Brought to you in glorious, gaseous fusion-surround. Intro to Stars Star Lives 1

Transcription

1 Brought to you in glorious, gaseous fusion-surround. Intro to Stars Star Lives 1

2 Stellar Evolution Stars are born when fusion reactions begin. Along the way they evolve, i.e. change. Stars die when fusion reactions end. Lives of stars are much to long for us to observe, so we look at stars in various stages and make inferences about how they evolve. Intro to Stars Star Lives 2

3 Collect data for many human beings as to their height and weight. Zeilik 6/e Intro to Stars Star Lives 3

4 Collect data for one person at different points in his life. The graph shows how a person s height and weight change with time. IF we understand how one person evolves over time, we can infer how many people evolve. Zeilik 6/e Intro to Stars Star Lives 4

5 Collect data for many human beings as to their height and weight. With stars, we ll work in reverse. We ll use the data from many stars to infer how a single star evolves. Zeilik 6/e Intro to Stars Star Lives 5

6 Consider an extended family: infants children teenagers adults elderly Most of the family members will be adults in mid-life because that is the longest period of life. => relative number in each stage reflects the relative length of the stage but! you must assume birth and death happen at a regular rate Intro to Stars Star Lives 6

7 Study the H-R Diagram most stars are on the MS => MS phase is the longest phase of life Main Sequence stars are busy converting H into He in their cores and they do so for the major part of their lives. What is the ONE characteristic that determines how a star evolves? Intro to Stars Star Lives 7

8 Star: huge, hot ball of gas, mostly H, heated by thermonuclear reactions in the core Stable star is in balance (MS star) - pressure outwards equals pressure inwards - gravity is pushing in - pressure from fusion reactions is pushing out Intro to Stars Star Lives 8

9 BALANCE!! Zeilik 6/e Intro to Stars Star Lives 9

10 Model for Stellar Anatomy: a stable star must be in balance a star must keep generating energy to stay in balance rate of energy produced = rate of energy radiated away balance must hold at every layer in the star What would happen if there were a backup, a bottleneck? energy transport must be even from core to surface more opaque the star is the slower the energy transport Intro to Stars Star Lives 10

11 To make a model: produces energy gravity pressure in balance energy has to be transported evenly matter behaves like an ideal gas thermonuclear processes mass, chemical composition temperature, pressure, density => model! Intro to Stars Star Lives 11

12 We use LUMINOSITY and TEMPERATURE to plot where the model star should be on an H-R diagram. What we understand about stars comes from the development of theoretical models. What is one BIG problem with the model? Intro to Stars Star Lives 12

13 Stars Evolve: radius temperature luminosity change in complicated ways => changes in placement on H-R diagram Plotting these changes results in an evolutionary track. Intro to Stars Star Lives 13

14 Surface temp. increases, luminosity stays the same. Luminosity Temperature What happens to the size? Intro to Stars Star Lives 14

15 Luminosity increases, temp. stays the same. Luminosity Temperature What happens to the size? Intro to Stars Star Lives 15

16 Intro to Stars Star Lives 16 Zeilik 6/e

17 What causes stars to evolve? Stars are happily burning H into He p-p reaction 4 H nuclei become 1 He nucleus => energy CNO cycle - produces MORE energy (stars > 1.5 M ) Zeilik 6/e Intro to Stars Star Lives 17

18 Eventually the core becomes almost entirely He p-p reactions stop CNO reactions stop fusion no longer keeps the temp. and pressure high What s next? Intro to Stars Star Lives 18

19 What s the conservation of energy trade-off when that happens? Pressure increases Temperature increases 100 million K => triple alpha process 3 He -> 1 C plus energy Helium burning Intro to Stars Star Lives 19

20 What happens when He runs out? Gravity wins, core collapses Pressure increases Temperature increases 600 million K => carbon burning fusing carbon into heavier elements Depending on the of the star, this cycle repeats until iron (Fe) is formed Intro to Stars Star Lives 20

21 This process is called nucleosynthesis - heavier elements are made out of lighter ones. It continues until iron is made and there the process stops entirely. Intro to Stars Star Lives 21

22 Theoretical Evolution of a 1 M Star Protostar - energy from gravitational collapse formed by contraction of cloud dense core forms, rest of cloud accretes on to it (1-4) Completely convective - bubbling ball of gas, very luminous, larger than it will be as a star, temp. is lower Higher luminosity than it will have as a star, temp. lower, opacity is high (2-3) Luminosity decreases, core heats up, 8 million K fusion reactions begin (3-4) Intro to Stars Star Lives 22

