# Astrophysics and Cosmology

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• Created by: dkoning00
• Created on: 29-05-18 15:27
State Newton's Law of Universal Gravitation
Every body in the universe attracts every other body with a force that is proportional to the product of their masses and inversely proportional to the square of the distance apart
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What is gravitational field strength
The force that would act on 1kg of mass placed at a point in a gravitational field
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Why was dark matter first theorised?
Because stars orbiting a galaxy at a greater radius should orbit more slowly than those closer to the centre but they actually orbit at the same velocity. This means mass of the galaxy increases with radius but the extra mass is not observable
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What is Kepler's Third law of planetary motion?
The square of the time period of orbit is directly proportional to the cube of the radius of orbit of celestial bodies
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Define a gravitational field
A region of space where masses feel forces because of their mass
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Define gravitational potential
The gravitational potential at a point is the work done in bringing a unit mass from infinity to that point
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Why are all values for gravitational potential negative?
Gravitational potential at infinity is zero
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State some similarities between electric and gravitational fields
Magnitude follows and inverse square relationship with radius, field lines show the direction of forces acting on a body, field is infinite in extent
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State some differences between electric and gravitational fields
Gravitational forces are always attractive so field lines only ever point in one direction, very different constants, electric fields only exert forces on charged objects, gravitational fields can't be shieled
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Describe the Doppler effect
The apparent change in frequency and wavelength of waves emitted from a scource due to the relative motion of the source and receptor
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Describe a main sequence star
A star in which nuclear fusion occurs in the core where hydrogen is fused into helium. The inwards gravitational force is balanced by outwards radiation pressure
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When can fusion occur?
When temperature and pressure in the core are high enough to give hydrogen enough kinetic energy in order to bring them so close together they overcome the Coulomb barrier. When this happens the strong nuclear force acts between quarks so they fuse
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Why do more massive stars spend less time on the main sequence?
Greater gravitational forces create greater density in the core so the collision rate is higher meaning the star runs out of hydrogen more quickly
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When does the main sequence stop?
When the amount of hydrogen in the core drops to a point when collision rate falls below a critical threshold, temperature in the core decreases and all fusion stops instantly
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What happens at the end of the main sequence?
There is no outwards radiation pressure to balance gravitational forces so the core collapses creating a red giant
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What happens in the red giant phase?
Core collapse increases temperature so helium fuses into carbon nuclei, the outer layers of the star expand and cool
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How is a white dwarf created?
Helium fusion in a red giant ceases like the end of the main sequence, the core collapses again but temperature is never high enough for more fusion, outer layers are ejected into space and the core becomes a white dwarf with high surface temp
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Why are white dwarfs not very luminous despite high surface temperature?
Very small surface area
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What happens to super massive red giants?
Greater mass causes greater gravitational forces which increases core temperature and leads to layers of fusion of heavier elements
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Why can't any elements heavier than iron be made by fusion in a star?
Iron has the highest binding energy per nucleon so fusing elements heavier than iron would be endothermic
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So where do heavier elements come from?
Collisions between neutron stars and super novae explosions
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What happens when all fusion in a massive star ceases?
The whole star collapses then explodes in a supernova
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What remains after a star goes supernova?
Either a neutron star or a black whole depending on the mass of the original star
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What is a black body?
A body which absorbs all radiation falling on it, like a star
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State Wein's displacement law
The peak wavelength emitted by a black body is inversely proportional to its absolute temperature
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What is the luminosity of a star?
Power output measure in Watts
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What two factors does luminosity depend on?
Surface area and surface temperature
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State the Stefan-Boltzmann Law
The power radiated by an object is proportional to the fourth power of its absolute temperature
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Describe the layout of the Hertzbrung-Russell diagram
Log graph of Luminosity against absolute temperature, temperature running backwards from 40,000K to 2500K
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Why might there be stars missing from top left of the main sequence?
By the time they are observed they have already left the main sequence
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Intensity - the power arriving per unit area
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What is parallax in relation to astrophysics?
The apparent motion of nearby stars compared to fixed background distant stars from which distance to nearby stars can be estimated
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Give the equation for trig parallax calculations
d=1AU/tan(p)
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What is a standard candle?
A class of astrophysical objects with known luminosity such as supernovae and cepheid variables
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How are standard candles used to estimate distances to stars in nearby galaxies
By knowing their luminosity and measuring intensity on earth, distance to objects in other galaxies can be calculated
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What is the ladder of distances?
Since each method of measuring distances to far away objects relies on the last, a change in the accuracy of one measurement has an effect on each that follows
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What is redshift?
Light from distant galaxies being shifted towards the red end of teh visible spectrum as a result of the doppler effect if they are moving away from us
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State Hubbles Law
The redshift (and hence recessional velocity) of a galaxy is directly proportional to its distance from earth
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How are Hubbles law and the Doppler effect used to calculate distances to the furthest galaxies?
Redshift is calculated as a ratio of the change in wavelength to actual wavelength from an absorption spectrum, this is roughly equal to the ratio of recessional velocity to the speed of light. From the Hubble constant, the distance can be found
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Where does cosmic microwaves background radiation come from?
After the big bang the universe consisted of plasma with no atoms, after 300,000 years the plasma cooled enough for atoms to form releasing vast amounts of high energy gamma radiation. This gamma has cooled and increased in wavelength to microwaves
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## Other cards in this set

### Card 2

#### Front

What is gravitational field strength

#### Back

The force that would act on 1kg of mass placed at a point in a gravitational field

### Card 3

#### Front

Why was dark matter first theorised?

### Card 4

#### Front

What is Kepler's Third law of planetary motion?

### Card 5

#### Front

Define a gravitational field