|
Topics to be learn :
- Properties of stars
- Birth of a star
- Stability of stars
- Evolution of stars
- End stages of stars
|
The universe is made up of innumerable galaxies that differ in structure and shape. Galaxies can be spiral, elliptical and irregular in shape.
Our solar system is situated in a galaxy. A galaxy is a collection of billions of stars, their planetary systems and interstellar clouds which are present in the empty spaces between stars.
Our galaxy is a spiral galaxy and is called the Milky Way and Mandakini. A spiral galaxy is show in figure.
Our galaxy has about 1011 stars. Its shape is like a disc with a bulge in the centre and its diameter is about 1013 km. The solar system is situated at a distance of 2 x 1017 km from its centre. The galaxy is rotating around the axis passing through its centre and perpendicular to the disc. Its period of rotation is about 2 x 108 years.
Astronomers study the universe through observation with the help of telescopes placed either on the earth or on the artificial satellites in space.
Properties of stars :
- One can see about 4000 stars with his naked eyes during night.
- Stars are gigantic spheres of hot gas.
Properties of the Sun :
Properties of the Sun : The star nearest to us is the Sun. Hence, it appears larger than all other stars.
- There are billions of stars in space which are higher or lower in mass, size and temperature than the Sun.
- The Sun is an ordinary star among other stars in space.
- 72% of the mass of the Sun is made up of hydrogen. Helium constitutes about 26% of the Sun’s mass. The rest 2% of the mass is made up of elements heavier than helium. ’
- Mass of the Sun (Msun) : 2 x 1030 kg. This is about 3.3 lakh times that of the earth.
- Radius of the Sun : 695700 km. This is about 100 times that of the earth.
- Surface temperature of the Sun : 5800 K
- Temperature at the centre of the Sun : 1.5 X 107 K
- Age of the Sun : 4.5 x 109 years
- Scientists have concluded that the properties of the Sun have remained unchanged over the past 4.5 billion years.
The masses of other stars are measured with respect to the mass of the Sun. This means that the mass of the Sun, written as Msun is used as the unit of mass.
[collapse]
Birth of a star : There are huge clouds of gas and dust in the empty spaces between the stars in a galaxy. These clouds are called interstellar clouds. The size of an interstellar cloud is about a few light years.
- When an interstellar cloud starts contracting due to some disturbance, its density and temperature increase.
- This results in formation of a dense sphere of hot gas and nuclear energy generation starts at the centre of the star.
- Therefore, the gas sphere becomes self-luminous.
- Thus a star is formed, i.e., a star is born.
- Contraction of a huge interstellar cloud may cause the formation of thousands of stars at a time.
Stability of a star :
Stability of a star : Properties of a star remain unchanged for quite a long time.
- The gravitational force and the force due to the pressure of the hot gas act together on a star.
- The gravitational force acts towards the centre of the star and tries to bring the gas particles close together.
- Hot gas shows the tendency to spread and its force acts away from the centre of the star.
- This force tries to disperse the gas particles.
- A balance between the gravitational force and the force due to the hot gas keeps the star stable.
However, if the magnitude of any one force is more than that of the other force, the star either contracts or expands depending upon which force dominates.
[collapse]
Evolution of star : Though the properties of a star remain unchanged for quite a long time, this situation is never static. A star passes through different stages. This process is called the evolution of a star.
Important stages of the evolution of a star :
Important stages of the evolution of a star are as follows :
(1) Initial stage of stability :
- The gravitational force and the force due to the pressure of the hot gas act together on a star.
- The gravitational force acts towards the centre of the star and tries to bring the gas particles close together.
- Hot gas shows the tendency to spread and its force acts away from the centre of the star.
- This force tries to disperse the gas particles.
- A balance between the gravitational force and the force due to the hot gases keeps the star stable, as long as the energy generation continues at the centre of the star.
(2) Burning of the fuel :
- As a star continuously emits energy, its energy constantly decreases. When the fuel at the centre of the star is exhausted, the energy generation stops and the temperature of the star starts decreasing.
- Decreasing temperature causes the gas pressure to decrease and the balance between the gravitational force and the force due to the gas pressure is no more maintained.
