Probing The Universe

Radio Telescopes

This remark of Haldane, a famous scientist, reflects, in a way, what most of us feel about this subject. The universe is a rather difficult subject to study. We cannot bring it to the laboratory to carry out experiments on it. We cannot compare,it with any other universe, this is the only universe we have. And finally, we are a part of it. We can study it only from within. We cannot go out of it and look at it from the outside.

So, how do we study it? It is here that the scientific method comes to our aid. You must understand that the study of the universe is a rather special example of the method of science, as we cannot experiment on it. However, the observations that we make about it provide us with an enormous amount of information that we can analyse and interpret in terms of the known laws of nature. Based on these laws, various theories and models of the universe are given by scientists.

Largest Radio Telescope

Observations are the pillars on which models and theories are based. You may ask: What observations can be made about the universe? And how are they made? We will now answer these questions. We will not discuss the underlying principles of The methods and instruments in detail. Our aim is to give you an idea of the vast variety of tools and methods available tor making observations about the universe.

Most of what we know about the universe has been learnt from a study of light, heat and other radiations like the radiowaves, X-rays, gamma rays etc. coming from the Sun and the stars. These radiations are detected by special instruments \ct up at astronomical observatories on the Earth and in orbit around the Earth. In the last few decades, we have been able to send to the neightbouring world. Many men have also visited the Moon and brought a lot of lunar material for study. Thus, there are a variety of ways for making observations and collecting information about the universe. However, before describing these methods, we will explain to you soom features of the radiations from space that bring the secrets of the universe-right to our doorstep.

 Visible and Invisible Radiation

 Light is very much a part of our existence. Without it we cannot see; It lends colour to the world around us. Light is also termed as visible radiation. There are other kinds of radiations in nature, that we cannot see. These are termed invisible radiations. Some examples of invisible radiations are the infrared and ultraviolet radiations, radiowaves, X-rays and gamma rays. We many come across all these radiations in our lives. For example, infrared (IR) radiation is given out by warm objects, such as our bodies, room heaters, buildings and the Earth after a warm day. Rattlesnakes detect infrared radiation very well. Ultraviolet (UV) radiation can kill germs. It is invisible'to us but can be detected by bumblebees. Radio waves are.emitted by TV and radio broadcasting stations and are received by our TV or radio sets through the antennas. Thus, they are useful in communication. They can also be detected by bats. X-rays are used in medicine, gamma rays are used in cancer treatment and are also emitted in nuclear explosions.

All these radiations-the gamma ray, the X-rays, ultraviolet rays, light, infrared rays, and radiowaves-are useful in astronomy. Actually they are different forms of the same kind of radiation called the electromagnetic radiation. Electromagnetic radiation is a form of energy. There are other forms of energy with which you must be familiar, like heat, sound or the energy stored in the spring of a watch. We usually think of electromagnetic radiation as being made up of waves that travel with the speed of light in vacuum. Now, the simplest examples of waves that you may know are waves of water in a pond or sea, waves on a string. You may have seen waves on a curtain fluttering in the air. Some people have wavy hair. We will not go here into the details of what waves are, or the special nature of electromagnetic waves.
But clearly, from their description given above, the various kinds of electromagnetic radiation do not seem to be alike. What is the difference between each of them?

 The difference lies in their wavelengths and hence in the energy they carry.

Telescope in North Pole

This is the usual way of showing a wave. The distance between two successive crests (hills) or two successive troughs (valleys) is defined as its wavelength. It is measured in  metres. The frequency of a wave is defined as the number of cycles it travels in a second. It is then measured in terms of cycles per second (cps) or Hertz. The product of the wavelength A and the frequency f of an electromagnetic wave is equal to its speed c:

Thus, if we know any two of these parameters, we can determine the third. The energy E carried by a wave of frequency f is given as:

                                                                  E=hf=hc/ A

where h is a constant number, known as Planck's constant. Thus, the higher the frequency of a wave or the lower its wavelength, the more energy it can carry across space. UV rays, X-rays and gamma rays carry huge amounts of energy. Therefore, constant exposure to them can prove very harmful. Luckily most of these harmful radiations are cut off by the Earth's atmosphere. All kinds of electromagnetic radiations arranged according to their wavelengths, constitute the electromagnetic spectrum.

