• CRN 10419
• Thursday 9-12.00 / Y5
• There will be 1 midterm
• Many quizes
• Many reading material
• Follow the website.
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
Contents of Astrophsics II
• Compact objects (white dwarfs,
neutron stars and black holes)
• Magnetohydrodynamics (MHD) and
• Galactic dynamics
• Accretion processes around compact
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
• Galileo-The First (or the second after Gilbert) Scientist
• Universal Law of Gravity
• The Chemistry of Stars-How the
spectroscopy challenges Aristotle.
Diurnal Motion Daily Motion
Stars move East
to West as the
West to East.
They cover 15
degrees per hour
which amounts to
360 degrees per
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
• 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.
Star Trails on the Equator (Kenya)
Constellations • Ancient people also
noticed that the
Preserve Their Form constellations (e.g.
preserve their form
during diurnal rotation,
hence they concluded
• 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
• This sphere is called
the celestial sphere.
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.
Globular Cluster These are stars physically
close to each other.
• Celestial Sphere
is still a useful
we only see the
on such a
• Measuring the
distances is a hard
Celestial Sphere=Gökküre=Felek (Çoğ:Eflak)
Seven Objects not Fixed to
the Celestial Sphere
• For the ancient people the celestial sphere was a
• All stars were fixed onto this sphere but there
were 7 objects moving independent of the celestial
• 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.
Retrogade Motion of Mars
Understanding the Retrogade Motion
Modern Science, to a large
extend, was born out of the
human desire to explain the
retrogade motion of the 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
Days of the Week
Thursday Tor Jupiter
Friday Frie Venus
Astronomical Names for the Days of the Week, Falk, M.., 1999, J. of the Royal Astron. Soc. of Canada, Vol. 93, p.122
• The Earth is a sphere at
the center of a spherical
• The Moon, Merkury, Venus,
the Sun, Mars, Jupiter,
Saturn and fixed stars each
rotate around their own
• These spheres are made of
crystal (so that they are
7 katlı gök
• Feleğin tekerine çomak sokmak
• Felekten bir gün çalmak
• Feleğin çemberinden (çeperinden?)
• 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
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.
Aristoteles: Objects on Earth and
Celestial Objects are composed of
• 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.
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
• Heavier objects fall more rapidly than the lighter
• Celestial objects eternally follow circular trajectories.
They do not change their speed during this motion.
• Each celestial object rotates around the Earth.
Comets According to Aristoteles
• The celestial objects
are eternal and the
sky is not subject to
• Hence comets must
be inside the sphere
of the Moon, i.e.
Aristarchus of Samos (310-230 BC)
• Proposed the heliocentric model
• This was not widely accepted because…
• …the parallax can not be measured with
• …the rotation of the Earth would throw
• …the Earth would leave behind the Moon
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
• Force is needed to keep objects in motion.
humans have been
believing that the
Earth is spherical
since long time.
It is a 19th century
think that the
the Earth was flat.
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
• 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
• 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!
• Mathematike Syntaxis
(13 Volume book of
• Arabic Scientists loved
the book and named it
"The Great Book"
• Today this book is
because of this.
Ptolemaic Model of the
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
• Ptolemy’s model had been used nearly
for 1500 years by the western and
Nasir al-Din Tusi (1201–1274)
• Resolved significant
problems in the
Ptolemaic system by
developing the Tusi-
couple as an
alternative to the
equant introduced by
• A mathematical device in
which a small circle rotates
inside a larger circle twice
the radius of the smaller
• 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.)
Size of the
to the size
The rest of the
size of the
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!”
• 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
• The crystal spheres are still there.
Tycho Brahe (1546-1601)
• Observed a supernova
• He thought this was a
• Surprised because he
thought the celestial
objects are eternal
and the sky does not
• The image in
this page is
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.
Tycho Brahe and the Crystal Spheres
• Tycho Brahe also understood that the
comet must have passed through the
• So the crystal spheres of Aristoteles
can not be real!
Around the Same Time in
• 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
• The Second Scientist after
• A strong refuter of the
Aristotelian world view.
• 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
• This refutes the Aristotelian notion of an
unchanging celestial sphere.
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.
A Celestial Object Defected?
• Directed his
telescope to the sky
• The Moon is not
perfect! It has
craters which are
• Discovered Jupiter’s 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
Galilei observed the Phases of Venus
• Heliocentric Model: All phases should be visible
• Geocentric Model: Only crescent and new phases
would be seen
Galilei and the Milky Way
As seen with
Milky Way is a
• 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
• Galileo himself tested the possibility by
observing hundreds of objects to see if
the instrument does anything except
The Sun also is not Perfect
Galileo's thought experiments and real
– 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
– 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
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
• limiting the scope of enquiry - consider only one question at a time;
• quantitative methods - did careful measurements of the motion of falling
– 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.
Three Quotations from Galileo
• In questions of science the
authority of a thousand is not
worth the humble reasoning of a
• Instead of philosophical arguments, I
prefer to discover a single fact.
• What has philosophy got to do with
• If the celestial objects are not
perfect their orbits also may not be
• Then introduce elliptic orbits into the
• Kepler’s laws for the motion of
• Gravitational attraction is between all bodies.
• The force that keeps the Moon in orbit is the
same force that causes the apple to fall
• Objects on the Earth and the objects in the
sky obey the same laws.
M m 11
F G 2 G 6.67 10 3 2
m /(s kg)
Aristotelian view: forces cause velocity
(force necessary to maintain uniform motion).
Newtonian view: forces cause acceleration
(force necessary to change motion)
• Successfully measured the parallax
of the star 61 Cygni.
• This was considered conclusive
evidence that the Earth was in
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
• Tycho Brahe supplied the accurate data about the
motion of the planets.
• Kepler analysing the data arrived at the Kepler’s
• Newton, trying to explain Kepler’s laws found the
general laws of motion which are not only valid in
the solar system but can also be applied to many
• 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
The Fate of Takiyuddin’s
Observatory in Istanbul
• Takiyuddin (1526-1580) came to Istanbul in
• 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
Istanbul Observatory (1577)
A comet at November 1577
Black Death in 1578
• Gossip that all this evil arising
because they have been observing the
legs of the angels in the observatory
• Şeyhülislam (Ahmed Şemseddin
Efendi): Observing the legs of the
angels from the ground with pipes is a
Black Death and the Şeyhülislam
• Şeyhülislam:İhrac-ı Rasad meş'um perde-i esrarı
felekiyeye küstahane itlak-ı cür'etin vehamet ve
akibeti meczumdur. Hiçbir mülkde mübaşeret
olunmadı ki ma'mur iken harap ve bünyan-ı 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
The End of the Observatory
• The king was scared and wanted the observatory
to be broken down.
• This was done by Kılıç Ali Paşa 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
End of the Lecture
• Be sure to read some of the material
on the web.