astrophysics definitions Flashcards
Astronomical Unit
The distance from the Earth to the Sun. It is equivalent to 1.5x1011 m.
Light Year
The distance that light travels in one year. It is equivalent to 9.46x1015 m.
Gravitational Field
Strength
The gravitational force per unit mass.
Gravitational Field
A region around a mass where another mass will experience a gravitational force.
Conservative Field
A field in which the work done by a force (e.g. the force of gravity) on a particle
that moves through any round trip in the field is zero i.e. energy is conserved.
Newton’s Law of
Universal Gravitation
Every object in the universe with a mass attracts every other object with a mass. It gives a measure of the gravitational force of attraction between any two objects and is an inverse square law.
Gravitational Potential
At a point in a gravitational field, it is the work done by external forces in
moving unit mass from infinity to that point.
Gravitational Potential
Energy
The energy that a mass has when it is a certain height above the Earth’s
surface.
Escape Velocity
The minimum velocity required by a mass, m, to just escape from a planet’s
gravitational field and reach infinity with zero velocity or zero kinetic energy.
Inertial Frame of
Reference
A frame of reference in which an observer is not accelerating with respect to
another. That is, an observer will move at a constant speed with respect to
another.
Non-Inertial Frame of
Reference
A frame of reference in which an observer is accelerating with respect to
another. That is, an observer will not move at a constant speed with respect to
another.
Equivalence Principle
Gravity pulling in one direction is equivalent to acceleration in the other.
That is, there is no way to distinguish between the effects on an observer of a
uniform gravitational field and of a constant acceleration.
Spacetime
The combination of space and time.
Gravitational Time
Dilation
Where the force of gravity is weaker, time passes more quickly
Gravitational Lensing
If an object has sufficient mass it can cause light which is travelling in a straight line to bend.
World Line/Geodesic
A curve in spacetime joining the positions of a particle throughout its existence.
It defines the position of an object in 3 dimensions (x, y and z) as well as the
fourth dimension of time (t).
Geodesic Path
The shortest distance between two points in spacetime.
Black Hole
A region in space where the pull of gravity is so great that nothing, not even
light, can escape its pull.
It is also the last stage in the life cycle of a very high mass star.
Event Horizon
The radius of a black hole below which nothing, not even light, can escape from
its gravitational pull.
From the perspective of a distant observer, time appears to be frozen at this
point.
Singularity
The centre point of a black hole.
Schwarzschild Radius
The distance from the centre of a black hole to its event horizon.
Stefan-Boltzmann Law
Gives a measure of the total energy being emitted at all wavelengths by a black body.
Black Body
A body that absorbs all the electromagnetic radiation incident on it and also
emits all wavelengths of electromagnetic radiation.
Luminosity
A measure of how bright a star actually is (i.e. the total power it emits), not
how bright it appears to us on Earth.
It is dependent on the star’s radius and surface temperature.
Apparent Brightness
A measure of how bright a star appears on Earth.
It is dependent on the star’s luminosity and distance from the Earth.
Charge-Coupled Device
(CCD)
A chip found behind the lens in a digital camera that stores images.
Hydrostatic Equilibrium
When the thermal pressure outwards from a star balances the gravitational
force inwards.
‘Proton-Proton’ Chain
A three stage process in which stars produce energy by converting hydrogen
into helium through nuclear fusion.
Hertzsprung-Russell
(H-R) Diagram
Relates a star’s luminosity to its temperature.
Stars are plotted on the diagram according to their size and luminosity and they
are found to fall into groups: main sequence, giants, supergiants and white
dwarfs.
White Dwarf
The last stage in the life cycle of a low mass (Sun like) star.
Neutron Star
The last stage in the life cycle of a high mass star.
