Chapter 9 Stars and Galaxies

Form 3 Science Chapter 9 Stars and Galaxies

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 × 10
34
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

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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)

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 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.

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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)

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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

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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