• Théory: 12 chapters • Exercises: ± 20 min talk on a chosen subject • Exam: oral, 2 questions

1. Astrophysics • Théory: 12 chapters • Exercises: ± 20 min talk on a chosen subject • Exam: oral, 2 questions • Final note: 2/3 exam + 1/3 talk

2. Structure of the course 1. The birth of astronomy 2. The solar system 3. Basic concepts in astrophysics 4. Astronomical observations 5. Energy sources 6. Interstellar matter and stellar birth 7. Stellar evolution 8. Life in the Universe 9. The Milky Way 10. Galaxies 11. The extragalactic Universe 12. Cosmology

3. The birth of astronomy • The Universe in prescientific civilisations • Astronomy, daughter of astrology • Astronomy in antique Greece • The heliocentric world

4. The Universe in prescientific civilisations Genesis 1.14 And God said, Let there be lights in the arch of heaven, for a division between the day and the night, and let them be for signs, and for marking the changes of the year, and for days and for years; 1.19 And there was evening and there was morning, the fourth day. The biblical world : (influenced by Babylon) Flat earth, floating on waters Firmament supported by pillars (mountains) The whole inside waters of heaven

5. The Universe in prescientific civilisations - 2 Creation of the world in babylonian mythology The female monster Tiamat (primordial chaos) is killed by Marduk (god of thunder) Tiamat is cut in two pieces: one half forms earth and the other forms heaven The blood of Tiamat’s partner gives birth to men Men’s mission : serve the gods

6. The Universe in prescientific civilisations - 3 Creation of the world in egyptian mythology (Heliopolis version ) Atum (primordial god) gives birth to Chu et Tefnu Chu (god of air) and Tefnu (goddess of humidity) give birth to twins: Geb and Nut Chu (god of air) separates Nut (goddess of heaven) from Geb (god of earth), → birth of the world as we know it

7. The Universe in prescientific civilisations - 4 Magical interpretation of events Example: the daily cycle of the Sun corresponds to the trip of the god Rê in the heaven, on his ‘million years boat’ During night, Rê enters the underworld where he has to fight against the forces of darkness; victorious, he rises again The unexplained phenomena are interpreted as individual actions of deities No ‘natural laws’

8. Astronomy, daughter of astrology Heaven/sky, domain of the gods • gods live in heaven • men’s life is subject to the whims of gods → watch the sky to find signs of men’s destiny All celestial bodies keep the same relative positions except: – the sun – the moon – the 5 wandering stars (planets) → their positions are signs

9. Astronomy, daughter of astrology- 2 The zodiac The ancients located positions in the sky with respect to arbitrary stellar groupings that seem to draw easily recognizable pictures: the constellations The apparent motion of the Sun and planets takes place in a zone of the celestial vault named zodiac That zone has been divided in 12 constellations (of slightly adapted size) corresponding to the 12 months in a year (one month ~ one lunar cycle)

10. Astronomy, daughter of astrology- 3 Babylonian and greek astrologies For the babylonians, the position of planets did influence the destiny of kings → it was important to predict the motion of sun, moon and planets in order to: – know their configuration at the time of king’s birth – predict their future positions → birth of astronomy The greeks adopt the babylonian ideas but generalize them to all people

11. Astronomy, daughter of astrology- 4 Precession of the equinoxes Planet earth is not perfectly spherical Solar attraction on the equatorial bulge causes an oscillation of Earth’s rotation axis with a period of 26000 years, around the perpendicular to the orbital plane (ecliptic) The equatorial plane also rotates → the intersection between the equatorial plane and the orbital plane also rotates → the constellations of zodiac shift by one astrological sign every 26000 / 12 = 2170 years, which is not taken into account by the astrologers

12. Astronomy in ancient Greece Thales of Miletus The ‘first scientist’ born ca. 625 BC Q : How is the world made? A : The first and basic principle of all things is water (common element that can be found in the 3 phases: solid, liquid, gas) Imagines the Earth as a disk floating on waters

13. Astronomy in ancient Greece - 2 Anaximander Student of Thales, born around 610 BC Replaces the single element of Thales by the 4 elements: – water – earth – air – fire + Earth is not floating on waters but is suspended in pace, ‘equally distant from all things’

