The document provides information about stars and galaxies. It begins by describing characteristics of the Sun such as its size, temperature, and composition. It then discusses the structure of the Sun and phenomena on its surface like sunspots, solar flares, and prominences. The effects of solar phenomena like solar wind on Earth are also outlined. The document concludes by describing galaxies like the Milky Way and theories about the formation, evolution and potential end of the Universe.

1. 1
FORM 3 SCIENCE NOTES
CHAPTER 9 STARS AND GALAXIES
9.1 The Sun
Characteristics of the Sun
 Located at the centre of Solar System.
 One of the many billions of stars.
 Huge mass of hot, glowing gases.
 Nearly 110 times bigger than Earth.
 Nearest star to Earth (Appears bigger and brighter)
 About 150 M km away from Earth.
 Sunlight takes about 8 minutes to Earth.
 Medium-sized star known as yellow dwarf.
 Largest object in Solar System.
 Mass of Sun is about 98% of total mass of Solar System.
 Its density is very much less than Earth (It is not a solid object but is made up of primarily gases)
 About 4.6 billion years old and will continue to exist for another 5.5 billion years.
Property Characteristic Characteristic compared to Earth
Diameter 1 392 000 km 109 times the diameter of Earth
Mass 1.989 × 1034
kg 333 420 times the mass of Earth
Density 1 485 kg m-3
0.27 times the density of Earth
Surface
temperature
5 500 – 6 000 C 227 – 273 times the temperature on Earth
Composition
About 70% hydrogen, 28% helium and
other elements such as carbon, nitrogen,
oxygen, silicon and iron which amount to
less than 2%
Earth’s atmosphere contains oxygen,
nitrogen, carbon dioxide, inert gases and
water vapour
Characteristics of the Sun

2. 2
Structure of the Sun
 Core 核心
- Inner most layer of Sun
- Extends from centre to about 25% of Sun’s radius
- 15 000 000 C
 Photosphere 光球
- Innermost atmospheric layer of dense gases
- Considered as the surface of the Sun
- Origin of visible light that reaches Earth
- 300 km thick, 6 000 C
- One of the coolest layers of Sun
- Experiences turbulence, much like surface of simmering pot of water
- This is because explosive eruptions of energy on surface of Sun
- Interactions with Sun’s magnetic field result in appearance of sunspots
 Chromosphere 色球
- Layer above photosphere
- 10 000 km thick, 6 000 C to 20 000 C
- Glows red (Hydrogen gives off a reddish colour)
- Invisible (Can only be seen during total solar eclipse)
- Turbulence in photosphere cause solar flares and eruptions which can be observed in
chromosphere layer
 Corona 日冕
- Extremely hot outermost layer above chromosphere
- Over 1 000 000 C
- Thickest layer, million kilometers
- Forms rings of whitish-blue light
- Can only be seen during total solar eclipse
- Prominences appear
Phenomena on the Sun’s surface
 Prominences 日珥
- Immense clouds of glowing gases that erupt from the upper chromosphere
- Allows some of the surface gases (Hydrogen and helium) escape into space
- Prominences escape into outer surface cool down and fall back to the Sun’s surface
 Solar flares 耀斑
- Result of violent energy explosions in complex sunspot groups.
- Release gases and charged particles
- 5 M C
- Emit electrons, visible light and radiation such as X-rays and UV rays
- Make the night sky above Earth’s poles appear colourful. (Aurora 极光)

