the sun and the stars
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The Sun and the Stars. The Sun and the Stars. The Sun and the Stars. Stellar evolution. i) Stellar birth. HST image of The Eagle nebula, a stellar nursery. Stars appear to be born in groups – why?. The Sun and the Stars. - PowerPoint PPT PresentationTRANSCRIPT
Dr Matt Burleigh
The Sun and the Stars
The Sun and the Stars
Dr Matt Burleigh
The Sun and the Stars
Stellar evolution
i) Stellar birth
HST image of The Eagle nebula, a stellar nursery
Stars appear to be born in groups – why?
Dr Matt Burleigh
Consider giant molecular cloud (H2 region), with mass M, radius R, no. of particles N,of average mass m, at temperature T
The potential energy of the cloud U, is given by
R
mNGMconstU
)(
NkTKE2
3
KEU
The total kinetic energy (KE) of the particles in the cloud is,
For collapse,
NkTR
mGMN
The Sun and the Stars
So,R
mG
kTM J
Where MJ is the Jeans mass
Alternatively, this can be written in terms of the Jeans density, J, where J is given by
So for collapse require low temperatures, and large masses
Dr Matt Burleigh
The Sun and the Stars
Molecular cloud (H2 region) collapses under its own self-gravity(free-fall internal pressure zero, no collisions)NB fragmentation likely.
[Exact trigger unknown - possibly density inhomogeneities shocks (SN), winds etc.]
As cloud collapses, PE converted into KE+ radiation in roughly equal proportion
Collapse continues provided:
KE dissipated Radiation escapes
If these conditions satisfied, gas remains cool, pressure remains low, collapsing material is a Protostar
Collapse continues until density of core high enough that 1, core optically thick.
Radiation trapped, core heats up, collapse stalls. Hydrostatic equilibrium established. Star is now a Pre-Main Sequence star
Dr Matt Burleigh
Cloud Collapse
Dr Matt Burleigh
The Sun and the Stars
Stellar birth cont’d
Because T low, opacity high, PMS star is fully convective
Heat transported rapidly to surface, R is large, therefore L is large (30xLsun, point B on next page)
PMS contracts slowly, core heats up, but T(sfce)constantL decreases slowly (point C). As core Temp rises opacity decreases, radiation becomes increasingly dominant transport mechanism and moves outward slowly from core.Once radn transport exceeds convection, sharp kink to left on H-R diagram (point D)
Eventually T core high enough (few million K) that nuclear fusion dominates energy production. Hydrostatic equilibrium now maintained by fusion, star stops contracting. Star is now aZero Age Main Sequence Star (ZAMS) E. Star radiative in core, convective in outer layers.
Takes ~20 million years to reach ZAMS from initial collapse.
A solar mass star will spend approx 10 billion years on main sequencequietly converting H He in the core via the PP-chain.
Dr Matt Burleigh
The Sun and the Stars
Evolutionary track for a PMS star
A core has developed
B PMS fully convective (L~30xLsun)
C core contracts without change in Tsfce
D radiative transport dominates
E T> few million K (TNR) contraction halted ZAMS star
It takes approximately 20 million years from the initial collapse for star to join main sequence
Dr Matt Burleigh
Star Formation
Dr Matt Burleigh
Star Formation
Dr Matt Burleigh
Star Formation
Dr Matt Burleigh
Star Formation
Dr Matt Burleigh
Stellar Lifecycle
Dr Matt Burleigh
The Sun and the Stars
Mass-luminosity relation
Eddington 1924 - for stars on the main sequence, a graph of mass versus luminosity follows a power-law, with slope 3
Msun
M
Lsun
L
In fact =2.3 for dim red stars, and breaks to =4 for more luminous stars, at a mass of 0.43 Msun.
This break in slope reflects
(i) differences in stellar interiors(ii) changes in opacity with temperature
Dr Matt Burleigh
The Sun and the Stars
Stellar lifetimes
M-L relationship for main-sequence stars
Stellar lifetime depends on its mass M, and the rate at which it consumes fuel, ie its’ luminosity L.
Therefore a stars lifetime t, relative to the suns lifetime tsun,
So, more massive stars have shorter lifetimes!!
