5.5 Astrophysics and Cosmology Flashcards
Define planets.
Objects with mass sufficient for their own gravity to force them to take a spherical shape, where no nuclear fusion occurs, and the object has cleared its orbit of other objects.
Define dwarf planets.
Planets where the orbit has not been cleared of other objects.
Define planetary satellites.
Bodies that orbit a planet.
Define asteroids.
Objects which are too small and uneven in shape to be planets, with a near circular orbit around the sun.
Define comets.
Small, irregularly sized balls of rock, dust, and ice. They orbit the sun in eccentric elliptical orbits.
Define Solar systems.
The systems containing stars and orbiting objects like planets.
Define galaxies.
A collection of stars, dust, and gas. Each galaxy contains around 100 billion stars and is thought to have a supermassive black hole at its centre.
Define nebulae.
Gigantic clouds of dust and gas. They are the birthplace of all stars.
How are protostars formed?
In nebulae, there are regions that are more dense than others. Over time, gravity draws matter towards them and, combined with the conservation of angular momentum, causes them to spin inwards to form a denser centre.
GPE is converted into thermal energy, which heats up the centre. The resultant sphere is very hot, dense dust and gas is a protostar.
How are main sequence stars formed from protostars?
For a star to form, the temperature and pressure must be high enough for hydrogen gas nuclei in the protostar to overcome the electrostatic forces of repulsion and undergo nuclear fusion to convert hydrogen into helium. When fusion begins, the protostar becomes a main sequence star, where the outward pressure due to fusion and the inward force of gravity are in equilibrium.
Describe how a low-mass main sequence star becomes a red giant.
Low-mass stars are classed as having a core mass between 0.5M☉ and 10M☉. As these stars have a smaller, cooler core, they remain in the main sequence for longer. Once the hydrogen supplies are low, the gravitational forces inwards overcome the radiation and gas pressures, so the core collapses inwards and the outer layers expand and cool.
The core of the red giant becomes hotter (as GPE becomes thermal energy) and begins to fuse helium into heavier elements (up to carbon), as hydrogen continues to be fused in the layers around the core.
Describe the evolution of a red giant to a white dwarf
When the star runs out of fuel, it expels its outer layers, creating a planetary nebula. The core that remains contracts further, becoming a dense white dwarf. The white dwarf has a temperature of around 30000K, and no fusion occurs. Photons which were produced earlier in the evolution leak out, dissipating heat. As the star core collapses, electron degeneracy pressure (caused as two electrons cannot exist in the same state) prevents the core from collapsing. As long as the core mass is below 1.44M☉, then the white dwarf star is stable – this is the Chandrasekhar limit.
Describe the evolution of a high-mass main sequence star into a red supergiant.
Where a star’s mass in is excess of 10 M☉, its evolution takes a different path. As hydrogen supplies deplete, the core contracts. Since the mass is greater, when GPE becomes thermal energy, the core gets much hotter than a red giant, allowing helium fusion into elements heavier than carbon (up to iron) to take place. The outer layers expand and cool, forming a red supergiant.
Describe the process of the death of a high-mass star.
When all of the fuel in a red supergiant is used up, fusion stops (as iron fusion does not release energy, it is unable to fuse further). Gravity becomes greater than the outward pressure due to fusion, so the core collapses in on itself very rapidly and suddenly becomes rigid (as the matter can no longer be forced any closer together). The outer layers fall inwards and rebound off of the rigid core, launching them out into space as a shockwave. The remaining core of a supernova is either a neutron star or black hole, depending on its mass.
Describe the evolution of a red supergiant to a neutron star or black hole
If the remaining core mass is greater than 1.44M☉, gravity forces protons and electrons to combine and form neutrons. This produces an extremely small, dense neutron star.
If the remaining core mass is greater than 3M☉, the gravitational forces are so strong that the escape velocity of the core becomes greater than the speed of light. This is a black hole, which even photons cannot escape.
