Download Lecture 15 Star Formation and Evolution 3/7

Star Formation
gas cloud
protostar
Star
PHYS 162
equilibrium
1
Star
Formation
protostar 
main sequence
star. Happens
faster if larger
mass
PHYS 162
2
Stellar Evolution
•
•
•
•
•
•
90% of its lifetime: star converts Hydrogen to Helium
p-p cycle Main Sequence
Helium builds up in the core, but not yet burning
gravity compresses which increases temperature
helium starts burning, more energy produced
Different equilibrium, less stable
NOT on main sequence
where on HR diagram is complicated (you don’t need
to know)
simplistically Red Giants=He burning
PHYS 162
3
SUN: Main Sequence  Red Giant
don’t need to know
PHYS 162
4
Helium Fusion  Red Giant
PHYS 162
5
Helium Fusion I
As mass Carbon12 (6p,6n) is less than the mass of 3 He4
(2p,2n) then combining 3 He into C releases energy
m( 4He)  4.0026u
3-Body Reaction
m(8Be )  8.0053u
12
m
(
C )  12.000u
C
He+He+He
OR two 2-Body Reactions
He+He
Be(4p,4n)
He+Be
C
PHYS 162
6
Helium Fusion II
•
•
•
•
Helium to Carbon burning is suppressed
3-body reactions are always suppressed
2-body Beryllium(8) is unstable. (It decays into 2
He nuclei in 10-16 seconds). An “accident” of
Nature. Need to have Be+He reaction occur before
the Be decays  slows up reaction
Larger electric repulsion than p-p as larger electric
charge (2 for He and 4 for Be). Therefore need
about 100,000,000 degrees K for He burning
 Stars like our Sun remain main sequence longer
due to this
PHYS 162
7
Our Sun  Red Giant
in ~5 billion
years, our Sun
will expand to
about the size of
1 AU = Earth’s
orbit
PHYS 162
8
Helium Fusion 
Red Giant
Change radius and
surface temperature
in late stages of
stars’ life cycles
PHYS 162
9
Stellar Evolution
• test out model of stellar evolution using
Star Clusters
• HR diagram of a cluster gives “snapshot” of
stars with the same age but different masses
• Birth  Main Sequence  Red Giant 
“live+die” faster if higher mass
• tell age of cluster by most massive star still
on Main Sequence
PHYS 162
10
Star Clusters
stars are usually near other stars - CLUSTER
• formed at the same time
• similar chemical composition
• about the same distance from us
Can classify by appearance and use to:
• study stellar lifetimes
• measure distances (earlier: spectroscopic parallax)
PHYS 162
11
Open Star Clusters - Pleiades
“Seven Sisters” chased by
Orion the hunter (Greek)
Subaru cluster (Japan) only 6
really visible to unaided eye
PHYS 162
Subaru Telescope12Hawaii
Open Star Clusters - Pleiades
7 daughters of Atlas and Pleinoe: Alcyone, Merope, Electra,
Caleano, Taygeta, Maia (mother
of Hermes), Sterope 13
PHYS 162
Globular Star Clusters
“fuzzy cotton ball” by eye or
with modest telescope
• usually dim red stars
• dense with 100,000 stars in 50-300 LY region with
less than LY separating stars
• no heavy elements. Just Hydrogen and Helium
• often outside plane of galaxy
Understood as group of old stars formed in early
history of the galaxy 3-12 billion years old
PHYS 162
14
Very Young Star Cluster
“moving” to
main
sequence
Note many
more low
mass stars
Some with
high surface
temp
(25,000)
PHYS 162
15
100 million year old Star Cluster
PLEIADES
largest stars
“moving”
off main
sequence to
become
giants
Modest
highest temp
(10,000)
PHYS 162
16
5 billion year old Star Cluster
largest stars are gone
stars little more
massive the Sun have
become giants
Highest temp on main
sequence now 6000
PHYS 162
17
Fate of Stars
INITIAL MASS
relative to Sun’s mass
M < 0.01
.01 < M < .08
0.08 < M < 0.25
0.25 < M < 12
12 < M < 40
M > 40
Final State
planet
Brown dwarf (not true star)
not Red Giant White Dwarf
Red GiantWhite Dwarf
Supernova: neutron star
Supernova: black hole
PHYS 162
18
White Dwarves
•
for “light” stars (less than 10 times the mass of the Sun)
burning of He to Carbon (Oxygen) is final fusion stage
• electrons’ pressure resisting gravity
• Outer Layers keep expanding (or oscillating) losing matter.
