Star Cycle

The Life Cycle of a

Star

Earth Science

Unit #1 - Astronomy

Stars

What is a star?

▶ A star is ball of

plasma undergoing
nuclear fusion.

▶ Stars give off large

amounts of energy
in the form of
electromagnetic
radiation.

X-ray image of the Sun

A Star is Born….

▶ Stars are formed in a

nebula

▶ A nebula is a very large

cloud of gas and dust in
space.

TED ED Video

Multiple Choice

A large cloud of gas and dust in interstellar space; the predecessor to stars...

• Constellation
• Black hole
• Nebula
• Galaxy

Protostars

▶ Dense areas of gas in

the nebula become
more dense due to
gravity.

▶ Soon the dense areas

of gas take on a
definite shape and
are called protostars.

Protostars

▶ As more gas is added to a protostar,

the pressure in its core increases.

▶ The increased pressure causes the gas

molecules to move faster, increasing
friction.

▶ As friction increases, heat is generated

and the temperature of the protostars
core increases.

A new star!!

▶ Once the core of a

protostar reaches
27,000,000o F,
nuclear fusion
begins and the
protostar ignites.

▶ The protostar now

becomes a star.

Multiple Choice

• Once the core of a protostar reaches 27,000,0000 F, ________________ begins and the protostar ignites. A star is born!

• gravitational force

• the sun

• a white dwarf

• nuclear fusion

Main Sequence Stars

▶ Once the star has ignited, it becomes a main sequence

star.

▶ Main sequence stars fuse hydrogen to form helium

through the process of nuclear fusion, releasing
enormous amounts of energy.

▶ The larger the star, the faster it will consume it’s

hydrogen. A blue giant will consume all the hydrogen in
about 100 million years. Our sun will consume all the
hydrogen in 12 billion years.

Nuclear Fusion

▶ Nuclear fusion is the process by which two

nuclei combine to form a heavier element.

▶ New stars initially will fuse hydrogen

nuclei together to form helium.

Balancing Act

▶ The core of a star is where fusion occurs

and creates energy. The radiative and
conductive zones move energy out from
the center of the star. (Outward Pressure)

▶ The incredible mass from all the the gas

causes the star to try to collapse on the
core due to gravity. (Inward Pressure)

Unbalanced Forces

As long as there is a nuclear reaction taking
place, the internal forces will balance the
external forces.

When the hydrogen
in a main sequence
star is consumed, the
forces suddenly
become unbalanced.
Gravity causes the
remaining gas to
collapse on the core.

Red Giant

▶ Collapsing outer layers cause core to heat up.

▶ Fusion of helium into carbon begins.

▶ Forces regain balance.

▶ Outer shell expands from 1 to at least 40 million

miles across. (10 to 100 times larger than the Sun)

▶ Red Giants last for about 100 million years.

Unbalanced Forces (again)

▶ When the red giant has fused all of the

helium into carbon, the forces acting on
the star are again unbalanced.

▶ The massive outer layers of the star

again rush into the core and rebound,
generating staggering amounts of energy.

▶ What happens next depends on how

much mass the star has.

Mass Matters

Red Giant

Mass < 3 x sun

White Dwarf

Black Dwarf

Mass > 3x sun

Red Supergiant

Supernova

Neutron Star

Black Hole

Multiple Choice

Stars can take different paths due to having different _____________

• temperature
• mass
• brightness
• color

Multiple Choice

Which of the following is NOT a final stage of a star. 

• black hole
• black dwarf
• neutron star
• red giant

Multiple Choice

After a supergiant, a high mass star explodes and forms a

• supernova
• red giant
• black hole
• planetary nebula

Multiple Choice

What is the correct order for the life cycle of our star the sun?

• nebula, fusion stage, red giant, black dwarf, white dwarf,
• nebula, fusion stage, red giant, white dwarf, black dwarf
• nebula, fusion stage, super red giant, black dwarf, white dwarf,
• nebula, fusion stage, red giant, super nova, black hole

White Dwarfs

▶

The pressure exerted on the core by
the outer layers does not produce
enough energy to start fusion.

▶

The core is now very dense and very
hot. (A tablespoon full would weigh 5
tons!)

▶

The stars outer layers drift away and
become a planetary nebula.

▶

A white dwarf is about 8,000 miles in
diameter.

▶

After 35,000 years, the core begins to
cool.

Planetary nebula around a
white dwarf star.

Black Dwarfs

▶ As the white dwarf cools, the light it gives

off will fade through the visible light
spectrum, blue to red to back (no light).

▶ A black dwarf still possesses gravity and

generates low energy transmissions (radio
waves).

Multiple Choice

The final stage of a low mass star is a 

• neutron star
• nebula
• black hole
• black dwarf

Mass Matters

Red Giant

Mass < 3 x sun

White Dwarf

Black Dwarf

Mass > 3x sun

Red Supergiant

Supernova

Neutron Star

Black Hole

Red Supergiants

▶ If the mass of a star is 3 times that of our

sun or greater, then the Red Giant will
become a Red Supergiant.

▶ When a massive Red Giant fuses all of the

helium into carbon, fusion stops and the
outer layers collapse on the core.

▶ This time, there is enough mass to get

the core hot enough to start the fusion of
carbon into iron.

Red Supergiants

▶ Once fusion begins again, the star will expand to be

between 10 and 1000 times larger than our sun.
(Out to the orbit of Uranus )

Supernova

▶

When a Supergiant fuses all of the
carbon into iron, there is no more fuel
left to consume.

▶

The core of the supergiant will then
collapse in less than a second, causing
a massive explosion called a
supernova.

▶

In a supernova, a massive shockwave
is produced that blows away the outer
layers of the star.

▶

Supernova shine brighter than whole
galaxies for a few years.

Gas ejected from a supernova explosion

Neutron Star

▶ The core will survive

the supernova.

▶ If the surviving core

has a mass of less
than 3 solar masses,
then the core
becomes a neutron
star.

6 miles in diameter

Black Holes

▶ If the mass of the surviving core is greater than

3 solar masses, then a black hole forms.

▶ A black hole is a core so dense and massive that

it will generate so much gravity that not even
light can escape it.