Evolution of the Solar System

Nebula A cloud of dust and gas attracted together by gravity

Birth of the Sun Nuclear fusion reactions take place at centre – hydrogen nuclei are fused together to make helium, releasing enormous amounts of energy

Planetary formation Inner planets made from more dense material (Rocky), outer planets are mainly gas (Gas Giants)

Stable stars Stars are stable because the inward forces of gravity are balanced by the outward force of radiation

Future of the Solar System – Sun becomes a Red Giant star Hydrogen fuel in core runs out Outward radiation pressure falls, so forces of gravity mean that centre of star collapses Rapid burning of remaining hydrogen Pressure and temperature now high enough in the core to fuse helium Sun expands out to the orbit of Earth, swallowing up the inner planets Cooler surface temperature – red colour

White Dwarf star Helium and hydrogen fuel in core is used up. Fusion ceases. The outer layers of star are “puffed away” into space The remaining centre of the star cools and gets smaller

Black Dwarf star Eventually star cools and fades This is the end of a life which has taken 10 billion years

What happens to stars much larger than the Sun? Stars that are >8 x mass of Sun will have much shorter lives, and burn much hotter A star 25 x mass of Sun gets through its life 1000 times faster After a time fusing hydrogen (‘main sequence’), these larger stars start fusing heavier and heavier elements in shells (like an onion) with the heaviest elements at the core The star becomes a Red Giant or Red Supergiant

Core of star

What happens after the Red Giant/Supergiant phase? Fusion of iron doesn’t release any net energy, so fusion stops at the centre of the core The star collapses A massive amount of energy is released This is a supernova explosion

A Supernova The enormous gravitational pressure compresses the core to a million million kilograms per cubic metre and raises its temperature to 10,000 million degrees Celsius The core collapses in 1/10 second from 12,000 km in diameter to 20 km – forming a neutron star The outer layer collapse in and rebound off the core releasing enormous amounts of energy in a Type II supernova explosion

Heavier elements (above iron) are formed in the supernova explosion For a few weeks the supernova is brighter than a whole galaxy For very large stars it is possible for the neutron core to collapse to become a black hole If not, the star, which is spinning, can be detected as a pulsar

SN 1987A – before and after SN 1987A – before and after

M1 Supernova explosion remnant from 1054 AD

Crab nebula pulsar Imaged by Chandra X-ray telescope

Black Holes and Pulsars Black Holes are caused when the concentration of mass is so large that the light itself can’t escape the pull of gravity. They can’t be observed directly but by their gravitational effect on other bodies. Material being sucked into a black hole gets accelerated to such high speeds that X-rays are emitted. Pulsars are rotating neutron stars that emit short bursts of radiation at very regular intervals. The radiation pulses are caused by the rotating magnetic field.

Pulsar

Black Holes Companion star X-rays Black hole Accretion disk

Neutron star Very dense star formed Some of these have strong magnetic field, which generate radiowaves As they spin they give off pulses of radiowaves – called pulsars

Black hole Very massive stars will end up as black holes where gravity is so strong that even light can’t escape