Story So Far
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![Page 1: Story So Far](https://reader034.fdocuments.net/reader034/viewer/2022052222/546e11afaf7959f83e8b5ee2/html5/thumbnails/1.jpg)
Story so far
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Ancient Greeks
• Had the Earth at the centre of the Universe.
• Evidence– Planets, stars ,sun, moon all appeared to rise in the East and set in the West.
• Conclusion– They must be going around us.
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• Ptolemy as the ancient Greeks was aware that the Sun, moon and 5 planets moved against the star background.
• This led to the two sphere model with the Earth at the centre.
• The variable movement of the planets and retrograde motion caused a problem with this model. Epicycles were used to explain retrograde motion.(p5)
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• Copernicus changed the model ,having the Sun at the centre. The planets moved in a circular path around it.
• This didn’t quite agree with observations of the planets motion.
• Kepler proposed a modification. Planets move in ellipses.
• He also proposed his laws, using Newton’s law of gravitation to find the relationship between T and r.
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• Galileo used a telescope to support Copernicus.
• He observed mountains on the moon– the heavens weren’t perfect.
• Observations of the Milky Way showing millions of stars supported the Copernicus idea that stars were at a great distance from the Earth.
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• Galileo discovered that Venus goes through a series of phases.
• Jupiter has 4 orbiting moons.
• These strongly supported a Copernican model.
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• Perturbations in Uranus’s orbit helped to find Neptune.
• Newton’s laws helped to find Neptune.• There was a problem with the stars which
were believed to be static.• They should cause the universe to
collapse.• It hasn’t so matter must be uniformly• spread through an infinitely large space.
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How do stars die?
• Sun type• E= mc² so any small change in the mass
of a star ,produces an enormous amount of energy.
• In the sun at 15 million ºK , hydrogen nuclei fuse to produce helium nuclei.
• 4H = He + 2 neutrinos + 2 positrons• 0.7% of the initial mass is converted to
energy
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• With a star
• 1. The energy produced by thermonuclear fusion exactly matches the energy radiated by the star. Temperature therefore is constant.
• 2.The thermal pressure outwards balances the gravitational forces inward keeping the size constant.
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Time on main sequence
• 0.5 x Sun’s mass 200 billion years
• Sun 10 billion years
• 3 x Sun’s mass 15 million years
• 25 x Sun’s mass 3 million years
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Death of a sun type
• Hydrogen burning ceases in the core.
• It contracts giving a loss in PE and a gain in KE so it gets hot. Expansion follows.
• Outer layers will cool as a result .
• A red giant is formed.
• As the core continues to contract, Helium burning occurs.(p38)
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• When this stops ,the core will collapse again .
• What happens next depends on the star’s mass.
• If it is below a critical mass like the sun, it will be peaceful.
• If more it will be spectacular and violent.
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• If less than 3 x the mass of the sun , there will be no more thermonuclear reactions.
• The star will be unstable and shed the outer layers ( about half its mass)
• The glow of this layer due to radiation from the core is a planetary nebula.
• The core will collapse until the electrons are closely packed enough to generate Fermi pressure.
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• This prevents any further collapse.
• We now have a star 1% the diameter of the sun called a white dwarf. Not very bright but very dense.
• There is an upper limit to the size of a white dwarf.
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• If a white dwarf has a mass 1.4 x the mass of the sun ( called the Chandrasekhar limit) then even the Fermi pressure will not stop it collapsing.
• Neutrons will be formed and this final stage takes only a few seconds. Giving a rapid rise in temperature.
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• A red giant of this mass doesn’t collapse immediately after it stops helium burning.
• Further thermonuclear reactions occur.
• Each one will produce a period of equilibrium. This will happen until all the fuel is exhausted.
• Then the neutrons will be compacted as far as they will go.
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• The intense radiation pressure from a very hot core causes the star to explode.
• A supernova occurs.
• It may emit as much radiation as a whole galaxy for a few days.
• It will then become a nebula.
• In this explosion, extreme pressures and temperatures can cause further fusion.
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• This can lead to elements more massive than iron being formed.
• The remnant can become a pulsar.
• A rapidly spinning neutron star with a strong magnetic field.
• This would accelerate charged particles leading to a beam of radio waves being emitted.
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• Hence as it spins, it produces regular pulses which can be detected.
• Some pulsars emit X rays. These can switch off for a few hours indicating a companion is blocking the radiation. It must be part of a binary system.
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What are Quasars?
• Radio brightness 10 million times the brightness of a whole galaxy.
• Massive red shift indicating a distance of 18 billion light years away.
• Optical brightness 100 x that of a galaxy.
• They often fluctuate in brightness .
• They must be a few light days in diameter.
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• Their huge power can only be explained by matter orbiting or going into a black hole.