When there were no city lights and skyscrapers depriving the eyes from the endless sky and countless stars that spread over this huge emptiness filling it down to the horizon, astonished man started to ask himself: Where the cosmos came from? Where it will go? Who created it? Or was it always there? Great thinkers and philosophers of the past have done a lot of speculation on these questions.
With the enormous development of science and technology in the recent years it has now become possible to think about the questions quoted above and about similar other questions in scientific terms. Starting with the ridiculous idea that earth, resting on the back of a tortoise (and there are turtles all the way down!), is at the central position of the universe and the entire cosmos including the sun and the other planets revolve around it in circular orbits we have come a long way.
We now know that all planets actually revolve around sun in the elliptical orbits. Our sun is an ordinary star on the inner edge of one spiral arm of our galaxy, the Milky Way. Our sun is not stationary but actually revolves around the galactic center with the speed 250 Km per second. There are ~1011 stars in our galaxy and there are ~1011 galaxies like our own!
There are two main theories of the universe, Steady State theory and Big Bang theory. We will see in brief the second theory, which has received wider acceptance in the present times.
The Big Bang theory :
The idea for a Big Bang can be traced back to an idea developed between 1927 and 1933 by a Belgian priest, Georges-Henri Lemaitre. Like most people, Einstein had assumed that the Universe was static, but Lemaitre did much to change Einstein’s view, by putting forward the idea that the Universe started from a primordial atom, as he put it.
This encouraged Arthur Eddington, Lemaitre’s old teacher, to express some concern: “It has seemed to me that the most satisfactory theory would be one which made the beginning not too aesthetically abrupt”. (The Big Bang is pretty abrupt, as beginnings of universes go!)
In fact from the 1930s through to the early 1960s a fierce debate raged between supporters of the Big Bang theory and the competing theory, the so-called “Steady State” theory. Fred Hoyle, a strong supporter of the steady state theory, who used it as a term of derision, coined the term “Big Bang”.
In 1948 three physicists, Ralph Alpher, George Gamow and Robert Herman calculated that if the Big Bang were true, then the early Universe must have been hot, but not too hot: they predicted a narrow temperature range of possibilities. As the Universe expanded this heat – radiation – would have become diluted, but it could still be detectable now: the so-called cosmic background radiation.
Yet in the 1950s and early 1960s the steady state theory was probably more popular: it seemed to make more sense and was easier to work with mathematically. The idea of the Big Bang seemed to make no sense.
In 1950 the scientist Martin Ryle, working at Cambridge (and ironically a colleague of Fred Hoyle) discovered, using new radio astronomy techniques, that the very, very distant galaxies, detectable only by radio waves (and thus called radio galaxies) were actually packed more closely together than the galaxies closer to us. Remember that when you look out a very, very long way you are looking back in time. What did this show? It showed that not only was the Universe expanding – which everybody knew – but it was also getting less dense as time passed. This agreed with the Big Bang theory, but not with the steady state theory.
And then in 1964 two American physicists found the “cosmic background radiation”. They detected a signal, with temperature equivalent of 3.5 degrees Kelvin, coming from all directions of the sky. It did not vary with the time of day, or season. The scientists thought at one point that the noise might be caused by pigeons nesting in the radio telescope, but no: it was the leftover energy from the Big Bang explosion.
This was the final straw for the steady state theory.
But if the Big Bang happened and everything was scattered, why is the Universe so lumpy, especially when the cosmic background radiation is so smooth? Shouldn’t the bits be scattered more evenly? Or rather: if the bits are lumpy, the background radiation should show some lumps too – just a few wrinkles, or fluctuations in temperature, caused by areas of higher density.
The COBE (Cosmic Background Explorer) satellite found these wrinkles in time in 1992. They are the wrinkles present in the fabric of time 300,000 years after the Big Bang. Stephen Hawking commented that this discovery was “the most important of the century, if not of all time”. It proved that the Big Bang was correct.
