IGCSE Physics\Stars & The Universe\Important Concepts
IGCSE PHYSICS
STARS AND THE UNIVERSE: IMPORTANT CONCEPTS
MEASUREMENT OF ASTRONOMICAL DISTANCES
Two different units are commonly used to express astronomical distances, i.e.
distances in the outer space, namely:
(a) astronomical unit (au): for distances within the Solar System
(b) light-year (ly): for distances beyond the Solar System
1 astronomical unit is equivalent to the average distance from the Sun to the Earth,
which is about 150 million kilometres.
Examples: Distance of the planets from the Sun
Distance from the Sun Distance from the Sun
Planet Planet
106 km au 106 km au
Mercury 58 Jupiter 780
Venus 108 Saturn 1430
Earth 150 1.0 Uranus 2870
Mars 230 Neptune 4510
1 light-year is the distance travelled by light in the outer space (vacuum) in one
year. Since the speed of light in vacuum is 3.0 x 108 ms-1, the distance in kilometres
equivalent to 1 light-year:
Test Yourself:
1. Determine the number of astronomical units that is equivalent to 1 light-year.
2. Proxima Centauri is the second nearest star to the Earth after the Sun. It is about
4.2 light-years away from the Earth. How far away is the star in kilometres?
______________________________________________________________________________
Nakazawa EduServ. Created by Jackson Tik, B.Sc. (NUS). ©All rights reserved.
1
, IGCSE Physics\Stars & The Universe\Important Concepts
STARS AND GALAXIES
A star is a luminous ball of plasma, mostly hydrogen and helium, held together by
its own gravity. The Sun is an example of a star.
In a stable star, energy is released due to fusion of
hydrogen nuclei into helium in the core of the star.
Two forces act within the star:
(a) Outward force due to high temperature and
pressure from the core
(b) Inward force due to gravitational attraction
towards the core
When the outward force and inward force are
equal to each other, the star is in hydrostatic
equilibrium and is hence stable.
The life cycle of a star:
(a) Gas and dust particles, mostly hydrogen, accrete due to gravitational
attraction, forming a nebula.
(b) As more gas and dust particles accrete, gravitational attraction increases,
causing the core of the nebula to collapse, resulting in a spinning ball of gas
and dust known as a protostar.
(c) As the protostar spins, the density and the temperature at the core increase
gradually to a point where fusion of hydrogen nuclei into helium is triggered,
forming a main-sequence star, releasing energy in the form of heat and light.
(d) Under high temperature, the star expand until the force due to outward
pressure becomes equal to the inward gravitational attraction, hence the main-
sequence star is a stable star.
______________________________________________________________________________
Nakazawa EduServ. Created by Jackson Tik, B.Sc. (NUS). ©All rights reserved.
2
IGCSE PHYSICS
STARS AND THE UNIVERSE: IMPORTANT CONCEPTS
MEASUREMENT OF ASTRONOMICAL DISTANCES
Two different units are commonly used to express astronomical distances, i.e.
distances in the outer space, namely:
(a) astronomical unit (au): for distances within the Solar System
(b) light-year (ly): for distances beyond the Solar System
1 astronomical unit is equivalent to the average distance from the Sun to the Earth,
which is about 150 million kilometres.
Examples: Distance of the planets from the Sun
Distance from the Sun Distance from the Sun
Planet Planet
106 km au 106 km au
Mercury 58 Jupiter 780
Venus 108 Saturn 1430
Earth 150 1.0 Uranus 2870
Mars 230 Neptune 4510
1 light-year is the distance travelled by light in the outer space (vacuum) in one
year. Since the speed of light in vacuum is 3.0 x 108 ms-1, the distance in kilometres
equivalent to 1 light-year:
Test Yourself:
1. Determine the number of astronomical units that is equivalent to 1 light-year.
2. Proxima Centauri is the second nearest star to the Earth after the Sun. It is about
4.2 light-years away from the Earth. How far away is the star in kilometres?
______________________________________________________________________________
Nakazawa EduServ. Created by Jackson Tik, B.Sc. (NUS). ©All rights reserved.
1
, IGCSE Physics\Stars & The Universe\Important Concepts
STARS AND GALAXIES
A star is a luminous ball of plasma, mostly hydrogen and helium, held together by
its own gravity. The Sun is an example of a star.
In a stable star, energy is released due to fusion of
hydrogen nuclei into helium in the core of the star.
Two forces act within the star:
(a) Outward force due to high temperature and
pressure from the core
(b) Inward force due to gravitational attraction
towards the core
When the outward force and inward force are
equal to each other, the star is in hydrostatic
equilibrium and is hence stable.
The life cycle of a star:
(a) Gas and dust particles, mostly hydrogen, accrete due to gravitational
attraction, forming a nebula.
(b) As more gas and dust particles accrete, gravitational attraction increases,
causing the core of the nebula to collapse, resulting in a spinning ball of gas
and dust known as a protostar.
(c) As the protostar spins, the density and the temperature at the core increase
gradually to a point where fusion of hydrogen nuclei into helium is triggered,
forming a main-sequence star, releasing energy in the form of heat and light.
(d) Under high temperature, the star expand until the force due to outward
pressure becomes equal to the inward gravitational attraction, hence the main-
sequence star is a stable star.
______________________________________________________________________________
Nakazawa EduServ. Created by Jackson Tik, B.Sc. (NUS). ©All rights reserved.
2
