STARS AND GALAXIES

By LUBEGA Shafiq

May 1, 2025

1 STARS
A star is a giant ball of hot, explosive gas (mostly hydrogen, few amounts of helium and other elements)
that makes its own light and heat energy by burning hydrogen in its core and turning it into helium through
a process known as nuclear fusion.

1.1 The sun as a star

The sun is a medium-sized star which consists of mainly hydrogen and helium. It is the major source of all
energy on earth. The radiation it emits is mostly infrared, visible light and ultraviolet light.

1.2 The source of energy in stars

Hydrogen is the fuel in the core of all newly born and mid-age stars. The high temperature and pressure in
star’s core squeezes the nuclei of hydrogen atoms together, to form helium. This process is known as nuclear
fusion.
A large amount of energy is released in the fusion process. Part of this energy mantains the higher temperature
in the core and the rest is released into space as electromagnetic radiation.

Activity 1
1. List the main types of radiations emitted by the sun.
2. Briefly describe the composition of our sun and the source of all the energy it releases into the vast space
surrounding our planet earth.

1.3 Importance of energy produced by the sun to the people on earth

• It is a source of heat energy to warm earth and all the creatures living on it. Without heat from the sun,
earth and all creatures living on it would freeze.
• The sun’s energy transports water through winds, ocean currents and clouds. Without the sun, water
would be stil with no flowing rivers and no hydroelectric power.
• The sun is a source of sunlight for plants to grow and humans survive on plants for food and oxygen.
• The sun is a source of solar energy. This can be transformed into several forms of energy like electrical
and heat energy.

1.4 Variation of stars

Stars vary in age, size, mass, surface temperature, colour and brightness.
1. Colour
The colour of a star depends on its surface temperature. A star with a very high surface temperature
appears white or blue. A star with a lower surface temperature appears red or yellowish.
2. Brightness

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 The brightness of a star depends on its surface temperature and distance from the observer. A star
with a very high surface temperature appears brighter. A star with a lower surface temperature appears
dimmer. Additionally, stars which are very close to us appear brighter than those very far away from
our home planet. Thats why the sun, our closest star, appears very big yet its just a medium-sized star.
3. Mass
Very massive stars, usually ten times our sun, are the hottest (with very high surface temperatures). As
a result, they appear blue or white in colour compared of medium-sized stars like our sun which appear
red or yellowish.

1.5 Origin of stars (How stars come into existance)

Stars form in large clouds of hydrogen gas and dust called molecular clouds. These molecular clouds collapse
due to extreme gravitational attraction. When this happens, friction makes the material to heat up resulting
into formation of a Protostar.
As more matter collects on the protostar, its mass and core temperature increase until its enough to sustain
nuclear fusion of hydrogen into helium. As soon as this nuclear fusion process begins, a star is born.
If the newly born star has a very large mass, it forms a blue or white star. If the newly born strar is a
medium-sized star, like our sun, it forms a red or yellowish star and this is common.

1.6 Life cycle of a star

1. Molecular clouds or Nebulae
A star forms from molecular clouds of hydrogen gas and dust. These clouds collapse due to gravitational
attraction to form a Protostar. When more matter collects on the protostar, its mass and temperature
increase untill it initiates nuclear fusion of hydrogen into helium. A main sequence star is born.
2. Main sequence star
A star spends most of its life time in this main sequence stage, squeezing nuclei of hydrogen atoms
together to form helium. In the main sequence stage, the core of the star is at equilibrium i.e., the inward
force of gravity that tends to shrink the core balances with the opposite outward force due to thermal
pressure.
3. Red giant or Red Supergiant
When the star runs out of hydrogen as a fuel for nuclear reactions, it becomes unstable. There is less
heat energy produced by nuclear fusion to sustain the outward thermal pressure. The inward force of
gravity exceeds the opposite outward force due to thermal pressure which makes the core of the star to
collapse inwards (becomes small).
At the same time, a fast burn-up of the hydrogen layer surrounding the core takes place which makes
the outer layers of the star to expand. The star’s surface temperature reduces making it turn into a red
giant (if it was originally a medium-sized star) or red supergiant (if it was originally a very massive star.)
In this red giant or red supergiant stage, the star’s core becomes hot enough and another stage of nuclear
fusion begins i.e., nuclear fusion of helium into carbon. The star spends some time in this stage untill all
the helium, its new fuel for nuclear reactions, gets used up.
NOTE:
Further stages in the life cycle of a star depend on its original mass (Low mass or High mass star). Let’s
continue . . .
For low mass stars (like our sun)
When the red giant runs out of helium (its new fuel), the core collapses further due to intense gravity pulling it
inwards. Energy is released during the process which makes the outer layers of the star to be expelled, leaving
behind a small core. This small core becomes a white dwarf in the center of a glowing shell of ionized gas
known as planetary nebulae. The white dwarf eventually cools into a cold black dwarf consisting mainly
of carbon. This will be the fate of our Sun.