To make any new idea in physics acceptable one now knows that one should have a consistent Quantum theory for it. Newton’s as well as Einstein’s theories about gravity are only provisional and cannot be final because they are classical theories. Let us see in brief what has happened with the theories of universe in this direction.
Some scientists, including Stephen Hawking, are working in a new area called quantum cosmology, trying to unite quantum mechanics with our current understandings of cosmology. The idea is that somehow our Universe might be the result of quantum mechanical fluctuation on a huge scale: only one possible Universe out of an infinite number of possible Universes.
In quantum mechanics particle has wave function, or state vector associated with it. Particles reside in infinite dimensional Hilbert space. Motion is nothing but generalized rotation of the state vector, describing everything that is possible to tell about the particle (complete information), in the infinite dimensional Hilbert space. In Hawking’s theories we have a wave function describing the set of all possible universes (wave function associated with the universe). So our starting point may be an infinite set of parallel universes, of which our Universe is just one when the wave function collapsed (when we, or something, made the observation).
So universe isn’t all that exists, it is all that can exist!
Matter, Energy, Space, Time are the main ingredients in the universe. There are a variety of objects undergoing different kind of motions under certain rules arrived at from different dynamical theories.
Classical Physics :
The dynamical theory that ruled the scenario for about three hundred years is mainly due to Newton. His theory was based on two main postulates; the absoluteness of space and time and the invariance of the laws of mechanics under the Galilean transformations. Newton’s theory was modified and made more realistic by new dynamical theories (Special and General theories of Relativity) by Einstein. (A far more violent restructuring took place after the advent of Quantum Mechanics developed in the collective efforts of the group of scientists at the beginning of this century.)
There are two theories of relativity; the special theory of 1905 and the general theory of 1915. These are the best possible classical theories of dynamics and gravity of today.
The Special Theory of Relativity :
The special theory proposes that the Lorentz transformations (and not the Galilean transformations) relate the inertial frames in a correct way. This theory is based on two simple postulates; the constancy of velocity of light for all inertial observers and the invariance of the laws of physics under the Lorentz transformations. When this idea was applied to bodies moving with uniform motion (that is, not speeding up or slowing down), it produced some shocking outcomes, contrary to the common sense, such as energy and mass are equivalent and so inter-convertible; so that the energy an object has, as a result of its motion, will add to its mass. In other words, the faster something travels, the heavier it gets. Of course at normal speeds this is change is not noticeable, but as you approach the speed of light you get noticeably heavier. At 10% of the speed of light, your mass is half a percent more than usual; at 90% of the speed of light, your mass is double. At the speed of light itself your mass would become infinite. Since the additional mass comes from energy, you cannot ever travel faster than the speed of light (because you would need infinite energy). Only light, or other waves with zero rest mass, can move at the speed of light.
This relationship is summarized in the famous formula
“c squared” is a very large number indeed (it is the speed of light squared), so a small amount of matter can release a huge amount of energy (e.g. energy released in an atomic explosion).
The General Theory of Relativity :
General Theory of Relativity is essentially the theory of gravity that results after the Newton’s theory of mechanics is replaced by more correct Special Theory of Relativity.
The theory is based on the principle of equivalence, which enables one to transform away the gravitational force, and the principle of general covariance, which assumes the locally Euclidean nature of the space-time and thus enables one to apply the special theory of relativity at least locally.
The theory applies to all bodies in the Universe, and explains gravity in a new way. The reason why the planets orbit around the Sun, the theory states, is because they are moving in space that has been curved by the mass of the Sun.
In fact the orbits of the planets in our Solar System, as predicted by Einstein, are almost identical to those predicted by Newton, except for Mercury (perihelion shift), which Einstein predicted more accurately than Newton (this was known before 1915 and therefore was an early confirmation of Einstein’s general theory).
