 |
EIT130 - Radio Astromy Project
Chalmers University of Technology - Autumn 1996 Group 18D:
Guide: Michael Lindqvist (Onsala Space Observatory) |
|
EIT130 - Radio Astromy Project
Chalmers University of Technology - Autumn 1996 Group 18D:
Guide: Michael Lindqvist (Onsala Space Observatory) |
| Abstract
Radio astronomy is a branch of science and technology that deals with radio waves from outer space. In order to receive the very faint radio signal, radio telescopes are used, consisting of a reciever in focus of a large parabolic reflector. The signal is then amplified and analysed. However the radio telescope has, compared to optical telescopes, very poor resolution. It can, however, be increased if the reflector is made bigger, or signals from two or more telescopes are combined by interference methods. With the last method a resolution comparable to the one obtained with optical telescopes can be achieved. We now know that radiation is emitted from interstellar matter as well as from stars and galaxies, and we can detect those signals in the wavelength band from 1 mm to 100 m. One important result of radio astronomy is the demonstration of the spiral structure of the Milky Way. This has been done through investigation of the 21 cm band. Unfortunately, radio astronomy has several threats. Mobile telephones, for example, use signals in the same bandwidth as radio astronomy, which makes it harder to detect signals from space. What we have tried to accomplish in this report is to find what radio astronomy is, what kind of tools a radio astronomer uses, and what the results have been so far. |
| Table of contents
1 ..... Introduction
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| 1. Introduction
In 1931, the American radio engineer K.G Jansky, investigated the noise that disturbs the reception from faint radio stations. He found that the noise intensity varied with the direction of the antenna in relation to the Milky Way. This shows that radio waves reach us from space. The discovery developed a new branch in astronomy, radio astronomy. Radio waves are detected by using different kinds of radio telescopes. A basic radio telescope system consists of the antenna, a feedline and a receiver. The antenna picks up radio signals. Most radiotelescopes make use of a parabolic surface that reflects all the signals to a certain point as with an optical telescope. This is, however, not a necessity for radio telescopes. Even a normal antenna can be used to detect signals from outer space. Most large antennas have small motors that can steer the antenna horizontally and vertically. When observing, these motors are constantly making corrections so that the antenna can point at the same radio source while the earth rotates. A typical radio source could be a star or a black hole. A feedline carries the signal to a receiver. The receiver is a device that tunes, amplifies and detects the signal. Often it is only one frequency that is particularly interesting. Then it is necessary to remove the other frequencies, as well as some natural radio disturbance. A recording system can be used to display and record the received information. Further processing can be done with a computer. Detailed three dimensional pictures can be created if the amount of data from an object is large enough. In space there are several different phenomena such as nebulas, galaxies, stars, black holes, pulsars, quasares and "ISM", InterStellar Media, that will attract the searching eye of astronomers at earth. The ISM contains everything from hot compact gas clouds to lowdensity dus with a temperature of ten to fifty degrees kelvin. In these different media the astronomers search for complex molecules that with the right circumstances could be the base for a possible life to evolve. Universe, where does it come from, what will be after and what was before. 2. The radio telescope 2.1. The telescope The Antenna A radio telescope collects radiation in an aperture or antenna from which it is transformed to an electrical signal by a receiver called a radiometer. Radio astronomy covers a frequency range from a few megaHertz (wavelengths of about 100m) to about 300GHz (wavelengths of about 1mm). Under favourable conditions, observations into the submillimeter range are possible. Observations are made both in the continuum and in spectral lines. Much of our knowledge of the structure of our Milky Way comes from radio observations of the 21-cm line of neutral hydrogen and from the 2.6 mm line of the carbon monoxide molecule 2.2. Facts and Examples The world's largest radio telescope is the Arecibo antenna in Puerto Rico. It's main reflector has a diameter of 305m. It's mirror is not parabolic, but spherical. The largest completely stearable radio telescope is at Greenbanks in the USA, and has a diameter of 105m. Here observations down to wavelengths of 4mm are possible. The oldest big radio telescope is the 76m antenna at Jodrell Bank at The University of Manchester, England. The Swedish-ESO (European Southern Observatory) submillimeter telescope at La Silla, Chile, operating down to wavelengths as short as 0.6mm. There are also two radio telescopes located at Onsala radio observatory with antenna diameters of 20m and 25m. The 20m telescope was built in the 1970's and it is more modern and more effective than the 25m telescope. It is surrounded by a "transparent" (for radio waves) protecting envelope (radom) consisting of a special type of plastic shut up in aluminium frames. This so called radom withstands wind forces up to 65m/s and it allows about 90%(depending on the wavelength being observed) of incoming radio waves through. 