Stars: HR Diagaram Stellar Evolution

Transcription

1 Stars: HR Diagaram Stellar Evolution Astronomy 1 Elementary Astronomy LA Mission College Spring F2015

2 Quotes & Cartoon of the Day Ancient stars in their death throes spat out atoms like iron which this universe had never known.... Now the iron of old nova coughings vivifies the redness of our blood. Howard Bloom In the vast cosmical changes, the universal life comes and goes in unknown quantities... sowing an animalcule here, crumbling a star there, oscillating and winding,... entangling, from the highest to the lowest, all activities in the obscurity of a dizzying mechanism, hanging the flight of an insect upon the movement of the earth... Victor Hugo

3 Announcements SS Homework posted, due 11/19 Schedule has been revised, final midterm now 12/3, Thursday after thanksgiving I will drop the lowest midterm grade If you miss midterm 3, that will become your drop grade unless you have a VERY good reason Very, very good

4 Last Class Intro to Stars Temperature, Color & Size Stellar Classification Intro to the HR Diagram LT HR Diagram

5 This Class Midterm Debrief Stellar Classification (review) Intro to the HR Diagram LT HR Diagram Stellar Evolution Main Sequence Red Giants and Supergiants Endgame (time permitting) Low mass stars planetary nebulae, white dwarfs High mass stars supernovae, neutron stars, black holes

6 Midterm Debrief Astronomy 1 Elementary Astronomy LA Mission College Spring F2015

7 Results Letter Current Course Grade Midterm 2 A 11 3 B 15 6 C D 1 12 F 5 13

8 Review of Classification Astronomy 1 Elementary Astronomy LA Mission College Spring F2015

9 The Harvard Computers The director of the Harvard Observatory from 1877 to 1919, Edward Charles Pickering hired women to process astronomical data. They were cheaper than men earned less than a clerical worker Willamina Fleming had been his maid

10 The Harvard Computers "Pickering's Harem" or the Harvard Computers included several now-famous astronomers Annie Jump Cannon, Henrietta Swan Leavitt Antonia Maury

11 Modern Classification Harvard spectral sequence Developed by Annie Jump Cannon Characteristic absorption lines determine stellar class Note: in Astronomy metal means anything heavier than He

12 Modern Classification Cecilia Payne (Payne- Gaposchkin) discovered this was actually a temperature sequence More precise than BB peak or color

13 Spectral Classification From hot to cool: O B A F G K M Each spectral class is further divided into 10 ranges according to temperature. 0 = hot, 9 = cool therefore O0 is the very hottest, and O9 is slightly hotter than B0 and M9 stars are very cool This is still a temperature-based classification

14 Let s Practice

15 Star Rue is Type K, Star Peeta is Type B, Star Katniss is Type F and Star Primrose is Type M. Which star is cooler than Rue? A. Peeta B. Katniss C. Primrose D. None of them

16 You observe a very bright, bluish star. It s spectral classification is most likely. A. B B. G C. M D. More information is needed to determine this

17 H-R DIAGRAM

18 Spectral Class isn t everything Spectral Class is not sufficient to uniquely identify a type of star The supergiant Arcturus and the red dwarf Proxima Centauri are both Type M & 3500 K They are definitely not identical!

19 Hertzsprung and Russell In 1911 Danish astronomer, Ejnar Hertzsprung, plotted the absolute magnitude of stars against their color Independently in 1913 American astronomer Henry Norris Russell plotted spectral class against absolute magnitude showed that the relationship between temperature and luminosity of a star was not random

20 The H-R Diagram H-R diagram plots Color and/or Temperature against Luminosity and/or Absolute magnitude Any data plotted like this is an H-R diagram, as is a theoretical version The H-R diagram is one of the most important tools in Astronomy

21 The Main Sequence The long strip from upper left to lower right is called the Main Sequence. (MS) Stars spend most of their existence on the MS 91% of nearby stars are MS stars. MS stars are fusing H into He in their cores.

22 Giants and Dwarfs

23 WARM-UP QUESTION

24 Star A has an absolute magnitude of -8.1 and belongs to spectral class B8. Star B has an absolute magnitude of 11.2 and also belongs to spectral class B8. Which star has the higher temperature? A. Star A B. Star B C. They have the same temperature. D. There is not enough information to determine which star is hotter.

25 LECTURE-TUTORIAL ON THE H- R DIAGRAM

26 Star A has an absolute magnitude of -8.1 and belongs to spectral class B8. Star B has an absolute magnitude of 11.2 and also belongs to spectral class B8. Which star has the higher temperature? A. Star A B. Star B C. They have the same temperature. D. There is not enough information to determine which star is hotter.