23 Most of energy is from fusion => a STAR! ZERO AGE MAIN SEQUENCE STAR 50 million years to get there Where a star ends up on the ZAMS depends on its. More massive is hotter and more. luminous Less massive is cooler and more. luminous Intro to Stars Star Lives 23

24 Main Sequence phase is about 80% of a star s lifetime. This phase ends when almost all the H in the core has been converted to He. temperature in the core gradually increases greater flow of energy to the surface luminosity increases Intro to Stars Star Lives 24

25 Journey Off the Main Sequence H is used up in the core, fusion reactions in the core stop, but carry on yet in a shell around the core Core contracts (why?) Heats up the shell of burning H Reactions go faster Radius increases, surface temperature increases Star next becomes Intro to Stars Star Lives 25

26 Star moves towards the upper right on the H-R diagram CORE : compressed to an extreme density no longer behaves like a gas degenerate electron gas - pushes outwards with a pressure Balance is re-established New temperature is high enough for a new fusion cycle, He -> C - triple-alpha process Rapid ignition spreads quickly - He flash! (only a few minutes) Intro to Stars Star Lives 26

27 Star adjusts to its new fusion cycle radius decreases luminosity decreases He burning phase once again, stable The End is Near Core is eventually all carbon Fusion stops in the core, continues in the shell Star again expands - a red giant 500 million years 3000 K, 1000 L, 100 R Intro to Stars Star Lives 27

28 Triple-alpha processes varies things happen in bursts (thermal pulses) every few thousand years luminosity varies rapidly heavy elements created in the star are carried outward (superwind), ripping off the envelope, leaving only a hot dense core Expelled material leaves a shell planetary nebula - eventually dissipates Planetary nebula - the funeral wreaths of the stars Intro to Stars Star Lives 28

29 M57 The Ring Nebula Green - Oxygen and Nitrogen Red - Hydrogen APOD In the constellation Lyra (summer sky) Intro to Stars Star Lives 29

30 APOD New view of the Ring Nebula from the Subaru Telescope Intro to Stars Star Lives 30

31 The Helix Nebula NGC 7293 APOD 450 Light Years away in the Constellation Aquarius 41 arcminutes in diameter Intro to Stars Star Lives 31

32 Cometary Knots in The Helix Nebula APOD Nitrogen is represented as red, hydrogen emission as green, and oxygen as blue. Intro to Stars Star Lives 32

33 APOD The Cat s Eye Nebula Intro to Stars Star Lives 33

34 APOD M2-9, a butterfly planetary nebula Intro to Stars Star Lives 34

35 NGC 2440 APOD Intro to Stars Star Lives 35

36 The Eskimo Nebula APOD Intro to Stars Star Lives 36

37 APOD M 27 The Dumbbell Nebula Intro to Stars Star Lives 37

38 APOD PKS Intro to Stars Star Lives 38

39 End of a 1 M Star core is a corpse, no more contration, no more fusion - white dwarf (75,000 years) luminosity is low temperature is high eventually cools to a black dwarf (billions of years) Our Sun will expand to about 1.1 AU Earth s air will be ripped off Earth s mantle will be vaporized We ll all be crispy critters. Intro to Stars Star Lives 39

40 Stars of differing masses have different fates Lower mass than 1M : similar to that of a 1 solar mass star takes much longer 0.74 solar mass star - MS lifetime 20 billion years - longer than the age of the Universe! below 0.08 solar masses - never became hot enough for fusion (Jupiter-like objects, brown dwarfs) Intro to Stars Star Lives 40

41 Evolution of Massive Stars 5-10 M very different from Sun-like stars reach higher temperatures in the core while on the MS, burn H=>He via the CNO cycle MS lifetime is about 400 million years Higher temperature => fuse carbon and even higher elements He core does not become degenerate no He flash Intro to Stars Star Lives 41

42 Even more massive stars 20 M stars such as O stars can possibly develop a degenerate carbon core (about 14 solar masses) carbon ignition - blows apart the star in a catacylsmic event - supernova outer layers blast into space Intro to Stars Star Lives 42

43 Observational Evidence examine stars at various stages with different masses with different compositions clusters of stars : groups held together by their own gravity Intro to Stars Star Lives 43

