The Physical Universe

The universe is vast. The Earth we live on appears to be just a speck of dust circling a small star in a remote corner of an obscure galaxy. If we are a speck in the immense space, we also occupy only an instant in the expanse of time. The universe is also very old. We now know that it is about fifteen or twenty billion years old, while we, the human beings have been around for only two million years or so. This vast and ancient universe is populated with a variety of objects. Let us now understand what objects constitute the universe and how they are distributed. In other words, what the physical structure of the universe is. In doing so, we will not go into its chemical composition or any other details. However, before we embark on this venture, we will give you an idea of the cosmic distances, so that you're able to appreciate what follows.

Cosmic Distances 

If we asked you what the distance between Delhi and Kanyakumari is, you would say that it is roughly 3000 km. Another way of answering the question would be that a train takes.about 50 hours to cover the distance. And if we know the average speed of a train, we would get a fairly good idea of the distance. The dimensions of the universe are so large that using familiar units of distance like kilometres would make little sense, Therefore, cosmic distances are measured in'"light years". One light year is the distance travelled by light. in one year. Now, light travels, about 300,000 kilometres in one second, i.e. its speed is 3 X 10*5 kms. At this sped it can travel seven times around the Earth in one second. A year has about 3 X 10*7 seconds (i.e. about 30 million seconds) in it.

Therefore, the distance light travels in one year is about 3 X 10%m/s X 3 X 107s=9 X 1W2 km, i.e. about nine trillion kilometres. This unit of length is called a light year, It measures not time but distances, enormous distances. Space and time are interwoven. We cannot look out into space without looking back into time. If we see a galaxy a billion light years away, we are seeing it the way it was a billion years ago. Thus, the distance in units of light years also tells us how far back into time we are looking.

 Solar Family

The Solar Family

Let us now take you on a journey across the universe. We begin from our planet Earth. It is our home. The Earth is active, lush and fertile. It is a place of blue skies, vast *oceans, cool forests, a world full of life. Its surface is cloaked by an atmosphere in which 'we can breathe, and which keeps the Earth's temperature quite-constant. It spins or rotates on its axis and revolves around the Sun, completing one orbit in one year. The Earth is not alone. It has a companion on its travels, the Moon which orbits the Earth once in 27.33 days. But the Moon is airless, waterless and lifeless; it is a dead world. From the Moon, the Earth appears as a beautiful bluish-white planet. The Earth is not the only planet orbiting the Sun. There are eight other planets that orbit the Sun. The nine planets and their satellites together with the Sun and many asteroids and comets make a family, the Solar System  The planets arc diverse in size, ranging from the giant Jupiter, eleven times the size of Earth, down to tiny Pluto, less than half the size of our planet. Each planet's distance from the Sun is Universe as a System b different, Mercury being the nearest and Pluto, the farthest. They are all different from each other in many respects.  Let's see what the space beyond contains.

 Night Sky 

 The Night Sky 

When you look up at the sky on a clear night, what do you-see? Most of it is dotted with twinkling stars. You may see the moon or Venus as a bright point or Mars as a reddish one. Venus is sometimes seen in the evening just after sunset and, at other times, in the morning before sunrise. Except for the planets, all other points of light are stars. From our planet Earth, on a really clear, dark night, we aan see .about 6,000 stars.

Stars and Constellations 

The night sky is interesting. The stars seem to fall into certain patterns. You must have seen the Saptarishi, also known as the Big Dipper or the Plough. It is a part of a bigger group of stars called the Great Bear or the Ursa Major which is seen in the northern sky. We can see many different star patterns like the Hunters, the Lion, the Dog, the Balance etc. These pictures are not there really in the sky. Our imagination has put them there. The early star gazers and the ancient astronomers traced out these star patterns and named them after gods and heroes, objects or creatures which these groups of sqrs resembled in their imagination.

The sky is divided into these patterns of stars or star groups which are called constellations. A constellation is an arbitrary grouping of stars and it merely defines an axa of the sky. All cosmic objects in a given region of the sky form a constellation. There are eighty eight constellations each having a definite boundary. Modern astronomers use thc ancient names of the constellations to refer to these eighty eight regions of the sky, and not to thc imaginary ligures of long ago.We show the star charts of bright stars and constellations visible in the Northern and Southern hemispheres. You could also look up the star charts published in many newspapers and magazines from time to time and familiarise yourself with the stars and constellations in the night sky.

