Life Cycle of Stars Science Notes
→ 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 a part of one such spiral galaxy and is called the Milky Way and Mandakini.
→ Astronomers study the universe through observation with the help of telescopes placed either on the earth or on the artificial satellites in space.
→ Our galaxy has about 1011 stars. Its shape is like a disc with a bulge in the centre and its diameter is about 1018 km. The solar system is situated at a distance of 2 × 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 × 108 years.
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
- 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: 2 × 1030 kg.
- (MSun): 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 × 107 K
- Age of the Sun: 4.5 × 107 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. Scientists have concluded that the properties of the Sun have remained unchanged over the past 4.5 billion years. Astronomers use the light year as a unit to measure the distance to and between the Stars.
Stability of a star:
- Properties of a star remain unchanged for quite a long time. The gravitational force and the stars.
- One light year is the distance travelled by light in one year.
- Light travels at 300000 km/s. One light year therefore equals 9.5 x 1012 km.
→ Light takes about 1s to reach us from the moon while it takes about 8 minutes to reach us from the Sun. It takes about 4.2 years to reach us from the star Alpha Centauri which is the star closest to the Sun.
→ 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.
→ When a gas sphere contracts, its temperature increases. This happens because of transformation of its gravitational potential energy into heat energy.
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.
Evolution of a Star
Though the properties of a star remain unchanged for a substantially long period of time, this situation is never static. A star passes through different stages. This process is called the evolution of a star. As a star continuously emits energy, its energy constantly decreases. This may cause the imbalance between the gravitational force and the force due to the hot gases.
If the temperature of a star remains constant, the star is more likely to remain stable and for the temperature to remain constant, energy must be generated inside the star. This energy is generated by burning of fuel at the centre of the star. Burning of the fuel at the centre of the star and hence a gradual decrease in its amount is the main reason of evolution of any star.
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 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.
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.
End-stage of a star
When the energy generation in a 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.
→ End stages of the 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. They appear reddish due to their lower temperature. Hence, they are called red giant stars.
At the end of the evolution, they 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.
- If there was no gas pressure in the Sun, it will collapse to a point in 1-2 hours.
- Gas pressure depends on the density and temperature of the gas. The higher the temperature and density, the higher is the pressure.
→ End stages of the stars having mass between 8 and 25 times the mass of the Sun (8 MSun < Mstar < 25 MSun):
→ These stars also 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.
→ 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 weigh 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.
→ 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.
→ End stages of the 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.