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59 Cards in this Set
- Front
- Back
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Similarities between Gravitational & Electric Field |
Unit Forces - g is force per unit mass, E per unit positive charge Inverse Square relationship
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Differences between Gravitational & Electric Field |
- Gravity always attractive, electric can be either - Objects can be shielded from electric fields but not gravitational fields - Size of the electric field depends on the medium between the charges (e.g. plastic or air). Makes no difference for gravitational fields
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Measuring distance/velocity within the solar system |
- RADAR: Radio telescopes send short pulses of radio waves towards object - Detects reflected waves and t taken measured - 2d = ct - Two pulses sent a certain interval apart can be used to measure distance moved in that time and hence the velocity of the object |
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One Astronomical Unit |
(AU)
The mean distance between the Earth and the Sun |
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One Light Year |
(ly)
The distance that electromagnetic waves travel in one year in a vacuum |
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AU / ly equivilence |
1 ly = 63,000 AU |
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One Parsec |
(pc)
The distance a star is away if its angle of parallax is one arcsecond (1/3600 of a degree) 3.26ly 206,265AU |
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Parallax |
Apparent change in position of near stars against the background of distant stars as the earth moves in orbit about the sun |
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Hipparcos Parallax |
Parallax measured by the orbital satellite Hipparcos. Much more sensitive readings as the image is not distorted by the atmosphere, allowing for measurement of greater distances. Now overlaps with Cepheid Scale |
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Luminosity |
The total energy a star emits per second |
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Radiation Flux |
The power (energy per second) of the radiation per square metre at a certain distance from a source |
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Standard Candles |
- Objects (e.g. supernovae & Cepheid variables) who's luminosity can be calculated directly - Flux used to find distance from Earth - If a SC is within a galaxy the distance to that galaxy can be found - Was how Hubble's Constant was calculated |
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Black Body definition |
A body that absorbs all wavelengths of electromagnetic radiation and can emit all wavelengths of electromagnetic radiation ('perfect' emitter/absorber) |
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Black Body graph |
y axis - power radiated x axis - wavelength - each line starts somewhere right of 0λ and slopes up to peak at a certain λ & power before sloping down and flattening to approach the x axis - Higher temperature lines peak at higher power and lower λ |
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Absorption Spectra |
- The radiation emitted from the star's core includes all wavelengths (black body) - As it passes through the outer layers of gas light of certain wavelengths are absorbed and re-emitted in all directions - This means a spectra of that star will have darker lines at specific λs characteristic of certain elements |
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Drawing Hertzsprung-Russell Diagram |
y axis - relative luminosity (non-linear scale) x axis - surface temperature (decreasing, non linear) Top right - Red Giants/Supergiants Top left to bottom right - Main sequence Bottom left - white dwarfs |
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Reason for H-R Diagram grouping |
Stars exist in these stable stages of their life cycle for long periods of time. Stars move through the transitional areas in between relatively quickly |
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H-R temperature group mnemonic |
Only Boring Astronomers Find Gratification Knowing Mnemonics (left to right) |
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Absolute Magnitude |
The magnitude (brightness) a star would appear if placed at a distance of 10 pc (32.6 ly) from Earth |
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Star Life Cycle - Stage 1 (common) |
1 - A gas cloud ~ 100ly across from which one or more stars will form. Condenses under gravity to 2 - A dark globule: Cold, contraction continues 3 - A protostar: Radiates strongly in IR. Very large (2 x solar system) Planets may form. Contraction continues, temp & pressure rise in core. 10,000,000k nuclear fusion starts 4 - hydrostatic equilibrium reached
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< 0.05 SM Star Life |
3 - Fusion never starts - eventually condenses to form a brown dwarf |
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Low Mass Star Life |
5 - Main sequence 6 - Red Giant 7 - Planetary Nebula & White Dwarf |
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High Mass Star Life |
5 - Main sequence 6 - Red Super-Giant (Increasing Size) 7 - Supernova 8 - Neutron Star (lighter) or Black Hole (heavier) |
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Main sequence |
Hydrogen Fusion in core. H fusion is more rapid in larger stars so MS period is shorter the more massive the star |
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Red Giant |
(~1 SM) - Amount of H becomes insufficient to allow H fusion to maintain hydrostatic equilibrium - Core contracts & heats up until hot (100Mk) & dense enough for He fusion, outer layers expand due to heat - He fusion releases large amounts of energy & star once again reaches new HE |
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Hydrostatic Equilibrium |
Where the pressure produced by fusion in the core balances the gravitational forces trying to compress the star |
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Red Super-Giant |
(~10-30 SM) - 1st stage same as Red Giant - Each time fusion wains core contracts to temp/density able to start fusing next element and the extra energy causes star to expand - Continues up to iron as fusion beyond iron is not energetically favourable and ceases |
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White Dwarf |
(~ 1 SM) - Once He fusion ceases further collapse begins - Core will not reach a temperature high enough for further fusion - Once core Earth sized electron degeneracy pressure opposes further collapse (if core mass < 1.44SM) - Very small & hot, cools to black dwarf |
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Planetary Nebula |
