Understanding the Universe

Universe

So far you have studied how information about the universe is collected.'lt is stored mainly in the form of photographs of the cosmic objects, and spectra of their light. The other radiations coming in from space are recorded in various ways. This information is analysed and interpreted to construct theories about the universe and the objects that constitute it.

The hypotheses and theories about the universe and its constituents are always open to change in the light of new discoveries. Quite often a given theory may turn out to be wrong. Observed data may also be misinterpreted. In fact, a given theory may never agree hundred percent with the observations about the universe. A good scientific theory about the universe is one that is extremely close to all relevant observations. At the other extreme are tentative and speculative theories. We will now present some theories and concepts which represent the best possible understanding of stars, galaxies and the universe as a whole. Let us see what can be known about stars on the basis of the information obtained.

Let Us Know about Stars

The point-like stars have always presented astronomy with many questions such as: Where are the stars? How bright are they really? What is their temperature, size, age, etc.? What are they made up of? The developments in astronomy have provided astronomers with an ability to interpret starlight correctly and answer such questions.

Stars

Where are the Stars?

Astronomers use various methods to measure the distances to stars. For determining the distances to nearby stars, the method of stellar parallax is used . For stars farther away, more sophisticated methods are used. We will not go into their details. The distance to astronomical objects situated very far away is found by measuring the 'red shift' of their spectral lines. As far away objects, such as galaxies and quasars, move away from us, the lines in their optical spectra are shifted towards the red end. This shift can be measured and their distances calculated by using appropriate formulae.

Fingerprinting the Stars

Maximum information about starlight can be derived from its spectrum. When a lens-sized prism is put over the front (or objective) end of a telescope, each star can be seen as a colourful spectrum. We can place a photographic film at the focal plane of the lens-sized prism. Then it becomes possible to register the spectrum of starlight. ! Ironically, the astronomer sees the spectra, not as brilliant rainbows, but as black and white patterns . Each star has its own characteristic spectrum-a fingerprint of its individual personality. From its spectrum, we can learn what elements a star is made up of, what its temperature is, how bright it is, how fast it is moving etc.

Stellar Motion

Stars are not fixed in the heavens. They are moving within the galaxies. The speed of a star moving toward or away from the Earth is indicated by a shift of its spectral lines. If a star is approaching the Earth, its lines shift towards the blue end of the spectrum. If it is moving away from the Earth, its lines shift towards the red end of the spectrum. The greater the star's speed, the more its lines shift.

You have read that there are many kinds of stars-blue, yellow or red, normal or giant, pulsating or releasing excessive energy. Most stars move together in groups. Only one out of four stars may travel alone. Of the rest, almost a third are double stars and the rest are groups of many stars. In a double star system, known as a binary, two stars , orbit one another. In a triple system; there are three stars-all three may move around each other, or two of them may move around the third. Then there are loose clusters, with a few dozen stars, to the large globular clusters containing hundreds of thousands of stars, all moving in many possible ways.

Galaxies

Having studied the stars in tams of their distances, colour, brightness, motion etc., astronomers and astrophysicists have turned more and more to questions like: How did the stars come to be as they are? The search for answers has revealed a grand picture , of stellar evolution. This picture tells us the story of how a star is born, how it evolves . and how it dies. We can now explain the diversity of stars as also the unusual species of stars, simply as different stages in the lives of normal stars.

But. in this brief observation span of a few decades, how has it been possible to construct the story of stellar evolution which takes place at a time scale of millions and billions of years? How this is done can be seen by a simple analogy. Suppose a visitor from outer space arrives on Earth for one hour and wants to know about us within that time. He lands his spaceship in a busy place and hurriedly videotapes the people there. After departing he looks at the tape. He observes that the majority of the people are of the same size, but some are quite small, some are so tiny that they need to be came. Still others, although of normal size, walk bent over with the help of canes. Being intelligent, our visitor soon realises that he is observing an ageing process: people are born very tiny, they grow up and spend most of their lives as active adults, and eventually they become old. Since the old people are few in number, he concludes that in the end they die.

Astronomers are in the same position with regard to the stars as our visitor from outside is with regard to people on the Earth. In a relatively short span of time, astronomers have observed more than a million stars. They have taken detailed spectra of their light, measured the brightness and surface temperatures. By carefully analysing this information they have deduced the story of stellar evolution. We will now relate this story.

