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 SphereGökküreFelek (ÇogEflak)
• Planets (GezegenSeyyare) 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
Diurnal Motion
Apparent Daily Motion of Stars Around the Earth
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 Majorbüyükayi)
  preserve their form during 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
Constellationtakimyildiz
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
Globular Cluster
These are stars physically close to each other.
12

• Celestial Sphere is still a useful concept
  because 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 SphereGökküreFelek (ÇogEflak)
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
16
(No Transcript)
17
(No Transcript)
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
Saturday Saturn
Sunday Sun
Monday Moon
Tuesday Tiw?Mars
Wednesday Odin?Mercury
Thursday Tor ? Jupiter
Friday Frie ? Venus
http//en.wikipedia.org/wiki/Days_of_the_week Astr
onomical 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 katli gök
23
Felek

• Felegin tekerine çomak sokmak
• Felekten bir gün çalmak
• Felegin 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
PtolemyBatlamyus (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
Ptolemys 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)
• Ptolemys model had been used nearly for 1500
  years by the western and eastern astronomers.

38
Nasir al-Din Tusi (12011274)

• 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 wasnt 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
Galileos 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 skys
enormous girth.
51
A Celestial Object Defected?

• Directed his telescope to the sky (1609).
• The Moon is not perfect! It has craters which are
  defects.

52
(No Transcript)
53
...Galileo

• Discovered Jupiters moons.
• This implies that (independent of whether the
  geoentric or heliocentric model is true) not
  every celestial object rotates around the Earth.
• This also obviates the argument against the
  Copernican sytem that if the Earth rotated around
  the Sun then the Earth and the Moon would get
  separated from one another.

54
Galilei observed the Phases of Venus

• Heliocentric Model All phases should be visible
• Geocentric Model Only crescent and new phases
  would be seen

55
Galilei and the Milky Way
As seen with the telescope Milky Way is a myriad
of individual stars.
56
Aristotelian Response

• Aristotelians refused to accept that what was
  seen through the telescope was real. (Some of
  them even refused to look through the telescope
  saying it is simply a nonsense device).
• Galileo himself tested the possibility by
  observing hundreds of objects to see if the
  instrument does anything except magnify.

57
The Sun also is not Perfect (Galilei 1613)
58
Galileo's thought experiments and real
experiments

• falling bodies
• according to Aristoteles, heavy bodies (contain
  more earth element) fall faster than lighter
  bodies
• observation fall equally fast if they have same
  shape and size
• Galilei difference in speed of differently
  shaped falling bodies due to air resistance
• thought experiment about two falling bodies -
  reductio ad absurdum
• consider two bodies, one light (L), one heavy (H)
  
• Aristoteles L falls more slowly than H ? L put
  under H should slow down fall of H ? H with L
  under it should fall more slowly than H alone
  but (L H) heavier than H alone ? should fall
  faster than H alone ? contradiction.
• ball rolling on inclined plane
• ball rolling down inclined plane speeds up
• ball rolling up slows down rate of slowing down
  depends on steepness of incline less steep ?
  longer distance travelled extrapolation to zero
  slope of incline ball will go on forever

59
Achievements of Galileo

• founder of modern science
• new methods introduced by Galilei include
• controlled experiments designed to test specific
  hypotheses
• idealizations to eliminate any side effects that
  might obscure main effects
• limiting the scope of enquiry - consider only one
  question at a time
• quantitative methods - did careful measurements
  of the motion of falling bodies.
• from observations and thought experiments,
  generalizes to two new laws
• LAW OF INERTIA
• without external influence (force) acting on it,
  a body will not change its speed or direction of
  motion it will stay at rest if it was at rest to
  begin with.
• inertia property of bodies that makes them obey
  this law, their ability to maintain their speed
  (or stay at rest)
• About FALLING OBJECTs
• if air resistance is negligible, any two objects
  that are dropped together will fall together
  speed of falling independent of weight and
  material.

60
Three Quotations from Galileo

• In questions of science the authority of a
  thousand is not worth the humble reasoning of a
  single individual.
• Instead of philosophical arguments, I prefer to
  discover a single fact.
• What has philosophy got to do with measuring
  anything?

61
Kepler

• If the celestial objects are not perfect their
  orbits also may not be perfect.
• Then introduce elliptic orbits into the
  Copernican system.
• Keplers laws for the motion of planets.

62
Newton (1687)

• Gravitational attraction is between all bodies.
• The force that keeps the Moon in orbit is the
  same force that causes the apple to fall down.
• Objects on the Earth and the objects in the sky
  obey the same laws.

63
New Mechanics

• Aristotelian view forces cause velocity
• (force necessary to maintain uniform
  motion).
• Newtonian view forces cause acceleration
  (force necessary to change motion)

64
Bessel (1838)

• Successfully measured the parallax of the star 61
  Cygni.
• This was considered conclusive evidence that the
  Earth was in motion.

65
Objects on the Earth and Celestial Objects are
made of same kind of elements

• Spectroscopy discovered in the 19th century
• Using spectroscopy astronomers understood that
  stars are made of mainly hydrogen and some other
  elements like Carbon and Oxygen that also make up
  the Earth.
• This is the last blow to the Aristotelian World
  View.

66
Short History

• Tycho Brahe supplied the accurate data about the
  motion of the planets.
• Kepler analysing the data arrived at the Keplers
  Laws.
• Newton, trying to explain Keplers laws found
  the general laws of motion which are not only
  valid in the solar system but can also be applied
  to many different phenomena.
• This was the physics behind the industrial
  revolution which took the Western civilization
  ahead of the Eastern world.
• The history unfolded differently in the Eastern
  world.

67
The Fate of Takiyuddins Observatory in Istanbul

• Takiyuddin (1526-1580) came to Istanbul in 1570
• Convinced the Ottoman King (III.Murat) to build
  an Observatory.
• By observations Takiyuddin was to prepare a zic
  in the name of the King.
• With this renewed catalogue whorshipping times
  would be more accurately determined.

68
Istanbul Observatory (1577)
69
A comet at November 1577
70
Black Death in 1578

• Gossip that all this evil arising because they
  have been observing the legs of the angels in the
  observatory
• Seyhülislam (Ahmed Semseddin Efendi) Observing
  the legs of the angels from the ground with pipes
  is a grand sin.

71
Black Death and the Seyhülislam continues

• SeyhülislamIhrac-i Rasad mes'um perde-i esrari
  felekiyeye küstahane itlak-i cür'etin vehamet ve
  akibeti meczumdur. Hiçbir mülkde mübaseret
  olunmadi ki ma'mur iken harap ve bünyan-i devleti
  zelzelenak-i inkilab olmaya.
• Observation brings illomens. The serious danger
  in attempting to understand the secrets of the
  celestial spheres should be obvious. In no
  country where observation is conducted there has
  ever been a case that the strong state had not
  been destroyed and the structure of the state was
  not shaken.

72
The End of the Observatory

• The king was scared and wanted the observatory to
  be broken down.
• This was done by Kiliç Ali Pasa in 1580 just
  after 3 years that the observatory started
  working.
• The second attempt to build a telescope in
  Ottoman Empire was in 1873 afer about 300 years.
• This also was broken down by the softas in 31
  Mart event in 1909.
• Kandilli observatory was built in 1911 by Fatin
  Gökmen.

73
End of the Lecture

• Be sure to read some of the material on the web.
• http//www.fizik.itu.edu.tr/eksiy/astro463/astro46
  3.html