astrophysics definitions

Question 1

Astronomical Unit

Answer 1

The distance from the Earth to the Sun. It is equivalent to 1.5x1011 m.

Question 2

Light Year

Answer 2

The distance that light travels in one year. It is equivalent to 9.46x1015 m.

Question 3

Gravitational Field
Strength

Answer 3

The gravitational force per unit mass.

Question 4

Gravitational Field

Answer 4

A region around a mass where another mass will experience a gravitational force.

Question 5

Conservative Field

Answer 5

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.

Question 6

Newton’s Law of
Universal Gravitation

Answer 6

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.

Question 7

Gravitational Potential

Answer 7

At a point in a gravitational field, it is the work done by external forces in

moving unit mass from infinity to that point.

Question 8

Gravitational Potential
Energy

Answer 8

The energy that a mass has when it is a certain height above the Earth’s
surface.

Question 9

Escape Velocity

Answer 9

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.

Question 10

Inertial Frame of
Reference

Answer 10

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.

Question 11

Non-Inertial Frame of
Reference

Answer 11

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.

Question 12

Equivalence Principle

Answer 12

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.

Question 13

Spacetime

Answer 13

The combination of space and time.

Question 14

Gravitational Time
Dilation

Answer 14

Where the force of gravity is weaker, time passes more quickly

Question 15

Gravitational Lensing

Answer 15

If an object has sufficient mass it can cause light which is travelling in a straight line to bend.

Question 16

World Line/Geodesic

Answer 16

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).

Question 17

Geodesic Path

Answer 17

The shortest distance between two points in spacetime.

Question 18

Black Hole

Answer 18

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.

Question 19

Event Horizon

Answer 19

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.

Question 20

Singularity

Answer 20

The centre point of a black hole.

Question 21

Schwarzschild Radius

Answer 21

The distance from the centre of a black hole to its event horizon.

Question 22

Stefan-Boltzmann Law

Answer 22

Gives a measure of the total energy being emitted at all wavelengths by a black body.

Question 23

Black Body

Answer 23

A body that absorbs all the electromagnetic radiation incident on it and also
emits all wavelengths of electromagnetic radiation.

Question 24

Luminosity

Answer 24

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.

Question 25

Apparent Brightness

Answer 25

A measure of how bright a star appears on Earth.
It is dependent on the star’s luminosity and distance from the Earth.

Question 26

Charge-Coupled Device
(CCD)

Answer 26

A chip found behind the lens in a digital camera that stores images.

Question 27

Hydrostatic Equilibrium

Answer 27

When the thermal pressure outwards from a star balances the gravitational
force inwards.

Question 28

‘Proton-Proton’ Chain

Answer 28

A three stage process in which stars produce energy by converting hydrogen
into helium through nuclear fusion.

Question 29

Hertzsprung-Russell
(H-R) Diagram

Answer 29

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.

Question 30

White Dwarf

Answer 30

The last stage in the life cycle of a low mass (Sun like) star.

Question 31

Neutron Star

Answer 31

The last stage in the life cycle of a high mass star.