Astrophysics Flashcards
Light or freely moving objects follow a particular path in spacetime. What is this path called?
Geodesic
By referring to ‘frames of reference’, in which situations would the following apply?
a) Special relativity
and
b) General relativity
a) Special relativity deals with motion in inertial (non-accelerating) frames of reference.
b) General relativity deals with motion in non-interial (accelerating) frames of reference.
When stars fuse hydrogen into helium via the proton-proton chain, which two particles are emitted during the fusion reaction?
positron and neutrino
What is the unit of luminosity?
watts (W)
What is meant by a conservative field?
The path taken between two points in the field does not affect the work done (energy) used
On a spacetime diagram, what is the name given to lines representing an object’s motion?
World-lines
Which two quantities does the luminosity of a star depend on?
The luminosity of a star depends on its radius and surface temperature
In this simplified Hertzsprung-Russell diagram, which types of stars can be found in areas A, B, C and D?
A- Main sequence stars
B- (Red) Giants
C- (Red) Supergiants
D- White dwarfs
What is the definition of escape velocity?
The minimum velocity required to allow a mass to escape a gravitational field to infinity (and have zero gravitational potential energy)
or
The minimum velocity required to allow a mass to reach infinity
Explain the stages of high mass star’s evolution from when it leaves the main sequence until its eventual fate.
- Higher mass stars can fuse elements in stages all the way up to Iron.
- These stars produce so much thermal pressure that they move into the supergiant region of the HR diagram.
- When fusion stops, the star’s core collapses suddenly, and rebounds outwards as a violent supernova explosion.
- The remaining core is so dense that it becomes a neutron star or black hole.
Describe the effect a mass has on spacetime.
Mass **curves **spacetime.
What is the unit of apparent brightness?
watts per square metre (Wm-2)
What is the definition of gravitational potential, at a point in space?
The work done in moving a unit mass from infinity to that point in space.
What is the definition of the gravitational field strength at a point?
The force exerted per unit mass (by a gravitational field).
Describe the effect of placing a clock at a lower altitude in a gravitational field.
The clock would run slower at low altitude - gravitational field is stronger and so curves spacetime more.
What is meant by the Schwarzschild radius of a black hole?
The Schwarzchild radius is the radius of a spherical mass where the escape velocity from the surface is equal to the speed of light.
OR
The Schwarzschild radius of a black hole is the distance from its centre (singularity) to its event horizon.
While in the main sequence for the majority of its lifetime, explain how a star remains stable.
The thermal pressure outward produced by fusion is balanced by gravitational attraction.
This is called gravitational equilibrium.
Explain the stages of a low to medium mass star’s evolution from when it leaves the main sequence until its eventual fate.
- Low to medium mass stars will stop fusing H to He, their cores will contract due to gravity and will start fusing He into heavier elements such as C and O.
- They will then move into the giants region of the HR diagram.
- When He fusion stops, the outer layers drift off to become a planetary nebula.
- This leaves behind a hot, dense core known as a white dwarf.
What is the unit of gravitational potential?
Joules per kilogram (J kg-1)
State what is meant by the ‘equivalence principle’.
It is impossible to tell the difference
between the effects of a uniform gravitational field and
of a constant acceleration.
OR
There is no experiment that can be performed that can distinguish between the effects of a uniform gravitational field and constant acceleration.
The world-lines on the following space time diagram represent the motion of different objects, A, B and C.
Describe the motion of objects A, B and C.
Object A is stationary
Object B is moving with a constant velocity
Object C is accelerating
How would you convert between
- astronomical units (AU) and metres?
- light years and metres?
- 1 AU = 1.5 x 1011 m (given in data sheet)
- 1 light year = 3 x 108 x 365.25 x 24 x 60 x 60 (= 9.46 x 1015 m) (NOT in data sheet!)
Derive the expression shown below for escape velocity.
Ek + Ep = 0
½mv2 + (-GMm/r) = 0
½mv2 = GMm/r
v2 = 2GM/r
v = √2GM/r
Shown is the relationship used to find the gravitational potential at a point in space.
How could you find the gravitational potential energy of a mass, m, at that point?
Multiply the gravitational potential by the mass:
Ep = V x m
Which quantity of a star determines its ultimate fate?
Its mass.
Describe the effect of placing a clock at a higher altitude in a gravitational field.
The clock would run faster at high altitude - gravitational field is weaker and so curves spacetime less.
What is the value of a field’s gravitational potential at infinity?
At infinity, the gravitational potential of a field is zero.
What is the main change in the fusion reactions of a star when it leaves the main sequence?
Nuclear fusion of hydrogen will cease in the core of the star.
Describe the fusion reactions happening in a main sequence star.
Fusion of hydrogen occurs in the core of main sequence stars.
Every star ultimately becomes a white dwarf, a neutron star, or a black hole.
Which one of these will our Sun become?
White dwarf
(our Sun is a low-mass star)