2.3. Techniques The different types of antennas shift with the different frequencies that are meant to be detected. In low frequencies the antennas are usually dipoles, similar to those used for radio and tv. In order to improve the resolution the collecting area must be increased by connecting several antennas in large fields of antennas, dipole arrays. The most common type of antenna, however, is a parabolic reflector. In lower frequencies the antenna surface does not have to be solid, which allows the surface to be made in the form of a metal mesh. But in higher frequencies, the surfaces have to be smoother. The main difference between radio and optical telescopes is that radio telescopes are not imaging telescopes. Radio telescopes have, at best, a resolution of 5 arc sec which cannot display an image worth the name. Instead radio telescopes can detect sources that do not send out light. The bigger the collecting area, the better the resolution becomes. There are therefore telescopes with diameters of up to 300 meters. In an effort to obtain a resolution as good as those obtained with optical telescopes, antennas with diameters of several kilometers are required, because the resolution is proportional to the wavelength ratio between radiowaves and visible light. Building telescopes larger than those of today's size is quite difficult. There are, however, possibilities to enlarge the diameter, which will be described later. 2.4. Antenna surface As mentioned earlier, it is of utmost importance that the surface of a radio telescope's antenna is as smooth as possible. To achieve a coherent amplification, the surface irregularities should be less than one-tenth of the wave length used. The larger the antenna's diameter, the better the resolution obtained can be but at the same time, it should not be forgotten that a "large perfect" parabola is much harder to produce than a "small perfect" parabola. There are principally two methods of determining the precision of an antenna surface. 2.5. The mechanical measurement method This method requires a reference point, usually located on the axis of the antenna, to determine the location of a point on the antenna. The location is determined by its angle and range from this reference point. The range information is provided by a surveyor's tape and the angle information by a theodolite. These instruments were however far from accurate and the theodolite was later replaced by a photo theodolite, while the tape was replaced by a laser ranging system. This method was still inadequate for the new generation of millimeter and submillimeter telescopes. 2.6. The holographic measurement methods This is an interferometric technique which has been very successful in recent years. It is mostly used for the measurement of large telescopes. This method applies the diffraction theory of electromagnetic waves. Geometric relations are then used to determine the surface errors of the antenna from the phase errors across the aperture. These waves/signals are received from different sources depending on the size of the antenna being observed and a few other factors. Satellites radiating high-frequency signals are however mostly used. Ground-based transmitters are quite common too. This method of measurement has proved to be very accurate and has quite a number of advantages over the other method. Using a ground-based transmitter at 86 GHz, a measurement accuracy of 5 micrometers rms has been achieved. 2.7. Controlling the Antenna To locate small signal sources, one must be able to move the telescope's beam over the sky with very high accuracy, which is enabled by using reference points in space that are easily detected and their locations well known. "Telling" the telescope where to point, (a process called "pointing") is given with the help of servos and an accuracy of about 1.5 arc sec is easily achieved. When the source is discovered the telescope can track the source with an accuracy of less than one arc sec over the period of approximately one hour. 2.8. Frontend Radio telescopes detects the signal, using a feed horn located at the antenna focal point. The horn transfers the signal along with information about wavelength and phase to an amplifier. Because the signal