Galaxies consist of millions of stars, gas and dust. Our galaxy, the Milky Way, is a medium-sized spiral galaxy located in the universe. It contains approximately 200 billion stars, and our solar system is situated in one of its spiral arms. Stars are formed from the gravitational collapse of large clouds of dust and gas called nebulae. As the core of the forming star grows denser through gravitational compression, the rising temperature and pressure triggers nuclear fusion, releasing energy and forming a star.

2. A galaxy is a set of bodies consisting of millions of stars with gas and
dust particles.
Galaxies come in many forms, such as spiral galaxies, elliptical galaxies
and irregular galaxies.
Our solar system is situated in the Milky Way galaxy.

3. Types of galaxies
Spiral Elliptical Irregular
THE MILKY WAY
•A medium large spiral galaxy
•Our solar system is located at the edge of
one of the spiral arms
•Consists of approximately 200 billion stars
and the Sub is one of it

5. Birth of Stars
Formed from nebulae (large clouds consisting of dust particles and gases such as hydrogen and
helium).
•The dust particles and gases in nebulae are pulled by a strong gravitational force that cause it to form a
globe.
•The strong gravitational force causes the globe of gas to shrink and compress until it becomes very
dense and forms a core.
•The core shrinks and becomes dense due to the increasing strength of the gravitational force
•The temperature and pressure in the core too high, a nuclear reaction will take place. Hydrogen gas
turns into helium. A huge amount of heat energy and light is released.
•The core will shine and a star (protostar) is formed.
•Star continues expand and becomes either an average star like the Sun or a massive star.
Death of Stars
•A lot of heat is generated in a star which will heat up the outermost layer of a star. Hydrogen within this
layer stars to burn. The star expand. During this stage, the stars appears red in colour and called a red
giant.
•A white dwarf is formed if the red giant no massive. Red giant that big enough will contracts so quickly
that a big explosion (supernova) occurs. Supernova is extremely bright. It can be seen in daylight. A
neutron star if formed in the explosion if the original star is a large star. A black hole is formed if the
original star is super-large star. It is called a black hole because light in it cannot escape and any matter
that enters it cannot escape too.

6. Relative Size Comparison between the Earth and the Universe

7. Characteristics of Stars
Stars have colours that correspond to its surface
temperature which range from a lower to higher
temperature.

8. Stars have different sizes; the really big ones are called supergiant stars,
big stars are called giant stars while the really small ones are called
dwarf stars. The brightness of a star depends on its size, distance and
surface temperature. The brightest stars in the sky are Sirius and Rigel.