- The vast space surrounding us is called universe. It is mostly empty space. The universe includes everything that exists: the most distant stars, planets, satellites, as well as our own earth and all the objects on it.
- Nobody knows how big the universe is or whether it has any limits. However, it is estimated that the Universe contains 100 billion galaxies, each of which comprises 100 billion stars.
- The sun which sustains all the life on our planet is only one of the billions and billion of stars that exist in this universe, whereas the planet earth on which we live is only a tiny speck in this vast space called universe. The earth is one of the eight planets, all of which revolve around a central star called sun.
- The billions stars which exists in the universe are not distributed uniformly in space. These stars occur in the form of clusters (or groups) of billions of stars called galaxies.
- Thus, in order to study the constitution of this universe we have to first discuss the objects like galaxies, stars, planets and satellites, etc. which are found in the universe.
Origin of the universe
Origin of the Universe
- The most popular argument regarding the origin of the universe is the Big Bang Theory. It is also called expanding universe hypothesis. Edwin Hubble, in 1920, provided evidence that the universe is expanding.
- As time passes, galaxies move further and further apart. The distance between the galaxies is found to be increasing and thereby, the universe is considered to be expanding. Here, the expansion of universe means increase in space between the galaxies.
- However, Scientists believe that though the space between the galaxies is increasing, observations do not support the expansion of galaxies in itself an alternative to this was Hoyle’s concept of steady state. It considered the universe to be roughly the same at any point of time. It did not have a beginning and did not have an end.
- However, with greater evidence becoming available about the expanding universe, scientific community at present favours argument of expanding universe.
Stages in Big Bang Theory
- In the beginning, all matter forming the universe existed in one place in the form of a “tiny ball” (singular atom) with an unimaginably small volume, infinite temperature and infinite density.
- At the Big Bang the “tiny ball” exploded violently. This led to a huge expansion. It is now generally accepted that the event of big bang took place 13.7 billion years before the present. The expansion continues even to the present day. As it grew, some energy was converted into matter. There was particularly rapid expansion within fractions of a second after the bang. Thereafter, the expansion has slowed down. Within first three minutes from the Big Bang event, the first atom began to form.
- Within 300,000 years from the Big Bang, temperature dropped to 4,500 K and gave rise to atomic matter. The universe became transparent.
Evidences in support of Big Bang Theory
The light from other galaxies is red-shifted.
The other galaxies are moving away from us.
The further away the galaxy, the more its light is red-shifted.
The most likely explanation is that the whole universe is expanding. This supports the theory that the start of the universe could have been from a single explosion.
Cosmic Microwave Background.
The relatively uniform background radiation is the remains of energy created just after the Big Bang.
- Stars are the heavenly bodies like the sun that are extremely hot and have light of their own. Stars are made up of vast clouds of hydrogen gas, some helium and dust. In all the stars (including the sun), hydrogen atoms are continuously being converted into helium atoms and a large amount of nuclear energy in the form of heat and light is released during this process.
- It is this heat and light which makes a star shine. Thus, a star is a hydrogen nuclear energy furnace, so big that it holds together by itself. The stars are classified according to their physical characteristics like size, colour, brightness and temperature.
- Stars are of three colours: red, white and blue. The colour of a star is determined by its surface temperature. The stars which have comparatively low surface temperature are red, the star having high surface temperature are white whereas those stars which have very high surface temperature are blue on colour.
- Some of the important example of the stars are: Pole (or Polaris), Sirius, Vega, Capella, Alpha centauri, Beta centauri, Proxima centauri, Spica, Regulus, Pleiades, Aldebaram, Arcturus, Betelgeuse, and of course, the Sun.
All the stars (except the pole star) appear to move from east to west in the night sky. This can be explained as follows: the earth itself rotates on its axis from west to east. So, when the earth rotates on its axis from west to east, the stars appear to move in the opposite direction, from east to west. Thus, the apparent motion of the stars in the sky is due to the rotation of the earth on its axis. Since we are ourselves on the earth, the earth appears to be stationary to us but the stars appear to be moving in the sky.
- Thus, it is due to the rotation of earth on its axis that we see the stars changing their positions in the sky as the night progresses.
