Stars Flashcards
Everything you need to know to get A* in OCR Physics A Level, closely following the specification and the textbook
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.
What relationship does a Hertzsprung-Russell Diagram show?
The relationship between luminosity on the y axis and temperature on the x axis.
Define luminosity
The radiant power output of a star, unit Watts
Can electrons exist between energy levels?
No, electrons exist only on discrete energy levels
Why are energy levels negative?
Because external energy is required to remove an electron from the atom.
What is the energy level value of an electron which is free from the atom?
Zero
What does it mean when an electron is in its ground state?
It means the electron is in the energy level with the most negative value
What does it mean when an electron is excited?
When an electron move from a lower to a higher energy level, with an atom in a gas, the electron has been excited
How does an electron get excited?
When photons that have an energy equal to the energy difference between the discrete energy levels, that’s when an electron gets excited
How can an electron lose energy once it has been excited?
When an electron moves from a higher to a lower energy level, it has become de-excited, and it loses energy and emits a photon with an energy equal to the energy difference between the energy levels
What are the 3 different types of spectra?
Emission spectra, continuous spectra and absorption line spectra
What is emission line spectra?
Each element produces a unique emission line spectrum because of its unique set of energy levels (Colourful lines against black background)
What is continuous spectra?
All visible frequencies or wavelengths are present. Atoms of a heated solid metal will produce this kind of spectra
What is absorption line spectra?
A series of dark spectral lines against a background of a continuous spectrum
How are emission line spectrum formed?
If atoms in gas are excited, then when the electrons drop back into lower energy levels, they emit photons with a set of discrete frequencies specific to that element.
How are absorption line spectrum formed?
When light from a source that produces a continuous spectrum passes through a cooler gas. As photons pass through gas, some are absorbed by gas atoms, exciting the atoms, creating dark lines on the spectrum. These lines show which photons have been absorbed
What is a diffraction grating?
An optical component with regularly spaced slits/lines that diffract and split light into beams of different colour travelling in different directions
Why are diffraction gratings better than double slits?
Using a larger number of slits produces a clearer and brighter interference pattern
What is the grating spacing, d?
The separation between slits on a diffraction grating
What is the grating equation?
dsinθ=nλ
How can a diffraction grating be used to determine the wavelength of light?
Measure the angle between several maxima and the zero order maximum, then plot a graph of sinθ against n. This produces a straight line through the origin, that has a gradient of λ/d.
Define a black body
An idealised object that absorbs all the electromagnetic radiation incident on it and, when in thermal equilibrium, emits a characteristic distribution of wavelengths at a specific temperature
What is Wiens displacement law?
the maximum wavelength is inversely proportional to the temperature, λ(max) x T = constant
How does the distribution of wavelengths change as the temperature increases?
The graph’s peak becomes higher and it moves to the left
How does the distribution of wavelengths change as the temperature increases?
The peak goes lower and the graph shifts to the right
What is Stefan’s law?
The total power radiated per unit surface area of a black body is directly proportional to the fourth power of the absolute temperature of the lack body
What is the units of luminosity?
Watts
What is the relationship between luminosity and the radius according to Stefan’s Law?
Luminosity is directly proportional to the square of the radius
What is the relationship between luminosity and the surface area according to Stefan’s Law?
Luminosity is directly proportional to the surface area of a black body
How can Wien’s displacement law and Stefan’s law be used to estimate the radius of a star?
determine the surface temperature of the star using Wien’s law. Use Stefan’s Law to determine the radius
