Mid term. Highest score without bonuses: 98 % (Alex) Re- +2 Astro- jeopardy: Final à Mean/Median: 80

Transcription

1 Mid term Highest score without bonuses: 98 % (Alex) Re- normaliza@on: +2 Astro- jeopardy: +2-3 Final Distribu@on à Mean/Median: 80 Weighing toward final grade: 0.2*80 = *102 = Solu@ons available on course home page

2 Admin HW 6, Posted, Deadline: * Wednesday 3.00 PM * Lab today -- parallax ** NIGHT LAB (if weather permits) TOMORROW Stay Tuned! ** Monday, Tuesday and (partly) wednesday intense learning about birth, lives, and deaths of stars! Ch. 12.2,13.1-3,14 STAY ALERT we ll finish with some fun discussions about the boarders between science, science fiction, and philosophy; wormholes, space-travel, time-travel, and will doomsday come First, some tips from the coach regarding exam: Ask does it make sense? And if you don t get a), try to do b) in any case 2015 Pearson Education, Inc.

3 Last Time. H-R diagram depicts: Luminosity Temperature Color Spectral type Luminosity Radius Temperature Recall formula from earlier lectures: L/Lsun = (T/Tsun)^4 (R/Rsun)^2

4 Last Time. The mainsequence turnoff point of a star cluster tells us its age. Open Cluster Pleiades ~ 100 million years Globular Clusters in Milky Way halo ~ 13 billion (!) years

5 Last Time Note from the coach: Mjeans = Mminimum in book In 1/cm^3

6 Last Time 1. Gravity causes gas cloud to shrink and fragment. 2. Core of shrinking cloud heats up. 3. When core gets hot enough, fusion begins and stops the shrinking. 4. New star achieves long-lasting state of balance

7 Last Time -- Orion Nebula; a Fly through a star forming region in the Milky Way Examine yourself on MA Interactive Fig !

8 How massive are newborn stars?

9 Very massive stars are rare. Low-mass stars are common.

10 Upper Limit on a Star's Mass Radiation vs. Gravity Photons exert a slight amount of pressure when they strike matter. Very massive stars are so luminous that the collective pressure of photons drives their matter into space. Observations have not found stars more massive than about 300M Sun.

11 Lower Limit on a Star's Mass Fusion will not begin in a contracting cloud if some sort of force stops contraction before the core temperature rises above 10 7 K. Thermal pressure cannot stop contraction because the star is constantly losing thermal energy from its surface through radiation. Is there another form of pressure that can stop contraction?

12 Degeneracy Pressure: Laws of quantum mechanics prohibit two electrons from occupying the same state in the same place.

13 Thermal Pressure: Depends on heat content The main form of pressure in most stars Degeneracy Pressure: Particles can't be in same state in same place Doesn't depend on heat content

14 Brown Dwarfs Degeneracy pressure halts the contraction of objects with <0.08M Sun before the core temperature becomes hot enough for fusion. Starlike objects not massive enough to start fusion are brown dwarfs.

15 Brown Dwarfs A brown dwarf emits infrared light because of heat left over from contraction. Its luminosity gradually declines with time as it loses thermal energy.

16 Brown Dwarfs in Orion Infrared observations can reveal recently formed brown dwarfs because they are still relatively warm and luminous.

17 Stars more massive than roughly 300M Sun may not be able to form due to too much radiation pressure Stars less massive than 0.08M Sun can't sustain fusion.

18 Some things we ve learned How do stars form? Stars are born in cold, relatively dense molecular clouds. As a cloud fragment collapses under gravity, it becomes a protostar surrounded by a spinning disk of gas. How massive are newborn stars? Stars greater than about 300M Sun would be so luminous that radiation pressure might blow them apart before they were born. Degeneracy pressure stops the contraction of objects <0.08M Sun before fusion starts.

19 13.2 Life as a Low-Mass Star Some goals for learning: What are the life stages of a low-mass star? How does a low-mass star die?

20 What are the life stages of a low-mass star?

21 A star remains on the main sequence as long as it can fuse hydrogen into helium in its core. Main-Sequence Lifetimes and Stellar Masses

22 Thought Question What happens when a star can no longer fuse hydrogen to helium in its core? A. Its core cools off. B. Its core shrinks and heats up. C. Its core expands and heats up. D. Helium fusion immediately begins.

23 Thought Question What happens when a star can no longer fuse hydrogen to helium in its core? A. Its core cools off. B. Its core shrinks and heats up. C. Its core expands and heats up. D. Helium fusion immediately begins.

24 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.

25 Red Giants As the core contracts, H begins fusing to He in a shell around the core; Luminosity increases; the increasing fusion rate in the shell does not stop the core from contracting; star climbs up on the HR diagram becomes a red giant

26 Helium fusion does not begin right away because it requires higher temperatures than hydrogen fusion larger charge leads to greater repulsion. (Recall Ch. 11) The fusion of two helium nuclei doesn't work, so helium fusion must combine three He nuclei to make carbon.

