story so far

Story so far

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.

• 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)

• 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.

• 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.

• Galileo discovered that Venus goes through a series of phases.

• Jupiter has 4 orbiting moons.

• These strongly supported a Copernican model.

• 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.

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

• 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.

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

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)

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

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.

• Their huge power can only be explained by matter orbiting or going into a black hole.