The History and Philosophy of Astronomy
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1 Astronomy 350L (Fall 2006) The History and Philosophy of Astronomy (Lecture 16: Birth of Astrophysics I) Instructor: Volker Bromm TA: Jarrett Johnson The University of Texas at Austin
2 Big Q: What is the Nature of the Stars?
3 Can we ever know the `Physics of the Stars? (= astrophysics) Auguste Comte ( ) founder of `Positivism - real knowledge only due to hard facts, e.g., laboratory science, measurements claimed that we will never know the nature of the stars - distant stars are forever out of our reach - we cannot conduct laboratory experiments with them
4 Which questions would an astronomer have asked about the stars in the early 1800s? How far away are they (stellar distance scale)? What are the stars made of (stellar composition)? How massive are they? How long do they live (stellar lifetimes)? By what mechanism do they shine? Is the Sun just a (nearby) star? A: Yes, already widely believed (Descartes, Newton)
5 The Hunt for Stellar Parallax!
6 The Hunt for Stellar Parallax! Resolution
7 Measuring the Distance to the Stars Friedrich Wilhelm Bessel ( ) highly talented in astronomy and mathematics ( Bessel functions ).. director of Konigsberg Observatory 1838: First stellar parallax
8 Q: How to select promising candidates? possible criteria: 1) brightest stars 2) most rapid proper motion Bessel s choice!
9 Stellar Motions on the Sky d proper motion 1/d
10 Q: How to select promising candidates? 61 Cygni: the `flying star (5 arcsec per year)
11 Great Success: First Stellar Parallax (1838)! Bessel: 61 Cygni - 1/3 arcsec 10.3 Lightyears almost simultaneously: - Wilhelm Struve: Vega - Thomas Henderson: Alpha Centauri Bessel s heliometer
12 The True Brightness of the Stars what we measure: flux = energy/area (`apparent brightness ) if distance (d) to star is known, can figure out true (intrinsic) brightness = Luminosity (L) L = 4 x pi x d 2 x flux ( inverse-square law )
13 What stuff are the Sun and stars made out of? scrutinize the light that we receive!
14 The Message of Starlight (Newton 1666) white light is composed of different colors!
15 The Spectrum of the Sun Joseph Fraunhofer ( ) master optician and telescope builder 1814: Discovery of Spectral Lines in Solar Light (= `Fraunhofer lines )
16 The Spectrum of the Sun (1814) Fraunhofer lines: dark lines Sun prism
17 What are the Spectral Lines? Heidelberg in 1850s and 60s: Gustav Kirchhoff ( ) and Robert Bunsen ( ) - discover the `Laws of Spectral Analysis Robert Bunsen (`Bunsen burner )
18 What are the Spectral Lines? `Flame test (Spectral Analysis): - each chemical element has a distinct fingerprint!
19 What are the Spectral Lines? `Flame test (Spectral Analysis): - each chemical element has a distinct fingerprint!
20 What are the Spectral Lines? Periodic Table: Dmitri Mendeleyev (1869) `Flame test (Spectral Analysis): - Kirchhoff/Bunsen discover new elements (Rubidium, Caesium)
21 What are the Spectral Lines? The Laws of Spectroscopy: - dark lines = absorption lines - bright lines = emission lines
22 What are the Spectral Lines? Niels Bohr ( ) Bohr s quantum model of the atom (1913)
23 What are the Spectral Lines? Bohr s quantum model of the atom (1913): - emission and absorption lines!
24 Classifying the Spectra of the Stars
25 Classifying the Spectra of the Stars Father Angelo Secchi (Jesuit, ) first scheme to classify stellar spectra
26 Classifying the Spectra of the Stars great classification effort at Harvard College Observatory, beginning in 1880s
27 Classifying the Spectra of the Stars the `women computers of Harvard
28 Classifying the Spectra of the Stars Annie Jump Cannon ( ) master classifyer instrumental in publishing the Henry Draper Catalogue , ~ 225,000 stars: - each with spectral type and brightness
29 The Harvard Sequence of Spectral Types Traditional mnemonic: Oh, Be A Fine Girl, Kiss Me! arranged in order of decreasing temperature on the surface of the star
30 Spectral Type and Temperature Red lower Temperature, blue higher T
31 Classifying the Spectra of the Stars
32 Chemical Composition of the Stars Cecilia Payne-Gaposhkin ( ) Harvard PhD 1925 hydrogen and helium are most abundant elements in the universe!
33 Chemical Composition of the Stars measured strength of spectral line (S) = abundance (A) x transition probability (P) -Metal lines (e.g., Ca): S= a x P -hydrogen lines: S= a x P Hydrogen is most abundant element!!!
34 Birth of Astrophysics (part 1) Measuring the Distance to the Stars: - Friedrich Wilhelm Bessel : First successful stellar parallax - 61 Cygni: 1/3 seconds of arc 10 lightyears - the greatest triumph which astronomy has ever witnessed (John Herschel) Figuring out the composition of the stars: - spectral analysis (absorption and emission lines) - Harvard classification: stars can be grouped according to spectral type (and thus surface temperature) - OBAFGKM - Hydrogen and Helium are most abundant elements in the Sun and the stars (Cecilia Payne)
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