CHAPTER 9 : STAR AND GALAXIES

9.1 The Sun

Characteristics of the Sun

1. At the centre of the Solar System, lies our very own star known as the Sun.

2. The Sun is nearly 110 times bigger than the Earth.

3. The Sun appears bigger and brighter than all the other stars. This is because the Sun is the nearest star to Earth.

4. The Sun is the largest object in the Solar System. The mass of the Sun is about 98% of the total mass of the Solar System.

Structure of the Sun

Figure 9.1 Structure of the Sun showing some phenomena on its surface

1. The core is the innermost layer of the Sun. The temperature of this layer can rise to 15 000 000 ̊C.

2. The surface of the Sun consists of three gaseous layers: photosphere, chromosphere and corona.

The photosphere1. The photosphere is the innermost atmospheric

layer of dense gases. It is considered as the surface of the Sun.

2. The visible light that reaches Earth from the Sun originates from the photosphere.

3. The photospheres experiences turbulence, much like the surface of a simmering pot of water. This is due to the explosive eruptions of energy on the surface of the sun.

4. Sometimes, interactions with the Sun’s magnetic field result in the appearance of sunspots on the photosphere.

The chromosphere1. The chromosphere is the layer above the visible

photosphere.2. Solar energy passes through this region on its

way out from the centre of the Sun. The

temperature of this layer ranges between 6 000 ̊C

and 20 000 ̊C.3. The chromosphere glows red because hydrogen

gives off a reddish colour at these high temperatures.

The corona1. The corona is the extremely hot outermost layer

above the chromosphere.2. Temperatures in the corona can reach over 1 000

000 ̊C.3. It is the thickest layer, stretching several million

kilometres into space.

Phenomena on the Sun’s surface

1. Prominences, solar flares and sunspots are examples of phenomena that occur on the surface of the sun.

2. These phenomena are believed to occur because of the changes in the magnetic field of the sun.

Prominences 1. Prominences are immense clouds of glowing

gases that erupt from the upper chromosphere. 2. The loops or arches of gases may shoot as high

as 100 000 kilometres from the surface of the Sun. This allows some of the surface gases, mainly hydrogen and helium, to escape into space.

3. Prominences can last for two to three months and can be seen during solar eclipses.

Solar flares1. Solar flares are the result of violent energy

explosions in complex sunspots groups. 2. Solar flares release gases and charged particles

far into space.3. Solar flares make the night sky above the poles

appear colourful . this is known as an aurora.

Sunspots 1. Sunspots are dark regions which are visible on

the photosphere.2. Sunspots appear dark because they are much

cooler than their brighter surroundings. The

temperature in these regions is about 4 000 ̊C.

Effects of the Sun’s phenomena on Earth

1. The phenomena that occur on the surface of the Sun have various effects on Earth.

2. a. The eruptions of prominences and solar flares release large amounts of solar material into space.b. Gases that escape to space carry a stream of electrically charged particles of energy.c. The continuous flow of these particles from the surface of the Sun causes a phenomenon known as solar wind.

3. When the solar wind blows past Earth, there are effects on the following.

4. Communications systemsa. Solar wind affects radio communication as it

causes radio signals to fluctuate.5. Navigation systems and compasses

a. Radio signals from transmitters are used by ships and aeroplanes to determine their locations.

b. Intense solar flares send out continuous streams of electically charged particles which interfere with the Earth’s magnetic field and compasses.

6. Satellites and astronautsa. The ultraviolet rays and X-rays given off by

solar flares heat up the Earth’s upper atmosphere. As a result, satellites have to orbit further up to prevent them from falling back to Earth. Their lifespans are also shortened.

7. Power generationa. The charged particles interfere with Earth’s

magnetic field and induce surges in the electric current along power transmission lines.

b. This overloads the power grids and causes blackouts over large areas.

8. Global climatea. Sunspots bring about changes in temperature,

humidity and atmospheric pressure, which affect the weather conditions on Earth.

b. Wind, land and sea breezes are a result of the effects of sunspots.

9. Formation of aurorae1. Aurorae result from a collision between the

charged particles of the solar wind and the gas molecules in the atmosphere of the Earth. This interaction causes them to emit a visible light.

Generation of energy by the Sun

1. Solar energy is generated deep in the core of the Sun through a process called nuclear fusion.

2. Nuclear fusion is a nuclear reaction in which several atoms of one element combine to form a different element.

3. In the nuclear fusion process at the Sun’s core, four hydrogen nuclei combine to form a helium nucleus and a large amount of energy is produced.

4. The energy generated is carried to the surface of the Sun and released as light and heat.

9.2 Stars and Galaxies in the Universe

Definition of a star

1. A star is a celestial body that releases its own heat and light. It is a big ball of hot gases.

2. Stars are made up of dust and gases like hydrogen and helium. They generate energy through nuclear reactions.

The Sun as star

1. The Sun is a medium-sized star. It is the closest star to Earth. Therefore, the Sun appears big, bright and hot, when compared to the other stars in the sky.

2. It is formed in a huge cloud of gas and dust called nebula.

3. 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

1. Astronomers use some of these characteristics to classify the stars.

The colour and temperature of stars1. The colour of a star indicates its temperature.2. The hottest stars are blue and the coldest stars

are red.3. The Sun, which is yellow in colour, is a star of

average temperature.4. Stars can be divided into seven classes based on

their temperatures. These are O, B, A, F, G, K and M.