44 OPEN CLUSTERS (Galactic Clusters) stars in a space a few 10 s of ly across as many as 20,000 in our Milky Way Galaxy distinctive H-R diagram family of stars all formed from the same cloud of gas and dust loosely bound gravitationally Intro to Stars Star Lives 44

45 The Pleiades 400 ly away 100 stars within a diameter ~ 10 ly APOD Distinctive H-R diagram for the cluster lower mass stars on the MS higher mass stars have turned off the MS Intro to Stars Star Lives 45

46 M 11 Wild Duck Cluster 3000 ly distant 150 million years old thousands of stars APOD Intro to Stars Star Lives 46

47 M ly away 20 ly across APOD Intro to Stars Star Lives 47

48 NGC 6791 An enigma! APOD Intro to Stars Star Lives 48

49 The Double Cluster NGC 869 and NGC 884. Also known as "h and chi Persei" APOD Intro to Stars Star Lives 49

50 GLOBULAR CLUSTERS 1,000,000 stars in a space ~ 100 ly in diameter at the center ~ 100 stars / cubic ly family of stars all formed from the same cloud of gas and dust tightly bound gravitationally distinct spherical shape, can see individual stars with a small telescope Distinctive H-R diagram MS turns to a red giant branch upper end of MS has disappeared horizontal branch coming back towards the MS Intro to Stars Star Lives 50

51 M 13 in Hercules over 100,000 stars over 150 light years across over 20,000 light years distant over 12 billion years old. APOD Intro to Stars Star Lives 51

52 M 3 APOD Intro to Stars Star Lives 52

53 47 Tucanae Closeup of its dense stellar center APOD Intro to Stars Star Lives 53

54 M 80 APOD Intro to Stars Star Lives 54

55 Stellar Populations Population I Stars found in Open Clusters bluish white (O, B supergiants) more luminous same chemical composition as the Sun (by mass) younger than Population II stars (i.e. of a newer generation) Population II Stars found in Globular Clusters reddish stars brightest are red giants much less metal in their composition than Pop. I older stars (i.e. of an earlier generation) Intro to Stars Star Lives 55

56 Quick model for origin of the Universe H and He created in the Big Bang with only traces of other elements All heavy elements we see today were made in the stars When a star dies, the heavy elements are blown off into the ISM (interstellar medium) Clouds form in the ISM, including these metals New stars form from these clouds The next generation of stars contains more metals than their predecessors Intro to Stars Star Lives 56

57 We know Pop. II stars formed earlier because they have less heavy elements. Population III star would be the original stars - made ONLY of H and He these have not yet been observed Intro to Stars Star Lives 57

58 Comparing Cluster H-R Diagrams Stars in a cluster are born at essentially the same time. They have differing masses which means?????. More massive stars evolve more rapidly than less massive stars. They become red giants more quickly. As the cluster ages, lower and lower mass stars move off the MS. Intro to Stars Star Lives 58

59 How far down the turn-off point is tells you how old a cluster is: lower down => older Globular cluster stars are, in general, older about 15 billion years old ± 2 billion years OLDEST stars known Open cluster stars are, in general, younger range 100 million years - 5 billion years Intro to Stars Star Lives 59

60 Variable Stars stars that are undergoing dramatic changes in their cores, luminosity varies dramatically, and over short periods of time On the H-R diagram, these lie in strips above the M-S. Vary regularly in the He burning phase. Intro to Stars Star Lives 60

61 RR Lyrae Stars vary in regular cycles of about 12 hours Pop. II stars (which means???) 100 L are about 5000 of them known Cepheid Variable Stars I period of 1-10 days II period of days Variation in luminosity comes from the star expanding and contracting - pulsating - star is unstable Intro to Stars Star Lives 61

62 Hot, Dim Stars core left after a red giant star blows off its outer layers - eventually cools to become a white dwarf Above and to the left of white dwarfs on the H-R diagram - HST measured one at 200,000 K Intro to Stars Star Lives 62

63 Synthesis of Elements To survive a star must fuse lighter elements into heavier ones to generate energy. More massive the star, the heavier the elements it can fuse. (Table 16.3) The waste product of one set of reactions becomes the fuel for the next set. Only the very massive can produce elements heavier than O, Ne, Mg, Na in their cores. There are only a few of these - these are responsible for distributing the heavy metals out into space for future stars to include in their formation. Intro to Stars Star Lives 63