25.2 Stellar Evolution

Textbook pages 707 - 714

What is a star, anyway?

• A star is an object in space that produces radiant energy through nuclear fusion.

• The size of a star does not matter.

• Star size ranges from over 1000 times the mass of the sun to smaller than the moon.

• A planet reflects light; a star produceslight.

•Stars have a “life cycle” that lasts for

billions of years.

•Scientists learn about stellar

evolution from observing stars in

different stages of their cycles and

piecing together their observations.

WHAT? STARS have LIVES?????

The Birth: Nebula

• A nebula is a cloud of gas & dust.

• This cloud has just enough gravitational

attraction between its molecules for it to

stay together.

• Most nebulae are about 70% hydrogen.

• This is the First Stage of Stellar Evolution.

• This is an

example of a

nebula.

• This photo was

taken by the

Hubble

Telescope.

What happens to the nebula?

• A force, such as an explosion of a

nearby star, or another nebula,

“bumps” into these particles.

• The nebula begins to CONTRACT.

• A region of denser material builds up.

Protostar: The Second Stage

• As this denser

material shrinks, it

spins more & more

rapidly, and flattens

into a disc.

Think of how a pizza

baker spins a ball of

pizza dough in the air

pulls it into shape…

This Hubble telescope photo shows the edges of

the forming protostar starting to flatten out, while

the center pulls in, or contracts.

• The center of the protostar gets hotter

and hotter, while its internal pressure

increases.

• After several million years, the

protostar’s gas becomes so hot it

forms PLASMA, or a state of matter in

which electrons are torn from their

parent atoms.

• At 10,000,000 C, nuclear fusion begins.

• Hydrogen atoms will fuse together to form heavier, helium atoms. Enormous amounts of radiant energy are released.

• Once nuclear fusion begins in a star, it continues for billions of years!

• Now we have a star!

Our sun is

producing

radiant energy

RIGHT NOW

by nuclear

fusion.

Other facts about nebulae…

• A nebula may produce more than one

star at a time. (Binary System.)

• A contracting nebula may also produce

planets at the same time it is creating

a star.

This “eagle nebula”

may be forming at

least three stars.

Stage Four: Main Sequence Stars

• The main sequence is the longest stagein the life of a star.

• Stars spend most of their existence within the Main Sequence.

• A star is in this part of its existence as

long as it is producing radiant energy through nuclear fusion.

Nuclear Fusion

• Radiant energy is generated in the core

of the star via nuclear fusion.

In Main Sequence stars,

hydrogen atoms fuse

to become helium

atoms.

When one kilo of hydrogen

is converted to helium,

enough energy is released

to keep a 100-watt light bulb

burning for 3,000 years!

Gravity vs. Energy

• Gravity keeps

the star from

expanding, as

long as the star

has enough

hydrogen to

fuse into

helium.

ENERGY

GRAVITY

Stage Five:

Giants and Supergiants

• When all the hydrogen with a star’s

core has fused into HELIUM atoms,

• The core contracts from GRAVITY, and

• The contraction RAISES the core’s

temperature.

• The higher temperature makes the

helium atoms FUSE into a shell of

CARBON.

Nuclear fusion is still going on

inside the shell, but the inside is

mostly HELIUM.

The carbon shell expands as energy is

released. It grows cooler, too.

This star is now a:

Red Giant (10x larger than the sun) or a

Red Super Giant (at least 100x larger

than the sun.

Our sun has converted

about 5% of its hydrogen in

5,000,000,000 ( or 5.0 x 109) years

Final Stages: White Dwarfs

• When helium fusion ends there is no

more energy.

• The red giant enters its final stages…

• The star loses its outer gases, and

reveals its core. This is called a

Planetary Nebula, and the hot, dense

core is a WHITE DWARF.

• It will shine for billions of years.

Planetary

Nebula

Rose Nebula

Spider Nebula

Ant Nebula

Southern Crab

Nebula

Black Dwarfs, Novas and Beyond

A Black Dwarf is a

completely dead star.

When a star is part of a binary system AND

it is a high-mass star, It may explode while

cooling. These are called NOVAS.

Novas can be up to one million times

brighter than the sun.

A nova lasts only a few days

before fading.

A white dwarf can explode

several times.

Supernovas

• A star of HUGE mass may produce such a HUGE explosion that it blows itself apart.

The amount of energy

radiated at that time

equals the amount of

energy radiated during

the star’s entire lifetime.

Neutron Star

• The core of a dead supernova.

• Incredibly DENSE

• Incredibly SMALL

A sugar cube’s worth of neutron matter

= 1,000,000,000 TONS on earth

20 km (12.5 miles) in diameter

Black Holes

• Sometimes a core is simply too

massive to become a neutron star.

• These stars contract with HUGE force,

leaving what appears to be a hole in

space.

• Not even light can escape from it.

NASA artist’s image of a black hole.

Image of largest black hole yet discovered.

Weighs about 18 billion Solar masses.

13.5 billion light years from sun.