Light (the thing that travels obeying the so called Least Action Principle) too bends while passing through curved three-dimensional space! Einstein actually predicted that the gravitational field near the Sun, causing the curvature of the space near the Sun, would cause a glancing ray of light to bend inwards. Two expeditions sent by the Royal Society, to Brazil and to the west coast of Africa, tested Einstein’s prediction during the eclipse of 29 May 1919. Arthur Eddington was in-charge of the African expedition. He said that the measurement of the first photographs of the eclipse was the greatest moment of his life: “Einstein’s theory is completely confirmed. The predicted displacement was 1″.72 and the observed 1″.75 plus or minus .06.”
By the time Einstein died clocks could measure time to a thousand millionths of a second. Putting a clock at the equator and a clock at the North Pole proved Einstein right: the equator clock lost time, because the equator is moving more quickly than the North Pole. (In a merry-go-round, the one sitting near the center is ageing faster than the one who is sitting near the rim with every turn.)
Einstein struggled for 3 years to find the mathematics to explain his theories. He eventually found the solution in Riemann’s lecture of 1854 (on non-Euclidean geometry) and his results become known as the general theory of relativity, which reveals that gravitational force is only a consequence of geometry! Gravity is nothing but the continuous effort of space to flatten itself!!
Relativity in a sentence :
Einstein once stepped off a boat in New York to be met by the press, who asked him to explain his theory of relativity in one sentence.
“Space tells matter how to move, and matter tells space how to bend.”
An expanding Universe! :
It was the scientist Edwin Hubble who discovered, in the 1920s, that the Universe was expanding. This was a revolution: almost nobody had thought of it before (until then nearly everybody had simply assumed that the Universe was constant in size).
Even Einstein, in formulating his general theory of relativity in 1915, had assumed that the Universe was static. In fact his theory predicted that the Universe was not static, but actually was expanding, so Einstein felt compelled to introduce an element into his calculations – called the cosmological constant – to enable the Universe to be static.
Einstein simply was unable to believe the predictions of his own theories – and had to introduce a fudge to keep the theory in line with what he thought – even though, on this, he was wrong!
Only a Russian, Aleksander Friedmann, had predicted, in about 1918, that the Universe was expanding, based upon Einstein’s own theories. In fact Friedmann predicted the very result that Hubble found.
The model of the Universe that scientists now talk about is often called the “Einstein-Friedmann model”. It is interesting to know that as per the outcomes of the Einstein-Friedmann model we (the universe) will either end in “fire” or end in “ice”. Everybody hopes that we end in fire, which keeps the chance open for the rebirth of the universe in another Big Bang. The universe that ends in ice is the universe weathering away into empty nothingness!
The most significant progress in our understanding of the large-scale universe came in 1929. In this year Edwin Hubble experimentally demonstrated for the first time that “Universe is not static as was previously thought but it is actually expanding!!!”
Edwin Powell Hubble (1889 – 1953) :
Edwin Hubble, son of a Missouri lawyer, took degrees in mathematics and astronomy at the University of Chicago and then went to Oxford to study law. After taking his law degree at Oxford he returned to Kentucky to practice law, until he joined the staff of the Yerkes Observatory in 1914. In the next decades Hubble carried out very important work in discovering more about the large-scale structure of our Universe – and about galaxies.
In 1924 he showed that ours was not the only galaxy, but that there were many others (like this one – distant galaxy NGC4261, 100 million light years away) – and all had huge areas of empty space between them. We now know that in way galaxies are the basic building blocks of our Universe – but big building blocks at that. Hubble also tried to measure the distances between these galaxies.
But what Hubble also discovered (in 1929) was that, wherever you look, most galaxies are moving rapidly away from us. And even stranger – the further away from us these galaxies are, the faster they are moving away. In fact Hubble found a straight line mathematical relationship between distance and speed. The further away a galaxy is from us, the faster the speed with which it is moving away from us. The constant that relates linearly the speed and diatance is now referred to as Hubble’s constant, and the law as Hubble’s law.
Based upon this new information our views of the Universe were once again revolutionized. We don’t live in a static and unchanging Universe after all. Clearly the Universe is expanding, with everything moving away from everything else at high speed. (This isn’t quite true. Some galaxies are moving towards each other. For example when we look at the Andromeda galaxy, we see it blue-shifted: it is moving towards us at high speed and may one day collide with our own galaxy! The collision of two galaxies generally results in their merger into a single bigger galaxy).
