To understand what radio astronomy is, we need to know what electromagnetic spectrum is. We detect the World with our eyes which can only work with visible light(radiation). Moreover, visible lights is only one small part of electromagnetic spectrum. . It consists of Radio waves, microwaves, infrared, visible, ultraviolet,x-rays and gamma rays. Actually, you do not have much chance to discover something in universe with visible light. Each part of it corresponds to certain frequencies. Thus, we can explain the electromagnetic spectrum as that it is the combination of all frequencies of electromagnetic radiation. Radio astronomy is a kind of sub-branch of astrophysics which studies stellar objects with radio frequency. These objects consist of galaxies,stars,quasars,masers and pulsars.
Different stellar object emits radiation with different frequency. This means that we can distinguish the source from others with the emitted radiation. Moreover, it can be determined whether the source of radiation is natural or not. Radio waves are the best way of sending message in the space. Because of that they have low energy, their interaction less than the other radiation types. Thus we can use them to get interact with other intelligence if they exist.. SETI project is a good example for it. So, if we know the special frequencies in radio astronomy, we can define any object in the space.
In 1860, theoretically , Maxwell’s equations show that the electromagnetic radiation is bound with magnetism and electricity. Moreover, it could be with any wavelength. In experimentally the first discovery of radio waves source is made by Karl Jansky, who worked in Bell Telephone Laboratories as an engineer, in 1930. He worked on interfered with short wave transatlantic voice transmissions with large antenna. He realised that his analog pen-paper system recorded a repeated signal of indistinct origin. He also realised that the signal peaked about per 24 hours. Thus, he thought that it could be sun. However there was a problem that the signal repeating cycle is 23 hours 56 minutes. At this point his friend, Albert Melvin Skellet, disclosed that this cycle was the certain lenght of the sideral day. So they decided that if the source was astronomical, it should be passing in front of the antenna once every Earth rotation. Then, Jansky found that rediation peaked whenever the antenna was aimed to the densest part of the Milky Way. The source was assigned Saggitarius A. He pronounce the discovery in 1933 and wanted to work on Milky Way in detail. However, Bell Labs take him to other Project. After Jansky, Grote Reber built a parabolic radio telescope in 1937 and observed the same things with Jansky. Moreoever, he conducted the first research in the radio astronomy. In 1942, British Army officer, J. S. Hey, announced his detection of Sun’s radio waves.
EM consist of all types of radiation. We can explain the radiation as a energy that moves and spreads out like a light come from lamp. EM can be shown in terms of wavelenght(m), energy(Ev) or frequency(Hz).