27 Let s Practice

28 A red giant of spectral type K9 and a red main sequence star of the same spectral type have the same. A. luminosity B. temperature C. absolute magnitude

29 What Type of Star is Aldebaran? A. Red Giant B. Main Sequence C. Supergiant D. White dwarf

30 What Type of Star is Vega? A. Red Giant B. Main Sequence C. Red Supergiant D. White dwarf

31 Stellar Lifecycles Astronomy 1 Elementary Astronomy LA Mission College Spring F2015

32 The H-R Diagram Reflects Evolutionary Stages Main Sequence is prime of life

33 The Main Sequence isn t forever! MASS is what controls ultimate fate NASA MASS is what controls MS lifetime

34 Life on the Main Sequence Astronomy 1 Elementary Astronomy LA Mission College Spring F2015

35 The Main Sequence Stars spend most of their lives on the main sequence fusing H to He in their core The pressure resulting from the energy released by the fusion balances gravity the star is stable Hydrostatic equilibrium

36 Life on the Main Sequence -- Hydrostatic Equilibrium Stars on the Main Sequence are in Hydrostatic Equilibrium The radiation pressure (outward) is balanced by the (inwards) pull of gravity Pressure tries to blow it apart, Gravity holds it together Copyrighted, by Nick Strobel

37 Energy Source Nuclear Fusion The Sun is a typical MS star Powered by Thermonuclear fusion Converts H to He and energy Requires extreme conditions Occurs only in star s core Astronomy 1 - Elementary Astronomy LA Mission College Levine F2015

38 nuclei have protons & neutrons protons + electric charge like charges repel closer they get, harder they push apart Must force nuclei very close, then nuclear force binds together Temp over 10,000,000 K required Nuclear Fusion Energy is released Image: /02/nuclear-fusion-and-why-its-awesome.html (for nuclei lighter than iron)

39 The Proton-proton Chain Image: (creative commons) Net reaction 4 1 H 4 He + energy He has less mass than 4 H about 0.7% missing Mass is converted to energy Einstein s mass-energy relation E=mc 2 In Sun, happens times per second

40 Rate of Mass-Energy Conversion 2015 by Sidney 1038 reactions per second consumes 6 x10 11 kg per second 600,000,000 metric tons Sun s mass is very large! (2x10 30 kg) % per million year

41 Low Mass and High Mass Stars Low mass: less than about 4x Sun s mass Sun is good model use the proton proton chain High mass: use the CNO cycle rather than the p-p chain same net result 4H->1He different set of reactions

42 Let s Practice

43 It takes extreme physical conditions to initiate nuclear fusion because. A. nuclei have positive electrical charge and repel each other. B. the nuclear forces that hold nuclei together are very short range C. Both of these D. Neither of these

44 If fusing H to He converts millions of tons of mass into energy every second, why aren t we worried about using up the Sun? A. The energy turns back into mass. B. The proton-proton chain only operates for a few seconds. C. A million tons of mass is a tiny fraction of the total mass of the Sun. D. The Sun is constantly manufacturing new H to replace the fused H.

45 The condition that keeps a Main-Sequence star relatively stable in size and temperature is. A. nuclear fusion B. the Jeans Instability C. hydrostatic equilibrium

46 The Sun s Luminosity comes primarily from. A. chemical burning B. gravitational contraction C. nuclear fusion D. nuclear fission

47 Stellar Evolution and Death Astronomy 1 Elementary Astronomy LA Mission College Spring F2015

48 The Main Sequence isn t Forever! Stars on the MS are stable/stationary Stars leave main sequence to become red giants or supergiants Mass determines maximum core temperature & fate

49 RED GIANTS & SUPERGIANTS

50 The Main Sequence isn t Forever! Low & Moderate Mass Stars become red giants Up to about 4x Sun s mass Up to about type A Max core temp supports fusion to He C,O

51 Core H Runs Out No more fusion in core. What do you think happens when the power plant shuts down? He ash core Hint: think about pressure & gravity Image from: senior/astrophysics/stellarevolution_mainsequence.html

52 Core H Runs Out pressure reduces star begins to collapse what happens when the star shrinks? It heats up! Area around core reaches 10,000 K Image H fusion starts again He ash core from: senior/astrophysics/stellarevolution_mainsequence.html

53 H fusion starts again Shell Causes outer layers to expand When they expand, they.. A Red Giant Emerges cool Red Giant Core continues to heat He fusing ash core at 100,000,000K He begins fusing C & O build up

54 A Red Giant Emerges H fusing shell expands He fusing shell forms Core stars to build up C,O ash He fusing core

55 Life as a Red Giant (Low Mass Stars) Red Giant is MUCH larger cooler more luminous short-lived: about 1/10 Main Sequence life Not as stable as a MS star Image from:

56 The Main Sequence isn t Forever! High Mass Stars become red giants & Supergiants Above 4x Sun s mass: OB and most A stars Much less stable & predictable Max core temp supports fusion up to Fe

57 Starts like low mass stars Red Giant/Supergiant Core temps continue to increase at ~1 billion K, C fusion stars layers of fusion up to Si (silicon) Fe (iron) Once Fe fusion starts, death (supernova) is imminent High Mass Stars

58 Let s Practice

59 If the red giant phase lasts 10% of a stars life on the main sequence, we would expect to find there are A. more main sequence stars than red giants. B. more red giants than main sequence stars. C. the same number of red giants as main sequence stars.

