STARS & UNIVERSE
QUESTIONS
1. What percentage of mass of sun makes the solar system?
99%
2. What force makes the planet revolve around the sun?
Gravitational force
3. What type of star is sun?
A. Small seized B. Medium sized C. Large Sized
QUESTIONS
4. Name the gases present in the sun?
Hydrogen and Helium
5. Which regions of EM spectrum does the sun radiate its energy?
IR,VL,UV
Temperature of stars
● Warm objects emit IR radiation and extremely hot objects emits visible light.
● The colour they emit depends on how hot they are.
● A star’s colour is related to its surface temperature.
NUCLEAR FUSION
● In stars hydrogen nuclei combine to form helium nucleus.
● Deuterium combines with tritium to form helium with the release of a huge
amount of energy.
● This provides pressure that prevents the star from collapsing under the
influence of gravity.
QUESTION
Light year
● We use light year because the astronomical distances are large.
● One light year is the distance travelled by light through space/vacuum in 1
year
● The diameter of the milky way 10^5 light years.
● Calculate 1 light year in metres.
FORMATION OF STARS
● All stars form from a giant interstellar cloud of hydrogen gas and dust called
nebula.
● Life cycle of stars can be divided into two based on its mass-
● Low mass stars and High mass stars.
Life cycle of a low mass star
Protostar
● Force of gravity pulls the particles closer together until it forms a hot ball
of gas.
● Density increases, more frequent collisions and temperature increases.
Main Sequence star
● Once the star initiates fusion reaction, it is called main sequence star.
● The star is stable and is in equilibrium.
● Inward force due to gravity is equal to the outward pressure force.
● Once a main sequence star is formed, its life cycle depends on mass.
Red Giant
After several billions of years all the Hydrogen that has been converting into
Helium reduced.
So fusion reaction in the core slows down and force of gravity becomes greater
than outward pressure so the core shrinks.
Helium will undergo nuclear fusion to form beryllium, which causes the outward
part of the star to expand.
A low mass star upto 8 times the mass of sun will become RED GIANT
Planetary Nebula
Once the second stage of fusion reaction is over, the star will become unstable
and eject the outer layer of dust and gas.
The layer of dust and gas which is ejected is called planetary nebula.
WHITE DWARF AND BLACK DWARF
The core which is left, collapse completely due to gravity to form white dwarf.
White dwarf will be cooling and the energy emitted decreases.
Once most of the energy has lost it becomes a black dwarf and it will continue to
cool until it disappears from sight.
Life cycle of a high mass star
Life cycle of a high mass star
● A high mass star, the one more than 8 times the mass of the sun becomes
RED SUPERGIANT
● Similar to low mass star the fusion reactions start to die down
● The core will undergo a series of shrinkage and heating up as a result
the outer parts will expand and contract.
● This time fusion reaction continues until iron is formed.
Life cycle of a high mass star
● Once the fusion reactions in the RED SUPERGIANT cannot continue, the core
collapses suddenly and cause gigantic explosion- this is SUPERNOVA
● At the centre of the explosion a dense body called NEUTRON STAR is formed.
● The remnants of supernova explosion will be ejected into the space and forms
new clouds of dust and gas (Nebula) which may form new stars with orbiting planets.
● For the biggest stars, the neutron star starts to collapse under gravity to
form black hole. Extreme dense point where light cannot escape.
