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Make a Radio Telescope PORTADARADIO ASTRONOMY MANUAL ALMA at School Contents 1. From Jansky to ALMA 1.1 Jansky’s Observations 1.2 The First Radio Telescopes 1.3 Basic Elements of the ALMA Radio Telescope 1.4 How Images are Formed by a Radio Telescope 1.5 One Telescope, Many Antennas 2. The Physics of Radio Astronomy 2.1 Electromagnetic Radiation 2.2 Radiation in Everyday Life 2.3 The Origin of Electromagnetic Radiation 2.4 How Waves Travel Through Space 2.5 Working at High Altitudes 3. Exploring Our Cosmic Origins 3.1 The Big Bang 3.2 The Chemistry of the Universe 3.3 Formation of Stars and Planets 3.4 Studying the Sun 3.5 The Sun and its Environment 4. Activities 5. Glossary The words highlighted in the text are defined may be found in the glossary at the end of the manual. 6. Slides Cover image Composition: Colorful view of ALMA. Credit: ESO/B. Tafreshi; three-dimensional view of gas outflow from NGC 253 as seen by ALMA. Credit: ALMA (ESO/NAOJ/NRAO)/Erik Rosolowsky Editing: Valeria Foncea, ALMA Education and Public Outreach Officer Design: Alejandro Peredo, ALMA Graphic Designer Scientific Advising: José Gallardo and Ignacio Toledo, ALMA Astronomers Pedagogical Advising: Pablo Torres, Sparktalents Foundation Based on: “The Invisible Universe” from Universe Awareness (UNAWE) and “Exploring Our Cosmic Origins - Educational Material about ALMA” from the European Southern Observatory (ESO) Translation: Kora McNaughton 2 Prologue Since the beginning of time, human beings have been fascinated by the sky and stars. However, it was only with the invention of the telescope in 1609 that humans could begin to study astronomical objects in detail, transforming astronomy and taking discoveries to unprecedented levels thanks to technological progress. Today we can study the Universe by observing types of radiation, different from visible light. Radio waves –including infrared, gamma rays, ultraviolet and X-rays– provide astronomers with glimpses into a completely new world: the “invisible” Universe. It will take many more generations of astronomers to reveal all of the secrets of the Universe. That’s why it is so important for the future of astronomy, and for science in general, to spark the interest of children and assist teachers in guiding their learning. This manual is designed primarily for teachers who want to expand their knowledge about radio astronomy in general and the ALMA Observatory in particular. It also contains activities that can be used in the classroom or as part of an extracurricular workshop. Although the manual provides recommendations for using the material in courses and grade levels, teachers should use their own judgment to determine how to integrate it into their planning, based on their knowledge of their students and their schools’ curricular plans. The text is organized into four chapters. The first provides a brief history of radio astronomy and general aspects of its physical properties, in comparison to optical telescopes. The second chapter contains a deeper examination of the physical concepts underlying radio astronomy such as refraction, reflection, resolution power and others, in order to help students understand the implications for observation. The third chapter explores the areas of radio astronomy investigation that ALMA can address and some conclusions that are expected to be verified. In other words, this chapter contains a brief description of the state of the art in radio astronomy research. Finally, the fourth chapter contains a series of activities for working with students. The activities are organized by level of complexity, starting with the simplest ones and ending with more in-depth activities, all of which are related to the ideas described in the previous chapters. The manual also provides recommendations on using the activities in natural sciences courses at different grade levels. This manual assumes that teachers have a basic understanding of concepts of physics, chemistry and algebra, and therefore does not explain elementary concepts that can be found in any science book. Nevertheless, some of these concepts have been included in the glossary for quick reference. Nor is the manual aimed at covering topics in depth or providing more complex technical details or theoretical models. Rather, it is designed to introduce these subjects and contribute a set of teaching activities as a first step. At the end of each chapter there is a set of questions to help students reflect on and understand what they have read. 3 1. From Jansky to ALMA Antenna at the array’s operations site. Photograph by Dave Yoder/National Geographic Yoder/National by Dave Photograph site. operations Antenna at the array’s ALMA, the Atacama Large Millimeter/sub-millimeter Array, is located high on the Chajnantor Plateau in the Chilean Andes, at 5,000 meters above sea level. Developed more than 80 years after radio waves from outer space were first detected, ALMA is a cutting-edge observatory that studies light emitted from some of the coldest objects in the Universe. 