received is very weak, highly sensitive receivers are used. These must bee cooled down to minimize the noise from the system which could otherwise mask the signal from the source. The signals detected are usually far smaller then the system noise, and extraction of the desired signal may take several hours. Radio observation falls into two general classes: spectral line observation and continuum observation. In spectral line observation the exact frequency of the observation is critical. Continuum observation is the measurement of the total amount of power emitted over a given bandwidth. Receivers used for these application fall into two broad types: Incoherent receivers and heterodyne receivers. The Signal Path in a Typical Heterodyne Reciever The signal from the antenna travels to the receiver where it is amplified in a radio-frequency (RF) amplifier with a gain in the order of 10 to 30 dB. In the next stage the signal is mixed with a strong local-oscillated signal producing an output signal on an intermediate frequency (IF). The local-oscillator signal is generated by Gunn oscillators or molecule lasers. There are different types of mixers such as the most widely used GaAs-Shottky diode mixer. The LnSb-Bolometer mixer is not as complexed and has not the same high performance. The SIS (Superconductor Insulator Superconductor) mixer is the latest type of mixer and has very good performance. The IF signal is then amplified with a gain in the order of 60 to 90 dB. For this HEMT (High Radio Interferometry Electron Mobility Transistor) or cooled GaAsFET amplifiers are used. The signal is then received by a detector and recorded on a data-recording system.
The 25-meter telescope at Onsala. 2.9. Backend Backend is the part of the telescope where the signal is processed and stored on magnetic tapes for later use. This is conducted by the use of powerful computers and a spectrometer. The spectrometer allows the scientists to observe the current signal. Radio Interferometry Interferometry is a term used among radio astronomers. The main aim of interferometry is to connect several radio telescopes to achive a high resolution. The received data from the telescopes is transferred to a computer which multiple the data from the different telescopes and form interference patterns.The structure of these patterns and how they change with time as the earth rotates, reflect the structure of radio sources on the sky. By using mathematical techniques we are able to transform these patterns to maps or images. To reach the same effect (same resolution) with a single radio telescope the required radius of the telescope is to be as long as the baseline between the interferometers. As a consequence of required higher resolution interferometers with global length baseline (more than thousands of kilometres) have been developed. A physical connection between these kind of "Very Large Baseline" interferometers is impossible. The problem is solved by using tape recorders. The signals received from the telescopes are recorded on tapes, synced with an atomic clock. Later all the tapes are played at the same time, which has the same effect as if the telescopes were connected physically. 3. Radio interferometry 3.1. VLBI VLBI (Very Large Baseline Interferometry) has a heritage of 25 years started by NASA. It is a technique which provides us with capability to receive images with a resolution of 1/10000 arc sec, which is in fact much better than the optical telescopes can perform.
The VLBI Theory VLBI is also used to measure our changing earth. VLBI makes a direct measurement of the Earth's orientation in space. It measures the time difference between the arrival of a radio wavefront (emitted by a quasar) at two Earth-based radiotelescopes. By observation from a global network of antennas, using a large number of time difference measurements from many quasars, VLBI determines the frame defined by the quasars and the precise positions of the antennas. The time difference measurements are precise to a few pico seconds. Because of that VLBI can determine the relative position of an antenna accurately to a few millimetres and the quasars positions to fractions of a milli arc sec.VLBI observations are sponsored by over 40 organizations located in 17 countries. Today with the VLBI technique we are able to determine the terrestrial reference frame (antenna locations on the Earth), the celestial reference frame (quasar positions on the sky), and Earth's orientation in space with unequalled accuracy. 3.2. Networks There are actually three big networks in the world. The EVN (European VLBI Network) in Europe, in the USA we find the VLBA Network and Russia has its own Russian Network. 