“What we know is a drop, what we don’t know is an ocean.” This quote by Isaac Newton perfectly describes the theme of today’s article. In the second chapter of this series, we studied the electromagnetic spectrum and its importance in astrophysics. What we see with our eyes is just a tiny portion of the vast spectrum: the VIBGYOR. But the universe isn’t just about the seven colors. It’s much more than that. So, in the 25th article of Basics of Astrophysics series, let us learn some concepts of radio astronomy, a branch of astrophysics that decodes the universe at the radio wavelength and helps in the search for the alien life.
The Atmospheric Window
The EM spectrum is spread across a long range of wavelengths. Our atmosphere, however, isn’t transparent to the entire range. It is selectively permeable to radio waves and the visible portion of the spectrum, as shown in the image above. But why is it opaque to other frequencies? Well, there is not a single reason. Each has a different mechanism.
Let’s start with the gamma rays. They are the most energetic waves of all. But if they are so energetic, then why can’t they penetrate the atmosphere? Well, because the atmosphere is very thick. Gamma-ray photons disintegrate into an electron and positron (known as pair production). Also, gamma rays have enough energy to break the chemical bonds. As they ionize the atoms, they lose energy. Hence, they do not reach the atmosphere.
Next are the X-rays. Just like the gamma-rays, they too are very energetic. They have the ability to knock out the inner-most electrons that are tightly bound by the nucleus. In this process, they lose energy. Even though the atoms in the atmosphere are widely spaced, the total thickness of the atmosphere is large and the total number of atoms is enormous. An X-ray photon passing through the atmosphere will encounter as many atoms as it would in passing through a 5 meter (16 ft) thick wall of concrete!
The UV light is stopped by the ozone layer. When high energy UV light from the sun hits a molecule of oxygen gas(O2), it breaks the oxygen bond holding the atoms together, thus creating two single oxygen atoms(O). In this process, the oxygen absorbs some of the UV light, but this still leaves a significant amount of UV light, which is where ozone comes in. The remaining UV light breaks apart the ozone molecule into one oxygen gas molecule(O) and an oxygen atom(O2), hence absorbing much of the remaining UV light.
Vibrational transitions of atmospheric molecules such as carbon dioxide, oxygen, and water vapors have energies comparable with those of mid-infrared photons and absorb most extraterrestrial mid-infrared radiation before it reaches the ground.
Then we have a window that allows the penetration of radio waves. However, the atmosphere is again opaque to the long range radio waves. The radio window is much wider than the visible portion.
So now we know why infrared telescopes (such as Spitzer), X-ray telescopes (Chandra), and gamma-ray telescopes (Compton) are out there in space. Radio and optical telescopes can work well on the ground. However, ground-based radio astronomy is increasingly degraded at wavelengths > 1 m by variable ionospheric refraction. The cosmic radio waves having a wavelength of more than 30 m are reflected into space by the ionosphere.
The Beginning Of Radio Astronomy
The field of radio astronomy was pioneered by Karl Jansky in August 1932. At Bell Telephone Laboratories Jansky built an antenna designed to receive radio waves at a frequency of 20.5 MHz.
After recording signals from all directions for several months, Jansky eventually categorized them into three types of static: nearby thunderstorms, distant thunderstorms, and a faint steady hiss of unknown origin. He spent over a year investigating the source of the third type of static. The location of maximum intensity rose and fell once a day, leading Jansky to initially surmise that he was detecting radiation from the Sun.
After a few months of following the signal, however, the brightest point moved away from the position of the Sun. Jansky also determined that the signal repeated on a cycle of 23 hours and 56 minutes, the period of the Earth’s rotation relative to the stars (sidereal day), instead of relative to the sun (solar day). By comparing his observations with optical astronomical maps, Jansky concluded that the radiation was coming from the Milky Way and was strongest in the direction of the center of the galaxy, in the constellation of Sagittarius.
