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Star Death
When stars die
13
Astronomy
Undergraduate 1
11/22/2008

Additional Astronomy Flashcards

 


 

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Term

Core Collapses (process)--1st

-When they start to die? 

Definition

Hydrogen fusion only occurs in core during main sequence.

 

As stars burn H to He, He builds up in the core.

 

Stars begin to “die” when they run out of hydrogen in their core.

 

In the absence of H fusion, the core collapses

 

Term

Core Collapse (process)--2nd

-Hydrogen shell burning? 

Definition

As the core collapses, its temperature increases (gravitational energy)

 

This start a hydrogen shell “burning"

 

Release of energy from burning shell causes star to enlarge.

 

Core continues to collapse.
Term

Core Collapse (process) --3rd

-Changes in Star

-How does it become stable?

Definition

Star expands, cools

 

Moves from main sequence, to red-giant branch on H-R diagram

 

Solar winds are strong

 

Pressure and temp increase

 

He fusion begins (He fuses to form carbon) in a “helium flash”

 

Energy output decreases

 

Star contracts.

 

Star becomes temporarily stable (horizontal branch)

Term

Core Collapse (process)--4th

Helium Fusion

Definition

During He fusion, carbon ash builds up

 

The star eventually runs out of He fuel

 

Core begins another collapse

 

What happens next in a dying star depends on the mass of the star…

Term
Low Mass Stars Death
Definition

Collapse ends when carbon atoms become “electron degenerate”

 

That is, the force of atomic electrons keeps atoms from further collapse

 

He shell around core stars to burn into C

 

Stellar position on the H-R diagram moves along asymptotic giant branch

 

expands and cools

 

luminosity increases

Term
Formation of Planetary Nebula
Definition

Shell burning is notstable –occurs in bursts

 

Bursts of fusion push off outer layers of star

 

All of the outer layers of the star are expelled

 

Forms planetary nebula

 

~60% of mass is lost in planetary nebula

Term
Planetery Nebula
Definition

Expanding shell of hot gas around a dead star

 

Emission Nebula ionized by hot, dense core

Term
White Dwarf
Definition

Small, dense, electron degenerate carbon core

 

1 tsp. of a white dwarf would weigh 5 tons!

 

~ size of the Earth

 

Slowly cools

 

Very small, less area to lose heat

 

Cools to become a black dwarf

 

White dwarf cannot be larger than 1.4 M

 

= Chandrasekhar LimitSirius

 

 

Term
High Mass Stars (M > 5 M)
Definition
High mass stars have:
 
  • More mass
  • Greater gravity
  • Higher core temperatures and pressures
  • Fusion reactions do not stop with Helium burning in the core as for smaller stars
Term
When Star becomes a giant (from a small mass star)
Definition

Star becomes giant (for small-mass star)…

 

  • Helium burning ends in core
  • Core contracts
  • Temp and pressure in core increase
  • He shell burning begins
  • Core continues collapse
  • Carbon fuses into higher-mass elements
Same process repeats
  • Fusion of different elements continues through neon, oxygen, silicon and finally iron
  • Note: all fusion occurs only in the very core
Star expands to become a Supergiant
 
Star moves back and forth on the HR diagram with each type of fusion.
 
Each stage of burning lasts for a shorter period of time…
Term
Death of a  High mass Star
Definition

Iron builds up in the core

 

Iron cannot be fused and produce more energy

 

What keeps iron core from collapsing?

  • Initially: electron degeneracy
After core has a mass greater than 1.4 M
 
  • (Chandresekharlimit) the electron degeneracy is not sufficient to stop collapse
  •  Electrons are forced to combine with the protons to create neutrons
  • Core collapses until pressure from physical force of neutrons bouncing against each other stops it
  •  Core rebounds and runs into outer material which is still falling inward.
Term
Super Nova
Definition

Collision produces huge shock wave pushing all material outward in an immense explosion

 

Explosion can be as bright as an entire galaxy (billions of stars) for a few days

 

Some energy creates elements heavier than iron…these elements are dispersed to the rest of the galaxy

 

Supernova leaves a large shell of expanding material around a central core (remnant)

Term
Evolution of a Cluster
Definition

All stars in a cluster are formed at the same time

 

Age of a cluster can be determined by looking at what point the stars depart the main sequence (the so-called “turn-off point”)

 

Age of Cluster = Lifetime of star at turn-off point

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