Birth and Evolution of a Star
The raw material for the formation of a star is mainly hydrogen gas and some helium gas. The life cycle of a star begins with the gathering of hydrogen gas and helium gas present in the galaxies to form dense clouds of these gases. The stars are then formed by the gravitational collapse of these over-dense clouds of gases in the galaxy.
Let us deal the various stages in the formation of star-
Formation of a Protostar
- In the beginning, the gases in the galaxies were mainly hydrogen with some helium. However, they were at a very low temperature of about, -173°C.
Since the gases were very cold, they formed very dense clouds in the galaxies. In addition, the gas cloud was very large, so the gravitational pull between the various gas molecules was quite large.
- Due to large gravitational force, the gas cloud started contracting as a whole. Ultimately, the gas were compressed so much that they formed a highly condensed object called a protostar.
- A protostar looks a like a huge, dark, ball of gas. The formation of protostar is only a stage in the formation of complete star. A protostar does not emit light.
The next stage consists in the transformation of this highly condensed object called protostar into a star which emits light.
Formation of a Star from Protostar
- The protostar is a highly dense gaseous mass, which continues to contract further due to tremendous gravitational force.
- As the protostar begins to contract further, the hydrogen atoms present in gas cloud collide with one another more frequently.
- These collisions of hydrogen atoms raise the temperature of protostar more and more. The process of contraction of protostar continues for about a million years during which the inner temperature in the protostar increases from a mere, -173°C in the beginning to about 107°C.
- At this extremely high temperature, nuclear fusion reactions of hydrogen start taking place. In this process, four small hydrogen nuclei fuse to produce a bigger helium nucleus and a tremendous amount of energy is produced in the form of heat and light.
- The energy produced during the fusion of hydrogen to form helium makes the protostar glow and it becomes a star. This star shines steadily for a very, very long time.
Final Stages of a Star̛ s Life
In the first part of the final stage of its life, a star enters the red-giant phase where it becomes a red-giant star.
After that, depending on its mass, the red-giant star can die out by becoming a white dwarf star, or by exploding as a supernova star, which ultimately ends in the formation of neutron star and black holes.
(1) Red- Giant Phase:
- Initially, the stars contain mainly hydrogen. With the passage of time, hydrogen gets converted into helium from the centre outwards. Now, when all the hydrogen present in the core of the star gets converted into helium, then the fusion reactions in the core would stop.
- Therefore, ultimately, the matter in the core of the star would consist only of helium. Due to the stoppage of fusion reactions, the pressure inside the core of the star would diminish, and the core would begin to shrink under its own gravity.
- In the outer shell or envelope of the star, however, some hydrogen still remains, the fusion reactions would continue to liberate energy but with much reduced intensity.
- Due to all these changes, the overall equilibrium in the star is upset and in order to readjust it, the star has to expand considerably in its exterior region (outer region).
- Thus the star becomes very big (it becomes a giant), and its colour changes to red. At this stage, the star enters the red-giant phase and it is said to become a red-giant star.
- Our own star, the sun, will ultimately turn into a red-giant star after about 5000 million year from now. The expanding outer shell of the sun will then become so big that it will engulf the inner planets like mercury and Venus, and even the earth.
- When a star reaches the red-giant phase, then its future depends on its initial mass.
Two cases arise:
- If the initial mass of the star is comparable to that of the sun, then the red giant star loses its expanding outer shell and its core shrinks to form a white dwarf star which ultimately dies out as a dense lump of matter into the space.
- If the initial mass of the star is much more than of the sun, then the red giant star formed from its explodes in the form of a supernova star, and the core of this exploding supernova star can shrink to form a neutron star or black hole.
(2) Formation of White Dwarf Star
- If the mass of red-giant star is similar to that of the sun, the red-giant star would lose its expanding outer shell or envelope because then the comparatively smaller amount of hydrogen fuel present in it will be used up rapidly, and only the core of the red-giant star will gradually shrink into an extremely dense ball of matter due to gravitation.
- Because of this enormous shrinking of helium core, the temperature of core would rise greatly and start another set of nuclear fusion reactions in which helium is converted into heavier elements like carbon, and an extremely large amount of energy will be released.
- When the mass of a star is similar to the mass of the sun (which is comparatively a small mass), then all the helium is converted into carbon in a short time and then further fusion reactions stop completely.