1.1 Jansky’s Observations It can take a long time to get to ALMA, not just because its location is almost inaccessible to human beings–at 5,000 meters above sea level and in the middle of the driest desert in the world–but also because it represents an enormous advance in this area of astronomy that observes the invisible. It is truly a milestone in radioastronomy. This journey began almost accidentally in 1931, when Karl G. Jansky, an engineer from the United States, made his first observations of extraterrestrialradio sources. During those years, Bell Labs had commissioned Jansky to study possible uses for short wave bands in communication, and needed to evaluate potential interference in the atmosphere. Using an antenna he designed and built himself, Jansky began receiving radio waves generated by natural sources such as electrical storms and lightning. The antenna was mounted on a rotating platform that could detect radio signals from any direction. Close Image 1. Jansky’s merry-go-round antenna. The wheels were used to to the antenna structure was a shed where Jansky kept a “pen and paper” recording system–similar to a rotate the structure and point it in seismograph–with which he recorded signals from nearby storms, distant electrical storms and a constant different directions. The antenna was designed to receive waves yet weak signal of unknown origin. Over the several months during which he recorded data, the unknown with a frequency of 20.5 MHz (1 MHz=106 Hz) and a wavelength of signal was always present. Jansky’s process consisted of rotating the antenna one full turn (360º) over 20 approximately 14.6 meters, located minutes. Thus, in one hour the antenna pointed in the same geographic direction three times. Image 2 in the shortwave (SW) band. In comparison, an FM radio can tune in shows one of the records of the signal over a two-hour period. waves from 88.0 to 107.0 MHz (3.4 to 2.8 meters, respectively). Credit: NRAO-Green Bank. 5 Image 2. Part of what Jansky Jansky initially believed the sun was the source of the unidentified signal, but as the days passed, he saw recorded in February 1932. The geographical direction is indicated at that the appearance of the crest was delayed by almost four minutes each day (see note on the difference the top: south (S), east (E), north (N) between a solar and a sidereal day). This time difference led Jansky to conclude that the source emitting and west (W). The time is indicated on the bottom. Each vertical division these radio waves was located in the Milky Way, whose greatest intensity was in its central zone, in the marks a period of five minutes. The arrows point to the crest or maximum Sagittarius constellation. observed every 20 minutes, as the antenna swept across the plane of our galaxy. This discovery was widely disseminated in the press, including a report in the New York Times published on May 5, 1933. Jansky intended to continue with his measurements to investigate these sources from outer space, but Bell Laboratories did not consider the signals to be a problem and he was assigned to another project, leaving this task to future astronomers. The unit of spectral flux density, in the International System, is now known as the Janksy. Note on the difference between the solar day and the sidereal day The definition of day seems simple: It is the time it takes meridian (am and pm in the 12-hour system: ante meridian a planet, satellite, or celestial body to complete a rotation and post meridian). around its axis. However, to describe the movement of a body we need a system of reference, that is, something with But the Earth’s orbit around the Sun is elliptical, at a speed respect to which we can say that a certain celestial body that varies during the year. As a result, the amount of time has completed a rotation, or more generally, that it moved. between two middays is never the same. To simplify this If we say the Earth rotates on its axis, with respect to what situation, assume that there is an imaginary Sun, with point can we say that it has completed a rotation? respect to which the Earth orbits in a circular pattern at a constant speed. That speed is its average speed in the This leads us to two definitions: the solar day and the elliptical orbit, resulting in a solar day of 24 hours. sidereal day. The former is based on the measure of time of the Earth’s rotation with respect to the Sun and the latter is The Earth’s revolution around the Sun is also the reason with respect to the stars.
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