Download Main-sequence stage Stellar lifetimes

Main-sequence stage
• e.g. the Sun:
‘envelope’
Convective
zone
Radiative
zone
‘core’
Tcore ~ 1.5x107 K
P core ~ 1011 atm
P surface ~ 0
Fusion
H -> He
• At Tcore, λmax ~ 0.1 nm -- hence γ-ray, X-ray photons
( + neutrinos).
• Photons diffuse outward slowly ( ~ 106 years)
through radiative zone; energy transport by
convection in outer 0.2 R.
• In high-mass stars, energy transport by
convection in inner part, by radiation in outer part.
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Stellar lifetimes
• H is the most abundant element
– Hence this energy source lasts longest.
– More massive stars are much more luminous (L ∼ M3.5 to 4.0), i.e.
energy output is much greater.
– Less massive stars, less luminous
– Lifetimes:
t ms = Fuel Energy use " M L
– e.g.
!
Mass Luminosity Main-sequence time
(M)
(L)
(106 years)
25
3
1
0.5
8x104
80
1
0.06
3
300
8000
80000
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The HR Diagram
• Very important for stellar evolution.
– describes the evolution of a single star, a group or a population
of stars.
100
Red
Luminosity
giants
L / L
1
White dwarfs
Main sequence
1/100
30,000
6,000
Temperature Teff (K)
3,000
• Stars prefer specific combinations of Teff and L.
–
–
–
–
e.g. the main-sequence, giants, white-dwarf regions.
other regions empty of stars, or nearly so.
either : stars never have such (Teff, L) combinations,
or : they do so only for a short time relative to stellar lifetime.
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The “human HR diagram”
• cf. analogous diagram for humans:
Height
(m)
2
Adult
(most time)
teens
1
child
40
Weight (kg)
80
• All stars spend most time as main-sequence stars,
with fusion reaction H -> He providing energy.
• Then they change fairly quickly to become other
types of stars.
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To red giants and beyond
• As H is exhausted in core:
– fusion stops so pressure support decreases
– core contracts, releasing gravitational PE.
– An H -> He fusion shell source around the core provides energy as
well.
• As core contracts, the outer parts expand from ~ 1 R
up to ~ 1 AU in size.
• Transition stage:
– Low-mass stars (~ 1 M) ~ 109 years, via subgiants .
– High-mass stars (> 5 M) ~ 105 years, quickly!
• Hence for an old (~109 y) group of stars, see
continuous line from MS, through subgiants, to giants.
• For a young (~107 y) group of stars, may see gap
between MS and giants -- the Hertzsprung Gap .
– cf. the CM diagrams of open and globular clusters.
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Fusion in red giants
• At the red-giant stage, new core fusion processes
start: He -> C and C + He -> O
– Requires Tcore ~ 108 K
– Can last up to ~20% of MS time
– Star still has (weak) H -> He shell source as well
• Low-mass stars become horizontal-branch stars
at this stage.
• High-mass stars move in blue-red-blue-red loops
upwards in supergiant stars.
• Theoretical isochrones (equal-time lines) and
ages of star clusters.
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Pulsating stars
• Stars with particular structures can become mildly
unstable , so that they pulsate - the surface of the
stars alternately expands and contracts radially.
• Pulsation occurs in a particular region of the HR -the instability strip.
– High-mass stars: the classical Cepheids are yellow supergiant
stars.
– Low-mass stars: the RR Lyrae variables are white horizontalbranch stars
– Type II Cepheids are also low-mass stars.
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End states of stars - I
• White dwarfs / neutron stars /
black holes
• Low-mass stars:
– At end of core He fusion, still have shell
sources of H and He fusion.
– Star returns to red as an asymptotic
giant-branch (AGB) star.
– Core cannot become hot enough for
further fusion.
– Within 104 to 105 years, star moves
through the envelope ejection phase -the planetary nebula stage.
– Leaves the remnant core: a hot white
dwarf object with mass < 1.4 M
– Typically M ~ 0.6 M, R ~ 0.01 R,
surface temperature ~ 100,000 K at first.
– Long-lived stage (> 109 years) with star
cooling down at a constant radius.
Cat’s Eye Nebula (HST)
Planetary Nebula
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End states of stars - II
• High-mass stars (now > 8 M):
– Only inner core hot enough for further fusion.
– Sources rapidly become more scarce. e.g. 25 M:
Reaction
Timescale Central core
temperature (K)
C -> Mg, Ne 600 years 0.6 x 109
Ne -> O
1 year
0.6 x 109
O -> Si
6 months 0.6 x 109
Si -> Fe
1 day
2.7 x 109
– No further nuclear fusion sources. Hence:
– Core collapses in seconds; central T rises again; γ-rays photodissociate 56Fe into 4He nuclei, then into protons and electrons.
– As densities rise, p + e–
neutrons + neutrinos
– Neutron-rich matter compresses to density ~ 1017 kg m–3 and
stops further collapse.
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Supernova!
• Following core collapse:
– Star’s inner regions collapse on to compressed core.
– They are then ejected together with the neutrinos out
through the star to produce a supernova event.
– > 90% of star’s mass may be ejected into interstellar
space.
– Energy released ~ 1043 J !
– Most likely remnant is a neutron star -- mass ~ 1.5 M
typically, radius ~ 15 kilometres !
– If remnant is massive enough (> 3 M ) then it will
become a black hole -- a singularity in space.
– Black : no light can escape!
– Hole : if you fall in, you can’t get out!
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Supernova 1987A
•
•
•
•
SN Type I: White dwarfs in binary systems
SN Type II Core collapse of M* > 8M
24 February 1987: Brightest SN in 400 years
SN Type II in Large Magellanic Cloud, 170
thousand light years from Earth.
• Blue supergiant called Sanduleak -69 202
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HST View of Supernova 1987A
Central ring from previous
mass loss from progenitor
star. Ionized by light from
SN explosion, glows brightly
But material from explosion
should eventually hit ring and
start brightening again…
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The ring is starting to brighten again due to SN debris hitting it.
Fireworks over next few years…
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SN1987a in 2003
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SN1987a: 1994-2003
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