The name astronomy brings to mind the wonders of the night sky. The beautiful view on a clear night from a dark site. Thousands of stars, planets, nebulae, clusters and an exciting galaxy that may perhaps be visible. It brings to mind the picture of a telescope. A large mirror and a sharp eyepiece giving brilliant magnification for the planets and an amazing clarity for nebulae and galaxies. The breathtaking view that one gets may rarely be challenged from its position of ‘best ever’.
But as you move into the realm of serious amateur astronomy, it is not just the view that you are interested in. You want to understand, comprehend and study what you see. You want to explain the phenomenon that you see. You want to know everything that there is to know about the objects you see. It is this curiosity of the human mind that has helped us survive in this world. It is this curiosity that helps us make breakthroughs in science and technology. And through such breakthroughs we learn more and more.
One such breakthrough gave us the world of radio astronomy. It gave us a look at the universe using a new eye; a radio eye. The human eye and most of the aids we use to observe operate in what is known as the ‘visible’ or ‘optical’ part of a wide spectrum. It was known for quite some time that there are other sections of this spectrum that exist but nobody had ever tried using these to observe the sky. In fact it was believed that there wouldn’t be any significant signal that one would receive if tried to observe in non optical regions. This combined with the absence of developed technology resulted in astronomy being carried out only in the optical region. But post World War with the development of the radar, radio astronomy shot to fame.
A radio astronomer doesn’t use telescope with a mirror and stare out at the sky just at night. He uses huge dish antennas with large receivers that stream radio signals that they catch from the sky onto computers and give him a radio signal. This radio signal is mostly in the form of a graph showing the amount of signal coming from the sky at any given time. Though the name radio astronomy might bring to mind a radio astronomer sitting under a radio telescope with headphones, listening to radio waves, this is not at all true. Radio astronomers rarely ‘listen’ to a radio signal. Radio waves are information carriers and they can carry sound as in the case of commercial FM radio channels or can carry visual signals as in the case of direct-to-home (DTH) satellite televisions. But the radio signals that come to a radio telescope from the universe mostly carry information about the conditions that prevail in that part of the universe.
So what does a radio astronomer look at, that an optical astronomer might miss?
Imagine a radio astronomer is scanning the sky for radio signals. He is moving his antenna from the direction of Orion constellation towards Auriga. Suddenly, as the telescope is passing the Taurus constellation, there is huge spike on his computer screen. The radio telescope found something! Wide eyed, he tries to figure out where the telescope was exactly. He stops it from moving and takes it back to that position in constellation Taurus. As the telescope slowly moves into position, there is a spike again. And again, and again! Like a heart beat. There are spikes on the screen at regular intervals. The spikes are actually coming really fast, but the computer is slowing it down for him. He checks the actual speed at which the spikes are coming. There are 30 spikes every second. “Wow! That is fast” he thinks. Like a heart that is beating really fast. Our astronomer has found the Crab Pulsar.
It is an extremely fast rotating object, also called as a neutron star because its made up of neutrons. It is one of the possible end stages of big stars. Stars much bigger than our Sun. When they finish their fuel, they tend to explode and shed their outer layers while a super dense core remains. This super dense core is about the mass of our Sun but only about 10 km in diameter. So pulsars are very small objects. They are rotating rapidly, have very strong magnetic fields and give out radio waves from their magnetic poles. The magnetic axis is most often not the same as the rotating axis and hence these radio waves reach us once or twice every rotation cycle. Like a lighthouse sweeping its light beam at us once every rotation. Pulsars are difficult to spot using optical telescopes but are fairly easily seen by radio astronomers.
Now our astronomer keeps observing the Crab Pulsar till it sets and starts searching the sky for more objects. As he goes east, the constellation Cygnus has risen. “Maybe there is something interesting here”, he thinks. And sure enough, his telescope spots something. It is not a simple signal. His computer analyzes the signal and gives him an image, a picture, of a small part of the sky in Cygnus. What he sees is an incredible view. Two large clouds of that seem connected to one another through a thin strand with a bright spot at the center. Is that a nebula? He checks the distance. It is outside our galaxy! It can’t be a nebula. It has to be another galaxy. It is a radio galaxy, with huge lobes on either sides of it. The bright spot in the middle is the galaxy, and from this galaxy large jets are streaming out on either side. These jets terminate and form the large lobes that are much bigger than the parent galaxy! “How does this happen?” he wonders. There is a super massive black hole at the center of this galaxy. And as this black hole eats matter from the galaxy, two large jets are created due to matter spiraling in so quickly and shoot out of the galaxy emitting strong radio waves. The name of this galaxy is Cygnus A.
Apart from pulsars and radio galaxies, the radio astronomers can also observe many different objects. Hydrogen gas clouds become visible in the radio region. Black holes can be inferred, planets studied and even the Sun can be an interesting radio source to observe. That is how a radio astronomer spends his day. Observing through a telescope and analyzing the data he collects for weeks till some new information is discovered.
Cygnus A - A lovely radio galaxy showing two massive lobes at the ends of two jets arising from the tiny looking galaxy in the center
Nikhil Pawar
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The Astronomy Journal 
Posted on August 25, 2008
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