Stars

Nebulae are…

Gigantic clouds of dust and gas (mainly hydrogen), often many hundreds of times larger than our solar system

Nebulae are often referred to as…

Stellar nurseries, as they are the birthplace of all stars

Nebulae are formed over millions of years as…

The tiny gravitational attraction between particles of dust and gas pulls the particles towards each other, eventually forming the vast clouds.

The steps in the formation of a protostar are…

Due to small variations in the nebula, denser regions begin to form. These regions pull in more dust and gas, gaining mass and getting denser and hotter as gravitational energy is transferred to thermal energy. This is a protostar.

A protostar is…

Not yet a star but a very hot, very dense sphere of dust and gas

For a protostar to become a star…

Nuclear fusion needs to start in it’s core

Fusion reactions produce energy in the form of…

Kinetic energy

To overcome the electrostatic repulsion between hydrogen nuclei in order to fuse them together to form helium nuclei…

Extremely high pressures and temperatures inside the core are needed

As nuclear fusion begins…

Hydrogen nuclei are forced together to make helium nuclei

Once a star is formed it remains…

In a stable equilibrium with almost a constant size

Gravitational forces act to compress a star, but…

The radiation pressure from the photons emitted during fusion and the gas pressure from the nuclei in the core push outwards

The force from the radiation and gas pressure balances…

The force from the gravitational attraction and therefore maintains equilibrium

Stars in the stable phase are described as being on their…

Main sequence

How long a star remains stable depends on…

The size and mass of its core

The cores of large, massive supergiant stars are much hotter than those of small stars, so…

They release more power and convert the available hydrogen into helium in a much shorter time

Really massive stars are stable for less time than…

Smaller stars

A planet is…

An object in orbit around a star with three main characteristics

The three main characteristics of a planet are…

It has to have a mass large enough for its own gravity to give it a round shape

It has no fusion reactions (unlike a star)

It has cleared its orbit of most other objects

A dwarf planet has one important difference from a planet which is…

Dwarf planets have not cleared their orbit of other objects

Asteroids are…

Objects too small and uneven to be planets, usually in near-circular orbits around the sun and without the ice present in comets

A planetary satellite is…

A body in orbit around a planet. This includes the moon and man-made satellites

Comets are…

Small, irregular bodies made of ice, dust and small pieces of rock. All comets orbit the sun in highly eccentric elliptical orbits

A galaxy is…

A collection of stars and interstellar dust and gas.

Our universe is…

Literally everything

The definition of fusion is…

A process in which two smaller nuclei join to form one large nucleus

During the main sequence…

The star is in a stable phase

Stars with masses less than 0.08Msun never become hot enough to burn hydrogen so never become main sequence. This means they become a brown dwarf which is essentially…

A failed star

A red giant star is…

An expanding star at the end of its life, with an inert core in which fusion no longer takes place, but in which fusion of lighter elements continues in the shell around the core

A red supergiant is…

A huge star in the last stages of life before it ‘explodes’ in a supernova

A white dwarf is…

A very dense star formed from the core of a red giant, in which no fusion occurs

A planetary nebula is…

The outer layers of a red giant that have drifted off into space, leaving the hot core behind in the centre as a white dwarf

Electron degeneracy pressure is…

A pressure created by the electrons in the core of a collapsing star

The Chandrasekhar limit is…

The mass of a stars core beneath which the electron degeneracy pressure is sufficient to prevent gravitational collapse - 1.44 solar masses

Supernova is…

The implosion of a red supergiant at the end of its life, which leads to the subsequent ejection of stellar matter into space, leaving an inert remnant core

A neutron star is…

The remnant core of a massive star after the star has gone supernova and the core has collapsed under gravity to an extremely high density

A black hole is…

The remnant core of a massive star after it has gone supernova and the core has collapsed so far thay nothing can escape it

When stars run low on hydrogen fuel in their core…

They begin to move off main sequence into the next phase

Stars between 0.5 solar masses and 10 solar masses will evolve into…

Red giants

The Pauli exclusion principle states…

Two electrons can’t exist in the same energy states

The electron degeneracy pressure is only sufficient to prevent gravitational collapse if…

The core has a mass less than 1.44 solar masses

Iron nuclei can’t fuse because…

Such reactions cannot produce any energy

If the mass of the core is greater than the Chandrasekhar limit…

A neutron star is formed after supernova

If the core has a mass of more than 3 solar masses…

A black hole is formed after supernova

Supernova create…

All the heavy elements. Everything above iron was created in a supernova

A singularity is…

A single point at the centre of a black hole which mass comtinues to collapse into