Astrophysics Flashcards
What is Kepler’s First Law?
Planets orbit the Sun in ellipses with the Sun at one focus.
What is Kepler’s second law?
A line connecting a planet and the sun sweeps out equal areas in equal times.
What is Kepler’s third law?
The square of the period is proportional to the cube of the radius.
Explain Kepler’s second law.
K2 is conservation of angular momentum so as the radius increases the velocity must decrease to maintain a constant angular momentum.
What is the virial theorem?
2KE + PE =0
How is the virial theorem shown?
Equate the graviational and centripetal forces and simplify.
How is the distance to a star measured using parallax?
What is the apparent magnitude of a star?
The brightness a star appears from Earth.
What is the absolute magnitude of a star?
The brightness a star appears from a distance of 10pc
How do you calculate the apparent magnitude?
m=const-2.5logF
How do you calculate the absolute magnitude of a star?
What is the ideal gas law?
How does radiation carry energy?
Photons carry energy between collisions with matter
How does convection carry energy towards the surface of a star?
The bulk of gas causes currents
How is the Virial Theorem applied to a star?
The kinetic energy is replaced with the interanl thermal energy U.
2U+PE = 0
Therefore the total energy
E=U+PE=U-2U = -U < 0
Therefore the star is gravitationally bound
How is nuclear fissions triggered?
Typical decay mode for some nuclei triggered by absorption of a neutron.
How are Newton’s and Kepler’s laws combined to get the Universal Theory of Gravitation?
What formula relates absolute magnitude, apparent magnitude and dist
What is Wien’s Law?
What is the overall equation for nuclear fusion?
4 1H ►4He + energy
Why does nuclear fusion release energy?
Helium is more tightly bound than hydrogen so the reaction yields energy.
How it the relative strength of the fundamental force measured?
Its is the ratio between the potential energy and the rest mass energy
What is the gravitaional coupling constant?
A dimensionless constant characterising the gravitational attraction between two elementary particles having non-zero mass.
What is the fine structure constant?
A dimensionless constant characterising the strength of the EM interaction between elementary charged particles.
What does the Grand Unification theory say?
The coupling strengths of the strong, electromagnetic, and weak force vary with energy (temperature) like this:
What is the potential energy of an object at a large distance (assume infinite) from the Earth?
Assuming the distance is infinite the potential energy would be zero so as it move towards the Earth it gains kinetic energy and loses potential energy.
What would the kinetic energy of an asteroid colliding with the earth be?
Equal to the change in gravitational potential energy from inifinity to the Earth’s surface.
What is evidence for a mass extinction caused by an asteroid
Enhanced iridium content of the soil layer indicating the end of the dinosaur period.
How could a magnetic field reversal be a catastrophe?
The weaker field during reversal could cause more UV radiation to be incident with the Earth’s surface.
The ozone layer could also be depleted by cosmic rays that aren’t blocked by the Earth’s magnetic field
What is the Solar constant?
The energy received from the sun per unit area per unit time at the distance of the Earth. S=1367W/m2
What is the albedo of an object?
The albedo of an object is the fraction of incoming radiation that is reflected.
e.g. White = 1
Black = 0
What is the Stefan - Boltzman law
How can the solar constant be derived from the Stefan - Boltzman law?
Why is the sky blue?
The sky is blue due to Raleigh scattering of sunlight. This scattering is inversely proportional to the fourth power of the wavelength so because blue light has a lower wavelength than red light it is scattered much more so the sky appears blue.