The Life Story of a Star

A young star is thought to be composed largely of hydrogen gas. Hence, the most likely place for a star to be born is in one of the numerous clouds of hydrogen gas that exist in the interstellar space. Stars are now believed to form inside large dense interstellar clouds of gas. It may happen that for some reasons, not fully known so far, a gas 'cloud starts contracting. Under the influence of gravitational pull of the gas, its contraction may continue further. Once such a process begins, a very large volume of gas clouds is affected. As gravity pulls in the clouds, the pressure in the cloud increases. Also, as the cloud contracts, the temperature at its. center increases. At this stage, it is called a protostar.

Origin of Star

When the temperature becomes sufficiently high (about 4 million degrees centigrade), a nuclear reaction starts in the protostar, in which the hydrogen nuclei fuse together to make helium nuclei  . In this process a large amount of energy is released. The energy travels to the surface of the star and is radiated in the form of light, heat and other electromagnetic radiation. This energy creates an outward pressure and force. The contraction of the star stops only when the inward pull of gravity is balanced by the outward force of this radiant energy. At such a time the star becomes stable in size and temperature.

The Sun has been in such a stable situation for the past 5 billion years. Nuclear reactions in the Sun convert about four hundred million tons (4X10t4 gms) of hydrogen into helium every second. It is expected that the Sun will remain in that state for another 5 billion years.

As the star consumes a significant percentage of the hydrogen fuel in its core, the  nuclear reaction decreases and the outforce of the radiant enem weakens. The core of the star further contracts because its gravitational pull becomes more than the out-force of radiant energy.

But this raises the temperature of the core. Meanwhile, the hydrogen nuclei 'burn' in the outer layer or shell surrounding the core. The extra heat from the core as well as the heat generated in the outer layers cause the star's outer region to 'boil' and expand.

Compare of Stars

The star becomes big and its brightness increases. But, as the outer layer expands farther away from the nuclear furnace, its temperature falls. The puffed-up star Looks red and cool. If it is many times more massive than the Sun, it becomes a red super giant like the Betelgeuse. If it is sun-sized or only slightly more massive than the Sun, it becomes a slightly swollen red giant.

The red giant stage of a star is a relatively short stage. In this stage, the star consumes its hydrogen at a very fast rate, piling'up helium in its core. As the fuel burns, the core contracts further, producing temperatures as high as 100 million". At this point the helium nuclei in the core fuse together in another nuclear reaction to form carbon nuclei. This is a critically unstable moment in a star's life with two layers of the star burning at the same time-an outer layer where hydrogen is being turned into helium and inner core where helium is being turned into carbon. Hereafter the fate of the star depends on the mass of its core. We will merely describe the process without going into the reasons.

Red Giant Star

If the mass of the core is less than 1.4 solar mass, where M, is the Sun's mass, the contraction of the core halts when it is about the size of the Earth. This limit of 1.4M, is known as the Chandrasekhar limit, after S. Chandrasekhar, the famous Indian scientist who won the Nobel Prize for this work. He had settled in the U.S.A. Such a star is known as a white dwarf. From the Earth it would be seen as very small and very faint but it is hotter than the Sun. One of the first stars recognised as a white dwarf was Sirius B, the faint companion of the bright star Sirius. Sometimes a white dwarf suddenly flashes millions of times more brightly. Such a phenomenon is known as 'nova'. As it cools, a white dwarf may turn into a black dwarf, disappearing from our vision.

If the core mass of the star is in the range 1.4M-3M, or the star mass is between 8M, to 15 M,, the core shrinks to a radius of about 10 km and a neutron star is formed. If a neutron star is born rotating very fast, it emits electromagnetic radiation, which astronomers detect as pulses of radiowaves. Such stars are called pulsars. Pulsars were discovered in 1967 and about 400 are now known.

 Pulsar

In stars with higher mass, the helium in the core turns into carbon. However, it continues burping in the outer layer. The core goes on contracting and becoming hotter. It sets the carbon nuclei burning to form oxygen nuclei. As each nuclear fuel is exhausted in the core, it contracts, increasing the temperature and sets the fuel in the core burning. Meanwhile, the earlier fuel keeps burning in the shell surrounding the core. Thus, the star contains several nuclear burning shells. This process may go on in massive stars all the way up to a core of iron.

