P7 3

P7 3

1. Fusion

   • Step 1
     • Hydrogen + hydrogen -> deutirium + positron + Gamma ray
   • Step 3
     • Helium-3 + Helium-3 -> Helium-4 + hydrogen + hydrogen
   • Step 2
     • Deutirium + hydrogen -> Helium-3 (unstable) + gamma ray
   • Overview
   • This releases a huge amount of energy using einsteins theory

2. Stars

   • Fusion only happens in the middle of the sun where it is hot enough
   • Outside layer of sun is only a few thousand degrees
   • Outer layer is called the photosphere
   • Main sequence stars only fuse hydrogens to heliums
   • A star will eventually run out of hydrogen fuel

3. Star Lifecycle

   • Red giant
     • When MS star runs out of hydrogen fuel fusion in the core will slow
     • As a result the core will be cooler and there will be less outwards pressure
     • The outer layers will fall inwards towards the core because of gravity
     • The surface will be cooler and as a result will be red
     • As the outer layers are pulled against the core, the core gets hotter
     • As a result new fusion reactions are possible and the star will start fusing heliums
     • From fusing heliums the star can fuse heavier elements like nitrogen oxygen and carbon
     • Our sun will only be able to fuse up to carbon - due to its size
     • Due to the high pressures from fusing heliums (which releases more energy) it will expand and get huge
   • Larger stars
     • Larger stars will form red supergiants when they run out of fuel
     • Because they get so big the core can get hotter and can fuse elements beyond carbon
     • Iron is the most stable element in the universe - a supergiant cannot fuse iron
     • No more fusion in red supergiants is possible after iron
   • Supernovae
     • In a red supergiant the core starts getting filled with iron
     • Then the star cannot fuse anything anymore
     • When the fusion stops the outer layer collapses in, it hits the iron core and bounces off and there is a huge explosion
     • This explosion is a supernova and has the capability to block off a galaxy from view for weeks
     • In supernovas there is enough energy to make elements heavier than iron
     • Everything heavier than iron is made by supernovae
     • Our star has traces of heavier elements so that means it is made from leftovers of other stars
   • Neutron stars
     • The center core of a star can survive a supernova
     • If the mass is smaller than 2.5x the size of our sun it will become a neutron star
     • A neutron star consists nearly entirely of neutrons compressed together in a size of just 30km
   • Black holes
     • A larger collapsed core will have more gravity
     • So it has so much gravity it can pull in light
     • So it is a black hole
   • White dwarfs
     • A small star will eventually build up a core of oxygen and carbon
     • After the outer layers are lost the core forms a white dwarf

4. Comparing stars

   • We don't see different types of stars just stars in different stages of their life
   • 90% of stars fall in the main sequence area
   • The sun is in the middle of the diagram

5. Gas Laws and Kelvin

   • Gases are made of particles widely spaced apart
   • They move in random directions
   • If a gas is trapped in a container the particles will collide with the walls and exert a small force - this is pressure
   • Pressure is measured in N/m2 or Pascals (Pa)
     • 1Pa is very small
   • 1 earth atmosphere = 101,225 Pa
   • Laws
     • Boyles law - pressure is inversely proportional to volume (constant temperature) - Pressure x Volume = constant
     • Charles Law - volume is directly proportional to temperature (constant pressure) - Volume/temperature = constant
     • Pressure law - pressure is directly proportional to temperature (constant volume) - pressure / temperature = constant
     • Temperature for these laws needs to be converted to Kelvin
   • Kelvin
     • An ideal gas is a gas that will reach -273 C without changing its state
     • No smaller pressure than 0 and no smaller volume than 0
     • If you back the LOBF of 2 graphs that show relationship of pressure and temperature and volume and temperature - pressure or volume reaches absolute zero at -273 C
     • To make the graphs directly proportional they invented a new unit - Kelvin
     • 0K = -273 C
     • 1 unit of degrees is equal to one unit of Kelvin
     • Kelvin = Celcius + 273
     • Celcius = Kelvin - 273
     • Our sun is 6000 C or 5727 K

6. How astronomers work

   • Ground based observatory
     • On surface of earth
     • Can only observe frequencies of radiation that are allowed through the atmosphere
     • Optical telescopes are housed within a dome to protect it from weather
     • Astronomical factors
       • Light pollution - all street lights etc - light gets scattered up in the atmosphere making it difficult to see light from the outside of the atmosphere
       • Scintillation - twinkling of stars - earths atmosphere is not completely uniform - as light goes through dense parts of the atmosphere it diffracts - to avoid this you need an observatory at a high altitude where the atmosphere is thinnest
       • Atmospheric conditions - clouds, air pollution and high water vapour
   • Finding a suitable location
     • One with minimal light pollution, high altitude, large percentage of clear nights per year
     • Examples
       • Chile
       • Hawai
       • Australia
       • Canary islands
     • Other factors
       • Cost
       • Environmental and social impact
       • Working conditions (altitude sickness)
   • Space telescopes
     • Can make distant observations of astronomical objects
     • No interferance from earths atmosphere
     • Can see frequencies of radiation that is blocked by the atmosphere
     • Dont have to put up with rain, water vapour and light pollution
     • Very expensive to put them in space and very expensive to send a maintenence team
   • International Cooperation
     • Unpredicable may be conflict between countries
     • 1 Collaberation between experts from different nations can make breakthroughs
     • 2. The nations can share the financial cost
     • 3. Possible to share facilities and information
     • 4. Cultural exchanges on a scientific and personal level
     • Eg Hubble was built by NASA and had contributions from ESA and is operated by STSI
   • Astronomers work
     • Spend a lot of time in offices
       • Analysing data
       • Creating computer programs to search through and analyse data
       • Writing research papers and going to meetings
     • They usually have to make a proposal to carry out an observation
     • They have a set time period to use the equipment - as some objects can only be seen at certain times in certain places
     • Some will accept applications from anyone, some will have criteria
     • Approved observation
       • Then get your time at the telescope
       • Then the data is sent by internet to office or by radio waves if it is in space
   • Controlling telescopes
     • Large telescopes are controlled by computers
     • Advantages
       • Much more precise and accurate
       • Due to the rotation of the earth, objects appear to move, computers can precisely track them
       • Dont have to be at the observatory to control it

7. Other life

   • Estimated 100,000,000,000 other stars in milky way
   • Many of these stars have planets
   • Other planets are called exo-planets
   • Our telescopes are so good we can detect a dip in radiation from from the sun which is a planet in orbit passing by
   • Planets have a small gravity pull on their sun, so the sun wobbles a bit - our telescopes can detect those wobbles
   • When a planet goes past a sun we can see the tiny area where the planets atmosphere lies and we can look at the absorbtion spectra to see if it contains oxygen
   • SETI (Search for Extra Terrestrial life)
     • They havent found anything, no evidence so far

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