So in earlier times everything would have been closer together. Actually we can draw a line back, to 10 or 20,000 billion years ago, when everything would have been in exactly the same place. This suggested that there was a time (called the Big Bang) when the Universe was infinitesimally small and infinitely dense. Then a huge explosion (actually, the biggest ever!) sent all the bits hurtling away in all directions!
Hubble’s law :
The Universe began probably between about 10 and 20 billion years ago in a violent explosion – the so called Big Bang. As a result all matter flew apart and even now galaxies are moving away from us in all directions as a result of this initial explosion.
This was first discovered – by observation – by Edwin Hubble, originator of Hubble’s law.
Hubble’s law is contained in the equation
(where “v” is the velocity of any one galaxy as observed from another, and “d” is the distance between them. “H0” is the value of Hubble’s constant).
The law states that distant galaxies are moving away from us, and that there is a direct relationship between the distance of a galaxy from us and the speed at which it is moving away. In simple terms a galaxy that is further away from us will be moving away at a higher speed than a galaxy that is closer to us.
The actual speed of any given galaxy at any given distance is that distance multiplied by the Hubble constant. The unit of measurement of the Hubble constant is “kilometers per second [of speed] per Mega parsec [of distance]. A Mega parsec is 3.26 million light years.
The problem is that we don’t really know what the value of H0, Hubble’s constant, is. Unlike some laws, or equations, H0 is a number that we can only derive; it seems, by observing the world around us and choosing a number that fits best with the available data. The biggest problem is that it is extremely difficult to measure distances between various galaxies accurately.
The original value for H0 derived by Hubble was about 500 kilometers per second per Mega parsec of distance, but more recent work has led to different values for the Hubble constant. One team has derived a value of about 50, and another team a value of about 100.
Now it is in order to understand about the “Objects” that dominate the Universe, let us see them in brief/
What are Galaxies? :
Well whatever it might seem, the Universe is not simply full of stuff that is spread out evenly all over the place, in no particular order, in fact quite the opposite. All the stuff in the Universe, the stars, the planets, the clouds of gas and dust, all tend to clump together, into “islands” of matter in space. These clumps, or groups of matter, are called galaxies. The gravitational force of the stars and other matter that makes up the galaxies holds them together.
Galaxies are not small. An average galaxy might have hundreds, or even thousands of billions of stars, and might be a hundred thousand light years in diameter. Really very, very big indeed!
But galaxies don’t appear to us to be that big. When we look up at the night sky, without a telescope, it’s possible to see some of these galaxies, but they appear to us almost like single stars. This is because they are so far away. Even our nearest galaxy, Andromeda (also known as M31), is about 2 million light years, or 700 kilo parsecs, away from us.
In fact it is only in this century that we have really come to understand very much about galaxies and the large-scale structure of the Universe. One man responsible for a lot of what we know now is Edwin Hubble, who did a lot of work in this area in the 1920s. Hubble carried out a classification of different types of galaxies, and we still use this classification today.
Galaxies come in a number of types, named after the way they look. So for example there are elliptical galaxies and disc galaxies, which are also known as spiral galaxies. It was Hubble who identified these two main types of galaxy. A lot of galaxies have strange names, such as the Clouds of Magellan, the Draco dwarf, the Fornax dwarf, Andromeda, the Pinwheel, the Whirlpool, Centaurus A, the Sombrero, and the Zwicky Antennae.
Spiral galaxies tend to be younger galaxies, made up of younger stars and with stars still being formed. Elliptical galaxies are older, with no gas present. The Milky Way is a spiral galaxy. There are also irregular galaxies, which have no bulge and no clear shape. The Magellenic Clouds are a good example of an elliptical galaxy.
The core of the distant galaxy, NGC4261, is 100 million light years away. This galaxy is believed to contain a super-massive black hole.
The spiral galaxy M100 is a member of the Virgo cluster (group of galaxies) which is made up of about 2,500 galaxies. Now let us consider our own galaxy, what is it?
