The Life cycle of a StarMuhammad JunaidBcs-A 22Im|sciences Peshawar

The Life cycle of a Star

Stars are born out of nebulae (a nebula is cloud of gas (mainly hydrogen) and dust.

Gravitational forces pull at the gas and dust compressing it into a slowly rotating globule.

The globule spins faster as it compresses

The spin-speed causes the pressure and temperature to increase .

The globule spreads out into a disk that becomes planets and a central core that becomes a star.

Fusion stage

When a temperature of about 27,000,000,000°F is reached, nuclear fusion begins.

This is the nuclear reaction in which hydrogen atoms are fused together to make helium atoms.

A lot of energy is given out as this happens.

The Stable stage

The fusion reaction produces radiation that causes an outward pressure called radiation pressure (blue arrows)

The Stable stage

The force of gravity that pulls all mass together causes an inward pressure (red arrows)

The star is in a stable state when the radiation pressure pushing outwards is equal to the gravitational pressure pushing inwards.

The bigger the Star the smaller its lifespan The most massive stars have the shortest

lives. Stars that are 25 to 50 times that of the

Sun live for only a few million years. Stars less massive than the Sun have even

longer life spans.

Death of a Star Size also determines what happens in

death …… When stars run out of fuel, they expand

to become Red Giants (larger ones into Super Red Giants!).

This happens because the radiation pressure is greater than the gravitational pull

Death of a Star

Larger stars then explode into a Supernova, smaller stars become a gas cloud.

Large stars then shrink into black holes whereas

Medium stars shrink into neutron stars and Small stars shrink into white dwarfs and

then cool into black dwarfs

Galaxies

Stars gather together in galaxies. There are billions of stars in each galaxy There are millions of galaxies in the

Universe Galaxies are often seen as spirals

with bright centres – the density of star material is greatest at the centre

a galaxy viewed from the edge

Galaxies are in constant motion