Transcript Document

Stellar Evolution
Where do gold earrings come from?
11
Goals
•
•
•
•
Where do stars come from?
How do stars evolve?
How does mass affect what happens?
How do stars die?
11
11
The Stuff Between Stars
• Space isn’t empty.
• Interstellar Medium – The gas and dust
between the stars.
All the interstellar gas and dust in a volume the size of the Earth only
yields enough matter to make a pair of dice.
11
Dust
• Space is dirty.
• Dust blocks or
scatters some light.
• Result: black clouds
and patterns against
the background sky.
11
The IR Universe
Orion - visible
Orion – by IRAS
11
Interstellar Gas
The Trifid Nebula – Gillian Schnider (Johnston, May 2003)
11
Orion Nebula –
copyright Robert Gendler
HII Regions
• For light: atoms must be excited.
• Energy comes from very hot stars.
11
Cold Dark Clouds
• If dust clouds block
light, then inside thick
dust clouds there should
be no light at all.
• Without light, there is
little energy.
• With little energy, gas
inside is very, very cold.
• Inside molecules form.
11
Gravity vs. Pressure
• Stars and other interstellar
material are in a perpetual battle
between forces pulling in
(gravity) and forces pushing out
(pressure).
• Gravity comes from the mass of
the cloud or star.
• Pressure comes from the motion
of the atoms or molecules.
– Think of hot air balloons.
– The hotter the air, the bigger the
balloon.
11
Star Formation
• Remember:
HOTTER
COOLER
• Cold interstellar clouds:
No heat = no velocity = no outward pressure.
Gravity wins.
• Gas begins to contract.
11
How to Make a Star
1
2
3
11
Orion Nebula – copyright Robert Gendler
11
•Visible and IR image of protostars in the Orion Nebula.
11
Concept Test
• A new star reaches the main sequence when
inward gravitational collapse is:
a. Halted by degeneracy pressure in the core.
b. Halted when the atoms are pushed up against
one another and contraction stops.
c. Finally balanced by outward thermal pressure
from nuclear reactions.
d. Finally balanced by radiation emitted in the
photosphere.
e. none of the above.
11
An H-R Life-Track
11
The Main Sequence
• For the Sun:
– While it took 40 – 50
million years to get
here, the new star will
spend the next 10
billion years as a main
sequence star.
• Bigger Stars:
– Everything goes
quicker.
• Smaller Stars:
– Everything longer.
11
Now what?
• The mass of the star
that is formed will
determine the rest of
its life!
• Recall: the more
massive the star, the
more pressure in the
core.
• The more pressure,
the more fusion.
• More fusion:
– More energy
produced
– Hotter
– Shorter life span
11
Open
Clusters
• These are the new
stars.
• Small groups of young
stars.
• Slowly drifting apart.
Jewel Box – copyright MichaelBessell
11
Concept Test
• Order the
clusters from
youngest to
oldest.
a. DBCA
b. ACBD
c. DCBA
d. ADBC
11
The Main Sequence
• A star is a delicate
balance between
the force of
gravity pulling in,
and pressure
pushing out.
• Stars on the main
sequence fuse
hydrogen in their
core to produce
thermal pressure.
• Longest phase of a
star’s life.
11
What then?
• When the hydrogen in the core is almost consumed
the balance between gravity thermal pressure
pushing out and gravity pushing in is disturbed.
• The structure and appearance of the star changes
dramatically.
• What happens then, depends on the star’s mass.
• Two cases:
– Low-mass (< 8 x mass of Sun)
– High-mass (> 8 x mass of Sun)
11
Helium
Ash
• Heavier elements, sink to the “bottom.”
• After 10 billion years, core is “choked” with helium “ash”.
• H  He continues in shell around non-burning core.
11
The Red Giant Branch
• Without fusion pressure in
core:
– Helium core collapses (no
counter to gravity)
– Density in core increases.
• 3He  C + Energy in core
• 4H  He + Energy in shell
• Extra energy results in extra
pressure. Star expands.
• The star gets bigger while
its outside gets cooler.
11
The Onion Sun
• Red Giant Stars
• Layers of:
–
–
–
–
Non-fusing H
Fusing H
Fusing He
Non-fusing C “ash”
11
Red Supergiant
• What happens when the
Sun runs out of helium
in its core?
• Same as before.
• Core shrinks, surface
expands.
• Radius ~ 3 AU!
11
Death
• Core is contracting and heating.
– Surface is cooling and expanding.
• Will it finally become hot enough in core for
Carbon to fuse?
• For the Sun: No.
• Gravity keeps contracting the core: 1000 kg/cm3!
• What stops it?
• Electron degeneracy pressure!
11
Electron Degeneracy
Pressure from motion of atoms
11
Electron Degeneracy
Pressure from electron shells
11
NGC3242 – HST – Bruce Balick
11
M57 – Ring Nebula
11
M27 – Dumbbell Nebula – copyright VLT, ESO
11
Cat’s Eye
11
Eskimo Nebula
11
Hourglass Nebula
11
White Dwarf
• Mass of Sun
• Radius of Earth
• Hot as Sun’s core
• A million times denser than lead
• Slowly cool off
NGC2440 – HST – Bruce Balick
11
11
High-Mass Stars
• Think back to the first
carbon core.
• How they get from
main sequence to the
carbon core stage is a
little different.
• Now however, there is
enough mass that it
becomes hot enough to
fuse carbon?
• Hot enough to eventually fuse lots of elements.
11
The Iron Core
4H  He + Energy
3He  C + Energy
C + He  O + Energy
The ash of one reaction, becomes the fuel of
the next.
• Fusion takes place in the core as long as the
end result also yields energy.
• This energy causes pressure which counters
gravity.
• But Iron doesn’t fuse.
•
•
•
•
11
Core-Collapse
•
•
•
•
•
•
Iron core – no outward pressure.
Gravity wins!
Star collapses rapidly!
Electron degeneracy can’t stop it.
Atomic structure can’t stop it.
Electrons and protons crushed together to
produce neutrons.
• Neutrons pushed together by force of gravity.
11
Supernova
11
Supernova
11
Supernova
11
Supernova
11
Supernova
• The result of the catastrophic collapse is the
rebound and explosion of the core.
• From start of collapse to now: 1 second!
• Matter thrown back into the interstellar
medium.
• Matter rushing outwards, fuses with matter
rushing inwards.
• Every element after Fe is made in the instant
of a supernova!
11
M1 – Crab Nebula – copyright VLT
11
Veil Nebula – Lua Gregory (English ’05)
11
NGC 4526 – 6 Million parsecs away
11
Concept Test
• Which of the following lists, in the correct
order, a possible evolutionary path for a
star?
a. Red Giant, Neutron Star, White Dwarf, Nothing
b. Red Giant, White Dwarf, Black Hole
c. Red Giant, Supernova, Planetary Nebula,
Neutron Star
d. Red Giant, Planetary Nebula, White Dwarf
e. Red Giant, Planetary Nebula, Black Hole
11
Homework #13
• For Wednesday a video
• For Friday read Chapter 10
• Do Chapter 10 Quiz
11