Light is the radiation to which human eyes arc sensitive, i.e., our eyes can.detect visible radiation. However, it fohns only a tiny part of the electromagnetic.spectrum. The colours in white light or light from the Sun can be seen when it is sent through a prism which splits it into the familiar spectrum of rainbow colours-violet, indigo, blue, green, yellow, orange and red. By definition, each colour of visible light has a specific wavelength, the violet light having the shortest and red light the longest.

Cosmic objects emit radiations of all wavelengths. For instance, visible light forms only 40% of the Sun's radiation. The rest is made up of the other kinds of electromagnetic radiation. However, as these radiations fall on the Earth, all except light and radiowaves get absorbed in its atmosphere. Only the visible light waves and radiowaves penetrate the atmosphere to reach the Earth's surface. These are, therefore, referred to as the two windows to the universe. We can view the-wavelengths lying outside these windows only if we move out of the Earth's atmosphere.

In Pursuit of Starlight

The easiest method of studying light from a cosmic object is to collect it through a telescope and record it on a photographic plate. Photographic films are exposed for long periods of time-sometimes night after night-to the light being collected by a telescope aimed at distant stars. Since the Earth rotates on its axis, the stars appear to move in the sky. The telescope is rotated following the daily movement of the stars at which it is aimed. Thus, its movement is synchronised with the movement of the stars being studied, stars, far too faint for human eyes, slowly begin to register on the plate. This method of collecting and investigating light from the cosmos is called optical astronomy.

Star light

Over the centuries, astronomers have refined the telescope from the first crude lenses of Galilee's day to giant 'telescopes in use today. Three simple pieces of glass, the lens, the mirror and the prism over the period of a few hundred years. have turned into sophisticated and powerful tools in human hands. Shouldn't we marvel at the ingenuity of the human mind'?

As of today, a huge optical telescope called the Hubble space telescope. after Edwin Hubble, is in orbit around the earth. Several large telescopes are stationed in the USA, Hawaii, Australia, Chile, Russia, U.K. etc. In India the major optical observatories are at Nainital, Gurusikhar (Near Mount Abu), Udaipur, Japal Rangapur (near Hyderabad), Kavalur and Kodai Kanal. Many smaller telescopes scan the skies every night, adding to our knowledge of the cosmos.

There are many other ways of learning about the heavens than by just studying the light coming from them. One of them is radio astronomy. Nowadays, scientists use very sensitive radio telescopes to tune in on the cosmic objects and study them. Let us see what this method is.

Tuning in on the Stars

The fact that stars emit radiowaves was discovered accidentally in 1932 by a young engineer Karl Jansky. He was trying to find the source of noise in a transatlantic telephonic link. He made an experimental radio receiver set to study this problem. To his surprise, he found that the disturbance was due to radiowaves coming from the Milky Way Galaxy. This was the beginning of radio astronomy, i.e. the study of cosmic objects through radiowaves emitted by them. The radio telescope, a basic tool of radio astronomy, collects radiations from space in the radiowave region. One of the largest radio telescopes in the world was designed and set up by Indian astronomers at Ootacamund. The other radio telescopes in India are stationed at Gulmarg, Ahmedabad, Gauribidanur near Bangalore.

Compare of Trapist System and Solar System

Radio telescopes may be tuned to receive radiowaves of the desired wavelength in the same way as we tune a radio to receive only the station we want. Radio telescopes not only give a 'view' of the invisible universe, but can also probe much deeper into space when compared with optical telescopes. Radiowaves can propagate through dust clouds in space, just as radio signals on the Earth can penetrate cloudy or foggy weather. Thus, they enable'radio astronomers to construct images of regions completely hidden from the view of optical telescopes. However, radio telescopes normally receive radiation within a narrow band of wavelengths.

Radio telescopes have led to the discovery of hundreds of cosmic objects that emit radiowaves. Most of these could be identified with the objects seen by optical telescopes. With the help of radio telescopes objects like pulsars were discovered. Pulsars are stars that send out pulses of light and radiowaves in regular bursts. For example, a pulsar in the centre of the Crab nebula at a distance of 6000 light years from the Earth sends out bursts of light and radiowaves 30 times a second.

