Transcript The Death of a Low Mass Star

The Death of a Low Mass Star
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Evolution of a sun-like star post heliumflash
– The star moves onto the horizontal branch
of the Hertzprung-Russell diagram
– Helium burning produces carbon and
oxygen “ash”
– Eventually, the helium concentration falls
too low to sustain burning in the core
Post Core Helium Burning
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Similar sequence of events to the end
of hydrogen burning
– Core contraction and heating
– Degenerate carbon/oxygen core forms
– Helium shell burning commences
Post Core Helium Burning
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External Appearance
– The star moves off the horizontal branch
and ascends the red giant region again,
becoming even larger and more luminous
– The star is now an Asymptotic Giant and is
on the Asymptotic Giant Branch
Asyomptotic Giants
Asymptotic
Giant Branch
106
Core helium
burning ceases
– Location on the
HertzprungRussell Diagram
Luminosity (L)
104
102
2 M
Zero age
main sequence
1
1 M
Termination of core
hydrogen burning
10-2
40,000
20,000
10,000
5,000
Temperature (K)
2,500
Asymptotic Giants
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Appearance and Structure
Orbit of Mars
AGB sun
dia. ~ 1.5AU
L ~ 10000
dia. ~ 1x Earth
Helium burning shell
Degenerate C/O core
Dormant hydrogen shell
Asymptotic Giants
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Material Redistribution
– Convection layers may reach to the core
– Carbon and oxygen brought to the surface
– In consequence, molecular absorption
bands often seen in the spectra of AGB
stars
– Soot coccoons may also form around such
carbon stars
Late Evolution
– As helium is consumed, the core contracts
and heats up.
– The hydrogen shell may re-ignite,
producing more helium which re-fuels the
temporarily depleted shell
– Helium shell burning re-ignites in a helium
shell flash, leading to a short-lived spike of
luminosity - a Thermal Pulse
– Luminosity rises by ~ 2
Late Evolution
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Such thermal pulses may occur a
number of times:
3x105 years
Late Evolution
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AGB stars produce strong stellar winds
– Typical mass loss ~ 10-4 solar masses per
year
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103 x a “normal” red giant
1010 x the sun
– Combined with the thermal pulses, such
winds drive off the outer layers of the star
– As much as 40% of a star’s mass may be
lost in this way
Late Evolution
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A number of shells of material now
surround the dying star
Central star in
opaque cocoon
Concentric shells
Note: the phase shown here is very brief - ~ 1000 years
See http://oposite.stsci.edu/pubinfo/PR/1998/11/b.html
for details
The Final Stages
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Ultimately, the hot carbon/oxygen core
is exposed
– Core surface temperature ~ 100,000K
– Sufficient UV produced to ionise and excite
the outer layers
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The spectrum is now characterised by emission
lines
Planetary Nebulae
– The emitted gases now glow in the
radiation of the exposed core, forming a
Planetary Nebula
Exposed core
Fluorescing gas
– Speed of gas ~ 10 kms-1
– Diameter ~1 ly
Planetary Nebulae
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Planetary nebulae often appear as rings
– actually spherical
– looking through a greater depth of material
at the edges
Core of
“dead” star
Partner star
Planetary Nebulae
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A disc of material around a star may
allow a bipolar nebula to form
Planetary Nebulae
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The planetary nebula phase is relatively
short lived
– The nebulae in the previous slides are
estimated to be only a few thousand years
old
– The material rapidly disperses, leaving the
central core
White Dwarfs
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Sun-like stars never achieve the core
temperatures and densities to ignite
carbon and oxygen
– After the planetary nebula has dissipated,
the hot core is left
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Degenerate matter
Mass ~ 1 solar mass
about the size of the Earth
about 100,000 K surface temperature
<10-2 solar luminosities
White Dwarfs
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–No further nuclear
reactions take place
Luminosity due to
contained heat only
No further contraction
takes place
Electron degeneracy
pressure supports the
star
Cooling curve of a
1/4 solar mass
white dwarf
104
–Cooling occurs over
many billions of years
Luminosity (L)
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102
1
10-2
40,000
20,000
10,000
5,000
Temperature (K)
2,500
White Dwarfs
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Bizarre properties:
– All as a consequence of the properties of
degenerate matter
– Higher mass white dwarfs are smaller
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hence dimmer
– Maximum mass ~1.4 solar masses
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the Chandrasekhar Limit
– These properties will be explored in a
future lecture
The Death of a High Mass Star
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High mass stars behave very differently
– Higher core temperatures and densities
imply burning beyond oxygen
– Final stages often violent, leaving remnants
even more bizarre than white dwarfs
– To be discussed in the next lecture