(~ 1 SM) - As core collapses it causes shockwaves that travel through - The star pulsates and throws off unstable outer layers into a planetary nebula - So named as they look for areas that looked more like planets than stars to astronomers |
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Neutron Star |
(~10-30 SM) - When fusion ceases in RSG core begins to collapse under gravity - If core mass >1.44 SM electron degeneracy pressure is overcome - Rapid core contraction (fraction of a sec) until neutron degeneracy pressure stops collapse
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Neutron Star Properties |
- Very dense (~ 4 x 10^27 kg/m^3) - Very small (~ 20km across) - Rotate very fast (up to 600 rev/second) - Emit radio waves in two beams as they rotate. When these sweep past Earth we observe radio pulses - these all called pulsars |
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Black Hole |
(~10-30 SM) - As with neutron star but neutron degeneracy pressure overcome (core mass > 2-3 SM) - All the mass collapses into a singularity which warps space-time around it |
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Electron Degeneracy Pressure |
- (or) electrostatic repulsion & application of the Pauli exclusion principle - When overcome: Proton fuses with electron to produce a neutron and an electron neutrino |
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Supernova |
- The halt of collapse caused by NDP causes collapsing outer layers to collide with surface of the core and produce massive shockwaves, blowing off the layers in a cataclysmic explosion - a supernova - Temperature/Pressure reaches high enough to fuse higher elements - Can briefly outshine an entire galaxy |
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Cosmological Redshift |
- Redshift as a result of the expansion of space between galaxies and the stretching of EM waves as they travel through it - Unlike normal redshift, which is a result of objects that actually are moving through space |
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Recessional Velocity |
How fast a galaxy is moving away |
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Condition for the redshift formula |
Velocity is much less than the speed of light |
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Hubble's Constant Units |
- km(/s)(/Mpc)
- SI: /s |
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The Hot Big Bang Theory |
The universe started off very hot and very dense (perhaps as an infinitely hot, infinitely dense singularity) and has been expanding ever since |
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Observable Universe |
A sphere (with Earth at the centre) with a radius equal to the maximum distance that light can travel within the age of the universe |
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Age of the universe |
- Depends on Hubble's Constant - If it has expanded at a constant rate then age = 1 / Ho - If Ho = 75km/sMpc then age = 13 billion years |
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Critical Density |
The density of mass in the universe that means gravity is just strong enough to stop expansion at t = infinity
Universe size approaches an asymptote |
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Density < Critical Density |
Gravity is too weak to stop expansion, the universe will expand forever |
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Density > Critical Density |
Gravity is strong enough to stop expansion and start the universe contracting again (ending in The Big Crunch) |
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Problems with estimating age/density of the universe |
- As the universe expands it becomes less dense so the more dense it is the younger it must be - Dark matter contributes to density but cannot be observed directly - Evidence found that the expansion of the universe is accelerating |
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Mass Deficit/Defect |
The difference between the mass of the separated nucleons and the combined mass of the nucleus |
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Binding Energy |
The energy needed to pull a nucleus completely apart, the same as is released when the nucleus formed. Equivalent to the mass deficit |
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Binding Energy per Nucleon Maximum |
around N = 50 |
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Binding Energy per Nucleon Graph |
- Very steep increase at start (Hydrogen) - Reaches maximum at N = 56 (Iron) (most stable) - Decreases with shallow gradient and almost straight line as N increases through the heavier elements |
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Nuclear Fission |
- When large, unstable nuclei with at least 83 protons (e.g. uranium) split into two smaller nuclei - Spontaneous: it just happens by itself - Induced: it is encouraged to happen - Energy released as the new nuclei each have a higher binding energy per nucleon |
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Heavier Elements |
- The larger the nucleus, the more unstable it will be - the more likely to spontaneously fission - This means spontaneous fission limits the number of nucleons a nucleus can contain (limits the number of possible elements) |
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Fusion |
Two light nuclei combine to create a larger nucleus and release energy as the new nucleus has a much higher binding energy per nucleon |
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Hydrogen Fusion |
H-2 + H-1 --> He-3 + energy
Deuterium + Protium --> helium-3 |
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Conditions for Individual Fusion |
- Nuclei must overcome the electrostatic repulsive force between them and get close enough for the strong interaction to hold them together - Requires about 1MeV of kinetic energy |
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Newton’s Law Of Universal Gravitation
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Every particle in the Universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them
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Gravitational field strength
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The gravitational force per unit mass acting on a mass placed at a point
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Gravitational potential energy (U) (at a point)
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The work done by an external force in a gravitational field in bringing a point mass from infinity to that point
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Gravitational potential (at a point)
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The work done per unit mass, by an external force in a gravitational field, in bringing a mass from infinity to that point
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Parallax Distance Formula
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d (parsecs) = 1 / p (arcseconds)
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