Dr Matt Burleigh
The Sun and the Stars
Stellar evolution I – evolution of 1 solar mass star (pop I)
After 10 billion years, most of Hydrogen in core exhausted core mostly He.
during this time, T (core) has increased slightly and star has expanded slightly, luminosity increases
A-B Once hydrogen in core used up thermonuclear reactions in core cease. Drop of pressure in core causes core to contract. Surrounding H is pulled into core region and raised in temperature. Burning occurs in H-shell around He core. Burnt H added to core, whose density increases
B-C Core contracts when too much material has been added to it. Energy generation in shell accelerates. Envelope must store more energy, and so it expands
radius of star increases, surface temperature decreases. (moves to right on H-R diagram)
A
B C
DE
Dr Matt Burleigh
The Sun and the Stars
CHeBe
BeHeHe1248
844
Stellar evolution I – evolution of 1 solar mass star
C-D Lower T higher opacity, energy carried by convection. Radius of star increases, surface temperature decreases further. Heat transport dominated by convection, v efficient. Luminosity increases rapidly
star moves up Red Giant Branch – winds remove a fraction of star’s atmosphere
Core continues to contract
When T reaches 100 million K , Helium burning occurs (triple-alpha process)
HeHLi 417 2Lighter elements are rare because they quickly combine with H to produce He nuclei, eg.
A
C
D
B
E
~0.65Msun
Dr Matt Burleigh
The Sun and the Stars
D Heat spreads rapidly through core.Triple-alpha reaction rate increases due to increased temperature, increases energy and hence temperature…. thermal runaway (Helium flash – heat spreads throughout core in few minutes, 60-80% He burntat this stage) Core pressure increases, core expands and cools.He core burning temporarily ceases
D-E Outer envelope and core contracts under gravity luminosity decreases, but surface temperature increasesStar moves down and to left on HR diagram. When core temperature sufficiently high core helium reignites
E Star burns He in core and H in layer around core (Subgiant)
Once core converted to C, core contracts again, burning layer of He around core, forces star to expand, star again becomes a giant. Star moves up Asymptotic Giant Branch (AGB).
Triple alpha-reaction v. sensitive to temperature, star becomes unstable – thermal pulses.
D
C
A
B
E
Dr Matt Burleigh
Thermal pulses – core contracts, causes burning shell around core to heat up, heats outer layers which expand and therefore cool, energy generation drops, core contracts…cycle repeats
Thermal pulses (every few thousand years) cause luminosity to vary by up to 50% on timescales of a few years. Energy transported rapidly to surface by convection
Star develops super wind removing outer layers, exposing core
Wind driven material form Planetary nebula (ionised by core). Core cools as a white dwarf.
The Sun and the Stars
Dr Matt Burleigh
Evolutionary phases of a solar mass star, post main-sequence
H-R positionH-R position StageStage Physical processesPhysical processes
ZAMSZAMS Core hydrogen burning beginsCore hydrogen burning begins
AA Evolution on main-Evolution on main-sequencesequence
Core hydrogen burning ceases; shell Core hydrogen burning ceases; shell hydrogen burning beginshydrogen burning begins
A-B-CA-B-C Evolution off main-Evolution off main-sequencesequence
Shell hydrogen burning continues; Shell hydrogen burning continues; convection dominates energy transportconvection dominates energy transport
C-DC-D Red giantRed giant Helium flash occurs; core helium burning Helium flash occurs; core helium burning beginsbegins
EE SubgiantSubgiant Core helium burning continues along with Core helium burning continues along with shell hydrogen burningshell hydrogen burning
Red giant again (AGB)Red giant again (AGB) Thermonuclear reactions then end; shell Thermonuclear reactions then end; shell helium and hydrogen burning continues helium and hydrogen burning continues
Planetary nebulaPlanetary nebula Star enters the planetary nebula stageStar enters the planetary nebula stage
White dwarfWhite dwarf All thermonuclear reactions stop; slow All thermonuclear reactions stop; slow coolingcooling
Dr Matt Burleigh
End of Main Sequence
Dr Matt Burleigh
The Sun and the Stars
Example planetary nebulae – note WD at centre of nebulae
Dr Matt Burleigh
Population I Stars
Typical stars are young, in galactic spiral arms where gas and dust found
Typically reside in open star clusters ~2% of mass elements heavier than H or He (ISM
enriched by supernovae) If M* a little > M energy generation is by CNO cycle Sun is population I
Dr Matt Burleigh
Population II Stars
First stars to be formed in Universe Have only 0.01% heavy elements Typically found in galactic bulge and globular
clusters Similar sequence of evolution but occupy different
region of H-R diagram during core He burning Significant temperature changes, heating and then
cooling