What is the Milky Way? :
The most outstanding galaxy – because it is the one we belong to – is called the Milky Way. You can see it on a clear night: it looks like a smear of light – or of stars – across part of the night sky. Actually, to be strictly correct, the Milky Way is not the name for our galaxy; it is our name for that part of our galaxy that we can see from where we are in it. (We could only see the whole of our galaxy if we were outside it, and we are not!)
We know that even ancient civilizations knew about the Milky Way, although they didn’t really know what it was. In fact it was only well into this century that we realized that the Milky Way is a galaxy after all, and that it’s the one that our Solar System belongs to. And we are pretty insignificant in our galaxy!
How insignificant? Well let’s start with our Sun, the biggest thing in our Solar System but only an average star. It’s about 1.4 million kilometers in diameter. How big is the Milky Way? Well, it’s about 30 kilo parsecs, across (so at the speed of light it would take nearly 100,000 years to cross it). That means that the Milky Way is about 925,000 million, million kilometers across, or more than 500 billion times bigger than the Sun!!
So if the Sun is the size of a full stop on our screen then the Milky Way would be about 237,000 kilometers across! (Really brain aching, isn’t it?)
The disk of the Milky Way has four spiral arms and is actually quite young. It’s made up of stars spanning an age range of between a million and ten billion years. (The Milky Way probably contains between about one and two hundred billion stars, but nobody is really sure because we haven’t counted them.) Our Solar System is about 30,000 light years from the center of the disc, slightly north of the central plane of the disc, and our Sun is just one of these couple of hundred billion stars, nothing special really.
Like most galaxies, the Milky Way is rotating. So the Sun, the Earth and all the other hundred billion stars are revolving around the center of the galaxy. In fact the Milky Way is rotating at about 250 kilometers per second and it takes about 250 million years to do one revolution. This is called the galactic year (just as the Earth year is the time it takes for the Earth to rotate once around the Sun). The Sun and the Earth have existed for about 18 galactic years.
Our Solar System:
The Solar System is the name we give to the Sun and the nine planets, the 61 satellites of the planets, the comets, minor planets, asteroids, gas and general debris that is held in orbit around the Sun by the force of gravity.
The planets are, in order, moving out from the Sun (and including the Sun, for comparison):
Sun: radius 697,000 km
Mercury: radius 2,439 km
Distance from the Sun 57,910,000 km
Venus: radius 6,052 km
Distance from the Sun 108,200,000 km
Earth: radius 6,378 km
Distance from the Sun 149,600,000 km
Mars: radius 3,398 km
Distance from the Sun 227,940,000 km
Jupiter: radius 71,492 km
Distance from the Sun 778,000,000 km
Saturn: radius 60,268 km
Distance from the Sun 1,429,000,000 km
Uranus: radius 25,559 km
Distance from the Sun 2,870,990,000 km
Neptune: radius 24,764 km
Distance from the Sun 4,504,300,000 km
Pluto: radius 1,160 km
Distance from the Sun 5,913,520,000 km.
As well as these planets there are lots of other smaller objects orbiting the Sun, including asteroids and comets. Many of the planets in our Solar System also have moons, or satellites, orbiting them. Earth has one satellite (called “the Moon”) but Saturn has around 30 and Jupiter has 16 moons.
So how does all this fit together? Well, all of these objects are held in orbit by the law of gravity; nothing is exempt from that law. The planets orbit the Sun with elliptical, not circular, orbits (although the orbits of Mercury and Pluto are very nearly circular). If we wanted to build a scale model of the Solar System, we might end up with the Sun say 1.5 meters in diameter; the Earth the size of a grape, 150 meters from the Sun. The Moon would orbit the Earth about 60 centimeters away, and even Jupiter, the largest planet, would be only the size of a large grapefruit. The nearest star would be about 9,000 kilometers away.