3.3. Geophysics Another very important aspect of radio astronomy is the application of VLBI-technology to measure distances between and within continental plates. This is achieved by measuring the distance between telescopes at known locations.The accuracy of this measuring method is very high with an error of a few cm per thousands of kilometers. Onsala observatory has taken part in an international project to study continental drifts. Measurements of the distance between Onsala and the Haystack-observatory in the United States east coast over a period of ten years show that these places glide away from each other with an average of 16mm/year. Changes in the position of the earth's axis and variations i earth rotation can also be studied with the help of radio astronomy. 4. Signal Disturbances 4.1. Atmospheric disturbances Atmospheric Disturbances The earth atmosphere is disturbing the radio signals coming to earth. Since the radio signals detected are very weak, even a rather small moderation in the atmosphere makes the detection of these signals a lot harder. Just as the waves from a radio station become weaker when going through a concrete wall, the radio waves from different molecules in space become weaker when going through the atmosphere. Clouds or just high humidity weaken the signals even more. Variations in air density both gather and spread the radio waves. Absorption in water- and carbondioxide molecules is also contributing to disturbance. It all depends on the signal radio frequency. When a new observatory is planned much research is done in finding the right site for the antenna. Different sites have shown that the best conditions for observing are obtained if the site is dry and is located high. To avoid these natural problems, or more correctly put, to minimize the effects of atmospheric factors, radio telescopes are normally located in dry places and at high altitudes. The best observation sites in the world are found in the Andes in Chile, The Canaries and on a volcano on Hawaii. The problem with refraction can be avoided by sending radio telescopes up into space. 4.2. Electric noise Detecting radio signals from outer space is getting harder and harder every day. There are different threats toward radio astronomy, which are troubling our astronomers. In search for higher living standard, we have contributed enormously to the major threats toward radio astronomy. Some astronomers proclaim that in the near future, we will cut ourselves out from the possibility to detect weak signals from distant radio sources. Weak radio signals from stars and galaxies sink in the presence of signals radiated by mobile telephones and T.V satellites. To own a mobile telephone has become as common as having your own car.The increasing number of mobile phones in use is contributing to a jungle of radio waves in the frequencies just under one gigahertz. When you make a phone call, from a a mobile telephone or even from an ordinary phone a series of different radio signals is transmitted through the air. From a mobile telephone, the signal is received at a base-station antenna which is connected to other stations by radiolinks. These links contribute to the amount of radiowaves with signals in the frequencies just above ten gigahertz. However, the signals from an ordinary phone when making local calls are transmitted by wires to a base-station which is connected to other telephones. On the other hand, if the call is a distant call, the base station is linked with a similar station in the same way as in the case of mobile telephones. If the call is to be transmitted over large areas the base stations use satellites which link the signals with other stations. This "radiowave pollution" makes it very hard for astronomers to distinguish between searched signals and "unwanted" signals thereby endangering the development of radio astronomy. In other words, as a result of the rapid development in satellite technology, it has become overcrowded on the frequency band in which astronomical objects radiate as a result of the rapid development in satellite technology. Methanol, for example radiates radiowaves in areas where stars are about to be formed. It is therefore of utmost importance for radio astronomers doing research in Starforming. However, methanol radiates at an already occupied frequency which complicates the jobs of these researchers thereby retarding their progress. Another problem which we hardly realize is the increased use of microwave ovens which send out strong radiowaves at very high frequencies. 5. Switching techniques When making radio astronomical observations, it is necessary to use some type of switching technique. The reason is that at radio and submillimeter wavelengths the atmosphere, the telescope and the receiver system are all strong sources of background emission. Particularly when observing continuum radiation, the weak astronomical signal has to be extracted from the very intense background. The unpredictable background emission can suddenly swamp a weak spectral line and continuum signals if it is not compensated for. 5.1. Load switching The first switching technique is called load switching. This is a method that can be used both for spectral line and continuum measurements. It is used to compensate for variations in the receiver, but does not compensate for the atmosphere and its variations. Load switching is therefore only used at low frequencies, where the atmospheric effects can be neglected. The receiver input is switched rapidly between the antenna observing the source and a very constant noise source. The receiver input is then the difference between the output from the antenna and the input from the noise source. 