14. Astronomy in ancient Greece - 3 Plato Born around 430 BC For him, true knowledge is acquired by reason (eye of the soul) and not by observation (eye of the body) Heavenly bodies must be perfect → they must move along perfect, immutable orbits The perfect geometric forms and the sphere and the circle The circular motion of heavenly bodies being perfect, it can go on forever

15. Astronomy in ancient Greece - 4 Eudoxus Student of Plato, born around 410 BC Imagines Universe as concentric spheres (Eudoxus spheres) Earth at world’s center Each sphere rotates with its own speed Only approximately explains the planetary motions

16. Astronomy in ancient Greece - 5 Eratosthenes Alexandria, 3rd Century BC Determines circumference of Earth June 21st at noon, the Sun is straight above Syene However, at Alexandria, its rays make a 7° angle with vertical Distance between Alexandria and Syene : 5000 stadia → circumference of Earth: 5000 × 360 / 7 ≈ 257 000 stadia Historians thinks that one stadium = 157.5 m → circumference = 40 500 km!

17. 7° Alexandria d → 7° Syene Astronomy in ancient Greece - 6 Did Eratosthenes prove that Earth is spherical? Eratosthenes model: spherical Earth, Sun very far away

18. 7° D 7° d Alexandria Syene Astronomy in ancient Greece - 7 Alternative model: flat Earth, nearby Sun d / D = tg 7° → D = d / tg 7° ≈ 40 000 stadia ≈ 6400 km

19. d D θ θ e Astronomy in ancient Greece - 8 Hipparcos (2nd Century BC) Determines Earth-Moon distance Max. duration of lunar eclipse: 2.5 h Moon synodic period: 708 h 2πD/e = 708/2.5 → D/e = 45 2θ = 0.5° = 1/114 rad (ΦSun) e + 2θD = d (1/45 + 1/114) D = d D = 32 d Modern value: D = 30 d

20. Astronomy in ancient Greece - 9 Retrograde motion of planets Just like the Sun and stars, planets rise East and set West They seem to move slightly faster than the stars → their Eudoxus spheres rotate faster However, one some occasions, a planet seems to move more slowly → moves back with respect to stars: retrograde motion How can it be reconciled with uniform circular motion?

21. epicycle deferent Astronomy in ancient Greece - 10 Ptolemy Born in Alexandria around 90 AD Modifies the Eudoxus system to explain the retrograde motion Each planet moves on a circle called epicycle The centre of the epicycle moves on another circle called deferent Earth is the center of deferent → reproduces the retrograde motion, with one epicycle and one deferent for each planet

22. epicycle equant deferent Astronomy in ancient Greece - 11 Ptolemy (2nd act) The original Ptolemy system does not reproduce accurately the measurements of Hipparcos (variation of angular velocity) → Ptolemy complexifies it to better match the observations: – the deferent centre is shifted with respect to Earth – the circular motion is uniform with respect to a point named equant, symmetrical to Earth with respect to the deferent centre

23. Astronomy in ancient Greece - 12 Unexplained coincidences • The centres of the Mercury and Venus epicycles are on the Earth-Sun line • For Mars, Jupiter and Saturn: `radius´ of epicycle parallel to the Earth-Sun line → tendency of planets to position with respect to the Sun

24. Astronomy in ancient Greece - 13 The legacy of the Greeks + recourse to reason and not to myths or revealed truths; freedom of thought – minor role of observation + they knew Earth is spherical (forgotten later) + might even have suggested Earth moved around the Sun (Aristarchus of Samos, 3rd Century BC) – the belief into `perfection´ of celestial phenomena (→ circular motions) blocked any progress in astronomy (and science in general) for more than 1000 years → contrasted legacy

25. The heliocentric world Nicolaus Copernicus (1473 – 1543) Born in Torun in a wealthy family, studies 10 years in Italy → gets in contact with `new ideas´ Back in Pologne, canon in Frauenburg cathedral Studies the texts of Ptolemy Builds a small observatory in a tower Uses the same measurements of planet positions as Ptolemy Shows that they can be interpreted in another way

26. The heliocentric world - 2 Copernicus world Central Sun Earth and other celestial bodies (Moon excepted) revolve around the Sun Circular orbits Simply explains the retrograde motion Does not account accurately for the Hipparcos measurements → reintroduces epicycles

