Astronomy 1 Fall 2016

Transcription

1 Astronomy 1 Fall 2016 Lecture11; November 1, 2016

2 Previously on Astro-1 Introduction to stars Measuring distances Inverse square law: luminosity vs brightness Colors and spectral types, the H-R diagram Masses of stars

3 Today on Astro-1 Massive stars are young stars Mass determines lifetime of a star. Star formation Where do stars form? How do stars reach the Main Sequence? How do protostars transition to the Main Sequence? Where do new stars form within galaxies? When did most of the stars in the Universe form?

4 The mass of stars determines their lifetimes

5 All main-sequence stars are made of the same stuff So why are more massive stars more luminous? Answer: They re turbocharged

6 H-R diagram with masses Greater mass means greater central pressure & temperature and so greater luminosity... and so shorter lifetime A star with 60 times the mass of the Sun... has 60 times as much nuclear fuel as the Sun but burns it 10 6 times as rapidly!

7 Stellar Mass Determines Main Sequence Lifetimes of Stars L L 0 (M/M 0 ) 3.5 Lifetime = Fusion Energy / (Luminosity) t 12 Gyr (M/M 0 ) -2.5 Massive stars lead short lives.

8 Stellar Lifetimes as a Tool for Understanding Galaxies & the Universe Massive stars lead short lives. 25 M 0 star burns up its core H before it has orbited around the center of the Milky Way galaxy even once. Regions of the galaxy with massive stars mark sites of recent star formation. The lowest mass stars have not had time to evolve off the main sequence M 0 star requires 25 Gyr to burn up its core H. We don t expect to find post main-sequence stars of this mass in a Universe that is 13.8 Gyr old.

9 Approximately how long will a 3-solar-mass star spend on the main sequence? A. 3 times the Sun s main-sequence lifetime B times the Sun s main-sequence lifetime C times the Sun s main-sequence lifetime D times the Sun s main-sequence lifetime E times the Sun s main-sequence lifetime Q19.2

10 Approximately how long will a 3-solar-mass star spend on the main sequence? A. 3 times the Sun s main-sequence lifetime B times the Sun s main-sequence lifetime C times the Sun s main-sequence lifetime D times the Sun s main-sequence lifetime E times the Sun s main-sequence lifetime A19.2

11 Where do stars form?

12 Orion Nebula is in an Interstellar Cloud Contains about 300 M 0 of gas and stars Illuminated by the five most massive stars UV light excites the atoms, which fluoresce

13 Emission Nebulae: Clouds of Excited Gas

14 Emission, Reflection, and Dark Nebulae in Orion

15 Dark Nebulae are Opaque Barnard 86 is about 1/7 th the angular diameter of the full moon. Dust completely blocks optical starlight. Radio waves pass right through. Cold. T = 10 K. Do dark nebulae emit light?

16 Dust also reflects and scatters light Reflection nebulae scatter and reflect light from the stars they surround

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18 Midplane of the Milky Way Galaxy

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20 Star-forming regions in another galaxy

21 From protostars to the Main Sequence

22 Stars Form within Dense Dark Nebulae (or Bok globules) About 1-3 pc across ~1000 M 0 Illuminated by nearby stars (of higher mass)

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24 Protostars collapsing gas cloud releases energy to make protostar shine collapse stops when fusion begins establishes equilibrium. The minimum mass to ignite fusion is 0.08 solar masses.

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27 The Clumps in Molecular Clouds Have a Range of Masses Evolution depends on mass

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31 Mass Loss from a Supermassive Star

32 Feedback from a massive star on its environment

33 Mass Loss and Accretion: T Tauri Stars (M < 3 M 0 )

34 Disks Enable Collapse by Transporting Angular Momentum

35 The Jets Feedback on the Environment

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38 Question (iclickers!) How does the temperature of an interstellar cloud affect its ability to form stars? A) Star formation is so complicated that it is not possible to say how one quantity, such as temperature, affects it B) Higher temperatures inhibit star formation C) Higher temperatures help star formation D) Star formation is independent of the temperature of the cloud

39 Question (iclickers!) How does the temperature of an interstellar cloud affect its ability to form stars? A) Star formation is so complicated that it is not possible to say how one quantity, such as temperature, affects it B) Higher temperatures inhibit star formation C) Higher temperatures help star formation D) Star formation is independent of the temperature of the cloud

40 Question (iclickers!) The major source of energy in the pre-main sequence life of the Sun was A) gravitational B) nuclear fusion C) chemical burning of carbon atoms D) nuclear fission

41 Question (iclickers!) The major source of energy in the pre-main sequence life of the Sun was A) gravitational B) nuclear fusion C) chemical burning of carbon atoms D) nuclear fission

42 Age-dating star clusters with the H-R diagram

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46 Star Formation Produces a Range of Relative number of stars Stellar Masses The minimum stellar mass is about 0.08 M 0. Why? The maximum stellar mass is roughly 100 M 0. Why? All star clusters show roughly the same distribution of stellar masses. Low mass stars are much more common than high mass stars.

47 Triggering Star Formation

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50 When did most of the stars in the Universe form?

51 Cosmic Star Formation History TODAY Sun formed Peak activity Gyr ago

52 Summary The Main-Sequence is a mass sequence High mass stars live fast and die young Stars form in clouds of cold gas, collapsing under gravitational instability Protostars are heated by gravitational collapse and often form disks and jets around them H-R diagrams can be used to age-date star clusters. Galaxies form stars in spiral arms Most of the action happened 7-9 x 10 9 years ago

53 Homework (Due Thursday 11/03/16) Do all review questions from chapter 18 on your own. For TAs, do How much does interstellar dust block starlight? review review speed = distance / time