60 Why might we observe a larger fraction of red giants compared to main sequence stars than the ~10% you would expect from the previous question? A. They live longer then main sequence stars. B. They are larger and brighter than main sequence stars. C. They are closer to us on average than main sequence stars.

61 ENDGAME OF LOW MASS/AVERAGE STARS STARS

62 From Red Giant to Planetary Nebula Low Mass Stars stop core fusion with He H,He fusion shells expand Push off outer layers planetary nebula

63 Planetary Nebulae

64 Final Remnant White Dwarf Core exhausts He Contracts & interior heats but not hot enough to fuse C to O Contraction stopped by electron degeneracy pressure resists putting 2 e in same place Core becomes white dwarf a hot, dense naked core ~25,000K at surface up to 1.4 x Sun s mass compressed to Earth size Artist s concept Image ESA/NASA

65 White Dwarf White dwarfs last a long time Held up by e- pressure Cannot shrink, cannot heat up, cannot increase pressure Stuck In isolation, very slowly cool and fade away Artist s concept Image ESA/NASA

66 Let s Practice

67 A white dwarf is hot and tiny. This implies it will have a color and a luminosity than its main sequence progenitor. A. redder, lower B. redder, higher C. bluer, lower D. bluer, higher

68 ENDGAME OF HIGH MASS STARS

69 Supernova Destroyer/Creater Space.com

70 The biggah boomah Type II Supernova Huge cataclysmic explosion about 10 billion times as luminous as the Sun fade over months or years Artist s Concept of Supernova Credit: ESA everything heavier than iron fused by explosion Leaves behind a neutron star or black hole

71 "Onion layers" of heavier and heavier elements in their interiors. Fusion in stellar core wont go heavier than iron (Fe) Fusing Fe uses up energy Fe core reaches 1.4 M. Chandrasekhar limit electron degeneracy pressure can t hold it up takes seconds! Core-collapse

72 Core-collapse The core collapses. Protons and electrons are pushed together form neutrons and neutrinos exert a tremendous outward pressure. observe neutrino outburst stops when neutrons getting packed too tightly neutron degeneracy outer layers fall inward outer layers crash into the core and rebound shock waves move outward Star explodes Type II Supernova

73 End result is either a Neutron Star or Black Hole NASA Depending on the...?

74 End result is either a Neutron Star or Black Hole NASA Depending on the...? Remaining mass in the core! mass is (almost) everything if you are a star...

75 The Neutron Star Route Collapsed core <2 or 3 M MS star about 8-20 M made of degenerate neutrons very intense magnetic fields VERY Dense 1.4 to 3 M compressed into a radius of about 10 km. As something spinning collapses, it spins faster. Artist s Concept of a Neutron star undergoing a starquake. Credit: NASA neutron stars spin very fast

76 Discovery of Neutron Stars/Pulsars Predicted in 1930s discovered 1967 A graduate student named Jocelyn Bell was monitoring radio emission from space discovered a really regular signal. Unlikely anything natural could produce such a regular, repeating signal. The source of the radio signals was dubbed "LGM1". (Little Green Men) Several more were discovered.

77 Pulsars Astronomers finally deduced that they were observing very rapidly rotating neutron stars -- pulsars. Charged particles move around the magnetic fields most intense around the magnetic poles. As the pulsar rotates, acts like a light house." When the radio emission is pointed at us, we see a "pulse."

78 Collapsed core >2 or 3 M The Black hole Route Progenitor star > about 20 M for objects more massive than a neutron star, there is nothing that can stop the inward collapse due to gravity The final result is a black hole, a very small, very dense singularity that warps spacetime sufficiently that not even light can escape. Artist s Conceptof a Black Hole Credit: NASA

79 SO WHAT IS A BLACK HOLE?

80 What is a black hole?

81 Journey into a Black Hole

82 ANOTHER SCENARIO (INVOLVING MORE THAN 1 STAR)

83 Type Ia Supernovae Ingredients : a white dwarf with a post-mainsequence companion Or 2 white dwarfs companion dumps material onto the white dwarf When the mass > 1.4 Msun, the degenerate gas cannot support the pressure resulting supernova completely obliterates the white dwarf Artist s Concept Image: NASA/CXC/M Weiss.

84 Importance of Type Ia SN Standard Candle SNIa explosions happen under very consistent conditions The light output of a Type Ia SN is therefore very predictable Can assume the same peak brightness is always produced

85 Let s Practice

86 What is the fate of a 10-solar mass star? A. Type Ia supernova B. Type II supernova C. white dwarf

87 What is the fate of a 5 solar mass remnant core after a Type II supernova has occurred? A. neutron star B. black hole C. white dwarf D. planetary nebula

88 If your black hole research spaceship approached the site of very massive star that had gone supernova, what would happen? A. It would inevitably get sucked into the black hole with no possibility of escape. B. It would detect the gravitational pull of the black hole and be able to go into orbit around it. C. It would be unable to locate the black hole because it s, well, black. D. It would be repelled by the black hole.

89 WRAP-UP

90 Topic for Next Class Galaxies & Our galaxy

91 Reading Assignment Astro:10&11 Astropedia:15&16

92 Homework HW SS Posted, Due 11/19