Edexcel IGCSE Physics - Stellar Evolution

what is the colour of a star caused by?

the visible light it emits

what determines how much visible light of each frequency a star emits?

its surface temperature

what colour light has the lowest frequency?

red

what colour light has the highest frequency?

blue

what frequency of light do very hot stars emit?

high

what frequency of light do cooler stars emit?

low

what are the 5 colours used to classify stars?

red, orange, yellow, blue, white

what is the order of star colours from hottest to coolest (1 is hottest)?

1. blue

2. white

3. yellow

4. orange

5. red

why are some stars white?

they emit roughly equal amounts of all colours of light

what are the 2 kinds of star evolution cycle?

• evolution of stars with a similar mass to the sun

• evolution of stars with a higher mass than the sun

what are the stages of evolution of stars with a similar mass to the sun?

• nebula

• protostar

• main sequence star

• red giant

• white dwarf

what are the stages of evolution of stars with a higher mass than the sun?

• nebula

• protostar

• main sequence star

• red supergiant

• supernova

• neutron star OR black hole

what is a nebula?

a cloud of dust and gas

describe the change from a nebula to a protostar

• the force of gravity pulls the dust and gas together to form a protostar

• the temperature rises as the star gets denser

describe the change from a protostar to a main sequence star

• when the temperature and pressure become high enough, hydrogen nuclei fuse to form helium nuclei

• this releases a lot of energy, keeping the core of the star hot

• a star is born

describe the stage of being a main sequence star

• the outward pressure caused by nuclear fusion balances the force of gravity pulling everything inwards

• this stage lasts for a very long time

• the heavier the mass, the shorter the stage

describe the change from a main sequence star to a red giant

• the hydrogen is used up and the centre of the star collapses due to the lack of outward pressure (force of gravity is greater)

• it becomes extremely dense and hot, so the energy and pressure cause the outer layers to expand greatly to become a red giant

• the star becomes red because the surface cools

describe the change from a red giant to a white dwarf

• the red giant star becomes unstable - outer layers of dust and gas are ejected

• the centre of the star compresses and it becomes very hot and dense

• the next stage is a black dwarf where the star cools and becomes more dense - only theoretical

describe the change from a main sequence star to a red super giant

• the hydrogen is used up and the centre of the star collapses due to the lack of outward pressure (force of gravity is greater)

• it becomes extremely dense and hot, so the energy and pressure cause the outer layers to expand massively to become a red super giant

• the star becomes red because the surface cools

describe the change from a red super giant to a supernova

• more fusion begins to take place to make heavier elements, going up to iron

• the stars expand and contract as the balance shifts between gravity and thermal pressure

• the stars explode in a supernova

describe the change from a supernova to a neutron star or black hole

• the supernova throws layers of dust and gas into space

• this leaves a very dense core called a neutron star

• if the star is massive enough, it will collapse and become a black hole - incredibly dense point that no light can escape from

what does apparent magnitude mean?

the perceived brightness of a star as seen from Earth - the closer the star the brighter it appears

what does absolute magnitude mean?

how bright a star would be if it was at a fixed distance from earth

what is the rough absolute magnitude of the sun?

+5

what is the relationship between absolute magnitude and brightness of a star?

the lower (more negative) the absolute magnitude, the more bright a star is

what diagrams are used to show different types of star?

Hertzsprung-Russel Diagrams

what are the axis on a Hertzsprung-Russel diagram?

• y-axis → (decreasing as upwards) absolute magnitude

• x-axis → (decreasing as right) temperature

where are main sequence, red giants/super giants, and white dwarfs located on a Hertzsprung-Russell diagram?

ignore luminosity and spectral class

where is the sun on a Hertzsprung-Russel diagram?