5.2. Position switching Position switching is the simplest switching technique which accounts for variations in the atmosphere. The telescope is moved between the source and a reference position in empty sky. Since atmospheric fluctuations can occur on very short timescales, and large telescopes cannot be moved that quickly, it appears that position switching cannot reduce the atmospheric error. Position switching can therefore not be used for continuum observations where the background emission has to be accurately known. However, by making spectral line observations this is not important. 5.3. Beam switching A third method is beam switching, which is also a source-reference observation technique. Instead of moving the entire antenna as in position switching, the telescope beam on the sky is changed. Much higher switching rates are possible than for position switching. Beam switching provides excellent compensations, but has the disadvantage that half of the observing time is spent looking at empty sky rather than the source. The angular separation between the two beams is limited by the antenna construction. After observing a data reduction procedure has to reconstruct the true source distribution. 5.4. Frequency switching Frequency switching is the most efficient switching technique. No observation of empty sky is made. Instead the receiver switches between two slightly different frequencies, and accumulates the difference between the two spectra in a spectrometer. The advantage of frequency switching is that it is time efficient. It is the only switching method in which we observe the source all of the time. Frequency switching, however, can only be used when observing spectral lines, as information on the continuum intensity, is being lost. 6. Telescopes in space The most important reason for putting telescopes into space is the lack of radio disturbances. As mentioned on earth something as trivial as a microwave oven can cause errors in the signal readings. 6.1. Sweden-ODIN Together with the space centres in Canada, Finland and France, Sweden is about to launch its first real satellite, ODIN. ODIN is a rather small radio satellite, which is going to be launched by a Pegasus launcher, a standard launcher for small satellites. At first there were plans of sending up a larger satellite, but that would have been to expensive, therefore it has been decided to make it smaller. ODIN has a life expectancy of two years, and will in that time do over 11,000 turns in its orbit. 7. Radio astronomy objects The scientists will focus on many different types of objects; The main object will be to study the interstellar medium, this by searching for the key species. Giant Molecular Clouds and Dark Clouds:
Comets:
Planets:
Nearby Galaxies:
Protostars:
Ozone in the Stratosphere:
7.1. ISM-The Interstellar Medium In the last few decades our apprehension about space been has deeply revised. From being referred to a big void, into something much more complex. The shock waves that travel through space, compress, heat and accelerate the interstellar gases. From within the big clouds that are formed, stars are born. In contrast to the hot gases of the stars, there are cold gases in the space between the stars. These cold gases were first detected in 1904, by a German astrophysicist named Hartmann. Until that point, it was a common belief that all particles in the universe were focused to the stars and the planets. This new discovery gave vital clues to the mystery about how the stars are formed. The most common molecules found in the interstellar medium are different compounds including hydrogen. Hydrogen is the lightest element known to man, and it is a part of the different processes that form all other elements. Today over a hundred different molecules have been detected drifting freely in the giant gas clouds. There is a limit at about 200 species, which is because the spectral lines at that point become to thin to be separated from eachother. 