Astrophysics SL Unit 13

Astrophysics SL Unit 13

1. Stellar radiation and stellar types

   • Energy source
     • The source of the Suns energy is nuclear fusion in the interior of the star. Inside the Sun, nuclei of hydrogen fuse together to form helium, releasing energy in the process. Beucase high pressures, the nuclei are suffciciently close to each other for collisions to occur. Owing to temperatures in excess of 10^8 K, the electrostatic repulsion hydrogen nuclei can be overcome and fuse together.
     • As Kinetic energy of the particles inside the Sun increase, the radiation pressure increases. Radiation pressure is the force exerted by solar radiation on the surroundings. It acts to stabilize the Sun against gravitational collapse, creating a kind of equilibrium.
   • Luminosity and Brightness
     • The luminosity of a star is defined as the energy emitted per unit time.
     • b= L/(4πd^2 )
     • Apparent brightness of a star is defined as the power incident on Earth perpendicular to unit area. To measure apparent brightness, the light reaching the Earth is collected with a telescope and detected using a charge-coupled device (CCD). The data is processed by a computer and the apparent brightness can then be worked out accurately.
   • Wiens Displacement law states that the peak wavelength of the emission of a black body is inversely proportional to its temperature.
     • Max. Wavelength = 2.9 x 10^-3 / T
     • If the energy emitted from a star is analyzed over a range of wavelengths and the peak wavelength is determined, then the surface temp. of the star can be determined.
   • Stellar Spectra
     • Star's absorption sepectrum has dark lines indicating absorption of light of specific energy/frequency/wavelength. The dark lines coorespnd to the emission line spectra of different elements / different elements have specific wavelengths absorbed.
     • Stars classified according to their spectra.
       • O, 30,000 - 60,000 K, Blue B, 10,000 - 30,000 K, Blue A, 7,500 - 10,000 K, Blue/white   F, 6,000 - 7,500 K, White G, 5,000 - 6,000 K, Yellow/white K, 3,500 - 5,000 K, Orange M, 2,000 - 3,500 K, Red
     • Types of stars
       • Binary stars: have one partner, rotate around a common center of mass.  Red giants: Extremely large, high luminosity, relatively low temp.  Red supergiants: White dwarf:  Cepheids: Visual binaries: Spectroscopic binary stars:

2. Cosmology

3. Stellar distances

4. E.1 The universe

   • E.1.1: Solar system: planets orbit sun, moons orbit planets in elipses.
     • Planet distance from sun: Smallest-->biggest, MeVEMJSUN
     • Radius compmared to sun: smallest-->biggest, MeMVENUSJ
     • Comets: made of ice and dust/rocks. Highly eliptical orbits around the sun, tail=melting ice. Tail always point aways from the sun.
   • E.1.2: Stellar cluster = group of stars fairly close to each other created by the collapse of the same gas cloud. Constelation= groups of stars which from Earth appear to be close but aren't in reality, form pictures.
   • E.1.3
   • E.1.4 Distances between starts within a galaxy = 30-100 light years. Distances between galaxies=5,9x10 ^ 7
   • E.1.5 Night period: stars appear to move in cricles, but its the earth rotating. Year period: constelations appear to change position and then return to the same position 1 year after, this is because of the revolution of Earth, e.g. the night will face a different side every night during one year.

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