5.5 Flashcards
Nuclear fusion
The process of joining together two nuclei and releasing energy from a change in the binding energies
What are comets?
Large rocky icy balls that travel in highly elliptical orbits around the Sun
Gravitational collapse
The inward movement of material in a star due to the gravitational force caused by its own mass. Star formation is due to the collapse of gas and dust
Why does gravitational collapse occur in mature stars?
When the internal gas and radiation pressure can no longer support the star’s own mass it under goes collapse
Radiation pressure
Due to the momentum of photons released in fusion reactions and acts outwards (in the direction of energy flow)
Main sequence star
A star in the main part of its life cycle, where it is fusing hydrogen to form helium in its core
Red giant
Star in the later stages if its life that has nearly exhausted the hydrogen in its core and is now fusing helium nuclei.
Bigger than normal star as the outer layers have cooled and expanded
White dwarf
End product of a low mass star, when the outer layers have dispersed into space.
Very dense, high surface temp, low luminosity
Planetary nebula
Expanding glowing shell of ionised hydrogen and helium ejected from a red giant star at the end of its life
Electron degeneracy pressure
Pressure that stops the gravitational collapse of a low mass star.
The pressure that stops a white dwarf collapsing
Chandrasekhar limit
The maximum possible mass for a stable white dwarf star, is equal to 1.4 x mass of our Sun.
Higher mass white dwarfs will further collapse to neutron stars/black holes
Red super giant
Star that has exhausted all the hydrogen in its core and has a mass much higher than the Sun
Supernova
Huge explosion produced when the core of a red super giant collpases
Neutron star
The remains of the core of a red super giant after it has undergone a supernova explosion, incredibly dense and composed mainly off neutrons
Black hole
Core of a massive star that has collapsed almost to a point
Very very dense and very small
Infinite gravitational field strength
Escape velocity > speed of light
Hertzsprung-Russel diagram
White dwarf: bottom left Main sequence: diagonal line through middle (y=-x) Nebula: bottom right Protostar: above nebula Red giant: middle right Red super giant: top right
Luminosity
Total energy a star emits per second
Continuous spectrum
Spectrum appears do contain all wavelengths over a comparatively wide range
Energy levels
Inside an atom they are the specific energies that electrons can have when occupying specific orbits. Electrons can only occupy these discrete energy levels and cannot exist at other energy values between them
Emission line spectrum
Individual elements have a spectrum of frequencies of electromagnetic radiation emitted due to electron transitions from a higher energy level to a lower one.
Bright lines against a dark background
Absorption line spectrum
Pattern of dark lines in a continuous spectrum from a light source caused by light passing through an absorbing medium such as a gas.
Dark lines are wavelengths that are absorbed
Transmission diffraction grating
Glass surface having a large number of very fine parallel grooves or slits, used to produce optical spectra by diffraction
Maxima
Regions of brightness which will be seen when the path difference between overlapping waves is a whole number of wave lengths
Wien’s displacement law
Max wavelength is inversely proportional to T
Used to estimate the peak surface temperature of a star from the wavelength at which the stars brightness is maximum
Stefan’s law
L = 4πr²σT⁴
Astronomical unit
The mean distance from the centre it the Earth to the center of the Sun
Parsec
Unit of distance that gives a parallax angle of 1 arc second (1/3600 of a degree)
Approx. 3.1*10¹⁶
Stellar parallax
Apparent shifting in position of a star viewed against a background of distant stars when viewed from different positions of the Earth in its orbit of the sun
Light year
Distance travelled by light in one year
Approx 9.5 x 10¹⁵
Doppler effect
Change in wavelength caused by the relative motion between wave source and an observer
Δλ / λ = v/c
Red shift
Apparent increase in wavelength of electromagnetic radiation caused when the source is moving away relative to the observer
Hubble’s law
Recessional velocity of a galaxy is directly proportional to its distance from the earth
Hubble’s constant
Constant of proportionality for Hubble’s Law
What temperature does the wavelength of CMBR correspond to?
2.7 K, the same as the average temperature of the universe
Cosmological principle
The universe is isotropic and homogeneous as long as a large enough volume is considered
Dark matter
Cannot be seen, does not emit or absorb EM radiation
Undetectable directly but its effect on the gravity of galaxies can be indirectly used to detect its presence
Dark energy
Type of energy that permeates the whole universe and opposes the gravitation between galaxies via the exertion of negative pressure
Detected by rate of expansion of universe