7.1.1. Starforming The stars are born inside molecule clouds in the ISM (interstellar medium) since the forming takes place inside the cloud it is not possible to actually see a forming star. Thus there are several of theories and no hard facts before the stars emerge from the molecule clouds. Large achievements are made with modern computers and sophisticated programs which can simulate what we do not see. Known is that stars constantly are formed and that the life-time of the stars are limited by several factors such as the mass, the surroundings and what type of star it is. As an evidence of that new stars are being born is that there are stars much younger than the sun and thus must have been formed at a later point in time than when the sun was formed. In the Milky Way alone there is molecule gas enough to form at least a billion new stars with the same size as the sun. Why this not happens is not known with certainty. In order to form the embryo of a star the molecules must be compressed 100 billion, billion times. Five billion years ago a part of a molecule cloud collapsed and formed our sun. The collapsed area was ten billion times bigger than the sun, and a thousand times bigger than our solar system. It took about a million years to form the sun and then another 100 million years before the sun was cleared from the cloud and visible to the nearest stars. Most of the planets were already formed at that time. 7.1.2. The forming There are three main theories why there are not as many stars born when there are so many star-forming molecule clouds. 1: The clouds rotate and this would prevent a collapse in angles vertical to the rotation axle. 2: Powerful turbulence caused by star-winds from newborn stars, supernova explosions or galactic rotation. 3: Magnetic fields are the most likely theory. The magnetic field stabilizes the cloud and when the cloud is compressed the magnetic field creates a force in the opposite direction of the contraction. A critical molecule cloud-mass must be reached in order for the cloud's gravity to exceed the magnetic field keeping the cloud from collapsing. When the collapse starts it takes half a million years to build up a solar mass from the incoming molecules if the temperature is constantly ten degrees Kelvin. This is extremely important for the process. If the temperature should rise during the contraction, which it does when static energy is transformed into kinetic energy, the inner pressure of the blob (the early stage of the star) would stop the contraction and the process stops. The gas is cooled by dust particles, CO molecules, and water molecules. They convert the kinetic energy to emission energy in infrared wave lengths. The rotation of the blob makes the collapse in the equatorial plane less effective leaving a disc of gas which creates the first planets orbiting the blob. The collapse decreases in the central parts of the blob when the gas in the blob becomes so dense that no emission is let out. This leads to an increase in temperature, the blob is heated up from inside and is now called a proto star. Now a massive star wind starts more than a million times more powerful than the solar wind. How the wind starts is not known but it stops the contraction by blowing away the incoming molecules. At this stage either a nuclear process starts and the proto star becomes a luminous star or the nuclear process fails and the proto star becomes a brown dwarf. 7.1.3. Evolved Stars After being born, a star can evolve into a variety of different types of stars. What color and how intense they will glow depends on how big mass they have. This part of the life of the star is called the Main serie. While on the Main serie, the star will have a stable fusion reaction and not change much in appearance. The stars stays on the Main serie for about 10 billion years, depending on their masses. The star that we know the most about is the sun. The sun is estimated to stay on the Main serie about 8 billion years. After being on the Main serie, stars change into other forms. Stars with about 1-10 solar masses, turn into red giants, Stars with bigger masses may turn into supernovas. 7.1.4. The star lifetime During the larger part of their lives the stars generate energy from fusion where four hydrogen nucleons form one helium nucleon. 0,7 percent of the original mass is then turned to energy. Stars contain 75 percent hydrogen but only 10 percent can be burnt. If also the stars luminosity (the effect of the photons emitted by the star) is known the lifetime can be estimated using Einsteins equation E=mc2, where E is energy, m is mass and c the speed of light. In the case of our sun the lifetime will bee ten billion years, and since it has shone for five billion years it has used half of its hydrogen. The sun burns 600 tons of hydrogen per second in order to maintain present luminosity. The luminosity depends on the square of the mass, thus a star with ten solar masses only has a lifetime of 70 million years. 7.2. Galaxies In 1924 E.P. Hubble managed to show that the Andromeda cloud is a galaxy like our Milky Way. He wanted to prove the existence of starsystems outside our own. What he did was to calculate the distance to different objects. He discovered that the distance to some objects was much larger than others. After his discovery it did not take long time before it became obvious that there were billions of other galaxies in the universe. The discovery had expanded the universe from just our galaxy to billions of galaxies. 