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 in Core + Hydrogen Fusion in Shell = Outer Layers Expand More  Helium fusion begins in the core.  T = 100 Million Kelvin  Helium Fusion = Carbon and Oxygen core Science Daily

1-3 M sun

Low Mass Star High Mass Star

Low MassHigh Mass RR Lyrae variablesCepheid variables Period = less than 1 dayPeriod = 1-50 days  The Red Giant pulsates.  Hydrostatic equilibrium is out of balance.

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 M sun 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. The Process of becoming a White Dwarf

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 M sun.  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 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

In 1 year  0.3 LY across In 100 years  Several LY across SN Remnant  The outer layers of the high mass star expand rapidly and collide with ISM.  ISM glows  May initiate new Star formation

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  parsecs across

 Star clusters demonstrate the evolutionary process of stars.

Practice Which HR diagram shows the youngest cluster?