The Solar System also contains comets and asteroids. Comets are lumps of icy material and dust, which orbit the Sun from a long way out (beyond the orbits of the planets Uranus and Pluto). They become visible as they approach the Sun, when a tail of evaporating material can be seen. Comets are usually quite small (the famous Halley’s comet is only about 15 kilometers by 10 kilometers in size) but their tails can extend for millions of kilometers.
As these comets orbit the Sun they can be seen regularly as they pass us by, although most comets return only once every hundred years or so (but there are some comets which we know take millions of years before they return).
Asteroids, the other “small objects” in our Solar System, are just lumps of rock orbiting the Sun. Most are found in orbit between the planets Mars and Jupiter, and it has been estimated that there are about a million asteroids, which are more than 1 kilometer across. Asteroids are probably just cosmic rubble left over after the formation of the Solar System.
Now we consider more worldly and stranger objects in the universe,
Black holes :
In 1783 a Cambridge don, John Michell realized that if a star was sufficiently massive, then its gravitational force would be so strong that nothing, not even light, could escape from the star. Any light emitted would be dragged back to the surface of the star before it could escape. So anybody outside this star would see nothing when looking at the star. They would be looking a what we call a black hole. But we would still feel the gravitational force of the star, or black hole.
According to Einstein’s theory of gravity, contained in his theory of general relativity (1915), at the end of a star’s life, when it has exhausted its nuclear fuel, the star will continue to grow smaller and smaller. Under the influence of its own gravitational force it will then get denser and heavier. When the star’s fuel is all used up it will undergo what we call “gravitational collapse”: it will get very small indeed, as all the parts of the star flow in towards the center.
We would normally expect this collapse to stop after a while, but Einstein’s equations have the peculiar result that this collapse can continue to go on forever, without the star ever stopping its collapse (until it becomes a point, or what scientists call a singularity).
A star that has collapsed so much that its gravitational pull stops even light escaping has become what we call a black hole. (If the Earth were to collapse to a black hole it would weigh the same but would be about the size of a golf ball.) What happens with a black hole is that the space in the region of the hole is so strongly curved that space and time become interchanged. Your space becomes your time and your time your space.
If you observed the collapsing star from outside you would see its motion slow down and stop because the direction of time inside the hole is perpendicular to the direction of time as seen outside the hole.
This is called the classical theory of the black hole. Built up from Einstein’s equations, it says that black holes are permanent and black; anything that falls in is “swallowed” without trace – including light – so all you see is a black nothing. But do black holes exist? They’re obviously hard to find because they don’t shine, so we can’t detect them just by looking (although that may not always be true: rotating black holes may in fact shine). Well, we are not yet certain that we have found any black holes, but there are some very strong contenders. For example quasars may well be black holes – or may have black holes at their center. We can’t think of anything else that could generate such massive amounts of energy. The galactic center around which the entire galaxy rotates could be a black hole.
There is “almost evidence” of a different kind, too. In 1967 Jocelyn Bell Burnell, working at Cambridge, in England, discovered objects in the sky that were emitting regular pulses of radio waves. We now think that these are rotating neutron stars. (This was the first real proof that neutron stars exist.) Neutron stars are stars that have collapsed, so that their mass (and hence their gravitational pull) has increased by a huge amount. Neutron stars aren’t black holes, but if they were a few times smaller (and denser) they would become black holes. If a star can collapse to become a neutron star, maybe it can collapse a bit more to become a black hole.
We can also “see” a black hole by looking at its gravitational effects on nearby stars. Are there cases where we can see a star being pulled in a particular direction (by gravity), but we can’t see what’s pulling it? If so, it might be a black hole that’s doing the pulling. For example what we know as Cygnus X-1 appears to be a black hole and a normal star together, with the star orbiting around the black hole.
The classical theory of the black hole (that black holes are permanent and black, so all you see is a black nothing) was generally believed to be a complete theory of black holes, until Stephen Hawking and Roy Kerr came along, with ideas about rotating black holes. Rotating black holes aren’t black! They emit radiation!!
