I will show some images of galaxies. This is a spiral galaxy as indicated by its shape. We should remember that the Solar System with the Earth, the Moon and all the Planets belongs to a galaxy named Milky Way that we all have seen during a starring night generally in summer when the temperature is warm and we can easily be outside. I think it is important to have an idea of the dimensions of these objects, we can then come back to this unit which is the speed of light and say that the Solar System is an infinitely small structure within the Galaxy: the light emitted by the Sun takes 8 minutes to reach the Earth, while the light emitted by a star at a border of a galaxy takes about 100000 years to cross the whole galaxy. This gives you an idea of the relative dimension of the Solar System with respect to that of a whole galaxy. Spiral galaxies are composed by a nucleus that I indicate here with this pointer, located in the central part, that in the most massive objects generally includes a black hole. We also see a diffuse structure that astronomers call “bulge”, a word that does not have a translation in Italian, and these spiral structures which give the characteristic shape to this object, showing at the same time a certain dynamic; these galaxies rotate with velocities up to 300-400 kilometres per second. Inside spiral arms there are clumpy regions, that in this image are white, which are regions where stars form through the collapse of gas. We also see these dark lanes which are composed of interstellar dust produced by stars here acting as a filter avoiding to the light emitted by stars to get out from the galaxy. These spiral galaxies are relatively thin objects; galaxies are distributed in the sky with a completely random orientation, when we see them face-on they have a circular shape, when we see galaxies edge-on they appear to be relatively thin discs such as a plate or a frisbee as shown in this image. Here we can see this swelling which is the bulge and the black dust lane which avoids to stellar light to get out of the galaxy. Galaxies are not only spirals, but there are also elliptical galaxies which, as the name indicates, have a much more spheroidal shape such as that of a rugby ball or an egg. Another characteristic of ellipticals is that of being generally poor in gas and dust, composed of old stars, and being non rotating systems. The motion of stars within ellipticals is generally chaotic such as that of a hot gas. Here again the speed of stars within these objects is of the order of 300 kilometres per second, at least in the most massive ones. There exist an intermediate class of objects in between elliptical and spiral galaxies which is that of the lenticular galaxies, characterised by stellar populations similar to those of ellipticals, generally poor in gas and dust. They do not always rotate and have a shape similar to a disc or a lens, it is for this reason that we call them lenticulars. Sometimes they have some dust absorbing the stellar light, as in this object, making the galaxy shape quite fascinating, as shown by the image. It is for this reason that this object is called the Sombrero galaxy. We astronomer try to understand how these objects work. Indeed these objects transform the “baryonic” matter produced by the Big Bang, the kind of “matter” that we know, which is principally hydrogen. Hydrogen has to transform and go through the molecular phase to cool down and collapse to form the first stars. This is a quite complex process. The newly formed stars are of different type: the Sun for instance is a relatively small star. There are also stars much more massive than the Sun, although they are rare, which can have masses up to 100 times the mass of the Sun, while the smallest ones have masses of the order of one tenth of the solar mass. The most massive stars at the end of the life (I remind you that stars live relatively long, and the most massive ones are those with the shortest lives), which lasts about ten millions years, explode into supernovae. What happens at this point: these massive stars consume all the available gas that they have previously transformed through nuclear fusion and thus they do not have any more fuel to continue their activity. There is thus a collapse, the matter first falls into the core of the star that later explode burning all the residual gas. The star then transforms into a very compact object, a neutron star or a black hole, and at the same time injects as an exploding bomb all the remaining matter through an extremely energetic process, indeed this is the most energetic process that we know in the Universe, into the interstellar medium with velocities that can reach up to 5000 kilometres per second. This matter is composed by nitrogen, carbon, oxygen and other elements called “metals”. These elements later aggregate to form dust particles, the same dust that we have seen in the previous images. This cycle is quite complex since dust later mixes with the gas, helping for instance the transformation of atomic into molecular hydrogen. What I consider interesting is that today we are able to observe all these components: we have built instruments able to observe the atomic hydrogen through the emission of a particular line in the radio domain using radio antennae, the molecular gas in the radio millimetric, stars depending on their age and colour, in the UV, in the visible or in the near infrared, while the dust in the far infrared. I will show you now a few images of telescopes. This is the Arecibo radiotelescope, built inside a depression in the ground on the island of Puerto Rico; the antenna is extremely large since it has a diameter of 300 meters; this instrument allows us to observe the atomic hydrogen. This other image is that of the Very Large Telescope, an array of 4 telescopes of 8.2 meters of diameter located in the Atacama desert in the north of Chile, which are at present the most advanced optical telescope for this kind of study. Just for comparison you can see the dimension of a man to appreciate the size of the telescope. Today with these instruments we are able to map galaxies at different wavelengths, thus to observe the different galaxy components. In these images we see the Andromeda galaxy in 6 different wavelengths. In the ultraviolet image (upper left) we see the youngest stars, in the visible (upper middle) stars of intermediate age, and in the near infrared (upper right) the oldest stars. This is the atomic hydrogen image (lower right), which shows the fuel that gives birth to stars. Since all images are on the same scale, you can see that the atomic