Astrophysics part 2: stellar evolution

Astrophysics 2

What kind of galaxy is our galaxy?

The Milky Way is a spiral galaxy

Stars can be classified based on their ___

Colour

What does the colour of a star tell us about it? What colour will a very hot, medium hot and cool star look like?

Their surface temperature

• A very hot star looks blue

• A medium hot star like the Sun looks yellow

• A cool star looks red

Why is a hot star blue?

Hotter stars emit shorter wavelength light which we see as blue and the shorter the wavelength the higher the frequency which means more energy

TRIPLE CONTENT: What does the brightness of a star depend on?

• Distance the star is from the Earth

• The luminosity of the star — how much energy is transferred from the star each second

Therefore taking account that, what are the three ways in which astronomers decide the brightness of a star?

• The apparent brightness/magnitude of a star

• The absolute brightness/magnitude of a star

• The luminosity of a star

What does it mean by the apparent brightness/magnitude of a star?

A measure of how bright a star appears in the night sky seen from Earth

What does it mean by the absolute brightness/magnitude of a star? What advantage is there when using this method?

A measure of how bright the stars would appear if they were all placed the same distance away from the Earth.

Scientists can make comparisons between stars

END OF TRIPLE: What does it mean by the luminosity of a star?

This measures how much energy in the form of light id emitted from a star’s surface every second

Evolution of a star of similar mass to the Sun

What are the stages?

• Nebula

• Star (main sequence)

• Red giant

• White dwarf

How is an initial form of a star created (nebula)?

• Stars form from large clouds of dust and gas particles we call nebulae and are drawn together by gravitational forces over millions of years

• As particles get closer the temperature and pressure increase

• Nuclear fusion reactions begin bc of this

  • Hydrogen nuclei join together to make a larger nuclei, a helium nuclei (eso pone en el textbook, en los summaries pone que a helium nuclei forms from the hydrogen nuclei joining together)

• This releases enourmous amounts of energy in the form of heat and light

• This very hot ball of gas is a very young star.

The appearance of a star changes gradually with time. These changes follow a pattern.

How does a nebula evolve into a star (main sequence)?

• When a star first forms (la nebula) the fusion produces forces that push the particles apart as the gas in the star expands outwards because of the high temps but the gravitational forces are always pulling the particles together.

• When these opposing forces are balanced, the star is in its main stable period and is called a main sequence star.

• This period can last for millions of years (this is the stage the Sun is in) and stays at a constant size and temp

How does a star evolve after the stage of star (main sequence)?

• Eventually there are less hydrogen nuclei so these fusion reactions stop.

• Gravitational forces are now the largest forces and compress the star.

• As the star shrinks in size there is a large increase in temp. So high that fusion reactions between helium nuclei begin.

• The energy released by these reactions causes the star to expand greately.

• The SA is so large as it expands that it becomes a little cooler so its colour changes to red (more of its light energy is emitted in the red part of the spectrum)

• The star is now called a red giant

How does a star evolve after the stage of the red giant?

• Eventually most of the helium nuclei have fused and new nuclear reactions begin but the gravitational forces are larger again causing the star to get smaller

• This contraction causes an increase in temp so the star changes colour as it emits more blue and white light.

• It is a white dwarf

• Finally as a white dwarf star cools, it changes into a cold black dwarf star

How does the matter from which a white dwarf star is made compare to matter found on Earth?

The matter from which the white dwarf is made is millions of times more dense than any matter found on the Earth

A star with a mass larger than the Sun follows a different path at the end of its life

Describe their evolution

• After the stable period, hydrogen nuclei runs out and the star contracts and heats up.

• Helium fusions begin and star expands into a red supergiant.

• When it runs out of helium nuclei, it contracts/collapses due to the gravitational force (other new reactions take place)

• Then it explodes throwing dust and gas into space to form a new stellar nebula.

• An exploding star is called a supernova.

• Any matter remaining will form a very dense neutron star (because it is made entirely from neutrons)

• If the neutron star has a mass that is 5x greater than that of our Sun or more, it collapses further to become a black hole

During the expansion of these stars, what elements are involved in the nuclear fusion reactions?

All the elements up to iron

TRIPLE CONTENT: The Hertzsprung-Russell diagram (HR diagram)

What does this diagram show?

The relationship between the brightness of a star, its surface temperature and the classification of a star.

Why is the HR diagram important?

By finding a star’s position in the diagram we know its internal structure (what elements are undertaking fusion reactions) and at what stage it is at in its life and predicting the final stages of its life.

We have to remember the general features of the HR diagram

Draw the HR diagram showing:

• The shape and position of the sequence

• the positions of the white dwarfs, red giant and red supergiants

• the axis

• colours

NOTE: the temperature scale is reversed. It is hotter towards the origin of the x axis (basicamente, blue = hot)