Transcript Introduction to Stellar Evolution

Hertzsprung Russell
(HR) Diagram
Spectral Types:
Proxy for Teff
Or surface temperature
Our Sun: 5,700 deg Kelvin
Or about 10,000 F
OBAFGKMLT
Not to scale: Red Giant 100 times bigger than Main Sequence Star
Pre main sequence evolution:
1)
Protostellar, self-gravitating
Gas cloud collapses
Then contracts in
Hydrostatic equilibrium
2)
Develops hot radiative core
3)
Reaches ignition point for
Hydrogen burning
Virial theorem: as cloud shrinks
Gravitational energy is released
Half of it goes into heating the
interior
Half of it is radiated away
as luminosity
(and its more luminous than it will be
When hydrogen is burning)
3) Start of main sequence
(hydrogen burning)
4) End of main sequence
(core is now all helium)
5) Climbing giant branch
hydrogen burning in shell
around inert helium core
hydrogen envelope swells up
6) Helium ignition in a “flash”
7) Helium burning to carbon/oxygen
7) Second climb up
Giant brach:
H- and He-shell burning
Around inert C/O core
(the pre white dwarf)
8) Ejection of
Planetary Nebula
exposes core
Which contracts and
Gets very hot as
Upper layers peeled off
Enters white dwarf phase
Near 100,000 deg Kelvin
9) White dwarf phase:
Just cooling off and
Getting dimmer
The Ring Nebula
in Lyra
The pre-white dwarf is
the very blue star
at the center !
A Planetary Nebula
Star ejects envelope
of unburnt hydrogen
Many cool pics
On Hubble web site
Sirius AB -- a Visual Binary
60 arcseconds = 1 arcminute = 1/60 of a degree
The Globular Cluster
Messier 5
Stages of Stellar Evolution:
A) Core hydrogen burning
= main sequence
H > He
when finished: helium core
B) Red giant branch
= hydrogen burning in shell
around helium core
C) Tip of giant branch
= ignite helium core
D) Core burns He > C, O
E) He shell burning
ejection of planetary nebula
F) Becomes white dwarf
Open / Galactic Star Cluster NGC6791 -- 8 Billion Years, 2-3 x solar metals
Isochrones
for
7, 8, 9 Gyr
Massive star evolution
(> 8-10 M)
After helium burning
To C,O
C ignites to O, Ne, Mg
Ignites to Si, S
Si, S to Fe peak elements
(Ni, Ti, lots of Fe)
Now stellar core can
do no more burning
to produce energy
So it collapses
and Explodes as a
Supernova
Remnant core:
Neutron Star or
Black Hole
Supernova in 1054 A.D.
Recorded by the Chinese
The Crab Nebula
Pulsar inside!
Pulses every 1/30 second
Pulsar:
Rotating,
Highly magnetic
Neutron star
Like a rapidly spinning
Searchlight beam!
An X-ray pulsar (also optical, radio)
Jocelyn Bell Burnell
British radio astronomer
Graduate student in 60s
Discovered first pulsars
In radio signals
With advisor Anthony Hewish
Sir Anthony Hewish
Nobel prize in physics
1974 for pulsars
Problems
• Impostors: contamination by other small stellar and substellar objects:
the radii of small stars, brown dwarfs and giant planets are similar
S
M
T
HJ
J
Sol
Gliese 229
Gliese 229b
HD209458
Jupiter
T=5800K
T~3400K
T~ 950K
T~1000K
T~200K
M=1000 Mj
M~300 Mj
M~50 Mj
M~1 Mj
M=1 Mj
R=10 Rj
R~3 Rj
R~1 Rj
R~1.4 Rj
R=1 Rj
Burrows and Liebert 1993 Reviews of Modern Physics
A new class of planets
• OGLE planets have
shorter periods than
RV planets.
• Transit searches find
a different population
because the selection
effects are different.
Fig. From the California-Carnegie team