His pioneering efforts in the field of radio astronomy have been recognized by the naming of the fundamental unit of flux density, the jansky (Jy), after him. Grote Reber was inspired by Jansky’s work and built a parabolic radio telescope 9m in diameter in his backyard in 1937. He began by repeating Jansky’s observations, and then conducted the first sky survey in the radio frequencies.
Radio telescopes are very different from optical telescopes. They need to be extremely large to receive a signal with low signal to noise ratio. Also, the presence of water vapors and nearby transmission devices can hinder the working of these telescopes. So the telescopes used for radio astronomy are built at high, remote and dry sites.
Obtaining a high resolution with a single radio telescope is difficult. This problem was solved using radio interferometry. It consists of widely separated radio telescopes observing the same object. This increases the total signal collected and is based on the constructive and destructive interference of the waves.
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This creates a combined telescope that is the size of the antennas furthest apart in the array. In order to produce a high-quality image, a large number of different separations between different telescopes are required (the projected separation between any two telescopes as seen from the radio source is called a “baseline”)
Radio Sources In The Sky
With the advent of radio astronomy, several new celestial objects have been discovered that could not be observed using the optical telescopes. Radio galaxies, pulsars, and quasars are some of them. These objects represent some of the most extreme and energetic physical processes in the universe.
Even the cosmic microwave background radiation, one of the key proofs of the big bang theory, was discovered using a radio telescope. Other astronomical radio sources are the Sun, Jupiter, merging galaxies, and Sagittarius A.
The 21 cm Line
In a neutral hydrogen atom, an electron orbits a proton. Both these particles have a magnetic dipole moment ascribed to their spin, whose interaction results in a slight increase in energy when the spins are parallel, and a decrease when antiparallel. The spins can only have parallel and anti-parallel orientation because the angular momentum in quantum mechanics is discrete.
The configuration in which the spins are anti-parallel has lower energy. When the electron ‘flips’ and makes its spin anti-parallel to that of proton, energy is released in the form of an electromagnetic wave. From Planck’s law, the wavelength associated with this energy is about 21 cm. This is known as the 21 cm spectral line or the hydrogen line and is observed in radio astronomy.
By calculating the Doppler shifts from this line, we can determine the relative speed of each arm of the galaxy. The rotation curve of our galaxy has been calculated using the 21 cm hydrogen line. It is then possible to use the plot of the rotation curve and the velocity to determine the distance to a certain point within the galaxy. The 21 cm line is widely used in cosmology to study the early universe.
Radio Astronomy And The Search For Alien Life
The 21 cm line and the 18 cm line in radio astronomy are important spectral lines in the hunt of alien life. The 18 cm line is emitted from the hydroxyl (OH) ion. The band from 18cm to 21cm wavelengths lies within the quieter zone of the spectrum. The idea is that water-based life-forms would recognize these important markers on the spectrum and use them in an attempt to communicate with the rest of the cosmos. For this reason, the band is called the waterhole — a place for life to meet and chat.
But the drawback of searching for the alien life around the 21 cm line is that it only works if the civilizations are broadcasting intentionally.
The Pioneer plaque, attached to the Pioneer 10 and Pioneer 11 spacecraft, portrays the hyperfine transition of neutral hydrogen and used the wavelength as a standard scale of measurement. For example, the height of the woman in the image is displayed as eight times 21 cm or 168 cm.
Despite being a very important spectral line, the 21 cm observations are very difficult to make. The ground observations are obstructed by the television transmissions and the ionosphere. The telescopes for 21 cm cosmology are hence situated at very secluded sites. Making these observations on the far side of the Moon is an option to overcome the terrestrial transmissions. 21 cm cosmology is an emerging branch of radio astronomy – and an exciting one too!
Radio astronomy helps us to explore the unseen. It is a very important subfield of astrophysics and a lot of research is going on in this field. Radio astronomy requires a lot of technical knowledge so if you are an engineer with mastery in electrical, instrumentation, communications, etc and you have an interest in astronomy too, then this field will suit you well. You may contact research groups in this field and see if your skills can be an asset. That’s because most of the research groups have people having mastery in a particular field. That’s how scientific collaborations work!