The Life & Death of Stars

Nuclear fusion

Nebula

Supernova

Black hole

Let’s review: What is a star?

• Giant ball of hot gases• The huge objects in

the universe• Our Sun is an example

of a star• Made of mainly

hydrogen and some helium

How do stars generate energy?

• Stars burn by a process called nuclear fusion• This process combines two hydrogen atoms

into one helium atom, which then fuse to form elements with even more protons

• This process gives off a lot of heat & energy• Gives the illusion of “burning”

Types of stars• 3 main sizes of stars

• Small/medium (like our Sun)• Large (aka giant)• Very large (aka supergiant or hypergiant)

• Different colours of stars indicate they’re different temperatures• Blue is hottest• Yellow is average• Red is ‘coolest’

• Usually, the bigger the star, the cooler it is• Most big stars are called Red Giants because they’re relatively cool• Blue giants are very rare

• The hotter the star, the shorter its lifespan

Sizes of stars1) small / med – most common2) large – rare3) extremely large – very rare

• The Sun is a med (average) sized star

• Small stars are called dwarf stars

• Large stars are called Giants

• Extremely large stars are called Supergiants

The Birth of a Star• All stars start out the same way, as

a nebula• A nebula is a giant cloud of gas

and dust• Stars are created in a nebula as the

gases contract due to the force of gravity, turning into a hot, dense clump

• As they become larger, they heat up until they reach a temperature of 10 000 000 C

• At this temperature, nuclear fusion begins

A Star’s Life • For the major part of a star’s life, it

undergoes nuclear fusion• Lower-mass stars can undergo this

process for up to 100 BILLION years!

• Stars like the Sun can last for about 10 billion years

• Higher-mass stars produce energy for only a few million years

• Fast-burning, hot (blue) stars only last for a million years… yikes!

A Star’s Death • A Star begins to “die” when the

hydrogen that fuels it runs out • H has fused into new substances (He)

• Outer layers swell• Once the core fuses into lead, fusion

stops and the star collapses inward, followed by an outward projection of particles

• Depending of the star’s size, its collapse is either in the form of a planetary nebula or a supernova

• After that, it then becomes one of the following:1) White dwarf (small/medium-sized stars)2) Neutron star (large stars)3) Black hole (extremely large stars)

Supernovas• In 1987, Canadian astronomer

Ian Shelton discovered one while working at an observatory in Chile

• His discovery was by accident!• He was looking at images of

stars when he noticed something unusual: one star looked MUCH brighter than the others

• Named it SN 1987A and it is 163 000 light years away• This means Shelton was looking at the

supernova the way it was 163 000 yrs ago!

• SN 1987A was the closest one to Earth since 1604

White DwarfsLife After Death of Small & Medium

Stars• When the star’s core

becomes lead it can not undergo fusion any longer

• This white hot ball of lead is called a white dwarf

• It cools over a loooong time to form a black dwarf star

Neutron Stars Life after death of a large star

• Sometimes a star is so heavy & under so much pressure that it cannot support its weight (even if it’s fused into lead!)

• The atoms collapse even further creating a ball of neutrons

• This forms a rapidly spinning neutron star that is only a few kilometers in size

• We see neutron stars from the radiation that shoots out of either end

Black holesThe Life After Death of Huge Stars

• Sometimes a star is so heavy that not even the neutrons can stay apart

• The neutrons crash together, forming a black hole

• After that we are not sure as this creates a black hole

• A black hole is an object so dense that not even light can escape it

• We can find black holes by looking for objects in space that are orbiting seemingly empty space

• These are very, very rare

How will our Sun die?

Death of a Biiiig Star

Life Sequence of StarsStar’s Size Phases of a star’s “life & death”

Small/Mediumnebula nuclear fusion red giant planetary nebula white dwarf black dwarf

Largenebula nuclear fusion red supergiant supernova neutron star

Extremely Largenebula nuclear fusion red supergiant supernova black hole

Life cycle of Stars… in pictures