7.2.1. The Hubble classification system This classification system divides the galaxies of the universe into four main classes with several subclasses. Several scientists have tried to make an evolutionary connection between different galaxy classes but did not succeed. The most common view today is that they have just been created under different circumstances and that there is no connection between them. Elliptic galaxies First we have circular and flattened elliptic galaxies. They are classified with an E followed by a flatness number from zero to seven. Galaxies with the class E0 are circular and the class E7 describes the most flattened elliptical galaxies. If we consider evolution these galaxies are among the oldest with many red giants. Compared to other galaxies an elliptical galaxy often contains a small amount of interstellar molecules because it has already been used to form stars. As the amount of interstellar molecules increases, so does the radiation from the galaxy. Elliptical galaxies are found in all kinds of sizes. Elliptical giant galaxies (gEx) can be as large as 1013 sun masses, but are very rare. Elliptical dwarf galaxies (dEx), however, contain relatively few stars and are probably the most common galaxies in the universe. Spiral Galaxies Next we have normal and bar spiral galaxies. Compared to elliptical galaxies a normal spiral galaxy is a just a thin disc with a lens-shaped centre. Normal spiral galaxies are classified with S0, Sa, Sb or Sc, where S0 is a transitional stage from elliptical galaxies. Sa-galaxies have a large centre with thin close arms, Sb-galaxies have smaller centres and larger more open arms and Sc-galaxies have small centres and wide arms. Bar spiral galaxies have arms that reach out from a stick-shaped structure. They are classified with SBa, SBb and SBc similar to normal spiral galaxies. The Andromeda galaxy and our own galaxy the Milky Way are examples of such galaxies. The size of spiral galaxies varies from 109 to more than 1011 sun masses and is formed by at least an equal number of individual stars. Irregular Galaxies Further, we have irregular galaxies which are more or less chaotic. Irregular galaxies are divided into blue Irr I galaxies and yellow Irr II galaxies. It is a common fact that the Irr I galaxies always have been irregular, while the Irr II galaxies were formed after a collision with another galaxy or an explosion in the centre. The highest percentage of interstellar molecules is found in Irr I galaxies. New stars are constantly being created. Sometimes galaxies are found that miss a centre and have a totally different amount of stars and stardust. Irregular dwarf galaxies (dIrr) are almost as common as the elliptical dwarf galaxies, but are much more difficult to detect. Other Galaxies Eventually there are always galaxies which fail to fit any classification system. A lot of them show rare, often symmetrical structures which can indicate that powerful phenomenons have occurred in their centres. These galaxies are just called peculiar. 7.2.2. Galaxy Accumulation When studying galaxies, you notice a tendency to galaxy accumulation. It seems as adjacent galaxies are affected by gravitation from each other. The Milky Way and 30 other known galaxies form a local galaxy accumulation. No galaxies have actually been detected in the nearest space outside our accumulation so far. Research has also shown the existence of super accumulation, which is an accumulation of several galaxy accumulations. Classification Galaxy accumulation can be described in two ways. First we have rich and poor accumulations.A rich accumulation can contain up to 10.000 members while a poor accumulation only has a dozen members. Secondly we have regular and irregular accumulations. Galaxy accumulations tend to be more irregular the less members. 7.2.3. Galaxy Creation Most galaxies were probably created 10-20 billion years ago. Nobody actually knows how a galaxy is born, but a common theory is that a big gravitation instability created a gigantic rotating cloud of interstellar molecules. When this cloud got colder it formed a protogalaxy, which consisted of only hydrogen and helium. Different types of galaxies are explained with different starting conditions considering mass, density and rotation velocity. 7.2.4. Radio Galaxies Observations with radio telescopes show that there are radiowaves coming from thousands of places in the universe. Most of these radio sources lie outside our galaxy. Often we find powerful radio sources in the centre of a galaxy, as in our own galaxy. If the radiation from a galaxy is much higher than normal we call it a radio galaxy. 