Astronomical Unit (AU)
The average distance from the Earth to the Sun (approx 1.5x10^11m)
Parsec (pc)
The distance at which the mean radius of the earth’s orbit (1AU) subtends an angle of 1 arc second (approx 3.1x10^16 m)
Light year (ly)
The distance light travels through a vacuum in one year (approx 9.5x10^15m)
Olbers’ paradox
For an infinite, uniform and static universe the night sky should be bright in all directions because of light received from stars
Hubble’s law
The recession speed of distant galaxies is directly proportional to their distance from the observer
Cosmological principle
The universe is homogeneous and isotropic and the laws of Physics are the same at all points within it
Critical density
The density of the universe which would cause it to be flat - i.e. the expansion rate would tend towards zero in infinite time
How does a star form?
- They begin as a large interstellar gas cloud consisting mainly of molecular hydrogen and a few other
elements (e.g. Fe) - Gravitational attraction between the atoms of the dust cloud causes the cloud to collapse.
- The gravitational collapse causes the gas cloud / protostar to heat up.
- The atoms have greater kinetic energy and move faster.
- The chance of fusion reactions becomes greater.
- Hydrogen nuclei fuse together to form helium nuclei at temperatures of 10^7K (Known as hydrogen burning)
- Fusion reactions increase the temperature of the cloud.
- A star of stable size is formed when the gravitational pressure balances out the radiation pressure
(from photons released in fusion reactions)
What happens when a star BEGINS to die?
When all the fuel is used up the radiation pressure decreases.
The increase in the gravitational pressure causes the helium nuclei in the outer layer to fuse together to form heavier elements.
The increase in the power production from the helium shell and hydrogen surrounding the core causes the outer layer of the star to expand due to radiation pressure
Death of a star less than 3 solar masses
- Surface area increases and surface temperature drops. It becomes a red giant.
- The core continues to collapse. When the temperature reaches 10^8K, the helium starts to fuse. About half the material surrounding the core is ejected away as a
planetary nebula. - The remnant core left is a White Dwarf. There are no further reactions inside. Glows brightly due to
photons produced from past fusion reactions leaking away. High density. - Prevented from further gravitational collapse by electron degeneracy or Fermi pressure. This
comes about because two electrons cannot exist in the same quantum state. The maximum mass of
a white dwarf is about 1.4 solar masses; this upper limit to the mass of a white dwarf is known as
the Chandrasekhar limit.
Death of a star greater than 3 solar masses
- Surface area increases and surface temperature drops. Becomes a super red giant.
- When the core collapses to form a white dwarf, it’s mass is greater than 1.4 solar masses. The
gravitational pressures are enormous and overcome the Fermi pressure. The electrons within the
core combine with protons to produce neutrons and neutrinos. The neutrinos escape and the
central core becomes entirely packed within neutrons. - The outer shells surrounding the neutron core rapidly collapses and rebound against the solid
neutron core. This generates a shock wave, which explodes the surface layers of the star as a
supernova. - The supernova blasts off heavier elements like iron and oxygen into space. (All elements originate
from supernovae) - For stars with mass in the range 3-10 solar masses, the remnant core is a neutron star. For stars of
above 10 masses, the neutron core continues to collapse into a black hole.
Closed shape of the universe
Density of the universe is greater than the critical density.
The gravitational force between matter is strong enough to decelerate and halt the expansion of the universe.
It will get hotter as it approaches the ‘big crunch’.
This model might oscillate between big bangs and crunches.
Open shape of the universe
If the universe density is less than the critical density.
The gravitational force between matter is too weak to decelerate or stop the expansions of the universe.
The universe will expand forever - eventually reaching absolute zero.
Flat shape of the universe
The universe will be flat if its density is equal to the critical density.
The rate of expansion tends to zero as the volume of the universe tends towards a certain limit. Most cosmologists believe the universe is flat.
What temperature does fusion begin in the core of a star being formed?
approximately 10^7k
