Astrophysics and Cosmoslogy

• 1 light year = the distance travelled by light ion one year = 9.5x10^15
• 360 degrees in a circle
• 1 minute of arc = 1/60 degrees
• 1 second of arc (1 arcsec) = 1/3600 degrees
• a star is 1 parsec away if the angle of parallax = 1 arcsecond
• 1 AU is the mean distance between the earth and the sun

• Trigonometric parallax can be used to calculate the distance from the sun to nearby stars.
• the angles are very small > ground based measurments only work for afew thousand stars.
• more can be done using measurments from satellites, which are closer.

• standard candles have an absolute luminosity which is known.
• The radiation flux on earth can be measured
• inverse square law can be used to find the distance
• this gives an idea of the scale of the universe.

• main sequence - radiation pressure from core balances gravitational force
•  Hydrogen in core runs out, > fusion stops > pressure outward stops
• core contracts + heats up
• helium begins to fuse (it is hot and dense enough)
• energy released causes expansion ( large SA, low surface temperature --> high luminosity)

Low mass star:

• collapses > not enough energy for further fusion > contracts  > outer layers ejected (as  aplanetary nebula) > hot dense core left.

High-mass star:

• spnd less time on main sequence as burn fuel more quickly
• fuse until core = iron
• star explodes in supernova > neutron star/ black hole produced

• spectra from galaxies all show red shift
• speed at which galaxy moves away can be calculated from this
• recessional velocity against distance > straight line > proportional
• this tells us that the universe is expanding
• uncertainty in the value for Hubbles constant - these distances are very hard to measure!
• if the universe has been expanding at the same rate for all of its life, age = 1/ hubbles constant
• it probably hasn't been expanding at a constant rate for all its life
• we also don't have an exact value for hubbles constant

All mass in the universe is attracted together by gravity. This tends to slow the rate of expansion.

The density of the universe is unknown.

• if the universe is critical density: gravity is just strong enough to stop the universe expanding at time = infinity
• if the universe is less than critical density: gravity is too weak to stop the expansion and the universe will expand for ever.
• if the universe is more than critical density: gravity is strong enough to stop expansion and start  the universe contracting again

Fission:

• large nuclei are unstable
• large nuclei split into smaller ones with greater binidng energy per nucleon

Fusion:

• lots of energy needed to overcome electrostatic forces of repulsion in order to get close enough for strong nuclear interaction to hold them together
• lots of energy is released as new heavier nuclei have higher binding energy per nucleon
• enegy released helps maintain temperature for further fusion reactions to happen

• The mass of the nucleus is less than the mass of its constituent parts
• energy and mass are equivalent according to e =mc
• decrease in mass is converted to energy

Binding energies of different nuclei can be compared using binding energy per nucleon:

• a high binding energy per nucleon means that more energy is required to remove nucleons from nucleus (it is more stable)
• if a reaction (e.g fission or fusion) increases the binding energy per nucleon, energy is released.