Astrophysics Flashcards
What is the work done independent on?
The path taken
How are satellites kept in orbit around a planet?
By the gravitational force, which provided the centripetal force
What is the gravitational potential of a point in space?
The work done moving 1kg from infinity to that point.
What is the gravitational force at infinity?
0N
What is escape velocity?
The minimum velocity required to allow a mass to escape a gravitational field to infinity with zero kinetic energy and zero potential energy.
What is the difference between special relativity and general relativity?
Special relativity occurs in inertial frames of reference
General relativity occurs in non-inertial frames of reference.
What does inertial mean?
Non-accelerating
What does non-inertial mean?
Accelerating
What is the equivalence principle?
It is not possible to distinguish between the effects on an observer in a uniform gravitational field or at a constant acceleration
What happens to the effects of gravity when an object in freefall experiences ‘weightlessness’?
The effects of gravity are exactly equilavent to the effects of acceleration
What curves space time?
Mass
What is gravity due to?
The curvature of space time
What can a large mass do to light?
Make light bend.
The light still travels in a straight line but space time has curved due to the large mass.
What is a geodesic?
The shortest distance between two points in space time that light/objects follow.
What does light do according to an observer moving at a high speed (due to the equilavence principle)?
Bend
Does light bend when there is a high or low gravitational field strength?
High g
What happens to time under the influence of greater gravity?
Time runs slower (dilates)
What does the altitude (height) of an object above a planets surface effect?
The time
“The lower, the slower”
Does time run slower at the front or rear of a spacecraft?
The rear
What is a line drawn on a space time diagram called?
A worldline
What causes a star to collapse into a very small radius?
It runs out of fuel and gravitational compressions cause it to collapse.
What is a black hole?
A compressed star with a very small radius
What do black holes have due to their high density/large mass?
A very strong gravitational field
They also severely curve/warp space time
What is the point of infinite density in the centre of a black hole called?
A singularity
What is the Schwarszchild radius?
The distance from the singularity to the event horizon
What is the escape velocity from the event horizon of a black hole equal to?
The speed of light
3x10^8
What does time appear to be at the event horizon of a black hole for a distant observer?
Frozen
What is the definition of luminosity?
The total energy radiated per second (or power radiated)
What is luminosity dependant on?
Radius
Surface temp
What is the definition of apparent brightness?
The amount of energy per second (power) detected per unit area
What assumptions do we make about stars?
They’re perfectly spherical
They have a uniform surface temperature
They are perfect ‘black-bodies’
What is a blackbody?
Absorb/emit all wavelengths of radiation
Where are stars formed?
Interstellar clouds (Nebulae)
How are stars formed?
In nebulae
Gravitational forces cause particles to be pulled into a central core
When the core is hot enough to sustain nuclear fusion, the outwards thermal pressure is balanced by inwards gravitational force
What process involves hydrogen nuclei in the sun undergoing nuclear fusion into helium?
The P-P chain
What are the three stages of the P-P chain?
Two hydrogen nuclei combine to form deuterium nucleus - positron and neutrino are produced
Deuterium combines with a proton to produce helium-3 and gamma rays
Two helium-3 nuclei combine to form helium-4 nucleus and two protons (which may cause further P-P chains)
What are H-R diagrams?
Scattergraphs of star luminosity against surface temp, they allow stars to be given classifications
What type of star accounts for 90% of stars in the universe?
Main sequence
What do main sequence stars do in their core?
Fuse hydrogen nuclei into helium nuclei
Explain the red giant branch
Hydrogen fusion in core of star supplies energetic to maintain the outwards thermal pressure that balances inwards gravitational forces
When hydrogen is depleted in core, nuclear fusion in core stops
Hydrogen in the shell surrounding core continues fusing, adding more helium to core
Gravitational forces cause core to contract, therefore heating core
Outwards thermal pressure increases, therefore volume (radius) of star increases
Surface temperature decreases as surface area increases, therefore star enters red giant/supergiant region of H-R diagram.
Explain helium core fusion
As the sun becomes a red giant, the mass of helium in the core will increase
Core will continue to contract
Core temperature will increase until helium begins fusing (into carbon)
What does the mass of a star determine?
It’s lifetime
Stars with a large mass have more fuel but deplete faster (moving from main sequence to red giant on H-R diagram)
What will every star ultimately become?
A white dwarf (low/medium mass star)
A neutron star (high mass star)
A black hole (high mass star)
