1. Nucleosynthesis and stellar lifecycles

2. Outline: What nucleosynthesis is, and where it occurs Molecular clouds
YSO & protoplanetary disk phase
Main Sequence phase
Old age & death of low mass stars
Old age & death of high mass stars
Nucleosynthesis & pre-solar grains
Stellar lifecycles

3. What nucleosynthesis is,
and where it occurs

4. Nucleosynthesis formation of elements Except for H, He
(created in Big Bang),
all other elements created
by fusion processes in
stars
Relative abundance

5. Stellar Nucleosynthesis Some H destroyed; all elements with
Z > 2 produced
Various processes,
depend on
(1) star mass
(determines T)
(2) age (determines
starting composition)
Z = no. protons, determines element

6. Stellar lifecycles: from birth to death
low mass
star (< 5 Msun)
high mass
star (> 5 Msun)

7. Stellar lifecycles: low mass stars
Stellar nucleosynthesis
2. Main Seq.
3. Red Giant
low mass
star (< 5 Msun)
1 & 5.
molecular
cloud
4. Planetary nebula
4. White dwarf
Nucleosynthesis possible
if white dwarf in binary system
(during nova or supernova)

8. Stellar lifecycles: high mass stars
Stellar nucleosynthesis
2. Main Seq.
(luminous)
3. Red Giant/
Supergiant
1 & 6.
molecular
cloud
high mass
star (>5 Msun)
5. Neutron star
4. Supernova
5. Black hole

9. Track stellar evolution on H-R diagram of T vs luminosity
Luminosity: energy / time

10. Distribution of
stars on
H-R diagram.
When corrected for
intrinsic brightness,
there are MANY more
cool Main Sequence
stars than hot.

11. On main sequence, luminosity depends on mass
L ~ M3.5

12. Molecular clouds: Where it begins & ends

13. Molecular clouds Mainly molecular H2, also dust, T ~ 10s of K
cold, dense areas in
interstellar medium (ISM)
Horsehead Nebula
Mainly molecular H2,
also dust, T ~ 10s of K

14. Famous Eagle
Nebula image.
Cool dark clouds
are close to hot
stars that are
causing them to
evaporate.

15. A larger
Interplanetary Dust
Particle (IDP)

16. Gravity in molecular
clouds helps promote
collapse of cloud
…and sometimes is
assisted by a trigger

17. Young stellar objects (YSOs) & protoplanetary disks (proplyds)

18. Molecular cloud fragments that have collapsed– no fusion yet
YSOs & Proplyds:
Molecular cloud fragments that have collapsed– no fusion yet
< Protoplanetary disk around
glowing YSO in Orion
Solar nebula:
the Protoplanetary disk
out of which our solar
system formed

19. Herbig-Haro
Objects--
YSOs with
disks & bipolar
outflows

20. Magnetic fields around
YSOs can create polar
jets and X winds

21. Collapse of molecular cloud fragments occurs rapidly
~105 to 107 yrs,
depending on mass
Protostellar disk
phase lasts ~106 yrs

22. Single collapsing molecular cloud produces many
fragments, each of which can produce a star

23. Main Sequence phase: Middle age

24. Star “turns on” when nuclear fusion occurs
main sequence star – either proton-proton chain or CNO cycle nucleosynthesis
P-P chain net: 4 H to 1 He

25. But– eventually the H runs out
Lifetime on main sequence = fuel / rate of consumption
~ M / L ~ M / M3.5
lifetime ~ 1/M2.5
So a 4 solar mass star will have a main sequence lifetime 1/32 as long as our sun

26. So, what happens when the core runs out of hydrogen?
Star begins to collapse, heats up
Core contains He, continues to collapse
But H fuses to He in shell– greatly inflating star
 RED GIANT (low mass)
or SUPERGIANT (high mass)

27. What happens next depends on stellar mass

28. Old age and death of low mass stars
Red Giant
Planetary nebula
White dwarf

29. Red Giant (RGB) star: H burning in shell

30. An AGB can lose its outer layers— Ultimately a planetary nebula forms,
leaving a white dwarf in the center
Planetary nebula
White dwarf

31. Planetary nebulas
Note: planetary nebula have nothing to do with planets!

32. Nuclear fusion
stops when
the star becomes
a white dwarf—
It gradually cools down

33. Old age & death of high mass stars
Super Giant
Neutron star
Supernova
Black hole

34. High-mass stars:
Progressive core fusion
of elements heavier than C

35. Includes: s-process nucleosynthesis as Supergiant,
r-process nucleosynthesis during core collapse

36. End for high mass star comes as it tries to fuse core Fe into heavier elements– and
finds this absorbs energy
STAR COLLAPSES & EXPLODES AS SUPERNOVA

37. --Fe core turns into dense neutrons
--Supernova forms because overlying star falls
onto dense core & bounces off of it

38. Supernova remnants

39. Crab Nebula
supernova
remnant.
A spinning
neutron star
(pulsar) occurs
in the central
region.

40. Nucleosynthesis &
pre-solar grains

41. Summary of nucleosynthesis processes
process main comment
products
H-burning 4He main seq.
He-burning 12C, 16O Red Giant
C-O-Ne-Si 20Ne, 28Si, 32Si, Supergiants
burning up to 56Fe
s-process many elements Red Giants, Supergiants
r-process many heavy supernova
elements