27. The heliocentric world - 3 How to choose between Ptolemy and Copernicus ? In favour of Ptolemy: • tradition (mostly religion) • common sense : if Earth moved, we would feel it (but Nicolaus de Cusa (1450) : passenger inside a ship) • lack of stellar parallax In favour of Copernicus: • simpler explanation of retrograde motion • decreasing amplitude of the Mars – Jupiter – Saturn retrogradations Ex-aequo : • similar complexity level • similar accuracy ( ≈ 5°)

28. The heliocentric world - 4 Tycho Brahe (1546 – 1601) Danish aristocrat, studies philosophy at university but is mostly interested in mathematics During an eclips, he is strongly impressed by the fact that such events can be predicted → studies astronomy 1572 : a Nova is observed in the Cassiopeia constellation Change in the heavens → contradiction with the ideas of the Greeks → attempt to measure its motion (celestial or atmospheric phenomenon?) Lack of accuracy → contradictory conclusions

29. The heliocentric world - 5 Tycho Brahe (2) Builds a 5½ feet sextant → shows that the Nova does not move → established reputation; King Frederick II of Denmark grants him a large sum of money + the Hven island where he builds an observatory During 20 years, Tycho: • holds court at the palace of Uranienborg • carries out measurements with an accuarcy never achieved before After the death of Frederic II, Tycho’s character causes troubles with the new king → exiles in Prague in 1597

30. The heliocentric world - 6 Johannes Kepler (1571 – 1630) Exiled in Prague because of religions wars Hired as assistant by Tycho Brahe, for analyzing his planetary positions measurements Believed that there was some sort of overall scheme in the Universe Spent a large part of his life searching for that scheme, which would reveal the ultimate beauty of nature

31. The heliocentric world - 7 First model of Kepler Based on the fact that 6 planets and 5 regular solids were known Heliocentric The 5 regular solids fit the space in between the 6 planetary spheres

32. f1 f2 The heliocentric world - 8 Kepler’s laws (1) Analysis of Tycho measurements → Kepler rejects both geocentrism and orbits based on circles and discovers 2 empirical laws (1609) 1st law: Planets move on elliptical orbits with the Sun at one focus of the ellips 2nd law: The line drawn from the Sun to the planet sweeps equal areas in equal times

33. b a The heliocentric world - 9 Kepler’s laws (2) Ten years later, he publishes his 3rd law: The square of the period T of a planet is proportional to the cube of the semi major axis a of its orbit 3rd law: T2 / a3 = Ct Contrary to the models of the Greeks, the Kepler laws are based on a careful and detailed analysis of observations

34. The heliocentric world - 10 Galileo Galilei (1564 – 1642) Born in an unwealthy family of italian minor nobility Reads about the invention of the telescope, builds one for himself and turns it toward the heavens → discovers: • mountains on the Moon • sunspots • phases of Venus • 4 satellites of Jupiter → challenges for the platonician / geocentric system

35. The heliocentric world - 11 Galileo’s problems with catholic church Becomes a strong advocate of the heliocentric system Gifted writer, in italian → popularizes that world model Dialog concerning the two world systems (1630) • Salvatio (partisan of Copernicus) • Simplicio (partisan of the greek system) • Sagredo (the one who seeks truth) Puts into Simplicio’s mouth many arguments advanced by the pope → Trial: Galilée, old and sick, is force to recant the heretical doctrine that the earth is moving

36. The heliocentric world - 12 Isaac Newton (1642 – 1727) Born in a relatively wealthy english family Studies natural philosophy at Cambridge university 1665 – 1666 : epidemy of pest Newton isolates himself in Woolsthorpe and invents or discovers: • the differential and integral calculus • the theory of colours • the theory of universal gravitation

37. The heliocentric world - 13 Philosophiæ Naturalis Principia Mathematica (1687) Newton shows that the whole mécanics can be deduced from a few basic principles: • the 3 laws of motion including the fundamental law of mecanics : F = m a • the law of universal gravitation

38. The birth of astronomy • The Universe in prescientific civilisations • Astronomy, daughter of astrology • Astronomy in antique Greece • The heliocentric world End of chapter…