Now, as the energy being produced inside the star stops, the core of star contracts (shrinks) under its own weight. And it becomes a white dwarf star.
- A great Indian scientist Chandrasekhar made a detailed study of the stars which end their lives by becoming white dwarf stars.
- Chandrasekhar concluded that the start having a mass less than 1.44 times the solar mass (or sun’s mass) would end up as white dwarf stars. The maximum limit of 1.44 times the solar mass (for a star to end its life as a white dwarf) is known as ‘Chandrasekhar Limit’.
- If, however, a star has a mass more than 1.44 times the solar mass or sun’s mass, then it will not die out by becoming a white dwarf star. This is because due to greater mass, it will have more nuclear fuel in it, which will not get exhausted in a short time.
- The stars having mass much more than solar mass (or sun’s mass) led to supernova explosions and end their lives by becoming neutron stars or black holes.
This point will become clearer from the following discussion
Formation of Supernova Star and Neutron Star
- When a very big star is in the red-giant phase, then being big, its core contains much more helium. This big core made up of helium continues to contract (shrink) under the action of gravity producing higher and higher temperature.
- At this extremely high temperature, fusion of helium into carbon takes place in the core and lot of energy is produced. Since the star was very big and contained enormous nuclear fuel helium, so a tremendous amount of nuclear energy is produced very rapidly which causes the outer shell (or envelope) of this red-giant star to explode with a brilliant flash like a nuclear bomb.
- This type of ‘exploding star is called supernova. The energy released in one second of a supernova explosion is equal to the energy released by the sun in about 100 years.
- This tremendous energy would light up the sky for many days. When a supernova explosion takes place, then clouds of gases in the envelope of red-giant star are liberated into the space and these gases act as raw material for the formation of new stars.
- The heavy core left behind after the supernova explosion continues to contract and ultimately becomes a neutron star (if mass of star was 1.44 time to 3 times the Sun) or Black Hole (if the mass of star was more than 3 times the sun).
- A neutron star contains matter in even denser form than found in white dwarf stars. Although a number of white dwarfs have been detected, but no one has yet observed a neutron star. This may be because neutron stars are very faint. A spinning neutron star emits radio waves and is called a pulsar.
- The first image of Black Hole: The Event Horizon Telescope—a planet-scale array of ground-based radio telescopes—has obtained the first image of a supermassive black hole and its shadow on 10th April, 2019. The image reveals the central black hole of Messier 87, a massive galaxy in the Virgo cluster.
First ever picture of a Black Hole
- A black hole is an object with such a strong gravitational field that even light cannot escape from its surface. A black hole may be formed when a massive object (very big object) undergoes uncontrolled contraction (a collapse) because of the inward pull of its own gravity.
- We will now describe how the black holes are formed from neutron stars after the supernova explosions of big stars. When a supernova explosion of a very massive star takes place, then the gaseous matter present in the outer shell (or envelope) of the star is scattered into space but the core of the star survives during supernova explosion.
- This heavy core of the supernova star continues to contract (shrink) and becomes a neutron star. The fate of this neutron star depends on its mass.
If the neutron star is very heavy, then due to enormous gravitational attraction, it would continue to contract indefinitely. And the vast amount of matter present in a neutron star would be ultimately packed into a mere point object.
Such an infinitely dense object is called a black hole. Thus black holes are formed by the indefinite contraction of heavy neutron stars under the action of their own gravity.
- The neutron stars shrink so much and become so dense that the resulting black holes do not allow anything to escape, not even light, from their surface. This is because the black holes have tremendous gravitational force.
- Since even light cannot escape from black holes, therefore, black holes are invisible, they cannot be seen. The presence of a black hole can be felt only by the effect of its gravitational field on its neighbouring objects in the sky.
- For example, if we see a star moving in a circle with no other visible stars in the centre, then we can conclude that there is a black hole at the centre. And it is the gravitational pull exerted by this black hole which is making the star goes in a circle around it.
- Dark matter is a type of matter hypothesized in astronomy and cosmology to account for a large part of the mass that appears to be missing from the universe.
- Dark matter cannot be seen directly with telescopes; evidently it neither emits nor absorbs light or other electromagnetic radiation at any significant level. Dark Matter is not exactly black hole.
- The composition of the constituents of cold dark matter is currently unknown. It could be group of black holes, dwarfs or some new particle.