See as planetary nebula  inside layers of star become
surface.
PHYS 162
19
White Dwarves II
• loss of over 1/2 star’s mass during Red
Giant phase
Mass being lost
core
At some point all that is left is the hot,
dense, inert (no fusion) C/O core about
size of Earth: WHITE DWARF
Will slowly cool down over time
PHYS 162
20
Planetary Nebula
• NOT planets (historic term) material ejected
by “pulsating” Red Giant.
• Can lose over half the star’s mass
Helix nebula
PHYS 162
21
Red GiantWhite Dwarves
white dwarf at center of
“planetary” nebula heats
up surrounding gass
PHYS 162
22
REMINDER
HertzprungRussell Diagram
Plot Luminosity
versus surface
temperature
PHYS 162
23
HertzprungRussell Diagram
Stars with larger sizes are
brighter then a smaller star
with the same surface
temperature
PHYS 162
24
Sun: Main SequenceRed GiantWhite Dwarves
white dwarf
PHYS 162
25
White Dwarves Mass vs Radius
• In WD, gravity is balanced by pressure due to degenerate
electrons
• A heavier WD will have smaller radius
• if Mass(WD) > 1.4 M(Sun) degenerate electrons can not
resist gravity  called Chandrasekhar limit and no WD
has a mass greater than this
• If WD can acquire mass from companion star and goes
over this limit  Supernova and (usually) a Neutron Star
PHYS 162
26
Stellar Supernova Explosions
For heavy white dwarves with a companion star
• acquire mass, if becomes > 1.4 M(Sun)
SUPERNOVA (Ia). p + e  n + neutrino
• Usually leaves neutron star
For high mass stars
• fusion continues beyond C,O
• core of degenerate electrons builds up - opposes gravity
• if Mass(core) > 1.4 M(Sun) core collapses in
SUPERNOVA (II)
• leaves either Neutron Star or Black Hole
PHYS 162
27
Supernova Explosions
1 billion times brighter
then the Sun for a few
months
PHYS 162
28
Supernovas
• 10-20 supernovas occur every1000 years in
a galaxy the size of the Milky Way (~200
billion stars) with ~15% being type Ia
• 8 observed in last 2000 years (185, 386,
393, 1006, 1054, 1181, 1572, 1604)
• Hard to observe if on “opposite” side of
Milky Way  all recent observed SN are in
other galaxies
PHYS 162
29
Degenerate electrons
not on tests
same as Pauli exclusion - two electrons which are
“close” to each other can not occupy the same
quantum state. Causes:
• Periodic table and different chemical properties
for different atoms
• why metals conduct electricity
• interior of stars and planets
electrons are forced to higher energy states and so
exert more pressure than normal
PHYS 162
30
Degenerate
electrons
Being “close” depends on
electrons energy. Easier
for lower energy to be
close (wavelength = h/p)
PHYS 162
31
HR Digram Worksheet
• For Sirius B use spectral class B1
• Vertical axis is Luminosity and a log scale so do .00001,
.0001, .001, .01, .1, 1, 10, 100, 1000, 10,000, 100,000,
1,000,000 with 2,3 or 4 lines separating each
• After labeling horizontal axes with spectral class B0M10, add on below them the surface temperature with:
B0= 24,000 A0=11,000 F0=7000 G0=6000
K0=5000 M0=4000
PHYS 162
32