Edexcel IGCSE Physics Unit 8 - Astrophysics

universe

Large collection of billions of galaxies.

galaxy

A large collection of stars.

Milky Way

The galaxy in which we live.

The solar system

Everything that orbits the sun, including planets, dwarf plants, asteroids, and comets.

Planet

Large object which orbits a star in an almost circular (but slightly elliptical) orbit.

The planets in our solar system, starting from the closest one to the Sun.

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

Dwarf planet

Planet-like object that isn't big enough to be a planet.

moon

An object which orbits a planet in an orbit which is almost circular.

natural satellite

An object which orbits a planet which is not man-made (e.g. a moon)

artificial satellite

An object which orbits a planet which is man-made.

asteroid

Lump of rock and metal that orbits the Sun.

comet

Lump of ice and dust that orbits the Sun, usually in a highly elliptical orbit.

circular orbit

Orbiting in a circle.

Elliptical orbit

Orbiting in a stretched circle, moving from close to a star to far away from the star.

instantaneous velocity

Velocity at a snapshot in time.

orbit

Circular motion of one object around another due to forces of gravity

geostationary

An object orbiting a planet at the same rate as the planet spins on its axis, so that it stays above the same point on that planet at all times.

orbital speed

The speed at which an object travels whilst orbiting another, more massive object.

orbital radius

The distance from which an object orbits another object.

Equation linking orbital speed, orbital radius and time period

v = 2πr/T

The size of the gravitational force between two objects depends on:

- their mass

- the distance between them

The gravitational field strength of a planet depends on:

- its mass (more mass, bigger g)

- its radius (bigger radius, smaller g)

The colour of the hottest stars

blue

The colour of the coolest stars

red

The colour of our sun

yellow

Star classifcation groups, from hottest to coolest

OBAFGKM

The four main parts of a H-R diagram

- main sequence

- white dwarf

- giants

- supergiants

Stages of a star similar to the Sun

nebula → main sequence → red giant → white dwarf

Stages of a star larger than the Sun

nebula → main sequence → red supergiant → supernova → neutron star (or black hole if very large)

nebula

A cloud of dust and gas.

protostar

A nebula collapsing under gravity, increasing in temperature

main sequence star

The long, stable period of a star's cycle, where the gravitational forces are balanced by the radiation pressure from the fusion of hydrogen into helium in the core. The phase the Sun is in, typically several billion years.

red giant

For a star about the mass of the Sun, when the hydrogen in the core runs out, it collapses until it becomes hot enough to fuse helium, and eventually elements as heavy as carbon. The outer layers expand outwards and cool, looking more red.

red supergiant

For a star much bigger than the Sun, when the hydrogen in the core runs out, it collapses until it becomes hot enough to fuse helium, and eventually elements as heavy as iron. The outer layers expand outwards and cool, looking more red.

white dwarf

At the end of its life, a star about the mass of the Sun ejects its outer layers as a planetary nebula, leaving a hot, dense, solid core. Fusion is no longer taking place.

supernova

For a star more than about 4 times the mass of the Sun, when it runs out of fuel for fusion, it explodes, throwing outer layers off and leaving a dense core.

neutron star

The very dense core left behind after a supernova.

black hole

The very dense core left behind after a supernova of a very massive star. It is so dense that not even light can escape.

CMBR

Cosmic microwave background radiation - radiation from all directions - remnant of the Big Bang

Red shift

Spectra of stars/galaxies have moved to the red end of the spectrum, showing they are moving away from us (doppler effect)

The doppler equation for velocity of a galaxy

change in wavelength ÷ reference wavelength = velocity of a galaxy ÷ speed of light