Astrophysics-I

1. Astrophysics-I • Fiz463e • CRN 10419 • Thursday 9-12.00 / Y5 • There will be 1 midterm • Many quizes • Many reading material • Follow the website.

2. Contents of Astrophysics I • Basic concepts in astrophysics • Astronomical distance scale • Observational instrumentation • Solar system (Sun, Moon, Planets, Comets and asteroids) • Stellar structure and evolution

3. Contents of Astrophsics II • Compact objects (white dwarfs, neutron stars and black holes) • Magnetohydrodynamics (MHD) and plasma physics • Galactic dynamics • Cosmology • Accretion processes around compact objects

4. Lecture I-History of Astronomy • Celestial Sphere=Gökküre=Felek (Çoğ:Eflak) • Planets(=Gezegen=Seyyare) and Their Retrogade Motion • Aristotelian World View • Ptolemian Astronomy • The Copernican Revolution-The Earth is a Celestial Object! • Galileo-The First (or the second after Gilbert) Scientist • Universal Law of Gravity • The Chemistry of Stars-How the spectroscopy challenges Aristotle.

5. Apparent Daily Motion of Stars Around the Earth Diurnal Motion

6. Diurnal Motion Stars move East to West as the Earth rotates West to East. They cover 15 degrees per hour which amounts to 360 degrees per day!

7. Ancient World View • Ancient people have been observing the sky and were well aware this circular trajectory of the stars. • Ancient people believed that the Earth was at the center of the Universe, motionless and non-rotating. • For the ancient people the circular trajectories of the stars were not just an apparent motion but was real, the stars were indeed rotating around the Earth.

8. Star Trails on the Equator (Kenya)

9. Constellations Preserve Their Form • Ancient people also noticed that the constellations(e.g. Ursa Major=büyükayı)preserve their formduring diurnal rotation, hence they concluded that... • The stars are not rotating by their own but they are fixed onto a sphere and the diurnal rotation of the stars is due to the rotation of this sphere. • This sphere is called the celestial sphere.

10. Constellations Constellation=takımyıldız The stars in the constellations are not physically close but their projections onto the celestial sphere appear to be close. Constellations help to locate objects on the sky.

11. These are stars physically close to each other. Globular Cluster

12. Celestial Sphere is still a useful conceptbecause we only see the projections of celestial objects on such a fictitious sphere. • Measuring the distances is a hard problem of astronomy (see Next Weeks lesson). Celestial Sphere=Gökküre=Felek (Çoğ:Eflak)

13. Seven Objects not Fixed to the Celestial Sphere • For the ancient people the celestial sphere was a real object! • All stars were fixed onto this sphere but there were 7 objects moving independent of the celestial sphere. • These are the 5 planets that can be identified by the naked eye (Mercury, Venus, Mars, Jupiter, Saturn), the Moon, and the Sun. • These objects are all located along the ecliptic and appear to be not fixed on the celestial sphere.

14. Retrogade Motion of Mars

15. Understanding the Retrogade Motion

18. Modern Science, to a large extend, was born out of the human desire to explain the retrogade motion of the planets.

19. Planets • Planetes means wanderers in Greek. • Planet = Gezegen = Seyyare • Notice they all carry the same meaning referencing their apparent wandering with respect to the “fixed” stars. • For the ancient people planets were gods and they gave their name to each day of the week.

20. Days of the Week http://en.wikipedia.org/wiki/Days_of_the_week Astronomical Names for the Days of the Week,Falk, M.., 1999, J. of the Royal Astron. Soc. of Canada, Vol. 93, p.122

21. Aristoteles (M.Ö.384-322) • The Earth is a sphere at the center of a spherical universe • The Moon, Merkury, Venus, the Sun, Mars, Jupiter, Saturn and fixed stars each rotate around their own spheres. • These spheres are made of crystal (so that they are not seen)

22. The 7 Spheres Above the Earth Below the Celestial Sphere 7 katlı gök

23. Felek • Feleğin tekerine çomak sokmak • Felekten bir gün çalmak • Feleğin çemberinden (çeperinden?) geçmek.

24. Aristoteles... • Dominated the philosophy all throughout the medieval ages. • The research at that time simply meant finding out what Aristoteles said about that research topic. No experiment, no questioning.

25. Today... There can be different models on any observed phenomena but there is no a priori authority accepted to be true without question. The validity of models is checked by experiment and/or observation whoever the proposer of the model is. The reference of truth is expriment. Hence we select between the models by asking the nature.

26. Aristoteles: Objects on Earth and Celestial Objects are composed of Different Elements • Objects on Earth (everything below the sphere of the Moon) are a mixture of 4 elements: Earth, Water, Air and Fire. Such objects are subject to change, decay and/or death and are defected. • Celestial Objects are made from a fifth element (Ether). Such objects are defectless/perfect, and eternal. They are not subject to any change. • Aristoteles was a student of Platon and envisaged the world of ideas in the sky.

27. Aristoteles: Objects on the Earth and the Celestial Objects Obey Different Laws • Each element has a natural place determining its natural motion:Earth belongs to the Earth. The natural place of Water is arround earth. Natural place of Air is above Earth & Water. And Fire is to be above the Air. A stone falls down because it belongs to the Earth. Fire tends to rise up because it wants to reach the greatest fire (the Sun), the bubbles in water rise up because air is to be above water, etc. • Apart from the natural motions there are forced (violent) motions. One has to apply force in order to keep objects in motion: The card stops when the horse stops. • Heavier objects fall more rapidly than the lighter objects. • Celestial objects eternally follow circular trajectories. They do not change their speed during this motion. • Each celestial object rotates around the Earth.

