Red Giant Phase to Remnant
(Chapter 10)
Student Learning Objective
• Describe or diagram the evolutionary
phases from the beginning of stellar
formation to remnant
Low Mass & High Mass
What happens during the red giant phase?
Nuclear fusion in the core stops.
All hydrogen has been used
It is not hot enough to fuse helium
The core and surrounding layers collapse.
Collapsing layers heat
Gravity “wins”
Inert Helium Core
Hydrogen begins burning
in a heated shell
surrounding the core.
Outer layers expand and
cool.
Pressure “wins”
Practice
1) What provides “normal” pressure in a star?
2) Why do collapsing layers heat?
3) Why do the outer layers cool?
Helium fusion begins in
the core.
 T = 100 Million Kelvin
 Helium Fusion = Carbon
and Oxygen core
Helium Fusion in Core
+
Hydrogen Fusion in Shell
=
Outer Layers Expand More
Science Daily
1-3 Msun
High Mass Star
Low Mass Star
The Red Giant pulsates.
Hydrostatic equilibrium is out of balance.
Low Mass
High Mass
RR Lyrae variables
Cepheid variables
Period = less than 1 day
Period = 1-50 days
Practice
1) Describe what is happening as a Red Giant pulsates.
2) What will happen to Earth as the surface of the Sun
approaches?
Diameter 100+ larger
Mercury and Venus in Sun
Earth at surface of Sun
What remains of a Low Mass Star
after nuclear fusion ends?
Stars less than 0.4 Msun become a Red Dwarf.
Extremely low mass stars can only transport heat by
convection.
Star accesses hydrogen from all layers
Fusion ends when all hydrogen is gone
Remnant slowly fades
A Red Dwarf with an Earth
NASA
Low mass stars like the Sun become a White
Dwarf.
Hot collapsed core (White Dwarf)
Surrounding ejected layers (Planetary Nebula)
White Dwarf
The Process of becoming
a White Dwarf
• Core contracts
• Outer layers expand and thin
• Pulsating star ejects outer
layers (Planetary Nebula)
• Planetary nebula glows (heat
excites gasses)
The White Dwarf will
fade over time into a
Black Dwarf.
A White Dwarf is the
compact core remnant of a
low mass star.
Electron Degenerate Matter
Ends core collapse of Low Mass star
Electron orbits are restricted
Orbits “hold up” the White Dwarf core remnant
Planetary nebula in constellation Lyra
Ring Nebula
The Littlest Ghost Nebula
Image Credit: APOD
White Dwarf Limit
The Chandrasekhar limit is 1.4 Msun.
 A “Sun” becomes the size of Earth
 As much as 40% of star ejected
What remains of a High Mass Star
after nuclear fusion ends?
A high mass star goes through several “Red Giant”
phases as it fuses heavier nuclei in the core and
surrounding layers.
Then it explodes! (SN Explosion)
Image Credit: Australia Telescope National Facility
Type II Supernova
High mass stars explode.
Energy production ends abruptly
Core cannot fuse iron (Fe)
Degenerate pressure cannot stop collapse
Gravity “wins”
The Explosion
A Type II SN explosion only takes milliseconds.
Core collapses
Entire star falls in on itself and rebounds
A pressure wave (shock wave) is produced
Outer layers are blasted into space
1028 Megatons of TNT released
Heaviest elements are produced
100’s to Millions times brighter than original star
Type II Supernova
Neutron Star
or
Black Hole
SN Remnant
The outer layers of the high mass star expand rapidly
and collide with ISM.
ISM glows
May initiate new Star formation
In 1 year  0.3 LY across
In 100 years  Several LY across
The Crab Nebula from VLT
Supernova Remnant Cassiopeia A (Hubble)
Kepler’s Supernova Remnant
NASA
What are the possible fates in a binary system?
Each of the stars in a binary system gravitationally
controls a volume of space called a Roche lobe.
Matter at the inner Lagrangian point, can
transfer to a companion object.
The object accreting matter may go nova!
Nova
A Nova is a thermal
nuclear explosion on the
surface of a core remnant.
 Red Giant fills Roche Lobe
 Core remnant companion
accumulates matter
 Nova
Nova Velorum 1999 (APOD)
Type Ia SN
A Type Ia Supernova is a Nova that destroys the
object accreting matter.
Object accumulates too much mass
Explosion of entire object (Type Ia SN)
Nothing remains
Practice
1) What is the primary difference between a Nova and a
Type Ia SN?
2) Can our Sun become a Type II SN? Why?
3) Can our Sun become a Nova? Why?
4) Can our Sun become a Type Ia SN? Why?
Why are star clusters important?
The HR diagram can show the age of the cluster.
Comparing relative ages leads to understanding
stellar evolution.
Open Clusters
Open clusters contain
young stars.
 100 to 1000 members
 See individual stars
 25 parsecs across
Globular Clusters
Globular Clusters
contain old stars.
 Millions of members
 Appears as single object
 10-30 parsecs across
Star clusters demonstrate the evolutionary
process of stars.
Practice
Which HR diagram shows the youngest cluster?