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HUMAN LIFE EXPECTANCY

• What determines how long you will live?

• Why do some people grow to be 100 years old and some only live to be 50 years old?

• What factors affect your everyday life expectancy?

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Star Life Expectancy

• The “life expectancy” of a star depends on the star’s MASS.

• Which star below would you expect to live longer? Why?• What are some similarities between a star’s life expectancy and a

human’s life expectancy?

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The Birth of a Star

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The Birth of a Star

When the temperature in the core of a protostar reaches roughly 15 million oC, nuclear fusion begins and a stable star is born.

A star is stable when its size remains constant over time. All stars have a stable period in their lives, the length of which is determined by their mass.

The star now enters its stable phase and is called a main sequence star.

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Stars are born in a nebula, or cloud of gas and dust

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The left pillar is approximately 4 ly in length.

Pillars of Creation

Part of the Eagle

Nebula, 7,000 ly from Earth, where many stars are forming

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A main-sequence star is ‘powered’ by nuclear fusion reactions taking place in its core.

In the Sun and most stars, hydrogen atoms fuse together to form helium. This provides the energy for life on Earth.

Nuclear Fusion

It would take 2,000 million nuclear power plants a whole year to produce the same amount of energy on Earth.

This process releases huge amounts of energy. Each second, the Sun produces 400,000,000,000,000,000,000,000,000 watts of energy!

Nuclear fusion involves light atomic nuclei joining together (fusing) to form heavier ones.

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Using nuclear fusion

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What makes a star stable?

Inside a star, the energy released by fusion produces

an outward acting force. This causes star expansion.

However, the star’s huge mass means that gravity is acting in the opposite direction, forcing

particles towards the core.

If these two forces are equal, then the star will not change its size. It is said to be stable.

explosive force of

fusion

gravitational

force

332,948

Earth masses

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The Stable Phase

Stars live the majority of their lives in this stable phase (a balancing act between gravity pulling fuel down to the core, and heat trying to expand it outward).

In this phase, stars give off energy and heat, and as a result, change color.

Our sun is about halfway through its 10 billion year stable phase.

Protostars that never get large enough for fusion to begin become BROWN DWARVES.

Material left over after the star forms becomes planets, asteroids, comets, etc.

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Stable star structure

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The end of stability

A star’s stable phase ends when most of its hydrogen has been consumed by fusion. The star enters it’s old age.

The star increases massively in size, becoming a red giant.

The reduced fusion causes the core to cool, lowering pressure causing the star to collapse upon itself under its own gravity.

As the outer layers contract, they heat up. This triggers the fusion of the remaining hydrogen. The increased energy output in the outer layers causes them to expand.

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What happens in the core?

The core continues to contract. This causes it to reach new extremes of temperature (over 100 million Kelvin) and pressure.

This gives helium atoms enough energy to fuse.

Thus heavier atoms, such as carbon and oxygen, are produced.

In the largest stars elements as heavy as iron can be produced. However such large stars will suffer a different fate to the Sun.

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The death of a star

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A white dwarf is formed at the end of the life cycle of a star that is about the same size as the Sun.

The outer layers of the star are ejected away from the white dwarf core and form a planetary nebula

What is a white dwarf?

This photograph was taken with the Hubble Space Telescope and shows ancient white dwarf stars in the Milky Way. The white dwarf stars are

shown ringed in blue.

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Red supergiants

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What is a supernova?

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However, not all elements are made in the early stages of a star’s life. Some of the heavier elements are only made when a massive star explodes in a supernova at the end of its life.

Nuclear fusion in stars produces new atoms.

How are elements made?

When all the hydrogen has been used up, other elements are fused together to make the heavier elements of the periodic table.

In the early stages of a star’s life, light elements such as helium are mainly formed.

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After a supernova the core of a of star more than 8 times the size of the Sun becomes a Neutron Star.

A neutron star is so dense that on Earth 1 tsp would weigh 1 billion tons!

What is a Neutron Star?

This is a baby neutron star in the remains of a supernova.

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After a really massive red giant collapses in a supernova explosion, it leaves an object so dense that nothing, not even light, can escape its gravitational pull.

Some scientists believe that there are black holes at the centre of galaxies.

If light cannot escape from a black hole, then how can a black hole be observed?

How are black holes formed?

The end of the life cycle of really massive stars is different to that of massive stars.

This is called a black hole.

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How can a black hole be ‘seen’?

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How do stars begin and end

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Lifecycle of small stars

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Lifecycle of large stars

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Terms about the lifecycle of stars

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Star Life Cycle

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1. What two elements make up most of the Sun?hydrogen and helium

Questions about stars

the Sun red giant white dwarf5. What is the fate of a star similar to the Sun?

A huge cloud of gas and dust from which a star is born.

2. What is a nebula?

nuclear fusion

3. What process has allowed the Sun to emit light and heat energy over thousands of million of years?

neutron star4. Which is the most dense, a white dwarf or a neutron star?