27 Helium Flash The thermostat is broken in a low-mass red giant because degeneracy pressure supports the core. The core temperature rises rapidly when helium fusion begins. The helium fusion rate skyrockets until thermal pressure takes over and expands the core again.

28 Life Track After Helium Flash Models and observations of star clusters show the late evolution of a low-mass star. Helium corefusion stars are found in a horizontal branch on the H-R diagram. After core helium fusion stops, He fuses into carbon in a shell around the carbon core, and H fuses to He in a shell around the helium layer. This double shell fusion stage never reaches equilibrium the fusion rate periodically spikes upward in a series of thermal pulses.

29 Planetary Nebulae Death of a low-mass star Double shell 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.

30 End of Fusion - End of Life Low Mass Star Fusion progresses no further in a low-mass star because the core temperature never grows hot enough for fusion of heavier elements (some He fuses to C to make oxygen). Degeneracy pressure supports the white dwarf against gravity à Very, very slowly this naked core of what once was a star cools and fades away. This will be the ultimate fate for our Sun!

31 13.3 Life as a High-Mass Star Some goals for learning: What are the life stages of a high-mass star? How do high-mass stars make the elements necessary for life? How does a high-mass star die?

32 Massive Monster Stars Tarantula Nebula reveals the Monster Star R136a1 Seems to be about 300 times more massive than the Sun. (Previuos upper limit ~150 Msun) 1/27/15

33 Massive Monster Stars Side note Could they be unresolved binaries? No shift detected what does the high-energy X-ray emission tell us? 1/27/15

34 Live Fast, Die Young! Rare, shortlived, but powerful Powerhouses: Shine a million times brighter than the Sun Shed an Earth mass per year Die in a violent explosion 1/27/15

35 Live Fast, Die Young!

36 CNO Cycle High-mass main- sequence stars fuse H to He at a higher rate using carbon, nitrogen, and oxygen as catalysts. A greater core temperature enables H nuclei to overcome greater repulsion. (recall pp-chain and discussion of fusion in the Sun in Ch. 11!)

37 Life Stages of High-Mass Stars Late life stages of high-mass stars: Hydrogen core fusion (main sequence) Hydrogen shell fusion (supergiant) Helium core fusion (supergiant) The greater mass makes it possible for fusion to progress longer à good for the Earth! Time-scales for later fusion stages shorter and shorter

38 How do high-mass stars make the elements necessary for life?

39 Big Bang made 75% H, 25% He stars make everything else.

40 Helium fusion can make carbon in low-mass stars.

41 Helium Capture High core temperatures allow helium to fuse with heavier elements.

42 Advanced nuclear burning, Multiple Shell Burning Advanced nuclear burning proceeds in a series of nested shells. Core temperatures in stars with >8MSun allow fusion of elements as heavy as iron.

43 Helium capture builds C into O, Ne, Mg

44 Advanced reactions in stars make elements such as Si, S, Ca, and Fe.

45 Iron is a dead end for fusion because nuclear reactions involving iron do not release energy. (Fe has lowest mass per nuclear particle.)

46 How does a high-mass star die? Supernova explosions! Iron builds up in the core until degeneracy pressure can no longer resist gravity. The core then suddenly collapses, creating a supernova explosion.

47 Supernova Explosion Core degeneracy pressure goes away because electrons combine with protons, making neutrons and neutrinos. Neutrons collapse to the center, forming a neutron star. If mass even higher, a black hole is formed. (Ch. 14)

48 Energy and neutrons released in a supernova explosion enable elements heavier than iron to form, including Au and U.

49 Massive Star Death: Supernovae In the first year of the period Chih-ho, the fifth moon, the day chi-ch ou, a guest star appeared approximately several [degrees] southwest of Thien-kuan. After more than a year it gradually became invisble. Chinese records from 11 th Century

50 Supernova Remnant Energy released by the collapse of the core drives outer layers into space. The Crab Nebula is the remnant of the supernova seen in A.D

51 Supernova 1987A The closest supernova in the last four centuries was seen in 1987.

52 Why does it shine so bright? One Supernova can rival the total luminosity of a whole Galaxy! Let s assume L SN L Sun ~ ly A star with L Sun is at a distance 10 ly from us. The supernova goes o ly away from us. Which one will shine brightest on our night-sky? b = L/(4 d 2 )

53 1/27/15 Cosmic Recycling

54 Summary - Role of Mass A star's mass determines its entire life story because it determines its core temperature. High-mass stars have short lives, eventually becoming hot enough to make iron, and end in supernova explosions. Low-mass stars have long lives, never become hot enough to fuse carbon nuclei, and end as white dwarfs.

55 Summary - Life Stages of Low and High Mass Stars Low Mass Star: Main Sequence: H fuses to He in core. Exhausting H in core à Red Giant. After fusing C in core, no higher elements are created à Planetary Nebula: leaves white dwarf behind High Mass Star: Main Sequence: H fuses to He in core à Red/Blue Supergiant. Massive, hot enough to fuse until Fe à Supernova (makes very heavy elements like gold) leaves neutron star or black hole behind.