the life cycle of a star the horsehead nebula – one of the most famous pictures in astronomy
TRANSCRIPT
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The Life Cycle of a Star
The Horsehead Nebula – one of the most famous pictures in astronomy
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Lesson Objectives
• All – To name the stages of a star
• Most – To describe the stages of a star
• Most – To describe the life cycle of a star larger than the Sun
• Some – To explain why a star moves from one stage to another
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Interstellar medium is the total mass of the gas and dust between the stars.
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• Stars form in clouds of dust and gas called stellar nebulae.
• Gravitational forces cause the large masses of gas to be squashed together.
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• A Nebula is a stellar nursery - a region of dust and gas where new stars are born.
• The Orion Nebula (M42) is the nearest nebula and can be seen with the naked eye.
• The first nebula ever photographed (in 1882)
• Its proximity (1500 light years) means that we know more about it than any other star-forming region.
• It is also in a very active stage of star formation.
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• Dense regions in the clouds collapse due to gravity
• As it gets smaller the protostar at its centre gets hotter
• Once the star contracts enough that its central core can burn hydrogen to helium, it becomes a "main sequence" star.
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• The Sun formed 4.5 billion years ago, as the Solar System coalesced from a cloud of gas and dust.
• The sun is a main sequence star.
• This is the longest, most stable period of a star’s life.
• It converts hydrogen to helium in its core, generating heat and light.
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• A stars expands as it grows old.
•As the core runs out of hydrogen and then helium, the core contracts and the outer layers expand, cool, and become less bright; forming a red giant.
• Our Sun will run out of fuel in ~5 billion years when it will expand, forming a red giant engulfing Mercury and Venus.
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• A planetary nebula occurs at the end of a red giant’s life.
• The outer layers of the red giant start to drift off into space.
• This is The Eskimo Nebula
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The Cat’s Eye Nebula
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The Helix Nebula
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When a red giant collapses, its outer layers shed off and what remains is white dwarf (a very hot, dense star).
This white dwarf is Shapley 1 about 1000 light years away from Earth.
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• A black dwarf is a hypothetical star, created when a white dwarf cools so it no longer emits heat or light.
• Since the time required for a white dwarf to become a black dwarf is longer than the age of the universe (13.7 billion years), no black dwarfs exist yet.
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• A giant star is much larger and brighter than a normal main-sequence star of the same surface temperature.
• Giant stars can be up to 100 times larger up to 1,000 times brighter than our the Sun.
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• After the hydrogen in a giant star's core has been
used up, they become red supergiants - the largest
stars in the universe in terms of volume.
• These stars have very cool surface temperatures
(3500–4500 K).
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• Betelgeuse is a red supergiant in the constellation of Orion.
• It is over 600 million miles in diameter (1,000 times bigger than the Sun but cooler).
• If Betelgeuse were at the centre of our Solar System, it would extend beyond the orbit of Jupiter.
• It is 520 light-years from Earth.
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• A supernova is the death of a large star. It is a spectacular explosion
• This supernova in the Large Magellanic Cloud (LMC) is the first to be visible to the unaided eye for almost 400 years.
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How are supernovae formed?• In the core of a red super
giant, lighter elements fuse to form iron.
• Iron nuclei absorb energy when they fuse and so the process slows down.
• Decreased pressure in the core, means the outer layers are not held up and so they collapse inwards.
• As the core is so dense, the outer material collides and bounces off, resulting in a huge explosion.
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What are supernovae? • A dying star that explodes violently, producing an extremely bright object for weeks or months.
• They emit visible, infra red and X ray radiation.
• Temperatures rise to 10 billion K.
• Enough energy to cause medium weight elements to fuse, forming heavy elements (up to Uranium in the Periodic Table).
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The remnants of a supernova in the constellation Cassiopeia, all that can be seen by astronomers.
Supernovae are rare – once every century in a typical galaxy.
But the core remains…
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• A black hole can be created when a giant star undergoes a supernova.
• A star with a mass greater than 20 times the mass of our Sun may produce a black hole at the end of its life.
• Black holes are objects so dense that not even light can escape their gravity and since nothing can travel faster than light, nothing can escape.
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It is made almost entirely from neutrons, compressed like a giant atomic nucleus.
If the mass of the core of a supernova is less than 2.5 solar masses, it becomes a neutron star.
Calvera, the closest neutron
star found in the constellation Ursa Minor
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