- As the magnitude of the gravitational force is now more than that of the force due to the gas pressure, the star starts contracting. This causes another fuel to start burning, e.g. on exhausting hydrogen, helium starts undergoing fusion.
- Availability of multiple fuels depends on the mass of the star. The higher the mass of the star, the more is the number of fuels used.
- The star either contracts or expands during the course of using these fuels. This may cause the imbalance between the gravitational force and the force due to the hot gas.
(3) Total exhaustion of the fuel :
- When all fuels are exhausted, the energy generation in the star finally stops completely and the temperature of the star starts decreasing.
- The balance between the gravitational force and the force due to the gas pressure can no more be maintained.
- The evolution of the star ends and the star proceeds to its end stage.
(4) End stage of a star :
- Once the fuel in the star is totally exhausted, the energy generation in the star stops and subsequently the gas pressure decreases, the star starts contracting and its density starts increasing.
- When the density becomes very high, some new types of pressures are generated which are independent of the temperature of the gas. In such a case, the pressure remains stable despite low temperature and absence of any energy generation and thus the star remains stable forever.
- This stage is the end stage of the star.
- Depending on the initial mass, stars can reach one of the three end stages.
- Stars having initial mass less than 8 times the mass of the Sun ultimately become white dwarfs.
- Stars having initial mass between 8 and 25 times the mass of the Sun ultimately become neutron stars.
- Stars having initial mass larger than 25 times the mass of the Sun ultimately turn into black holes.
[collapse]
Evolution of stars based on their mass and their end stages :
(1) End stages of stars having initial mass less than 8 times the mass of the Sun :
(1) End stages of stars having initial mass less than 8 times the mass of the Sun (Mstar < 8 MSun):
These stars undergo huge expansion and their radius increases by a factor of 100 to 200 during their various stages of evolution. These stars appear reddish due to their lower temperature. Hence, they are called red giant stars.
- At the end of evolution, these stars explode, their outer gas envelope is thrown out and the inner part contracts roughly to the size of the earth. Hence, the density of the star becomes very high.
- In this stage, the pressure due to electrons becomes independent of temperature and sufficient to balance the gravitational force forever. Such stars look white and due to their small size they are called white dwarfs.
[collapse]
(2) End stage of the stars having mass between 8 and 25 times the mass of the Sun :
(2) End stage of the stars having mass between 8 and 25 times the mass of the Sun (8 MSun < MStar < 25 MSun) :
These stars pass through the red giant stage and later supergiant stage, during which their size may increase to 1000 times.
- The huge explosion, called the supernova explosion, occurs in the last stage of the evolution.
- It is very powerful and very high energy is given off in this case. As a result, the stars are visible even during the day.
- Later their central portion contracts to about 10 km.
- In this stage, the stars are completely made up of neutrons and are called neutron stars.
- The pressure of these neutrons is independent of temperature and sufficient enough to balance the gravitational force forever.
[collapse]
(3) End stages of stars having mass larger than 25 times the mass of the Sun :
(3) End stages of stars having mass larger than 25 times the mass of the Sun. (Mstar > 25 MSun) :
- After the supernova explosion, no pressure can balance the gravitational force.
- Hence these stars contract continuously and their gravitational force and density increase exponentially.
- All nearby objects get attracted towards these stars and not even light can come out of them.
- Light falling on these stars is completely absorbed by the star. We cannot see these stars.
- A very minute black hole is formed at the place of such a star. This is the end stage of these stars.
[collapse]
| Interesting facts :
1. As the size of the white dwarfs is similar to that of the earth, their density is very large. One spoonful material of the white dwarf will weight a few tons. As neutron stars are much smaller than the white dwarfs, their density is even higher and one spoonful material of these stars will weigh as much as the weight of all living beings on the earth.
2. A star in our galaxy exploded about 7500 years back. As the star is about 6500 light years away from us, the light emitted in the explosion took 6500 years to reach us. It was first seen on the earth by the Chinese in the year 1054. It was so bright that it could be seen during the day also for 2 years. After 1000 years of the explosion, the gases emitted during the explosion are seen to be expanding with velocities higher than 1000 km/s.
|
Yes want
Thanks for notes this notes are very helpful….