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Parts of a MS star
What holds a star up
while it is on the MS?
On the Main Sequence
How does energy
get out?
Radiation & Convection
May take a million years to reach the surface.
The smallest stars
Brown Dwarfs : Stars with core mass
< .08 Msun (failed stars).
Brown Dwarfs do not get hot enough
to fuse H, but they do fuse Deuterium
for a very short time. Deuterium is an
isotope of H, with a neutron. About
1,000 Brown Dwarfs have been found.
They radiate in the infrared
wavelength.
Red Dwarfs : Stars with a core mass
of .08 to 0.4 solar mass
Coolest and dimmest of all MS stars.
They remain on MS hundreds of
billions of years. When all the H is
converted to He fusion ceases, they
cool down, moving down and to the
right in the H-R diagram.
Red Dwarfs are very low
mass stars with no more
than 40% of the mass of
the Sun and represent the
majority of the stars.
.
They
have relatively low temperatures in their
cores; red dwarfs transport energy from the
core to the surface by convection.
A low-mass main-sequence star of spectral
classes M and L. Red dwarf stars range from
about 0.6 solar mass at class M0 down to 0.08
solar mass in cool M
Death of Low Mass Star
Core Mass
0.5 - 1.4
Final State
White Dwarf
Thermodynamics
When Fusion stops, core shrinks & temperature of
core rises.
When the Envelope expands its temperature cools
down.
Evolution of Low-Mass Stars
0.5 - 1.4 Msun
Main Sequence Phase
Energy Source: H fusion in the core
Using P-P cycle H fuses to He
Slowly builds up an inert He core
He
Fusing
Envelope
When the H in the core is almost completely
converted into He, H fusion stops in core.
The left over H is pushed out into a shell ring
around the He core.
The core collapses & heats up.
The increasing temp
will cause the H shell
to fuse forming He
that will join the core
Outer layer expands
and cool forming a
Red Giant
(On the H_R Diagram)
•The star gets brighter and redder, climbs up the
Giant Branch. (Takes 1 Byr)
At the top of the Giant Branch, the star’s
envelope is about the size of Venus’ orbit
• The core will contract until it gets hot
enough to fuse the He in the core into
Carbon & Oxygen.
When the fusion begins, the burning occurs
rapidly because of the H shell burning and
the He burning in the core.
This is called the “Helium Flash”. Some of
the outer layers are blown outward causing
the star to loose mass.
The star gets hotter, and moves onto the
Horizontal Branch
Once the He fuses and forms C & O, the core
contracts.
•C-O core collapses and heats up
•He burning shell outside the C-O core
•H burning shell outside the He burning shell
The core never gets hot enough to fuse the Carbon
& Oxygen
Outside:
Envelope swells &cools
because of H & He
burning
Climbs the Asymptotic
Giant Branch
Climbs the Giant Branch again, slightly to the left , and
higher, becoming a super red giant. .
Core and Envelope separate, takes ~100,000 yr
C-O core continues to contract:
With weight of envelope taken off, core
never reaches Carbon fusion temp of 600
Million K
Outer envelope gets slowly ejected . This is a
non-violent ejection; a series of puffs or
burps.
Expanding envelope forms a ring nebula
around the contracting C-O core.
A Planetary Nebula forms
Hot C-O core is exposed, moves to the left
Becomes a White Dwarf
Planetary Nebulae
Butterfly
Nebula
Fig. 13.16c
Planetary Nebula
Ejection not
explosive
Nebula shell
expands
Outer shells
of red
supergiant
“puffed off”
•After ~ 50,000
years, the nebula
spreads so far that
the nebulosity
simply fades from
view.
The nebula is
ionized, and
heated by
the. Ultraviolet
radiation
from the hot
star
Hot dwarf left behind
Cools down to form a WD
Contraction of the core is stopped by electron
degeneracy. The electrons repel each other as
they are pressed closer together and a White
Dwarf forms.
White Dwarfs have a mass that is less than 1.4 Mo
They will shine for a long time but no fusion is
taking place.
•One teaspoon
weighs about 5 tons.
Electron energy levels
• Only two electrons (one
up, one down) can go into
each energy level.
• In a degenerate gas, all
low energy levels are filled.
White Dwarfs are planetary in size, but have a stellar
mass
Radius (a little smaller than Earth!)
