|
Lecture
20: Star Clusters
|
| Astronomy
101/103 |
Terry
Herter, Cornell University
|
|
Lecture
Topics
|
- Stellar
Clusters
- Seeing
stellar evolution at work
- Globular,
Galactic, and Open Cluster
- Stellar
Explosions
- White
Dwarfs
- Summary
of Stellar "End-Products"
|
|
Star Clusters
and
Associations
|
Groupings
of stars which may stay together for a long time (>
1010 years) or a short time (< 108
years).
Clusters
are places where we can see the results of stellar evolution
A
Spiral Galaxy
|
|
Galactic
Clusters
|
- Located
in the disk of our galaxy
- Young
stars (Population I)
- ~50
to 10000 stars per cluster
- ~10
pc in diameter
- Star
density ~ 0.1 to 10 stars/pc3
- Irregularly
shaped
- ~1000
in the Galaxy
- Examples:
Pleiades, Hyades, Praesepe
|
|
Globular
Clusters
|
- Located
in the halo of our galaxy
- Old
stars (Population II)
- ~104
to 106 stars per cluster
- ~20-100
pc in diameter
- Star
density ~ 0.5 to 103 stars/pc3
- Roughly
spherically shaped
- ~150
in the Galaxy
- Examples:
M3, M13 (Hercules), M15
|
|
O-B Stellar
Associations
|
- High
metal abundances
- Interstellar
gas and dust may be present
- Located
in the spiral arms of our galaxy
- Young
stars (Population I) w/ associated interstellar material.
- ~100
to 1000 stars per cluster
- ~100
- 200 pc in diameter, Irregularly shaped
- Star
density ~ 0.01 stars/pc3
- ~80
in the Galaxy
- Example:
Orion O-B Association
|
|
Star Cluster
Evolution
|
- Stars
in a cluster are all thought to be born at the same time.
- This
means the high mass stars evolve off the main-sequence
first.
- We
can tell the age of a cluster by the highest mass stars
present.
|
|
Cluster: Birth
|
Model
Evolution of a Cluster
- H-R
diagrams are plotted for a cluster of stars at different
times after its formation. The size of the symbol is proportional
to log(R/Rsun).
Cluster
at Birth
|
|
Age = 8x106
years
|
|
Age = 256x106
years
|
|
Age = 3x109
years
|
|
Age = 8x109
years
|
|
Fusion
Times
|
Fusion
Time in a 20 Msun Star
|
Fuel
|
Time
(years)
|
|
H
|
7,000,000
years
|
|
He
|
500,000
years
|
|
C
|
600
years
|
|
O
|
0.5
years
|
|
Si
|
1
day
|
|
Note
that the reaction times get much faster as we go to heavier
elements. This is basically a runaway effect.
|
|
Stars
Explode!
|
- Mild
Explosion => Planetary Nebula
- Ejection
of the outer layers of the red giant
- Strong
Explosion => Nova
- Eruptions
in a binary star system
- Catastrophic
Explosion => Supernova
- Blasting
away the outer parts of a star
|
|
Explosion
Results
|
- Explosions
put the processed stellar material back into the interstellar
medium for the next generation of stars to use!
- In
a supernova, neutrons bombard nuclei and
build up very heavy elements, e.g. Gold,
Uranium, etc.
|
|
Stellar
End-Products
|
- White
dwarfs
- Light
up planetary nebulae for a while
- Eventually
cool and fade away. It becomes to faint to see.
- Pulsars
=> cold neutron stars
- Black
Holes => ???
|
|
Importance
of
Mass
|
- The
fate of a star is linked to its mass when it nears the
end of it's life.
- The
depends upon
- Its
initial mass
- How
much mass it loses along the way.
|
|
White
Dwarfs
|
- For
Mcore < 1.4 Msun, the core is
stable.
- A
white dwarf forms
- Size
of the earth but mass of the sun!
- As
the star cools we might expect it to get smaller and smaller.
- …
It doesn't!
|
|
What Stops
Core
Collapse?
|
- The
Pauli Exclusion Principle:
- No
two electrons can be at the same place at the same
time with the same energy.
- Electrons
cannot move closer together because they have nowhere
to go.
- The
strong repulsion caused by the Exclusion Principle is
called
- Electron Degeneracy Pressure
|
|
The
Winner is?!
|
- Electron
degeneracy pressure can balance gravity when Mcore
< 1.4 Msun.
- When
Mcore > 1.4 Msun, collapse continues
as the electron get "assimilated" into the nuclei
to create a Neutron Star.
- When
Mcore > ~ 4 Msun, even nuclear
"pressure" can not halt the collapse. Gravity
creates a Black Hole.
|
|
The
Death of
Stars
|
|
Stellar Mass
|
Nature
of collapse
|
Size
of Radius (km)
|
Density
(g/cm3)
|
End
Product
|
|
Mstar
< 1Msun
|
Slow
gravitational contraction
|
---
|
---
|
White
Dwarf
|
|
1
Msunto ~5 Msun
|
Mild
core collapse
|
7000
|
107
|
White
Dwarf
|
|
~5
Msun to 15 Msun
|
Fast
core collapse
|
20
|
3x1014
|
Neutron
Star
|
|
Mstar
> 15 Msun
|
Very
fast core collapse
|
4
|
1016
|
Black
Hole
|
|
|
|
|
|
