Radio Astronomy: this mysterious
Radio astronomy studies the celestial bodies by analyzing their electromagnetic emission in the spectral range of the radio waves with the radio telescopes (typically in the range of frequencies between 20 MHz and 300 GHz).
It's called radio source any body characterized by measurable radio emission.
The radio sources, as a function of their specific and prevalent emission mechanism, may exhibit very different physical and radiative characteristics from each other, falling into two basic types: thermal and non-thermal radiation.
The analysis of the received signals consists to determine the intensity of cosmic radiation picked up by the different directions of space and for different wavelengths, in addition to their degree of polarization.
The spatial distribution of cosmic radiation is represented as a "radio map" of the sky that show the trend of the brightness isophotes along lines (or isothermal case is highlighted in the distribution of brightness temperature), corresponding to steps along which the "radio-brightness" of the sky stay constant.
The radio sources located in the continuous spectrum are classified in according to the performance of their spectral characteristic.
We distinguish thermic sources that emit as blackbodies to a constant and uniform termperatura, characterized by an increasing trend of the flux density with frequency (their spectrum follows the law of blackbody radiation Planck - see chart below) and non-thermal sources radiating (generally) on the issue of sincroctrone. In this case the radiation is due to the orbital motions of high-energy electrons, trapped in the magnetic field produced by radio source: the speed of the particles is relativistic, then the emitted radiation is concentrated in the direction of their instantaneous motion. This mechanism characterizes the majority observable radio sources.
It is possible, in practice, to distinguish the different emission mechanisms thanks to the fact that the intensity of the radiation varies in a different way with the frequency: increasing for the thermal radiation, descending for the radio sources of non-thermal type.
Spectral variation of broadcasting: the image at the left shows the spectral pattern (such as varies the intensity of radio-emission as a function of frequency) of the most important radio sources.
As in optics, radio astronomy in the study of received energy may be classified in photometry (study of the issue in the continuous spectrum) and spectroscopy (the measurement and study of a spectrum).
The power incident on a radio telescope is generally in the form of continuous radiation, with a spectrum that shows slow variations with the frequency and can be considered constant with respect to the bandwidth of most instruments.
There are, peculiar phenomena of discrete radiation (spectral lines) generated in correspondence of specific frequencies by atomic and molecular processes.
There are three components:
1. continuous widespread radiation, with very large spatial and spectral characteristics;
2. radiation from sources located in the continuous spectrum;
3. discrete radiation that consists of spectral lines in emission or absorption.
Filtering characteristics of the atmosphere and artificial interference
Unlike what occurs in conventional communication systems, electromagnetic wave received by a radio source is not observed any type of modulation: the signal manifests the characteristics of a "noise", its intensity varies random over time. Generally is interested measuring the energy received and record the intensity of the incident radiation.
If we study variable sources over time (such as, for example, the pulsar) will need to keep the bundle pointing of the antenna on the source and record the changes in intensity over time, while if you want to get the radio equivalent of a photograph will be necessary "sweep" the region of the sky which affects the intensity and record the signal in function of the position of antenna pointing. Another important difference between the radio and television broadcasts and the cosmic radio emission is that the first transmit on frequencies specified, with very narrow frequency bands (in order to optimize the allocation of frequencies with numerous programs from many different stations), while the second radiating, in general, a continuous spectrum signal simultaneously on all frequencies. Exceptions to this rule are the clouds of matter characterized by diffuse emission in very narrow band due to atomic and molecular transitions (for example, the interstellar MASER).
Not all of the energy emitted by cosmic radio can reach the Earth: at lower frequencies (about 15 MHz) the earth's ionosphere behaves like a impenetrable screen to effect the absorption of the ionized gas, while at higher frequencies (above a few tens of GHz) the water vapor content in the troposphere completely absorbs the radiation coming from the outside.
This is called "radio window" that selects the range of frequencies within the radio receptions are possible from the ground. A much heavier limitation of radio astronomy receptions comes from artificial radiation generated by human activities: the various transmitting stations broadcasting all over the globe radiating high powers that interferes destructively with any attempt to measure the weak cosmic radiation. Something similar would happen if you tried to observe the night sky surrounded by bright floodlights.
To resolve this problem, in an attempt to find a solution that reconciles the needs of science with those of communications, have been assigned the appropriate frequencies reserved for radio astronomy research. The increasing pressure to apply for new radio channels makes, however, the radio astronomy observations increasingly difficult: the human interference can seriously affect the future of this science.
Difference between optical astronomy and radio astronomy
- The ability to resolve fine spatial details (resolving power) is dependent on the wavelength of the radiation.
- An optical telescope with 10 cm aperture is able to resolve details that would require the use of a radio telescope with a diameter of the order of 40 Km (at a frequency of 1420 MHz)!
- The sensitivity of the instrument, regardless of the operating wavelength, is proportional to the effective antenna.
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