Temp. – anywhere from 100,000 to 2500 K.
White dwarfs shine by leftover heat, no fusion.
WD will cool off and fade away slowly, becoming a
"Black Dwarf“.
White Dwarf’s mass < than the Chandrasekhar mass
(1.4 Solar Mass)
Takes ~10 Tyr to cool off , so none exists yet.
White Dwarfs are so small, that they can only be
seen if close-by, or in a binary systems.
Sirius B
The most famous W.D. is Sirius’ companion .
Sirius B
Temp. 25,000 K
Size: 92% Earth's diameter
Mass: 1.2 solar masses
Sirius B
The mass of a star, in the size of a planet.
A lone white dwarf is a cooling corpse but a white dwarf
in a binary system can be revived
There is more !! A White Dwarf in a binary
system…
I
White Dwarf
Evolving (dying) star
Roche Lobes
II
Evolving (dying) star
White Dwarf
Accretion Disk
III
Evolving (dying) star
Roche Lobe filled
W.D. can take on material but, if the W.D.
exceeds 1.4 solar masses (Chandrasekar limit)
powerful explosions take place and they could
happen more than once. The star will get
down below 1.4 solar mass.
Type 1a
super
NOVA!!
Since the Type 1a supernova is always a white dwarf they
can be used to judge very great distances (using the inverse
square law). Type Ia: No hydrogen lines in the spectrum
Type II: Hydrogen lines in the spectrum
There is a
further
subdivision of I
into Ia, Ib, Ic
Sun
Red Giant
Becomes Red Giant
when H is almost gone
Orbit out to
almost Venus
Red Super Giant
Becomes a Red
Super Giant
Low Mass Stars
Envelope separates
from core and forms a
planetary nebula
If the White Dwarf is a
binary star, a Supernova
type 1a can form, if its
mass becomes greater
than 1 ¼ solar masses
Core forms a
White Dwarf
White Dwarf becomes a Black
Dwarf (dead star)
Only H , He in shells, C & O in core left C & O do not fuse
Wanted
Of course you know the relationship is just
going to end in a Type 1a supernovae...but I
suppose its better to have transferred mass
and exploded than to have never transferred
mass at all...
Crab Nebula
Supernova
Remnant
Stellar Graveyard
High Mass Stars
Final Core Mass
1.4 < M < 3.0
Final State
Neutron Star
Evolution of Massive
Stars
Massive stars have the same internal
changes as we saw in low mass stars ,
except :
massive stars evolve more rapidly
due to rapid nuclear burning, and
massive stars produce heavier
elements
Evolution of High-Mass Stars
High-Mass Stars
O & B Stars core mass >1.4 and <3 Msun
•Burn Hot
•Live Fast
•Die Young
Main Sequence Phase:
•Burn H to He in core using the CNO cycle
•Build up a He core, like low-mass stars
•But this lasts for only ~ 10 Myr
Red Supergiant Phase
After H core exhaustion:
•Inert He core contracts & heats up the
H burning in a shell .
• Envelope expands due to the burning H
shell and cools
•Envelope ~ size of
orbit of Jupiter
Moves horizontally across the H-R diagram,
becoming a Red Supergiant star
Takes about 1 Myr to cross the H-R diagram.
Core Temperature reaches 170 Million K
Helium Flash : Helium ignites
This Helium flash is not as explosive as the
one for low mass stars.
Helium Fusion produces C & O in core:
Star heats up and becomes a Yellow
Supergiant.
Star becomes a Yellow Supergiant.
Yellow
When He exhausted in core
•Inert C-O core collapses & heats up the
H & He burning in shells. Star expands
and becomes a Red Supergiant again
C-O Core collapses until: Tcore> 600 MillionK
• Carbon in the Core ignites.
C fuses to form : Ne , and O
•Core at the end of
• Carbon Burning
•Phase:
Nuclear burning continues past Helium
Things happen fast!
1. Hydrogen burning: 10 Myr
2. Helium burning: 1 Myr
3. Carbon burning: 1000 years
4. Neon burning: ~10 years
5. Oxygen burning: ~1 year
6. Silicon burning: ~1 day
Finally builds up an inert Iron core
End of the line!!