Table 9.1 the colour and temperature of stars

Class ColourSurface

Temperature ̊C Example

O Blue More than

25 000Spica

BWhitish-

blue11 000 – 25 000 Rigel

A White 7 500 – 11 000 Sirius

FYellowish

- white6 000 – 7 500 Procyon

G Yellow 5 000 – 6 000 The Sun K Orange 3 500 – 5 000 ArcturusM Red Less than 3 500 Betelgeuse

Size

1. Stars are of different sizes. They can be classified as neutrons, dwarfs, giants and supergiants.

2. The smallest star is the neutron star which has a diameter of about 10 kilometres.

3. The Sun is a medium-sized star. Most stars are about the same size as the Sun. A dwarf is a star

that is about the same size as the Sun. A white dwarf is a star smaller than the Sun.

4. A giant star is a star about twenty times bigger than the Sun.

5. A supergiant star is a star that is more than 100 times the size of the Sun.

Brightness 1. Long ago, the brightness of a star was known as

the apparent magnitude. This was determined by naked eye.

2. The brightness of a star depends on factors such as the surface temperature , size and distance from Earth. Classifications using the naked eye are not accurate.

Formation of stars

1. Stars are formed within huge clouds of gases and dust called nebulae.

2. A nebula consists mainly of gases such as hydrogen and helium and dust that collect as a result of the pull of gravity between the particles.

3. A star is formed when a nebula is pulled inwards towards the core until it becomes compact. As the nebula collapses, it starts to spin.

4. The gravitational force increases and this causes the material within the nebula to condense. As a result, the temperature and pressure of the gases and dust particles at the centre increase.

5. a. When the temperature reaches 15 000 ̊C , nuclear fusion takes place at the core of the nebula.2. Hydrogen atoms fuse together to form helium

atoms, releasing a large amount of heat and light energy.

6. The ball of gas starts to shine and a new star is born.

7. The star continues to generate heat and light energy through the nuclear fusion reactions that take place in its core.

8. Once a star is stable, its size remains constant.

Deadth of stars

1. The lifespan of a star depends on its size.2. a. A star with a small mass has a longer lifespan

when compared to a star with a big mass.

b. Small stars ( smaller than the Sun ) may finally die after more than 10 billion years but super-large stars may not survive more than 100 million years.

3. Once the hydrogen fuel is used up and nuclear fusion is completed, the core of the star starts to shrink. The star is said to be dying.

4. a. The star cools, undergoes further changes depending on the mass of the star and then eventually dies.b. A star will become either a white dwarf, a

neutron star or a black hole when it dies.

Figure 9.2 Formation and death of Star

Galaxies

1. A galaxy is a group of millions or billions of stars held together by gravity.

2. There are millions of galaxies scattered at random throughout the Universe. Each galaxy has its own shape, size and luminosity, and contains different bodies.

3. There are three basic types of galaxies.a. Elliptical galaxies b. Spiral galaxiesc. Irregular galaxies.

The Milky Way

1. On a clear night, we can see a band of light spreading across the sky. This is our own galaxy, the Milky Way.

2. It is home to Earth and the Solar System.3. The Milky Way is a Spiral Galaxy. It is shaped

like a flat disc and has projections.

4. The centre of the Milky Way has many old stars while its spiral limbs have a lot of dust, gases and young stars.

Figure 9.3 The Milky Way

The Universe

1. The Universe consists of matter, energy and space.

2. The Universe is everything you see around you. It is unimaginably huge.

3. The origin of the Universe is still unanswered but many astronomers support the Big Bang theory.

4. Astronomers believe that the Universe is still expanding and galaxies are also breaking up.

5. Until today, astronomers still do not know the exact size of the universe.

Position of the Solar System in the Universe1. The Milky Way and millions of other galaxies

make up the Universe.2. Thus, the Solar System is only one tiny part of

the whole Universe.

9.3 The Universe as a gift from God

The extent of the Universe

1. No one really knows the extent of the Universe.2. The Universe that is observed through the most

powerful telescope we have today may be only a small portion of the real physical Universe.

3. If the current Big Bang theory is correct, the Universe is 15 billion years old. We can assume that the size of the Universe is about 15 biliion light years.

The Universe is not permanent1. All the stars in the galaxies in the Universe

generate light and heat energy through nuclear fusion.

2. The stars shine by burning hydrogen. When the source of hydrogen in the Universe is exhausted, all the stars will burn out and die.

3. The gravity of the Universe may become so great that it will bring all matter in the Universe back together and the Universe will collapses into black holes.

4. It is, therefore, true to say that everything that exists in the Universe is not permanent.

The importance of the Sun to life on Earth

1. Earth and other planets are linked to the Sun by a gravitational force. The gravitational force of the Sun is always present to keep Earth in the right position to receive light and heat from the Sun.

2. Without light and heat from the Sun, life cannot exist on Earth.

3. The Sun provides light energy and heat energy directly to living organisms.

4. All energy on Earth originates from the Sun.5. Heat energy from the Sun can lead to the

formation of clouds, rain, wind and droughts.6. Green plants carry out the process of

photosynthesis by absorbing and converting the solar energy from the Sun into chemical energy. This energy is stored in the food synthesised by plants.

7. During photosynthesis, green plants also remove carbon dioxide from the air and increase the supply of oxygen which is needed for life to exist on Earth.

8. Solar cells convert solar energy into electrical energy for our daily use.

The importance of the Moon to life on Earth

1. The Moon influences life on Earth directly on indirectly.

2. The most obvious influence of the Moon on Earth is ocean tides.

3. When the Moon orbits close to Earth, the pull of its gravity results in high tides. This, in turn, causes erosion and the displacement of the Earth’s surfaces which affect life on Earth.