The Evolution of Stars :
Why does the Sun, our star, burn but never run out of things to burn? When we light a fire it goes out eventually, as the coal or wood, or whatever it is that is burning, is burned away. How come stars are different? (And yes, it does burn! Just look.)
The answer is that the Sun, like all stars, creates its heat by a different process called thermonuclear fusion. Let’s go back a bit and look at the life of an average star.
A star starts out as a large, fairly cool ball, or cloud, of gas. Because of the effects of gravity this cloud of gas will begin to shrink to a smaller size, becoming more dense in the process. As this cloud of gas shrinks it becomes hotter as a result of the gravitational energy released.
Once the central temperature of this cloud of gas reaches a particular value (about 15 million degrees Kelvin, for our Sun), then what is called nuclear fusion begins to take place. This is the process of merging two hydrogen nuclei in the star into a helium nucleus, a process that generates immense amounts of energy (which is why all stars, including our Sun, have burned, and will continue to burn, for so long).
The process of nuclear fusion also releases heat, which creates an outward pressure (radiation pressure), stopping the star from collapsing any further.
A star will generally start out being made up of about 75% hydrogen and 25% helium, so the process of nuclear fusion involves gradually burning up all the hydrogen by converting it to helium. In fact the bigger a star is, the faster it has to burn up its hydrogen – its fuel – in order to stop itself collapsing. So the life of a star depends solely on its mass, or how much hydrogen it has. The more mass, the less time it “lives”. The Sun has been burning for about 4.5 billion years, and it will probably continue for the same period again. A star twenty times the size of the Sun will last for perhaps “only” a million years before it has burnt up all of its hydrogen.
During the last half million years of a star’s life, when it has burnt all its hydrogen, it will burn helium in its core, with a hydrogen shell surrounding the core. This star is called a red giant. Stars will not spend long as red giants: no more than about a fifth of the time they spent as hydrogen burning stars.
If the star is massive enough it will then burn carbon, creating neon. Then it burns neon and oxygen, to make sulphur. So there are onion layers of elements in the star, all burning and fusing into heavier elements. At the center of the star there is an iron core, which slowly grows bigger: this iron cannot burn (falling of the binding energy curve begins at iron).
What happens to a star next depends on its mass. If it is a relatively small star then it will settle down as what we call a “white dwarf”. This type of star has a radius of a few thousand miles and a density of hundreds of tons per cubic inch. A white dwarf has no more reactions taking place; it is just losing its heat gradually over time. Eventually it will cool down to become a black dwarf.
Or our star (if massive enough) may implode into a small, very dense star – a neutron star. This would have a radius of about ten miles and a density of hundreds of millions of tons per cubic inch. Such a collapse generates a shock wave of energy that blows the star apart, generating a flash of light that can outshine a galaxy. This is what we call a supernova. A supernova can temporarily outshine an entire galaxy of 100 billion stars.
A neutron star is very dense indeed. Neutron stars are almost invisible, but they emit radiation, as they rotate, like a lighthouse in space. We see them as a blinking star, or pulsar. Well over 400 pulsars have been observed since their initial discovery in 1967.
But bigger stars, with larger masses, would collapse dramatically, almost to a single point – zero size (This was first calculated in the 1924 by an Indian scientist, Subrahmanyan Chandrasekhar. His theory produces a limit on the mass of a star relative to that of the Sun, called Chandrashekhar’s limit. The value of this limit for the star under the consideration decides the fate of the star after its nuclear fuel gets consumed). As this kind of bigger star increases its density (by getting smaller), its gravitational forces increase, getting stronger and stronger, until eventually even light is pulled back in by gravity. Such a star will have collapsed to become a black hole.
I take an opportunity at this juncture to conjecture another kind of star as a possible candidate. It will have mass heavier than neutron star and lighter than a black hole. It should come into existence due to balancing of the inward pull due to gravity and outward pull due to quantum degeneracy pressure of fermions called quarks. Let us call this star the quark star.
