5.5 Astrophysics and Cosmology Flashcards
What is the definition of a planet?
A planet is an object in orbit around a star with three characteristics:
- large enough mass for its own gravity to give it a spherical shape
- no fusion reactions
- has cleared its orbit of most other objects
What is the definition of a planetary satellite?
A planetary satellite is a body in orbit around a planet. This includes moons (natural satellites) and man-made satellites.
What is the definition of a comet?
A comet is a small, irregular body made up of ice, dust and small pieces of rock. They orbit stars in elliptical orbits.
What is the definition of a solar system?
A solar system contains a star and all the objects that orbit around it.
What is the definition of a galaxy?
A galaxy is a collection of stars, interstellar dust and gas.
What is the definition of the universe?
The universe includes all of space-time (energy, matter etc.).
How does a main sequence star form from interstellar dust and gas?
- Star formation begins in a nebula.
- The dust and gas is pulled closer together by gravity, and undergoes gravitational collapse.
- Due to variations in the nebula, denser regions begin to form.
- Protostars form at the centres of these dense regions.
- During collapse, gravitational potential energy is converted into thermal energy, causing the protostar to become hot and dense.
- Nuclear fusion begins when the core reaches a temperature and pressure sufficient for hydrogen nuclei to undergo fusion, forming helium.
- The protostar transitions into a main sequence star.
- A main sequence star maintains equilibrium through a balance between radiation pressure (from nuclear fusion) and gas pressure (from high temperature and density) pushing outwards against gravity, resisting gravitational collapse.
How does a low-mass star evolve from a main sequence star to a white dwarf?
- Stars between 0.5 and 10 solar masses are low-mass stars.
- As hydrogen in the core runs out, the core of the star collapses. The core is inert.
- The pressure and temperature in the outer shell increases, hydrogen fusion begins to occur and the outer shell of the star expands.
- The outer layers continue to expand and cool until they eventually drift off into space as planetary nebulae, leaving behind the hot dense core (white dwarf).
- The white dwarf does not undergo fusion, it emits light from its previous fusion reactions.
What are the characteristics of a white dwarf and how does it not collapse due to gravity?
- A white dwarf is very hot and dense, although small.
- The Pauli exclusion principle states that two electrons cannot exist in the same energy state, so the electrons in the star cannot be squeezed together any more by gravity.
- This is called electron degeneracy pressure, and prevents the gravitational collapse of the white dwarf.
- However, this only applies to cores with mass below the Chandrasekhar limit (1.44 solar masses). Electron degeneracy pressure is not enough to prevent the gravitational collapse of more massive stars.
How do high-mass stars evolve from main-sequence stars to neutron stars or black holes?
- High-mass stars have mass greater than 10 solar masses.
- Due to more force from gravity and higher pressure and temperature in the core, the stars run out of Hydrogen more quickly than low-mass stars.
- When hydrogen in the core runs out, the core collapses.
However, since the core is very hot, fusion of helium nuclei into other elements occurs. - Changes in the core cause the star to expand into a red supergiant.
- Fusion of heavier elements occurs in the inner layers, and lighter elements in the outer layers, until an iron core develops.
- Iron nuclei cannot fuse, so makes the star unstable.
- The outer layers collapse and the star explodes as all the outer layers bounce off the iron core, ejecting all the core material into space (this is a supernova).
- If the mass of the core is greater than the Chandrasekhar limit, but less than 3 solar masses, a neutron star forms.
- If the mass of the core is greater than 3 solar masses, gravitational collapse continues and the star becomes a black hole.
What are the properties of neutron stars and black holes?
- Neutron stars are very dense, but small, and almost entirely made up of neutrons. They are prevented from collapsing by neutron degeneracy pressure, (also described by Pauli’s exclusion principle).
- Black holes are formed when the star collapses into a small enough point that the resulting gravitational field is so strong that the escape velocity is greater than the speed of light.
What is the Hertzsprung-Russell diagram?
- The Hertzsprung-Russell diagram is a graph of stars with luminosity on the y-axis and average surface temperature on the x-axis.
- The temperature axis increases to the left.
- Main sequence stars are found in a curved line going diagonally downwards across the entire graph.
- Red giants are just above and to the right of the centre of the graph, splitting from the main sequence line.
- Red supergiants are in a horizontal line at the very top of the graph on the right, very bright but not very hot.
- White dwarfs sit in the bottom left corner, very hot, but dim.
What are energy levels of electrons in isolated gas?
- When bound to atoms in a gas, electrons exist at discrete energy levels.
- These energy levels are negative, because external energy is required to remove an electron from an atom.
- an electron with 0 energy is free from the atom.
- The energy level with the most negative value is called the ground state/level.
How do emission spectral lines form in terms of energy levels and transitions?
- When an electron moves to a higher energy level, it is said to be excited.
- This requires the input of external energy.
- When the electron moves back to a lower energy level, it loses this energy.
- The energy lost is emitted as a photon with an energy equal to the energy change of the electron’s transition between energy levels.
What equations describe the emission of an electron from a higher to lower energy level?
E=hf
E=hc/λ