3. 3
 Sunspots 太阳黑子
- Dark regions which are visible on photosphere
- As a result of high concentration of magnetic fields generated by Sun
- Appears dark (Much cooler than their brighter surroundings)
- 4 000 C
- Appear in groups and different sizes
- Small sunspot normally last several hours while a major group of sunspots may last for a few
months.
Effects of the Sun’s phenomena on Earth
 Eruptions of prominences and solar flares release large amounts of solar material into space. Gases
that escape to space carry stream of electrically charged particles of energy. (Solar wind 太阳风)
 Communication systems
- Earth’s atmosphere reflects radio signals.
- Solar wind causes radio signals to fluctuate 波动.
 Navigation systems and compasses
- Radio signals from transmitters are used by ships and aeroplanes to determine their locations.
- Solar wind disrupts the radio signals, resulting in inaccuracies in navigation systems.
- Intense solar flares send out continuous stream of electrically charged particles which interfere 干扰
with Earth’s magnetic field and compasses.
 Satellites and astronauts
- UV rays and X-rays given off by solar flares heat up Earth’s upper atmosphere. Satellites have to
orbit further up to prevent falling back to Earth.
- Lifespans are shortened.
- High energy particles released by intense solar flares increase radiation hazards and pose threat to
health of astronauts in space.
- Excessive radiation damages satellites.
 Power generation
- Charged particles interfere with Earth’s magnetic field and induce surges in electric current along
power transmission lines.
- Overloads power grids and causes blackouts over large areas.
 Global climate
- Sunspots bring about changes in temperature, humidity and atmospheric pressure, which affect
weather conditions on Earth.
- Wind, land and sea breezes are affected by sunspots.
 Formation of aurorae
- Aurorae – Bands of coloured light in night sky, especially at polar regions on Earth.
- Result from collision between charged particles of solar wind and gas molecules in atmosphere of
Earth.
- Fluctuations in solar wind can cause them visible and lower altitudes.

4. 4
Generation of energy by the Sun
 Solar energy is generated deep in core of Sun though nuclear fusion.
 Nuclear fusion – Nuclear reaction in which several atoms of one element combine to form a different
element.
 Temperature and pressure in core are intense that nuclear reactions take place all time.
 Four hydrogen nuclei combine to form a helium nucleus and large amount of energy is produced.
 Energy generated is carried to surface of Sun and released as light and heat.
9.2 Stars and Galaxies in the Universe
Definition of a star
 Star – Celestial body 星体 that releases its own heat and light.
 Big ball of hot gases
 Made up of dust and gases
 Distance between stars and Earth is measured in light years.
 One light year = Distance light travels in one year = 9,460,528,177,426.821 km
 Not distributed uniformly in Universe but are grouped together in galaxies.
The Sun as a star
 Sun was formed in a huge could of gas and dust called nebula.
 It is a huge sphere made up mainly of hydrogen and helium. It generates heat and light and releases
energy through nuclear fusion.
Various types of stars
 Colour and temperature
- Colour of star indicates its temperature.
Class Colour Surface temperature ( C) Example
O Blue More than 25 000 Spica
B Whitish-blue 11 000 – 25 000 Rigel
A White 7 500 – 11 000 Sirius
F Yellowish-white 6 000 – 7 500 Procyon A
G Yellow 5 000 – 6 000 The Sun
K Orange 3 500 – 5 000 Arcturus
M Red Less than 3500 Betelgeuse
The colour and temperature of stars

5. 5
 Size
Type of star White dwarf Dwarf Giant Supergiant
Relative size (Sun = 1) 0.1 10 10 – 100 > 100
 Brightness
- Brightness of star is known as apparent magnitude.
- Star with an apparent magnitude of 1 is the brightest and star with an apparent magnitude of 6 is the
dimmest.
- Brightness of stars depends on:
 Surface temperature
 Size
 Distance from Earth
Name of star Distance in light years
Sirius 9
Canopus 98
Alpha Centauri 4.3
Arcturus 36
Vega 26
Capella 45
Rigel 90
Procyon 11
Achernar 118
Beta Centauri 490
Ten brightest stars (In order of brightness)

