Fusion and Life Cycles of Stars

Fusion and Life Cycles of Stars

Space is filled
with the stuff to
make stars.

Stars start from clouds

Nebula
consist of
Hydrogen
Helium and
other
elements

Multiple Choice

What are nebulae?

• groups of exploding stars

• clouds of gas and dust in space

• a collection of stars

• groups of stars forming a pattern

Gravity collapses the Nebula

Protostars are
formed

Nuclear Fusion in Stars

Special Isotopes of Hydrogen fuse together to form Helium

Energy is created from mass! E = mc2

A Balancing Act

Energy released

from nuclear
fusion counter-acts
inward force of
gravity.

Throughout its life,
these two forces
determine the
stages of a star’s
life.

Stellar Evolution

The Mass of a star determines its life cycle

Stages of a Star

Low mass stars, like the sun, evolve
differently than high mass stars

Stages of a Low Mass Star

Hydrogen fuses
to form Helium

Low mass star

Higher mass elements
fuse causing the star
to expand

Star runs out of elements to fuse.
Gravity collapses the star.

Stages of a High Mass Star

Hydrogen fuses
to form Helium

High mass star

Higher mass
elements fuse

Star runs out of
elements to fuse

Stars that are 4 times as massive as the sun
will eventually become Supernovae

Supernova !

What’s Left After the Supernova

Neutron Star (If mass of core < 5 x Solar)
• Under collapse, protons and electrons

combine to form neutrons.

• 10 Km across

Black Hole (If mass of core > 5 x Solar)
• Not even compacted neutrons can

support weight of very massive stars.

Nucleosynthesis

Stellar processes produce complex elements

H-R Diagrams

Show temperature (color) vs. Luminosity (brightness) of stars

Color & Temperature

The color of a star indicates its temperature

H-R Diagrams

As stars evolve we can chart their path

The Beginning of the End: Red Giants

After Hydrogen is exhausted in core ...
Energy released from nuclear fusion
counter-acts inward force of gravity.
• Core collapses,

∙ Kinetic energy of collapse converted into

heat.

∙ This heat expands the outer layers.

• Meanwhile, as core collapses,

∙ Increasing Temperature and Pressure ...

The end for solar type stars

Planetary Nebulae

After Helium exhausted, outer layers of star expelled

White dwarfs

At center of Planetary Nebula lies a

White Dwarf.

• Size of the Earth with Mass of the Sun “A

ton per teaspoon”

• Inward force of gravity balanced by

repulsive force of electrons.

Fate of high mass stars

After Helium exhausted, core collapses again

until it becomes hot enough to fuse Carbon
into Magnesium or Oxygen.

∙12C + 12C --> 24Mg

OR 12C + 4H --> 16O

Through a combination of processes,

successively heavier elements are formed
and burned.

The End of the Line for Massive Stars

Massive stars burn

a succession of
elements.

Iron is the most

stable element
and cannot be
fused further.

∙ Instead of

releasing energy,
it uses energy.

Periodic Table

16O + 16O 32S + energy

4He + 16O 20Ne + energy

Light Elements Heavy Elements

4 (1H) 4He + energy

3(4He) 12C + energy

12C + 12C 24Mg + energy

4He + 12C 16O + energy

28Si + 7(4He) 56Ni + energy 56Fe

C-N-O Cycle

Supernova Remnants: SN1987A

a b

c d

a) Optical - Feb 2000
• Illuminating material

ejected from the star
thousands of years
before the SN

b) Radio - Sep 1999
c) X-ray - Oct 1999
d) X-ray - Jan 2000
• The shock wave from

the SN heating the
gas

Supernova Remnants: Cas A

Optical

X-ray

Elements from Supernovae

All X-ray Energies

Silicon

Calcium

Iron

Supernovae

Supernovae compress
gas and dust which lie
between the stars. This
gas is also enriched by
the expelled material.

This compression starts
the collapse of gas and
dust to form new stars.