stars and astrophysics Flashcards
define a planet?
objects with mass sufficent for their own gravity to force them to take 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 billions 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 the 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 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 depleted in the core, the gravitational forces inward overcome the radiation and gas pressures so the core collapses inwards until the pressure is great enough for fusion of hydrogen to helium to occur in a shell around the core (not in core itself as no hydrogen there ). as result the fusion radiation will cause the outer layers to expand and then cool, which gives them the red colour
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 3000K, and no fusion occurs. photons which were produced earlier in the evolution leak out, dissipating heat. as the star 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 is stable - this is the Chandrasekhar limit.
describe the evolution of a high mass main sequence star into a red supergiant?
it occurs when stars mass is in excess of 10M. as the hydrogen supplies deplete, the core contracts. since the mass is greater, the core gets much hotter than a red giant, allowing helium fusion into elements heavier then carbon ( up to iron) to take place. the core then expand (due to increased fusion radiation and heat) and this process continues until there is an iron core as iron nuclei’s cant fuse
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 and suddenly becoes rigid (as matter can no longer be forced any closer). the outer layers fall inwards and rebound of the rigid core, launching them out into space as a shockwave. the remaining core of a supernova is either a neutron star or a black hole depending on its mass
describe the evolution of a red supergiant to a neutron star and 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 is greater than 3M, the gravitiational forces are so stron that the escape velocity of the core becomes greater than the speed of light. this is a black hole which even photons cant escape
describe the process of electrons exciting in discrete energy levels?
electrons bound to an atom can only exist in certain discrete energy levels. the electrons cannot have an energy value that is between two levels. each element has its own set of energy levels. when an electron moves from a lower energy state to a higher energy state, it is excited. this requires the input of external energy ( e.g heating or absorbing a photon of the exact energy required)