If a star starts with a mass of more than 20 M,, its contraction continues. Then the core of the star collapses to become a black hole.
Its gravity is now so strong that nothing, not even light, can leave it. Obviously, we cannot see a black hole. Sometimes, massive stars (with the core mass between 3 M,and 15 M,) explode, releasing a tremendous amount of energy. Such explosions are called 'supernova'. The brightest of the supernova hurl out almost as much light as the entire galaxy-its brightness becoming equal to hundred million Suns. A supernova was seen in the Milky Way Galaxy as recently as in 1987 A.D.


The evolutionary stages of any real star involve many factors which are not completely known. Nature may not adhere strictly to the sequence described above. The description of the final stages in the life of a star is purely theoretical and, therefore, open to change. For example, in a supernova explosion it is very uncertain whether what remains will be a neutron star, a black hole or nothing at all. Every new set of observations makes the life cycle of a star clearer to astronomers.

Dwarf Stars

Stars may be regarded as laboratories that allow scientists to study many natural phenomena like the synthesis of heavy elements or the triggering of interstellar cloud collapse. Such events resulted in the formation of the Solar System and life on Earth.

Stars, as you know, are themselves part of galaxies. Do galaxies evolve the same way as the stars? Scientists think that the answer is yes, but they have yet to work out a very convincing theory of evolution of galaxies. However, the study of galaxies has provided one very interesting piece of information about the universe-that it is expanding.

 The Expanding Universe

Hubble's observations had proved the existence of galaxies. After mapping as many galaxies as could be seen by the telescopes then'available, he turned his attention to the motion of galaxies. He was motivated to do this by a puzzling report of V.M. Slipher, an American astronomer. He had discovered in 1912 that many of the faint nebulae were moving away from the Earth at very great speeds. Their spectral lines exhibited large shifts towards the red end (what is called as red shift). This seemed peculiar because stars in the Milky Way Galaxy move at much smaller speeds, some moving away from us with others moving towards us. Slipher had made these observations a decade before galaxies were discovered. Then it was thought that the nebulae were objects in our own galaxy. He did not know what to make of his observations.

Observable Universe

But as Hubble knew that these nebulae were galaxies, he began a systematic study of the relation between their speeds and their distances along with his colleague M.L. Humason. What they found was very interesting. To put it simply, his observations showed that;
i) all galaxies were moving away from us;
ii) the farther away a galaxy was from our Galaxy, the greater was the speed at which it moved away.

Hubble's discovery put forth the picture of an expanding universe. But if all the galaxies are moving away from us, are we at the centre of the Universe? No. If we were situated in another galaxy, even then the other galaxies would seem to move away from us. You can understand this picture of an expanding universe if you study Fig 10.7 and also perform a simple activity.

Try This

Take a balloon and mark a few points on it with a pen. Inflate the balloon. What can you say about the movement of points with respect to each other? Did you observe that each point on the balloon moves farther away from the other as you inflate it more and more? We can picture the movement of galaxies in a similar ' fashion. However, this is a rather simplistic experiment because here you are viewing the balloon from outside, whereas, when we observe the universe, we are within one galaxy. This was just to give you an idea of how the universe is expanding.
Now, if the universe is expanding, then it was much more compact million of years ago Exploring the Universe and the galaxies were much nearer to each other. Does it set you wondering what the universe was like in the beginning? What caused it to expand like this? We will now describe what theories cosmologists have given about the origin and evolution of the universe.

Closing in on Creation

The most important current theory for the origin of the universe is the Big Bang theory.According to this theory, the universe started with'a huge explosion. It was not an explosion like the ones with which we are familiar, which start from a definite centre and spread out. It was an explosion which occurred everywhere in space at the same time. It filled all space from the beginning, with every particle of matter rushing apart from every other particle. This was not a burst of matter into space but rather an explosion of space itself. Every particle of matter rushed away from every other particle. It is so far impossible to 'picture' the first moment of 'creation' of the universe.

