1. A105 Stars and Galaxies Today’s APOD ROOFTOP TONIGHT AT 9 PM
HAND IN HOMEWORK
Exam coming on Nov. 2

2. Upcoming Events
Orionid meteor shower peaks Saturday night, view from 11:45 onward – if weather is clear, watch for at least 20 minutes from a dark site
Transit of Mercury
Nov. 8, 2:15 PM – Sunset
From Sample Gate

3. Let’s talk about…
Mid-Term Grades
The Next Exam

4. Temperature, Diameter, and Brightness

5. Star formation brings stars to the main sequence
…What happens next?

6. Explaining the HR Diagram
Energy
Gravity
Energy Transport

7. Review: Why was the Sun’s energy source a major mystery?
Chemical and gravitational energy sources could not explain how the Sun could sustain its luminosity for more than about 25 million years
Why does the Sun shine?
The Sun shines because gravitational equilibrium keeps its core hot and dense enough to release energy through nuclear fusion.

8. How does nuclear fusion occur in the Sun?
The core’s extreme temperature and density are just right for nuclear fusion of hydrogen to helium through the proton-proton chain
Gravitational equilibrium acts as a thermostat to regulate the core temperature because fusion rate is very sensitive to temperature

9. Stellar Mass and Fusion
The mass of a main sequence star determines its core pressure and temperature
Stars of higher mass have higher core temperature and more rapid fusion, making those stars both more luminous and shorter-lived
Stars of lower mass have cooler cores and slower fusion rates, giving them smaller luminosities and longer lifetimes

10. Massive Stars
Sun-like Stars
Low Mass Stars

11. Star Clusters and Stellar Lives
Our knowledge of the life stories of stars comes from comparing mathematical models of stars with observations
Star clusters are particularly useful because they contain stars of different mass that were born about the same time

12. Evolution of a Very Low Mass Star (~0.3 solar masses)
The entire star is convective.
As hydrogen is consumed, the core shrinks and heats, the luminosity rises along the main sequence.
Since convection occurs through the whole star, all the star’s hydrogen is burned.
Leaves a helium remnant ๏
Lifetime: 300 Billion Years

13. What are the life stages of a Sun-like star?
A star remains on the main sequence as long as it can fuse hydrogen into helium in its core
What happens next?

14. Life Track after Main Sequence
Observations of star clusters show that a star becomes larger, redder, and more luminous after its time on the main sequence is over

15. Sun-like stars become red giants
When the helium core contracts, the surrounding
hydrogen puffs up and the star becomes a red giant.

16. Broken Thermostat
As the core contracts, H begins fusing to He in a shell around the core
Luminosity increases because the core thermostat is broken—the increasing fusion rate in the shell does not stop the core from contracting

17. Helium fusion does not begin right away because it requires higher temperatures than hydrogen fusion—larger charge leads to greater repulsion
Fusion of two helium nuclei doesn’t work, so helium fusion must combine three He nuclei to make carbon

18. Once helium burning begins the “thermostat” starts to work again
Once helium burning begins the “thermostat” starts to work again. Helium burning stars neither shrink nor grow because core thermostat is temporarily fixed.

19. End of Fusion
Fusion progresses no further in a Sun-like star because the core temperature never grows hot enough for fusion of heavier elements
Electron pressure from quantum mechanics supports the core against further gravitational contraction

20. The End of Solar-type Stars
Main
Sequence
Planetary
Nebula
Red
Giant
White
Dwarf
When the carbon core reaches a density that is high enough, the star blows the rest of its hydrogen into space.
The hot, dense, bare core is exposed!
Surface temperatures as hot as 100,000 degrees
The hot core heats the expelled gas and makes it glow

21. Planetary Nebulae
Fusion ends with a pulse that ejects the H and He into space as a planetary nebula
The core left behind becomes a “white dwarf”

22. Planetary Nebulae!

23. Life Track of a Sun-Like Star

24. Earth’s Fate
Sun’s luminosity will rise to 1,000 times its current level—too hot for life on Earth

25. Earth’s Fate
Sun’s radius will grow to near current radius of Earth’s orbit

26. Summary The life stages of a Sun-like star
H fusion in core (main sequence)
H fusion in shell around contracting core (red giant)
He fusion in core
How does a Sun-like star end?
Ejection of H and He in a planetary nebula leaves behind an inert white dwarf

27. Life Stages of High-Mass Stars
Late life stages of high-mass stars are similar to those of low-mass stars:
Hydrogen core fusion (main sequence)
Hydrogen shell burning (supergiant)
Helium core fusion (supergiant)

28. What about Massive Stars?
Massive stars continue to generate energy by nuclear reactions until they have converted all the hydrogen and helium in their cores into iron.
Once the core is iron, no more energy can be generated
The core collapses and the star explodes
SUPERNOVA!

