Whats is Radio Astronomy?

Introduction: What is radio astronomy [reference: pictortelescope.com]

 

Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. Unlike traditional telescopes that observe the sky in the visible part of the electromagnetic spectrum (400-700 nm), radio telescopes observe the sky in wavelengths ranging from centimeters to a few meters.

 

A diagram of the electromagnetic spectrum, showing various properties across the range of frequencies and wavelengths.

Screenshot 2022-05-29 at 2.23.31 PM.jpg

But why should we observe in radio wavelengths?

 

There are several reasons to observe the sky at radio frequencies. The most common reason radio astronomers point radio telescopes to the sky is to study sources that produce radio emissions: radiation that is invisible to the human eye, but is capable of providing compelling information to astronomers & astrophysicists.  

A significant advantage radio astronomers have over IR, UV and high-energy astronomers is the atmospheric window: our atmosphere is completely transparent to radio waves, so we don’t need to send large radio antennas to space (like we usually have to do with IR/UV/high-energy astronomy satellites) in order to efficiently expose our instruments to the sky.

 

A diagram showing the amount of absorption of each wavelength of light by the Earth’s atmosphere, highlighting the atmospheric windows.

Screenshot 2022-05-29 at 2.39.15 PM.jpg
Screenshot 2022-05-29 at 2.39.21 PM.jpg

It is also worth noting that radio observations can be carried out during both daytime and nighttime, and even under poor weather conditions (e.g. clouds)!

 

Last but not least, we can utilize the techniques of radio interferometry and aperture synthesis using large arrays of antennas to achieve extremely high (angular) resolution observations, which is how the first image of a black hole became possible (the combination of several radio telescopes around the world allowed the Event Horizon Telescope team to create a virtual telescope the size of the Earth)!

 

The Very Large Array (VLA) is one of the most sophisticated radio telescope arrays in the world, consisting of twenty-seven 25-meter radio antennas that can function as a high-resolution radio interferometer.

 

So… What can I observe with a radio telescope?

 

There are plenty of sources in the sky that present radio emissions, including galaxies, supernova remnants, nebulae, radio galaxies, quasars, pulsars, masers and more. Since the radio telescope you’ll be using is primarily used to detect the neutral hydrogen concentration of the spiral arms of the galaxy, we will focus our attention on the hydrogen line.

 

The 21-centimeter hydrogen line

 

The hydrogen line refers to the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation is at the approximate frequency of 1420.4 MHz, which is equivalent to the wavelength of approximately 21 cm ( λ =c/v). This wavelength falls within the radio region of the electromagnetic spectrum,  and it is frequently observed by radio astronomers.

The exact mechanism under which hydrogen atoms emit electromagnetic radiation with a wavelength of 21 cm is slightly complicated.

Why observe at radio wavelengths?

There are several reasons to observe at radio wavelengths. Below we will show you the advantages and disadvantages:

Advantages

Weather
Radio astronomy can be done from the earth without being too much affected by the weather (although the quality of the observations is better with good weather)! However, there is now also a radio antenna in space, to further improve the resolution of the observations.

Day and night
Radio telescopes observe day and night (although for some observations the influence of the sun is negative!)

Hydrogen
About 90 % of the visible matter in the Universe is Hydrogen (wavelength: 21.106114 cm). With a radio telescope, we can study the most abundant element in the Universe.

No absorption
Radio waves are not affected by absorption. Optical waves are absorbed by e.g. dust clouds that are floating between the stars (like a sort of interstellar fog ). Radio telescopes see straight through these dust clouds.

Disadvantages

Resolution
On the negative side, to get good quality images that show all the details of the celestial objects it is more complicated than, e.g. at optical wavelengths. This has to do with long wavelengths of radio waves. To get good angular resolution requires large telescopes.

Complicated procedure
A complicated procedure is required to produce the images of the observed objects (in other words, the observer does not see the images straight away). This procedure uses very powerful computers and it is necessary because of the way the observations are done.