28. Comets According to Aristoteles • The celestial objects are eternal and the sky is not subject to change. • Hence comets must be inside the sphere of the Moon, i.e. they are atmospheric events.

29. Aristarchus of Samos (310-230 BC) • Proposed the heliocentric model • This was not widely accepted because… • …the parallax can not be measured with naked eye. • …the rotation of the Earth would throw us away. • …the Earth would leave behind the Moon

30. Summary: For the Ancient People... • Objects on the Earth and the Celestial Objects have different structures. • Celestial objects are perfect while the objects on the Earth are defected and are subject to decay. • Different laws in the sky and on the Earth. • The Earth is at the center of the Universe (obviously!) • Force is needed to keep objects in motion.

31. Flat Universe? The Universe of the ancient people was very small Though this picture is misleading because humans have been believing that the Earth is spherical since long time. It is a 19th century misconception to think that the ancients believed the Earth was flat.

32. Hipparchus & Epicycles • Retrograde motion of planets could be explained by a combination of circular motions; • the planet moves in a small circle called an epicycle • the centre of the epicycle moves around a larger circle called the deferent • If the planet moves around the epicycle faster than the epicycle moves around the deferent, retrograde motion will occur in some regions of the orbit

33. Ptolemy=Batlamyus (Claudius Ptolemaeus-140AD) • Ptolemy expanded upon Aristotles geocentric model to predict the motion of planets accurately. • Following Hipparchus, he assumed that planets moved around circular epicycles. The centres of the epicycles moves around the Earth in circular deferents. • He added a number of refinements to the old model to obtain better agreement with observations. In particular, he offset the centre of the deferent from the centre of the Earth!

34. Ptolemy’s Epicycles

35. Ptolemaic System • Mathematike Syntaxis (13 Volume book of Ptolemy) • Arabic Scientists loved the book and named it al-kitabu-l-mijisti, i.e. "The Great Book" • Today this book is called “Almagest” because of this.

36. Ptolemaic Model of the Universe

37. The Ptolemaic Model • Ptolemy was able to predict the motions of the seven celestial objects to great accuracy by introducing more and more epicycles (equivalent to Fourier analysis) • Ptolemy’s model had been used nearly for 1500 years by the western and eastern astronomers.

38. Nasir al-Din Tusi (1201–1274) • Resolved significant problems in the Ptolemaic system by developing the Tusi-couple as an alternative to the physically problematic equant introduced by Ptolemy.

39. Tusi-Couple • A mathematical device in which a small circle rotates inside a larger circle twice the radius of the smaller circle. • Rotations of the circles cause a point on the circumference of the smaller circle to oscillate back and forth along a diameter of the larger circle. Tusi's diagram of the Tusi couple (Vatican Arabic ms 319, fol. 28v; 13th. c.)

40. Size of the Earth Comparable to the size of the Heavens The rest of the Universe was comparable to the size of the Earth.

41. Nicolaus Copernicus (1473-1543) • “Who in this most beautiful of temple would put his lamp at a better place than from where it can illuminate them all?. Thus the Sun sitting as on a Royal throne, leads the surrounding family of stars!”

42. Copernican Model • The Sun is at the center of the Universe (not just the Solar System) • The orbits are circular (and still there are epicycles though they are significantly less than the geocentric model) • The crystal spheres are still there.

43. Tycho Brahe (1546-1601) • Observed a supernova explosion (1572). • He thought this was a new star. • Surprised because he thought the celestial objects are eternal and the sky does not change.

44. Stella Nova • The image in this page is from Tycho Brahe's "Stella Nova“. http://www.texts.dnlb.dk/DeNovaStella/Index.html

45. Tycho Brahe and Comets • Observed a comet • Made collaborations with an observer at a different location to find that the comet was nearly in the same position with respect to the background stars for both observers (i.e. no parallax). • Concluded that the comet was at least six times farther away than the Moon.

46. Tycho Brahe and the Crystal Spheres • Tycho Brahe also understood that the comet must have passed through the spheres. • So the crystal spheres of Aristoteles can not be real!

47. Around the Same Time in Istanbul! • Takiyuddin founded the Istanbul Observatory in Tophane (1577) • He had similar instruments as with Tycho Brahe. Some of the measurements of Takiyuddin are even more precise. • However, Takiyuddin wasn’t able to continue his observations as long as Tycho did.

48. Galileo (1564-1642) • The Second Scientist after Gilbert • A strong refuter of the Aristotelian world view.

49. Galilei Supernova • In 1604 he observed the SN studied by Kepler. • He thought this was a new star. • The new star showed no motion accross the sky compared with the other stars (i.e. No parallax) • Gave series of well recieved lectures arguing that it must be as far away from the Earth as the other stars. • This refutes the Aristotelian notion of an unchanging celestial sphere.

50. Galileo’s Poem for the New Star No lower than the other stars it lies And does not move in other ways around Than all fixed stars-nor change in sign or size. All this is proved on the purest reasons ground; It has no parallax for us on Earth By reason of the sky’s enormous girth.