6. 6
Formation of stars
 Nebula 星云 consists mainly of gases such as hydrogen and helium, and dusts that collect as a result of
pull of gravity between particles.
 Star is formed when nebula is pulled inwards towards core until it becomes compact. As nebula
collapses, it starts to spin.
 Gravitational force increases and causes material within nebula to condense.
 As a result, temperature and pressure of gases and dust particles at the centre increase.
 When temperature reaches 15 000 C, nuclear fusion takes place at core and hydrogen atoms fuse to
form helium atom, releasing heat and light energy.
 Ball of gas starts to shine and new star is born.
 Star continues to generate heat and light energy through nuclear fusion reactions in core.
 Once a star is stable, its size remains constant.
Gases and dusts in
nebula are pulled
by strong gravity
forces until it
becomes compact.
A very hot and dense
core is formed.
Core becomes hotter.
Nuclear reactions
take place. A star is
formed.
Formation of a star

7. 7
Death of stars
 Star with small mass has longer lifespan.
 Once hydrogen fuel is used up and nuclear fusion is completed, core of star starts to shrink (Dying).
 The star cools, undergoes further changes depending on mass of star and eventually dies.
 Star will become white dwarf, neutron star or black hole when it dies.
 Death of medium-sized star
- Heat generated will heat up the outermost layer and causes it to expand and become brighter.
- Become bigger and red giant star is formed.
- Outer layer breaks and drifts into space.
- Core cools down to become white dwarf 白矮星 and uses helium as its nuclear fuel. When helium
is exhausted, it will fade into a dark body called black dwarf 黑矮星.
 Death of large star
- Expands become red supergiant which collapses rapidly and causes gigantic explosion called
supernova 超新星.
- The dense core left is called neutron star 中子星. It will eventually lose all its heat.
 Death of super-large star
- Expands become red supergiant and collapses rapidly and causes gigantic explosion called
supernova. (Same as large star)
- Contracts and becomes very dense that even light cannot escape from it. (Black hole 黑洞)
Formation and death of medium-sized star, large star and super-large star

8. 8
Galaxies
 Galaxy 星系 – Group of millions or billions of stars held together by gravity.
 Elliptical galaxies
- Flattened oblong shape
- Little gas or dust (Very few new star are formed)
- Consists mostly of old stars
- Core is bright but edges are dim
 Spiral galaxies
- Disc-shaped with arms spiraling outwards
- Brightest galaxies in Universe
- Young, hot stars, dust and gases are concentrated in spiral arms
- Percentage of young stars is high (A lot of dust and gases)
- 30% of galaxies are spiral galaxies
- Example: Milky Way, Andromeda Galaxy
 Irregular galaxies
- Do not have specific shapes (May change in shape)
- Smallest galaxies
- Contain a lot of dust and gases
- Consists of mainly new stars and nebulae
- 10% of galaxies are irregular galaxies
- Example: Large Magellanic Clouds, Small Magellanic Clouds
Elliptical galaxy Spiral galaxy Irregular galaxy

9. 9
The Milky Way
 Milky Way appears on a clear night, with a band of light spreading across the sky.
 It is a spiral galaxy.
 It is shaped like a flat disc and has projections 投影.
The Universe
 Universe 宇宙 consists of matter, energy and space.
 It is everything we see and is unimaginably huge.
 Origin of Universe is still unanswered but many astronomers support the Big Bang theory 大爆炸原理.
 Astronomers believe that Universe is still expanding and galaxies are also breaking up.
 Until today, astronomers do not know the exact size of Universe.
9.3 The Universe as a Gift for God
The extent of Universe
 If Big Bang theory is correct, Universe is 15 billion years old.
The Universe is not permanent
 Stars use nuclear fusion to generate heat and light energy.
 When hydrogen is exhausted, all stars will burn out and dies.
 If outward expansion continues, all matter will be too far apart and eventually disappear.
 Gravity of Universe may become great that it can bring all matter together and Universe collapses into
black holes.
The importance of the Sun to life on Earth
 Keep Earth in right position to receive light and heat.
 Provides light energy and heat energy to living organisms.
The importance of the Moon to life on Earth
 Ocean tides
 When Moon orbits close to Earth, pull of its gravity results in high tides.
 This causes erosion and displacement 移位 of Earth’s surfaces which affect life on Earth.
Position of solar system in Milky Way
Milky Way during the night