Origin of Universe

One-hundredth of a second after the creation of the universe is the earliest time about which scientists can speak with any confidence. At this instant, the temperature of the universe was about a hundred billion degrees centigrade. This-is much hotter than in the centre of even the hottest star. At such temperatures none of the components of ordinary matter, atoms, molecules, or even nuclei of atoms, could have held together. Instead, the matter rushing apart in the explosion consisted of various types of elementary particles. The particles most abundant in the early universe were the electrons, positrons and neutrinos. There were also some protons and neutrons. The rest of the universe was filled with energy. It was a kind of a cosmic soup.
As the explosion continued, matter and energy rushed apart, the universe expanded and the temperatures dropped, reaching 30 billion (3~ 10"') degrees centigrade after about one-tenth of a second; 10 billion degrees after about one second; and 3 billion degrees after about fourteen seconds. At the end of the first three minutes, the universe became cool enough (about 1 billion "C) for the protons and neutrons to begin to form into simple nuclei. The first to be formed was the nucleus of heavy hydrogen which was made up of one proton and one neutron. There were also helium nuclei made of two protons and two neutrons. It was still too hot for atoms to hold together, they were ripped apart as soon as they were created. This matter continued to rush apart, becoming steadily cooler and less dense.

Many thousands of years later, it became cool enough for electrons to join with nuclei L to form atoms of hydrogen and helium. Soon, the resulting gas began to form clumps under the influence of gravitation. These clumps ultimately condensed to form the galaxies and stars of the present-day universe, almost 5 billion years after the Big Bang.

There is another theory about the origin of the universe known as the steady state theory. This theory holds that the universe has always been just about the same as it is now. As it expands, new matter is created continuously to fill up the gaps between the galaxies. Thus, the problem of the origin and early moments of the universe is banished: there was no early universe.

Expanding Universe

However, the Big Bang theory is the most favoured by the astronomers and astrophysicists. Why is it so? This is due to the evidence based on observations which lend support to the 'Big Bang' universe.

Evidence Favouring the Big Bang

One piece of evidence comes from the expansion of the universe which we have i already described. The expanding universe suggests that the matter was packed much more densely in the early stages of the universe. The proof for this also comes from the distant objects quasars. When we 'look' at quasars situated 6 to 8 billion light years away, wc are looking at them as they existed then. If the universe were more dense in b i that epoch, we should be able to sce some evidence of that density in the quasars. We do see such high density among thc quasars.

Big Bang

Another substantial hit of evidence for the Big Bang theory comes from thc cosmic background radiation. For many years the astronomers believed that if there was a . ' cosmic explosion long ago, radiation from that event should still exist within the universe. This radiation may be weak, it may have lost its energy due to the expansion and cooling of the universe, but it should exist. Radio-astronomers have, indeed, discovered faint signals-a constantly present background radio noise that pervades all space. Calculations done by astrophysicists show that this radiation, called the cosmic microwave background radiation, is a relic of the ancient past when the universe was in its first throes of creation in the Big Bang.

An additional discovery made by astronomers in the past two decades is that of the primordial abundance of elements, i.e. the elements hydrogen and helium first created in the aftermath of creation are found to be most abundant in the universe. By examining the light coming from the various parts of the universe, astronomers have found out that, out of every 100 atoms, almost 93 are hydrogen atoms and seven are helium atoms. Elements heavier than helium are present in traces only. This suggests that the universe started out with a Big Bang from a very hot and dense state and quickly cooled as it expanded. The hot and dense conditions lasted long enough for some hydrogen to fuse into helium. But they did not last long to allow other heavier elements to form in significant amounts. These were made much later in the interior of massive stars.

Quasar

In conclusion, we find ourselves amidst an expanding universe .which throws up puzzles with amazing regularity. With each question answered. many more questions arise. For example, an important question today is about the future of the universe. Will it go on expanding like this? Or, will its expansion stop some day? What will happen then? Will the universe remain as it were then or will it start contracting? We also find ourselves amidst an explosion of ideas and techniques being applied to study the universe. In this unit we have tried to give you a brief glimpse of the enigmatic nature of the universe and our undaunted efforts to understand it. Don't you feel that this brief insight was worthwhile? There is a quote of Edwin Hubble from his last paper:

'From our home on Earth we look out into distances and strive to imagine the sort of world into which we are born. Today we have reached far out into space. Our immediate neighbourhood we know rather intimately. But with increasing distance our knowledge fades .......... until at the last dim horizon we search among ghostly errors of observations for landmarks that are scarcely more substantial. The search will continue The urge is older than history. It is not satisfied and it will not be suppressed.