29. Core then suddenly collapses, creating supernova explosion
Iron builds up in core until degeneracy pressure can no longer resist gravity
Core then suddenly collapses, creating supernova explosion
Death_seq_of_high-mass_star.swf
Download a good supernova explosion movie from the Chandra Science Center (chandra.harvard.edu)

30. A “Recent” Supernova in Our Galaxy
A new star in Taurus observed by the Chinese in 1054 A.D.
Visible in the daytime
Gradually faded; gone after about two years
The Crab Nebula is a supernova remnant

31. The Crab Nebula Continues to Expand
The Crab Nebula is about 7000 LY away
The Nebula is about 10 LY across
Expanding at a speed of about 1,400 kilometers per second
The Crab Nebula - Then and Now
Images taken in 1973 and recently

32. The Large Magellanic Cloud
Distance: about 150,000 LY
Part of the Local Group
“Irregular” galaxy
Lots of star formation

33. Super- nova 1987a Feb. 1987 Star previously known – 18 solar masses
Study formation of supernova remnant

34. Rings around Supernova 1987A
The supernova’s flash of light caused rings of gas around the supernova to glow

35. Summary
The life stages of a high-mass star are similar to the life stages of a low-mass star
Higher masses produce higher core temperatures that enable fusion of heavier elements
A high-mass star ends when the iron core collapses, leading to a supernova

36. Is life on Earth safe from harm caused by supernovae?
Earth is safe at the present time
because there are no massive stars
within 50 light years of the Sun.

37. Sun-like Star Summary Main Sequence: H fuses to He in core
Red Giant: H fuses to He in shell around He core
Helium Core Burning:
He fuses to C in core while H fuses to He in shell
4. Planetary Nebula leaves white dwarf behind
Not to scale!

38. Life Stages of High-Mass Star Main Sequence: H fuses to He in core
Red Supergiant: H fuses to He in shell around He core
Helium Core Burning:
He fuses to C in core while H fuses to He in shell
Multiple Shell Burning:
Many elements fuse in shells
5. Supernova leaves neutron star behind
Not to scale!

39. Role of Mass
A star’s mass determines its entire life story because it determines its core temperature
High-mass stars with >8MSun have short lives, eventually becoming hot enough to make iron, and end in supernova explosions
Sun-like stars with <2MSun have long lives, never become hot enough to fuse carbon nuclei, and end as white dwarfs
Intermediate mass stars can make elements heavier than carbon but end as white dwarfs

40. The Evolution of Stars

41. The Composition of Stars
90% hydrogen atoms
everything
else
Helium
10% helium atoms
Less than 1%
everything else
(and everything
else is made in stars!)
Hydrogen

43. Abundance of Elements in the Galaxy
Goals:
Know how chemical elements are created
in the Early Universe
in Stars
in Supernovae
Know how the Galaxy is enriched in chemical elements

44. The Origin of Elements
The process by which elements (nuclei) are created (synthesized) is called nucleosynthesis
Nucleosynthesis has occurred since the creation of the universe and will essentially go on forever
The elements created come together to form everything material we know, including us

45. Primordial Nucleosynthesis
Hydrogen and helium were created during the Big
Bang while the Universe was cooling from its initial
hot, dense state.
About 10% of the lithium in the Universe today was
also created in the Big Bang. We’re still not sure
where the rest comes from.
The first stars formed from this material.

46. Hydrogen Burning Stars burn hydrogen in their interiors to
produce helium.
Hydrogen burning also rearranges carbon,
nitrogen, and oxygen.

47. Helium
Burning
Three helium atoms
combine to form carbon

48. Light
Elements

49. The Iron Peak Metals In the cores of massive stars just before
supernova explosions, atomic nuclei
exchange protons and neutrons to form
the iron peak metals.

50. Making Elements Up to Iron
Hydrogen – from big bang nucleosynthesis.
Helium – from big bang and from hydrogen burning via the p-p chain and CNO cycle.
Nitrogen – from CNO cycle.
Carbon, Oxygen – from helium burning.
Light elements (Neon, Magnesium, Calcium – from carbon and oxygen burning.
Iron metals – from the final burning
Making Elements Up to Iron

51. Heavy Metals All heavier elements are formed
when iron peak elements capture
neutrons

52. Elements Heavier than Iron …
Once iron is formed, it is no longer possible to create energy via fusion.
Elements heavier than iron require a different process (Iron is atomic number 26.)
The heaviest naturally occurring nucleus is uranium (atomic number 92). How do we get to uranium then?
Elements heavier than iron are created by
neutron capture
The neutron is converted into a proton and added to the nucleus, increasing the atomic number to make the next element in the periodic table.

53. Making Heavy Metals in Stars
In low mass stars like the Sun, heavy metals are created when the star is a giant
Massive stars make heavy metals when they become supernovae

54. Stellar Nucleosynthesis
We know now that all chemical elements heavier than atomic number 5 (Boron) were produced in stars.
The light elements are essentially ashes of nuclear burning during the normal stellar evolution process.
The heavier elements are produced in the envelopes of giants and during explosive nucleosynthesis that occurs during supernovae.

55. Chemical Enrichment of the Universe
We know now that massive stars act as factories for creating heavy elements
Massive stars end their lives in supernova explosions
The explosion scatters the new elements into interstellar space
Elements synthesized inside stars are also brought to the surface and expelled via stellar winds
A new generation of stars recycle this material, enriching it further

56. The Galaxy (and the universe) is gradually enriched in heavy elements
Despite all the nucleosynthesis that has occurred since the creation of the universe, only 2% of the ordinary matter in the universe is now in the form of heavy elements. Most is still hydrogen and helium

57. EXAM NOV. 2nd Star Death – Units 67, 68, 69 News Quiz on Tuesday
For Next Week...
Star Death – Units 67, 68, 69
News Quiz on Tuesday
Homework Due EACH THURS.
EXAM NOV. 2nd