hydrogen is distributed on a disc much larger than the stellar discs. This means that galaxies have in their external regions a gas reservoir able to sustain the formation of new stars for some billion years. The molecular gas (lower middle) is instead distributed in compact regions, with a distribution comparable to that of the youngest stars. This is reasonable since we know that stars form into molecular clouds rich in gas. The dust shown by the infrared image (lower left) is also distributed as the young stars, and this is because dust is produced and heated by these newly formed stars. We thus have today a quite complete view of these, making their study quite accurate. Galaxies are distributed in the Universe in a non homogeneous way: there are regions of different density. The study of galaxies with these new instruments has shown us that even when galaxies seem isolated, they might be interacting systems. In this ultraviolet image (left panel) we see a system of galaxies, with M82 (up), which is a galaxy extremely active in star formation, M81 (low) and a small, dwarf galaxy (left). When we look at this sytem in the ultraviolet or in the optical, it looks like composed of non interacting objects. However if we observe them in the atomic hydrogen (right panel) we see M81, M82, a third object and a dwarf galaxy, but we see also prominent tails of gas linking the different objects, indicating that matter goes from one to another object. This is due to the fact that galaxies, through the same gravitational forces mentioned in the beginning able to induce huge tides up to 15 meters as observed in the Channel because of the relative position of the Earth, the Sun and the Moon, the same forces are able to strip material from one object and bring it to another, feeding the activity of star formation with this new fuel. I show you now a movie of a simulation done at the computer: we now know these physical processes, we know how they work and we are now able to reproduce them with a computer. We can thus simulate to artificially reproduce these galaxies. I now present you a short movie showing two spiral galaxies that meet each other, interact and later merge to form an elliptical galaxy. This is another example of the time evolution of this kind of objects. This movie is quite rapid, since it reproduces an event that in nature takes about 500 millions – 1 billions years, a relatively short event compared to the age of the Universe. Let’s start the movie: here we see a spiral galaxy that turns unperturbed, and we see entering from the right side another spiral galaxy. They meet each other. This encounter induces strong perturbations in the distribution of stars and gas, producing bridges of matter connecting the two objects. They then fall one on the other, loosing their rotation, forming an object similar to an elliptical galaxy. This is indeed what we observe in nature. I show you now some real images of objects strongly resembling to the images of the simulated galaxies done at the computer. This is a binary system in interaction where we see two galaxies that have already interacted, forming these long tidal tails, that in this case are quite evident, but also displacing some matter now connecting the two galaxies. This process is extremely violent and can destroy the morphology of these objects. This is another example of interacting galaxies. In this case the main galaxy, this big object, is interacting with this dwarf galaxy, which will be soon swallowed by the main galaxy. Here again we see these long tidal tails, whose size can even be much larger than the size of the galaxy itself, producing objects with spectacular shapes and morphologies. Galaxies can be isolated, can be in small systems such as those that we have just seen, or can also be located in rich clusters of galaxies. Clusters of galaxies are groups of galaxies that can include some thousands of objects. This is a spectacular image of a relatively nearby cluster where we see that its members are principally elliptical galaxies. These yellow features are indeed mainly composed by old stars, and this is probably due to the fact that in these dense environments the interactions have been so violent to transform most of the galaxies into ellipticals, rendering them rather different than normal, isolated objects. There exist however a few spiral galaxy. I will end describing an experience that has been done a few years ago that I consider as one of the most interesting exercise ever done, that brought us a unique information on the evolution of galaxies and of the Universe itself. This has been done using the Hubble Space Telescope, that you all probably know, which is a 2.4 meters diameter telescope put in orbit in 1990. This telescope has been pointed in a sky region that we knew was almost empty considering the images available at that time, a dark sky region. It has been done an extremely long exposure, the longest exposure ever done. Imagine to have a photographic camera with an objective of 2.4 meters diameter in the space, thus without the problem of the filter due to the atmosphere, and open the shutter for 10 consecutive days without interruption. This is the resulting image, the Hubble Ultra Deep field, the deepest image ever done. This field, that we thought empty, in reality is covered by galaxies, indicated by all these dots. There are only a few stars. As a first approximation the largest objects are the closest galaxies, the smallest the farther. In reality cosmology is a bit more complex since it slightly deforms these kind of simple relations. These smallest objects have only about 10% of the present age of the Universe, thus they are only one or two billion years old. I end here with this beautiful image. Thanks (Applause)

Video Details

Duration: 19 minutes and 17 seconds
Country: Italy
Language: Italian
Producer: TEDx
Director: Gerolamo Saibene
Views: 113
Posted by: tradottiinitaliano on Dec 22, 2010

Alessandro Boselli ha effettuato i suoi studi presso la Facoltà di Fisica dell’Universita di Milano, per poi ottenere un dottorato di ricerca in Astrofisica all’Osservatorio di Paris-Meudon. Dopo un soggiorno al Max Planck Institut fur Kernphysik di Heidelberg, è ricercatore dal 1996 al CNRS presso il Laboratoirio di Astrofisica Spaziale di Marsiglia ( ). La sua attività di ricerca è centrata sullo studio della formazione ed evoluzione delle galassie. È uno specialista della materia interstellare, dell’attività di formazione stellare e delle proprietà strutturali di galassie appartenenti a diversi ambienti.

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