7.3. Quasars Quasars or QSO (quasi-stellar object) are extremely luminous star-like sources and as incredibly distant as 1000's of Mpc. They are 100 times more luminous than an entire galaxy. Quasars are probably active galactic nuclei powered by supermassive black holes. A comparison of the optical and radio maps of the same section of the sky shows that for many quasars, there are radio sources nearby. These sources can be divided in the following two classes: D - A double source consisting of two well-separated regions. C - A complex source consisting of three or more components. The majority of quasars belong to the D class. The first identified quasar was 3C 48 in 1960. By examining photographic plates dating back to 1887, H. Smith and D. Hoffleit determined that the optical brightness of 3C 273 was highly variable. The presence of surprisingly quick changes in light started a controversy about the nature of quasars that still continues. The current model of quasars is a supermassive black hole (10 billion solar masses) at the centre of a young galaxy. Enormous amounts of gas and dust are falling into the black hole. As it falls in, this matter converts its gravitational potential energy into kinetic energy, which then converts into electromagnetic energy via collisional and magnetic interactions. Because the gravitational field is so strong, extremely large amounts of energy can be released in this way. 8. Conclusion Radio Astronomy has already proved to be a most valuable complement to standard astronomy. Although a radio telescope has very poor resolution compared to optical telescopes, the use of radio telescopes has, and will be very useful. The discovery of many galaxies, star formations, how stars are born, how stars die, what quasars are and several other areas are important results coming from the study of radio signals. Still there is very much to be done. For each answer we receive, several questions are asked, and the research goes on. However, the threats against radio astronomy are severe and could perhaps destroy this whole branch of astronomy. Can radio astronomy survive? Will there be enough useful results from radio astronomy to hold back all the threats against it? Well, there are a few questions still to be answered. |
| 9. Annotated Bibliography
(1): Kraus, John D. (1986). Radioastronomy, 2nd edition Radio Astronomy embraces a wide range of topics from astrophysical phenomena to reciever and antenna design. (2): Olofsson, Hans (1989). Stjärnornas uppkomst, utveckling och död. Astronomisk tidskrift, vol 22 no 3 The article explains the birth, life and the end of stars. (3): Verschuur, Gerrit L. (1992). Interstellar Molecules, Sky and telescope, vol 83 no 4 It is a study of molecules in interstellar materia and how you detect and analyse them. (4): Payne, John M. (1989). Millimeter and Submillimeter Wavelength Radio Astronomy, Proceedings of the IEEE, vol 77, no 7 Technical readout of millimeter and submillimeter telescopes and how they function. (5): (1996). Are We Killing Radio Astronomy?, New Scientist, vol 2044 August The threats against radio astronomy and how we can avoid them. (6): Olberg, Michael (1994). Odin: A Swedish Submillimetre Wave Satellite for Astronomy and Aeronomy A text describing ODIN, a forthcomming swedish satellite. For submillimeter wavelenght radio astronomy. (7): Phillips, Thomas G. and Rutledge, David B (1986). Superconducting Tunnel Detectors in Radio Astronomy, Scientific American, May 1986 An article about a sensitive new radiation detector for recieving radio signals in the millimeter and submillimeter bands. (8): Smith, Robert C. (1995). Observational Astrophysics One chapter is dealing with Radio telescopes and techniques. Describing filled and unfilled aperture telescopes, very long baseline interferometry and radiospectroscopy. (9): Hjalmarson, Åke and Winberg, Anders (1983). Interstellära molekylmoln och stjärnbildning Kosmos, no 7 An article dealing with how stars are formed and the different stages of the star from beginning to end. And also about astrochemistry and moleculephysics. (10): Knee, Lewis B.G. (1992). Observational techniques and calibration Dealing with some fundamentals in radioobservation, such as switching techniques, calibration, antenna temperature and the effect of the atmosphere. (11): Olofsson, Hans (1989). Stjärnornas uppkomst, utvecling och död, Astronomisk Tidsskrift vol 22 no 3 (12): Åke Hjalmarsson och Anders Winnberg (1983). Kosmos: Interstellära molekylmoln och stjärnbildning, svenska Fysikersamfundet (13): Hans Olofsson (1996). Kosmiska moln gömmer livets gåta, Forskning och Framsteg no 3 (14): Thomas G. Phillips and David B. Ruthledge (1986). Superconducting tunnel detectors in radio astronomy, Scientific American May (15): Göran Pilbratt (1986). High Resolution Observations of Superluminal Radio Sources, Technical Repoert no 161 (16): Brett Holman. Quasars, Astronomy 177 (17): Öystein Elgaröy & Öyvind Hauge (1990). Damm: Fra strålende objekter til sorte hull, Del 2 Detailed information about galaxies. (18): Patrick Moore (1991). Atlas over universet General knowledge about the universe. |