Quasar

Certain radio sources like 3c273, detected by radio telecopes and later examined by optical telescopes, were named quasars . Quasar, an abbreviation of 'quasi-stellar radio source', is a star-like object situated billions of light years away. Not all quasars are radio sources. Since the electromagnetic waves from quasars are being detected on the Earth, they must be sending out huge amounts of energy. Quasars are comparatively small in size, only about a light month across. That is, if you imagined the Milky Way Galaxy to be a football field, a quasar would appear like a grain of sand. But it emits 100 times more energy than the entire Milky Way Galaxy.

Scientists have also found that many elliptical galaxies that seemed unimportant when seen throug6 optical telescopes, were powerful sources of radiowaves. These galaxies were named radio galaxies. Often, the centre of a galaxy is a powerful source of radiowaves. Violent movements of huge quantities of matter and gas take place in the central part of the galaxies, emitting radiowaves in the process. Radio telescopes also showed that organic molecules exist in interstellar space.

Messengers from the Sky

Light and radiowaves are not the only messengers from tht: sky to our planet Earth. There are others; like the meteorites entering the Earth's atmosphcrc from time to time. They bring us many messages about the cosmic object  from which they were chipped off. Earth is also constantly bombarded by cosmic ray which, as you've read earlier. are beams of electrons. proton and helium nuclei that cruise through space, at very high speeds, approaching the speed of light. Their origin and their travel through space is a puzzle that scientists have not yet been able to solve completely. Onv- it is solved, we will get to know a lot more about interstellar gas clouds, the stars and the galaxies.

Sky

Ventures in Space

Sometimes the atmospheric conditions distort the light or radiowaves coming in from space. For instance, there may be a storm disturbing the radiowaves. Or clouds may obscure light. Then it is not possible to study the universe in these regions of the electromagnetic spectrum. Even otherwise, modern science and technology have given astronomers several new ways and means of probing the universe. We will briefly describe each one of them.

Observatories in Space

With the coming of the Space Age, observatories equipped with telescopes and cameras could be placed right in space, beyond the Earth's atmosphere. An observatory in space may be in the form of an orbiting satellite like the Unmanned Orbiting Solar Observatories, Orbiting Astronomical Observatory, Skylab, Einstein Observatory, IRAS (Infra Red-Astronomy Satellite) and many others. An observatory may also be stationed on the Moon or any other planet having suitable temperature and other conditions. Instruments are also put aboard high flying balloons, rockets and aircrafts to record observations. These observatories can record radiation from a cosmic object in the regions of the spectrum such as the IR, UV, gamma rays and X-rays that do not penetrate the Earth's atmosphere.

Visiting the Neighbouring Worlds

As space research came of age, it became possible for us to send spacecraft to other planets and even land men and instruments on the Moon. These ventures also provided a rich stock of information about the Solar System. For instance, astronauts of the Apollo mission to the Moon in the nineteen seventies brought back lunar rocks and soil samples, photographs of the lunar surface' and left several instruments there for further study

We have been able to send spacecraft, also called probes, across the Solar System to know more about our planetary neighbours. Space probes have visited a number of planets and a host of their moons, and successfully landed and operated on the surfaces of Mars and Venus.

The American spacecraft, Pioneer-10, crossed the orbit of Neptune in 1983, and, thus, became the first man-made object to leave the Solar System. With the hklp of observations from the Earth and the data sent by these probes, scientists have been able to arrive at a better theoretical understanding of the origin and evolution of the Solar System.

To sum up, in this section we have given you a bird's eye view of the wide variety of tools and methods that astronomers use to make observations about the universe, We gave this brief description so that you may appreciate the importance of observations in astronomy. The'universe is far more complex than we can imagine. Whatever hypotheses or theories we come out with, must be validated by observations. This is the reason why astronomers devise newer and better.techniques of observation, to know more about the universe, to test their hypotheses and theories.

Other
. Information about the universe can be gathered in many ways through optical astronomy, radio astronomy, space observatories, space probes etc.
. The information is analysed and interpreted to understand various phenomena occurring in the universe and construct theories or models to explain them. One such theory is about stellar evolution
. By observing the red shift in the spectral Lines of starlight from millions of galaxies, it has been established t t the universe is expanding
. The most important theory of the origin of universe is the Big Bang theory; According to this theory the universe was created in a gigantic explosion which occurred everywhere in space at the same time
. Another theory about the origin of the universe known as the steady state theory holds that the universe was always the same as it is now and will remain the same. There was no early universe.
. The evidences such as the expansion of the universe, cosmic background radiation, primordial abundance of elements support the Big Bang theory.