Stars are classified according to their spectral groups using the famous Hertzsprung-Russell diagram, the plot of Luminosity verses decreasing Temperature. The seven groups are: O (ionized Helium lines are seen) B (Neutral Helium lines) A (Hydrogen lines at maximum strength) F (Metallic lines become noticeable) G (solar type spectra) K (metallic lines dominate) and M (molecular bands are seen). When the spectral types are ordered in a sequence of decreasing effective temperatures, they read
What are Quasars? :
Quasars are the most luminous and the most baffling objects in the Universe. They are all at great distances from the Earth, but we see them in the sky as little points of light, like stars.
Until recently it was generally thought that quasars were indeed nearby stars. They certainly look like that, as this image of quasar PKS2349 shows. But in 1963 the scientist Maarten Schmidt detected an object – named 3C 273 – which he assumed was a star. But this star had a red shift of 16%, which meant that it was around two billion light years away. (We can calculate, or rather estimate, this distance, based upon our knowledge of Hubble’s constant and our theories about the Big Bang.) This was a very long way away for something that appeared so brightly in the sky. Surely it couldn’t be just a star?
Then Schmidt found other similar objects. Another star – named 3C 48 – had a red shift of 37%. This meant that it was departing at about one third of the speed of light, and was about five billion light years away – yet it was very, bright! Obviously these things couldn’t be stars – but what were they? Scientists started to call them “quasi-stellar objects”. This was shortened to the name quasars, which in 1970 became their official name.
Between 1963 and 1965 Schmidt found five quasars altogether; one was red shifted by a massive 2.01, or 201%. Quasars are in fact the most red-shifted objects in the Universe, so they would appear to be the most distant objects that we can see.
We now end our discussion with some recent ideas forwarded in the recent terminologies like super-strings, super-gravity, quantum gravity etc.
What are Super-Strings? :
The scientist Freeman Dyson gave a description of a super-string as “a wiggly curve which moves in a ten-dimensional space-time of peculiar symmetry”. Super-string is a kind of mathematical abstraction.
The theory of super-strings proposes that space-time is ten-dimensional, with six of those dimensions wrapped up very tight, very small. This space-time is filled with a mass of super-strings; not individual strings, but groups in which super-strings may be distributed. We don’t have enough of the picture yet to know how it might work in detail – or whether it will work. It might be a clunker, a grand cul-de-sac, or it might be a huge step forward. It might even be the answer to everything!
Freeman Dyson again: “The theory passed several crucial tests which other theories had failed. To have found a theory of the Universe which is not mathematically self-contradictory is already a considerable achievement”.
Why the name super-strings? Einstein’s great discovery in 1915 was his theory of gravity. 60 years later a new version was discovered, called super-gravity. About the same time a new theory of particle interaction was discovered – string theory, because particles are represented by one-dimensional curves or strings. The same mathematical trick, which turned gravity into super-gravity, was then used to turn strings into super-strings.
If the super-string theory is wrong we can probably prove it wrong quite quickly. If it is right we’ll need to prove it is right. That is harder and will take longer. But every year it is not proved wrong can make us feel a bit more confident.
The Big Meets the Small :
At the moment our equations governing the different types of forces that we know about are completely distinct; they bear no resemblance to each other. However many scientists believe that the four forces can be unified into a single force, a single equation or set of equations to explain everything. In this way we could unite all our theories to produce a theory of everything.
Many scientists also think that we are very near finding this solution, although it’s OK if you want to be skeptical. (After hearing one theory Niels Bohr replied, “we are all agreed your theory is crazy. The question which divides us is whether it is crazy enough”.)
Many scientists also believe that the solution to this theory lies in higher dimensional space: that is, a world that has more than our three dimensions (or four, if you include time). The focus is on super-string theory, but the problem is that nobody is smart enough to be able to solve the math. It requires techniques that we just don’t have yet.
Another approach is to try to unite quantum theory with relativity in a quantum theory of gravity. At the moment we have two partial theories: the general theory of relativity (which describes the force of gravity and the large scale structure of the Universe) and quantum mechanics, which deals with the very, very small. Unfortunately these theories are inconsistent with each other. At least one of them is wrong, but we don’t know which!