Stars Flashcards
Everything you need to know to get A* in OCR Physics A Level, closely following the specification and the textbook (53 cards)
Definition of planets
An object in orbit around a star with a mass large enough for its own gravity to give it a round shape, that undergoes no fusion reactions and that has cleared its orbit of most other objects
Definition of planetary satellites
A body in orbit around a planet - may be natural (moon) or artificial
Definition of comets
A small, irregular shaped body made of ice, dust and small pieces of rock in a highly eccentric orbit around the Sun.
Definition of Solar systems
A planetary system consisting of a star and at least one planet in orbit around it
Definition of galaxies
A collection of stars and interstellar dust and gas bound together by their mutual gravitational force
Definition of the universe
Everything that exists within space and time
Definition of Nebulae
A cloud of dust and gas
How are stars born?
Nebulae are formed when tiny gravitational attraction between dust particles pull them closer together. As dust gets closer, gravitational collapse accelerates. Due to tiny variations in the nebulae, denser regions form. These regions pull more dust and gas, gaining mass and getting denser and hotter. In one part of the star, a protostar forms.
What happens after a protostar forms?
For protostar to become star, nuclear fusion must start in its core. As more and more mass is added to the protostar, it grows so large and core becomes so hot that kinetic energy of hydrogen nuclei is large enough to overcome electrostatic repulsion. Thus a star is born.
What happens once a star is formed?
It remains in stable equilibrium with almost constant size. Gravitational forces compress the star but radiation pressure from fusion reactions and the gas pressure from the nuclei push outwards, maintaining equilibrium. This is described as a main sequence star. This is the longest period in the lifetime of a star
What is gas pressure?
The pressure of the nuclei in the stars core pushing outwards and counteracting the gravitational force pulling matter in the star inwards.
What happens to a low mass star when it runs out of hydrogen in the core?
Low mass stars (Mass between 0.5 and 10 solar masses) evolve into red giants, then the outer layers drift off an form a planetary nebula, and the core remains and becomes a white dwarf.
What happens inside the star when it turns into a red giant?
The reduction in energy given out by fusion means the gravitational force is stronger, so the core of the star collapses and as it shrinks, pressure increases, so star begins fusion in the shell around the core. The core remains inert. This causes the star to expand, and cool
What happens after the red giant phase of a star?
Most of the layers of the red giant around the core drift off into space as a planetary nebula, leaving behind the hot core as a white dwarf.
What are the characteristics of a white dwarf?
It is very dense, with a mass similar to the sun, and a volume similar to earth’s. Surface temperature can be around 30 000K. Only stars that have a core with a mass lower than the Chandrasekhar limit can form white dwarfs
What is the Chandrasekhar limit?
1.44 solar masses
What is the electron degeneracy effect?
2 electrons cannot exists in the same energy state. When core of star begins to collapse due to gravitational force, electrons are squeezed together, creating a pressure that prevents core from further gravitational collapse. This is the electron degeneracy pressure.
What happens to a large mass star after the main sequence period?
Become red supergiant, supernova occurs, then can either form a neutron star or a blackhole
Describe all the processes that happen when a star turns into a red supergiant
The star starts to expand. Inside, the temperature and pressure are high enough to fuse massive nuclei together, forming a series of shells inside the stars. This process continues until star develops iron core, which cannot fuse.
What happens after nuclei have been fused to iron?
A supernova occurs, where the layers bounce off the core, leading to a shockwave that ejects all the core material into space.
What are the two things that could happen after a supernova?
Either a neutron star forms, or a black hole forms
What is a neutron star, and what are the conditions for it to form?
If the mass of the core is greater than the Chandrasekhar limit, gravitational collapse continues, forming neutron star, which are entirely made up on neutrons and are very small and dense.
What is a black hole, and what are the conditions for this to form?
If the core has a mass greater than 3 solar masses, gravitational collapse continues to compress the core. The result is a gravitational field so strong not even light can escape.
What are the characteristics of black holes?
Nothing, not even photons, can escape black holes due to their extremely strong gravitational fields, and they are thought to be at the centre of most galaxies.
