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Summary IB Physics HL - Option D: Astrophysics (Notes)
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Detailed notes on IB Physics HL Option D (Astrophysics) with diagrams and bulletpoint explanations.
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Solar System- Each planet is kept in its elliptical orbit by the gravitational attraction between the Sun
and its planet
Asteroid: a small rocky body that drifts around the Solar System. There are many orbiting
the sun between Mars and Jupiter - the asteroid belt. An asteroid on a collision course with
another planet is known as a meteoroid.
Comets: mixtures of dust and ice in very elliptical orbits around the Sun. Their ‘tails’ always
point away from the Sun.
Nebula: a giant cloud of dust and gas in space.
Binary stars: consist of two stars that rotate about a common centre of mass.
Diff between constellations and stellar clusters
- Stars in cluster gravitationally bounded, constellations not
- Stars in cluster originated from the same gas cloud, constellations not
Speed of light = 3.0 x108m/s
1 lightyear (ly): the distance travelled by light in one year.
1 Astronomical Unit (AU): Average distance between the Earth and Sun
The nature of stars
- Stability of a star depends on the equilibrium
between two opposing forces (gravitational
force & radiation pressure).
- The gravitational force causes the star to
collapse while the radiation pressure causes
the star to expand.
- The equilibrium is gained through nuclear
fusion which provides energy so that the star’s
radiation pressure is high enough to oppose
gravitational force.
,Stellar parallax
- A term used to describe the distance between two objects in space.
- Limitation: if a star is too far away from Earth, its parallax will be too small to be
measured with accuracy.
How it is measured:
Luminosity and Brightness of stars
Luminosity (L): a measure of the total power radiated by a star or other celestial object
per second (unit: watts (W))
Apparent brightness: The total power received on Earth per m2 per second, by a star.
(unit: Wm-2)
𝐿
𝑏= 2 whereby b= apparent brightness, L= luminosity, d= distance (m)
4π𝑑
, 4
𝐿 = σ𝐴𝑇 whereby L= luminosity, σ = Stefan-Boltzmann constant [5.67 x 10-8 W m-2 K-4], A=
area 4xpixd^2(m2), T= temp (K)
whereby ε = emissivity (a number between 0 & 1) that is
characteristic of the material. Very black surfaces have emissivity = 1., and ∆Q /∆t = rate at
which energy is absorbed.
Stellar characteristics and evolution
Black body
- A perfect thermal emitter and absorber. Its spectrum depends only on its temperature
- A surface that absorbs all radiant energy falling on it. Absorbs radiation of all
wavelengths.
- The radiation wavelengths have a maximum intensity relationship which can be
determined by using Wien’s Displacement Law:
λmax T = constant
Thus, the temperature T of a star in kelvin is given by:
−3
2.9×10 𝑚𝐾
𝑇= λ𝑚𝑎𝑥
Stellar spectra - Link 1
- Uses the absorption spectrum
- : a continuous spectrum that passes through a cool gas
and has specific spectral lines removed. The missing wavelengths in a star’s
absorption spectra correspond to the absorption spectrum of a number of elements in
the star.
Hertzsprung-Russell (HR) diagram
- Shows the relationship between absolute magnitude, luminosity, classification, and
surface temperature of stars.
- Plots the temperature of stars against their luminosity.
- Depending on its initial mass, every star goes through specific evolutionary stages
dictated by its internal structure and how it produces energy.
- Each stage corresponds to a change in the temperature and luminosity of the star,
which can be seen to move to different regions on the HR diagram as it evolves.
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