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Structure of the Universe. Galaxies. A galaxy is the basic structural unit of matter in the universe.

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A galaxy is the basic structural unit of matter in the universe.

A galaxy is a collection of stars (usually 100 billion or more), gases and dust held together by gravity. It is believed that the universe is made up up of more than 100 billion galaxies.

Galaxies are classified by their shape: elliptical, irregular or spiral.

The Milky Way Galaxy is spiral-shaped.


A star is the most visible piece of matter in a galaxy; it is a large ball of gas that produces a large amount of energy. This ball of gas is held together by gravity.

Stars and their Energy Production

Most of the energy produced by a star is a result of nuclear fusion. Nuclear fusion is the combining of smaller elements to make larger one. In this process, some of the mass is converted into energy.

In our sun, hydrogen nuclei is converted into helium. Less than 1% of this mass is converted to energy.

Nuclear fusion requires a tremendous amount of heat and pressure.

The energy from the sun (through nuclear fusion) is radiated into space in the form of electromagnetic radiation.

In our sun, hydrogen is combined to make helium.


RT = Pg. 15

Characteristics of Stars

The chart shown is a tool used by scientists to make comparisons between stars based on their surface temperature and luminosity.

Luminosity is the rate at which a star emits energy with relation to our sun if all stars were the same distance from an observer.

The chart shows a pattern between the surface temperature of a star and its luminosity.

As a star “cools”, the color of the star will change.


RT = Pg. 15

Star Types

Super Giants

Very Bright!

100-1000x larger than the sun.

End stage of star development; usually followed by a supernova.


10X larger or more than the sun

Red, orange, yellow in color

Main Sequence

Most of a star’s life span is in main sequence.

Average size

White Dwarfs

Very small (Earth-sized)

Very hot; not so bright!

End stage of star development; usually followed by a black dwarf


Star Origin and Evolution

Gas and dust

Matter originates from “Big Bang”.

Star Evolution for stars more massive than the Sun.

Matter collects, gravity forms and nuclear fusion begins

Main Sequence Star

Super Giant forms


Star Evolution for stars of similar mass to that of the Sun.

Neutron star forms; very dense

Red Giant forms

White dwarf forms as fuel is used up.

Black hole forms; extreme gravity field created

“Death” of a star; Black dwarf