Massive star at the end of Silicon Burning:
Onion Skin of nested nuclear burning shells
Protons & electrons form neutrons &
neutrinos. Collapse is final.
At the start of Iron Core collapse:
•Radius ~ 6000 km (~Rearth)
•Density ~ 108 g/cc
A second later!! , the properties are:
•Radius ~50 km
•Density ~1014 g/cc
•Collapse Speed ~0.25 c !
Supernova explosion
Neutron degeneracy pressure halts the
collapse
Material falling inwards rebounds.
Outer layers of the atmosphere, including
shells, are blown off in a violent explosion
called a supernova.
The star will outshine all the other stars in
the galaxy combined.
Elements heavier than Lead are
produced in the explosion and ejected
into space. Stars do recycle.
The ejected material often attain speeds of
100,000 km/sec.
Close to 150 supernova remnants have been
detected in the Milky Way.
There are smaller numbers of massive stars
and so smaller amount of explosions.
The Famous Supernova SN 1987A
type II Supernova
At maximum
Before
Supernova remnants
Cas A in x-rays
(Chandra)
Cygnus Loop (HST):
green=H, red=S+,
blue=O++
Vela
Remnant of SN386, with central
pulsar (Chandra)
SN1998bu
The
rings of
SN
1987A
are from
previous
mass
loss
1a is binary with a White Dwarf
Type II : Hydrogen lines in the spectrum
Supernova explosion
1, The iron
core collapses
2. Neutrons stop
the collapse
3. The rebound of the core
sends shock waves causing
an explosion that blows the
outer atmosphere into space
as a super nova
• The Crab Nebula.
• A supernova that, according to the Chinese,
exploded in 1054.
• Despite a distance of ~ 7,000 light-years, the
supernova was brighter than Venus for weeks
before fading from view after nearly two years.
•Even today, the nebula
• is still expanding at
• more than 3 million
•miles per hour.
Structure of a Neutron Star
•Diameter~ 12 km in diameter
•Mass -about 1.4 times that of our Sun.
•One teaspoonful of material would weigh a billion
tons! Rotation Rate: 1 to 100 rotations/sec
Lighthouse Model:
Spinning magnetic
field generates a
a strong electric field.
The magnetic axis is miss-aligned with the rotation
axis of the neutron star .
The star's rotation sweeps the beams outward as it
rotates.
If we are in the sight path, will see regular, sharp
pulses of light (optical, radio, X-ray.)
Pulsars: emitted sharp, 1 millisecond-long
pulses every second at an extremely
repeatable rate.
A typical pulsar signal, received with a radio telescope
The connection between pulsars and
neutron stars was the discovery of a pulsar
in the crab nebula.
Iron
Proto-stars
(born in cool gas GMC)
Main Sequence Stars
Core Mass (CM) 0.5- 1.4 MO
Brown
Dwarf
Red
Dwarf
0.08 0 .5 MO
CM<0,08
Black Dwarf
Core Mass (CM) > 1.4 MO
Red Giant
Red Super Giant
Red Super Giant
Planetary Nebula
CM 0.5 – 1.4 MO
White Dwarf
(H Fusion)
White Dwarf
Binary can
produce
Type ia
supernova
Yellow Super Giant
Red Super Giant
Supernova (Type II)
CM > 1.4 & < 3
Neutron Star
CM > 3
Black Hole
Massive
star
Outer layers of the atmosphere,
including shells, are blown off
in a violent explosion called a
supernova
Neutron Star
Red Supergiant
Becomes
a Red
Supergiant
when H
exhausted
Yellow
Supergiant
Becomes Yellow
Supergiant when
He exhausted
Orbit size of
Jupiter
Red Supergiant
Becomes Red
Supergiant
Black Hole
Massive Stars
Black Holes
We know of no mechanism to halt the collapse of
a compact object with mass > 3 Msun.
It will collapse into a single point – a singularity:
=> Becoming
a
Black Hole!
Massive stars form the following:H, He, C, Ne, O,
Si, Fe . Iron will not fuse. Low mass stars form
only H, He, C, O
Honeycutt H
To memorize
this sequence,
use this :
Has
He
Caused
C
No
Ne
Oxford
O
Student
Si
Injury
(Iron) Fe
Thanks to the following for allowing me to
use information from their web site :
Nick Stobel
Bill Keel
Richard Pogge
NASA