The stars and constellations help explorers and navigators. In the ancient tiril*s, they helped the seagoing ships. By watching the position of the constellation with respect to the horizon night after night, the early sailors could determine the ship's latitude. And now the spacefaring ships find their bearings in space with the help of stars and constellations. lt we extend the line joining the last two stars of the Saptarishi called the Pointers, we can see'the Pole star. It is a bright star situated in the North, almost on the Earth's axis of rotation. Therefore. lt appears like a fixed s.tar. Its position gives the geographical north. Thus, the Pole star also helped the earliest navigators in finding their way at night.

As the earth revolves around the Sun, different constellations appear, disappear and I reappear at different times of the year. Thus, there are different constellations in different seasons. The motion of planets, when viewed against these constellations, I appears as if the planets were entering or leaving a constellation at a specific time of I the year. Such movements of planets and stars have also been

 The Milky Way Galaxy 

The white band stretched across the night sky is, in fact, a partial view of the Milky Way Galaxy. Being inside the Galaxy, we can see it only in parts. We cannot see the whole of it, the way we see the other galaxies. Visualising the whole Milky Way Galaxy and determining its shape has not been easy. By watching a large number of galaxies distributed in all directions in dozens of views as far as modem telescopes can see, scientists have been able to form a picture of what our galaxy must look like from outside. In ihis, they have also been helped by the observations about the stars in our galaxy, their distances and motion, etc. The picture of the Galaxy constructed by the astronomers is shown in . Doesn't it look somewhat like a disc or a gramophone record with a swollen centre? The Milky Way Galaxy contains about 100 billion stars. The stars are not uniformly distributed.

In the edge-on view, the Galaxy consists of two basic parts: the disc and the halo. The disc consists of stars, as well as clouds of gas and dust called nebulae. It has a diameter of 100,000 light years, and a thickness of about 5,000 light years. This collection of gas and stars rotates about the centre (also known as the nucleus) of the Galaxy, with each part moving at a different speed. The Solar System at a distance of about 30,000 light years from the centre, in the outskirts of the Galaxy, also revolves. Moving at a speed of 250 km per sec., it takes roughly 200 million years to complete one revolution around the centre of the Galaxy. There are individual stars like the Sun as well as groups of stars, called galactic clusters, that move together in the disc. Astronomers have identified about 1000 galactic clusters in the disc, each containing 10 to 1,000 stars.

The diw of the galaxy contains spiral arms, which are about 2,500 light years wide. The distance between the adjacent arms is about 1,500 light years. The spiral arms are seen clearly because that is where the brightest stars and gas clouds are found. Dark clouds of dust and gas line the inner rlms of the arms. The other stars in the disc are not arranged In any conspicuous pattern. The Sun, for instance, lles between two spiral arms.

The halo is spherical and has its centre at the nucleus of the Galaxy. The central redon of the halo consists of a vast concentration of stars that form the nuclear bulge of the disc. Elsewhere the halo consists of very little gas and widely separated stars and about 120 globular clusters. The globular clusters are compact spherical systems each containing aiiy number of stars from ten thousand to several millions. It is observed that the halo, or the spherical component of the Galaxy, does not rotate with the disc.

From a study of the distribution of globular clusters, we may conclude that the Sun cannot be at the centre of the Galaxy. Because if it were so, the globular clusters would have been distributed around the Sun. They are not, and hence we may surmise that the Sun is not at the centre of the Milky Way Galaxy. The centre of the Galaxy is, in fact, the centre of the distribution of these globular clusters.

Beyond the Milky Way Galaxy 

Let us now move away from the Earth and venture into the space beyond. If we were at a point far out in space we would see scattered in space, a large number of faint, wispy tendrils of light. These are all galaxies. Galaxies are made up of billions of stars, and clouds of gas and dust. The universe is full of galaxies. Some of them are solitary wanderers. Most of them moving clusters. drifting endlessly in the great cosmic dark.

Shapes of galaxies

The galaxies are usually found in three shapes: spiral galaxies, elliptical galaxies and irregular galaxies. Further refinements have been suggested in recent years, but we will not go into those details. The elliptical galaxies are so called because they have an elliptical shape on a photographic plate. Elliptical galaxies in general do not have much gas or dust from which to form new stars, and they consist of old stars. The irregular galaxies do not show any coherent structure. 'he number of elliptical and spiral galaxies is almost equal, whereas the irregular ones comprise about 10% of all galaxies.

