Astrophysics 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 objects have cleared their orbit of other objects
Define dwarf planets
Planets where the orbit has not been cleared of other objects
Define Planetary satellites
Bodies that orbit planets
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 center.
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 denser 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 -> thermal energy which heats up the centre. The resultant sphere of 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 star begins to
collapse inwards. It evolves in to a red giant
Describe the evolution of a red giant to a white dwarf
As helium nuclei run low, the red giant evolves into a white dwarf. The outer shells begin to
drift off into space as a planetary nebula, and the core remains as a very dense white dwarf.
The white dwarf has a temperature of around 3000K, and no fusion occurs. Photons that 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 is in excess of 10 M☉, its evolution takes a different path. As hydrogen
supplies deplete, the temperature is high enough for helium fusion into heavier elements to take
place, forming a red supergiant. The red supergiant has layers of increasingly heavy elements
produced from fusion, with an inert iron core (as iron fusion does not release energy, it is
unable to fuse further).
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. Gravity becomes greater than the outward pressure due to fusion, so the core collapses in on itself very rapidly. The outer layers fall inwards and rebound off the rigid core, launching them 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
If the remaining core mass
is greater than 1.44M☉, protons and electrons combine to form neutrons. This produces an
extremely small, dense neutron star