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For high mass stars
When the red supergiant runs out of helium (its new fuel), the core collapses further due to intense gravity
pulling it inwards. The core becomes smaller and hot enough to initiate another stage of nuclear fusion i.e.,
nuclear fusion of carbon into oxygen, nitrogen, and iron.
When the red supergiant runs out of carbon, nuclear fusion then stops and the energy of the star is released
in a supernova explosion. The centre of the supernova collapses to a very dense neutron star, which spins
rapidly and acts as a pulsar.
If the red supergiant is very massive, the remainder at the center of the supernova is a black hole. A black
hole is a region with strong gravitational attraction that nothing can escape from its surface, even light.

Did you know?

A massive black hole, weighing about 4 million suns sits at the center of our own galaxy, the milk way galaxy.
The more matter a black hole swallows, the more it widens. One day, our home galaxy will be swallowed by
this black hole. It will broaden and swallow all matter in the galaxy (planets, comets, the sun, meteors (space
rocks), moons e.t.c). Finally, the whole solar system will be a large black hole! Don’t worry, me and you won’t
be in existance.

Why do stars appear to twinkle as observed from earth?

The twinkling of stars is due to the refraction of starlight as it passes through the turbulent atmosphere.
The atmosphere has several layers of different temperature and optical densities (refractive indices) which bend
starlight and change its direction. As the light continues its journey to your eyes, it keeps changing paths
slightly, making the star appear to flicker or twinkle.
Shooting stars
Shooting stars aren’t stars at all—they’re meteoroids (tiny pieces of rocks, comets, asteroids, or even debris
from collisions in the solar system). When they come closer to Earth, gravity strongly pulls them and they
slam into the atmosphere at very high speeds. As a meteoroid plunges into the atmosphere, it compresses and
heats the air in front of it, and friction causes the meteoroid itself to heat up dramatically. This intense heat
vaporizes the meteoroid, leaving behind a glowing streak of light that we observe as a shooting star.

Activity 2
1. Write a report to members of your community who believe that stars are heavenly bodies and eyes
through which the creator observes the rest of happenings in our universe. Help them overcome this
misconception.
2. Theories say that one day the sun, our nearest star, will engulf earth and mercury. Explain the series of
stages the sun must go through before such a distressful event occurs!
3. Write a brief explanation to members of your community who believe that stars fall from the sky as
“shooting stars” when they reach the end of their life cycle.
4. Draw a cycle diagram summarizing the stages in the life cycle of a massive (very large star)star.
5. Draw a cycle diagram summarizing the stages in the life cycle of a low-mass (medium-sized) star.

2 GALAXIES
A galaxy is a collection of billions of stars. As well as containing stars, galaxies consist of clouds of gas, mostly
hydrogen and dust. Galaxies move in groups known as clusters, many rotating as spiral discs. There diameters
are enormous that it is measured using a new astronomical unit known as light-year. A ligh-year is the
distance travelled in (the vacuum of) space by light in one year.

2.1 Examples of galaxies

• The milk way galaxy (where our solar system and home planet belong)

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 • Andromeda galaxy
• Cygnus A

2.2 Variation of galaxies

Galaxies vary in size and number of stars. In a galaxy there are very many stars arranged in recognisable
shapes called constellations. Different galaxies have got different constellations. Some are elliptical, spiral
and others have no particular shape. Our milk way galaxy has a spiral constellation.

2.3 Location of our planet

Earth’s location in the universe is difficult to pinpoint because the universe is believed to have no center or
edge. But let’s try . . .
Earth is located in theVirgo supercluster, in the local group (a smaller group of galaxies within the Virgo
supercluster), in the milky way galaxy (the second largest galaxy in the local group), in the Orion Arm (one
of the Milk way’s spiral arms), in the solar system, and the third planet from the sun.

Research work
Using any smart device (tablet, laptop, smart television, or smartphone), you can get understanding of how
the universe (galaxies, solar systems, planets, moons, and everything including the non-observable universe)
came into existance.
***THE End***