Before going too deep into the space, let's take a closer look at, what astronomers on' Universe as a System Earth like to call, the Local Group of Galaxies. Its cross-section is several million light years, it is made up of around twenty galaxies. The nearest galaxies to the Milky Way Galaxy are the Large and Small Magellanic Clouds, visible in the Southern hemisphere. They are irregular in shape. The galaxy Andromeda'lies nearly two million light years away and is visible to the unaided eye. It is a spiral galaxy, three times bigger and brighter than ours.

As we move further out, we find that such groupings, or clusters of galaxies, are extremely common. There are some hundreds of billions of galaxies in the universe which form clusters of all kinds. There are rich clusters containing as many as ten thousand galaxies and poor clusters having only a few galaxies. Our own galaxy is a member of a poor cluster. The nearest rich cluster, at a distance of about 70 million light years, is Virgo. It is irregular in shape and is huge, extending 7 million light years from end to end. Like galaxies, clusters are also shaped like spirals, ellipses or they may be irregular.

In recent years, one more step has been added in understanding this physical structure of the universe. There is evidence to suggest that the clusters of galaxies, rich and poor, in turn form superclusters or super galaxies (i.e. clusters of clusters) that are 200-300 million light years in diameter. They may be made up of about a 100 member clusters. The clusters, Local Group and Virgo, are members of the same supercluster. The superclusters are very much alike. They are rather evenly distributed in space. Thus, on a larger scale than this, the universe appears uniform, that is, it has the same structure and composition everywhere, it looks the same in all directions.

The structure of the universe that we have described above is not static. It is changing. New stars and new galaxies are being born. Stars, galaxies and clusters move. Sometimes they collide giving rise to new galaxies. All these stars, galaxies, clusters and superclusters have a story to tell about ancient events on the largest possible scale. We are only now beginning to read this story.

Interstellar and Intergalactic Space 

The space between the stars and the galaxies looks empty, doesn't it? But this 1s not true. In the great dark between stars in the galaxies, and galaxies in the clusters, there are clouds of gas and dust. The gas clouds are mainly made up of hydrogen atoms and cannot be seen by the unaided eye. Only the modem astronomical instruments have been able to detect these particles. Cosmic dust is made up of bigger particles. These clouds of dust are revealed when they reflect the light of stars falling on them . Cosmic dust and clouds of gas in a galaxy are found to play a great role in the formation of a star.

In the gas clouds and cosmic dust we also find traces of different kinds of ordinary molecules, like water, ammonia, carbon monoxide etc. There are many organic molecules, like methane, methanol (also known as wood alcohol), formic acid (the substance that gives ant and bee bites their sting), and many more. The organic molecules are the matter out of which our kind of life arose on the planet Earth. The abundance of such organic molecules in the interstellar space suggests that there might be life somewhere out there, perhaps in a different form. We may not be the only ones, after all!

Another major constituent of the universe are cosmic rays. Cosmic rays are beams of charged particles, such as the electrons, protons and helium nuclei etc., that freely travel in space at nearly the speed of light. These particles carry large amounts of energy across space.

Let us now end our brief journey of the universe, and summarise what we have discovered. The matter of the universe is concentrated in large superclusters of galaxies, each measuring 100 to 200 million light years across and each containing millions of galaxies. The galaxies are grouped in rich or poor clusters, have different shapes and are distributed in different ways. The galaxies contain stars and clouds of gas and dust. Stars may be grouped in clusters or they may be individual stars like the Sun having planetary systems like our Solar System. Our planet Earth is a part of the Sun's family.

Don't the diversity and the expanse of the universe seem truly amazing? Lost somewhere in the vastness of space and immensity of time is our tiny planetary home. This is a humbling thought. Yet, our species is young, curious and brave. We would rather not feel overwhelmed by the expanse of universe in space and time. We would like to end this unit with the words of Carl Sagan from his book 'Cosmos' (p.1):

"The Cosmos is all that is or ever was or ever will be ........... In the last few years, we have made the most astonishing and unexpected discoveries about the Cosmos and our place within it, explorations that are exhilarating to consider. They remind us that humans have evolved to wonder, that understanding is a joy, that knowledge is a ' prerequisite to survival. I believe our future depends on how well we know this Cosmos in which we float like a mote of dust in the morning sky."