The objects in the sky such as Sun, planets, stars, pulsars and many other
sources radiate in all bands of electromagnetic spectrum (see section
1.1.2). Some objects can be weak in the visible band and
can't be detected using optical telescopes.
However, the same objects can be strong in radio bands and hence can be
detected in the longer radio wavelengths. In 1931, a radio engineer Karl
Jansky, while working on the arrival direction of
thunderstorm static, at 
found a few sources caused by the thunderstorms
but was still left
with a continuous 'hiss' type static. He initially hypothesized that the
source could be the Sun, but after studying its coordinates and the fact
that it comes back again after a sidereal day and not a solar day, it was
found that the position coincides with the center of our Galaxy, the Milky Way.
Later, another radio engineer, Grote Reber, observed at wavelengths
of 
and 
and later at 
, using a parabolic reflector
antenna 
in diameter. He could not detect radio emissions in the first
wavelengths, but detected emission at and found that most of this
emission comes from the plane of our galaxy. Later, an astronomer, Prof.
Jan Oort got interested in Reber's work. Apart from stating that the
detected radiation was part of a continuum, he also predicted that if a
monochromatic (single wavelength) line radiation could be found within
the radio window, the emission could be strong enough for detection by
radio telescopes. A young colleague of Prof Oort , Dr. van de Hulst, took
the challenge up with great enthusiasm to find a mechanism that might
produce such radiation. He reported that neutral Hydrogen, an atom that
is a major constituent of the Universe, had a hyper fine energy
transition corresponding to a wavelength of 
(1420MHz).
(Ref : Kraus, 'Radio Astronomy'). This was soon detected and has become
an extremely powerful diagnostic tool in radio astronomy research.