W onders of the C osmos

DR. AJITH KUMAR

ASSISTANT PROFESSOR & HEAD
DEPARTMENT OF PHYSICS
ST. PIUS X COLLEGE, RAJAPURAM
 “Look again at at dot. at s here. at s home. at s us. On it
everyone you love, everyone you know, everyone you ever heard of,
every human being who ever was, lived out eir lives. e a rega of
our joy and su ering, ousands of con dent religions, ideologies, and
economic doc ines, every hun r and forager, every hero and coward,
every crea r and des oyer of civiliza on, every king and peasant,
every young couple in love, every mo er and fa er, hope l child,
inven r and explorer, every acher of morals, every corrupt poli cian,
every superstar, every supreme leader, every saint and sinner in
e his ry of our species lived ere-on a mo of dust suspended in a
sunbeam”.

Carl sagan
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Cosmos and Cosmolog

Cosmos: Refers to the universe as an ordered and harmonious system

Derived from the Greek word "kosmos," meaning "order" or "world."
Encompasses all matter, energy, galaxies, stars, planets, and space itself.

Cosmology: The scienti c study of the origin, evolution, and ultimate fate of the universe
Focuses on understanding the universe's large-scale structure and dynamics.
Explores key events such as the Big Bang, galaxy formation, and cosmic expansion.

Importance: Provides a framework for understanding our place in the universe and the
fundamental laws governing its behavior.
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Introduction to Astronom
Astronomy is the study of celestial objects, such as stars, planets, and galaxies,
and the interactions between them.

It aims to understand the history of the universe, from its birth in the Big Bang to
its current state and future evolution.

Modern astronomy evolves continually with advancements in technology,

allowing for deeper exploration and understanding of the cosmos.
y

The Scienti c Method in Astronom

Observation: Gathering data through telescopes and instruments.

Hypothesis Formulation: Proposing explanations for observed phenomena.

Modeling: Developing mathematical or conceptual models to describe the observations.

Testing and Peer Review: Verifying models through experiments and critical
evaluation by the scienti c community

Example: Transition from geocentric (Earth-centered) to heliocentric (Sun-centered) models of the

universe, driven by improved observations and evidence.

Signi cance: Science is a dynamic process that adapts with new discoveries and data.
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The Laws of Natur

Universal "rules" that govern the behavior of the cosmos, applicable everywhere.
The same physical laws apply universally, enabling predictions about distant celestial phenomena.

Evolution of Models: Example: Einstein’s theory of general relativity revolutionized our

understanding of gravity and predicted phenomena like black holes.

Role of Mathematics: Mathematical equations provide precise descriptions of natural laws,

essential for advancing scienti c understanding.
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Astronomical Measurements and Scale

The universe's immense distances and sizes require innovative measurement methods.

Scienti c Notation: Simpli es the representation of vast numbers.

Light-Year: The distance light travels in one year (~9.46 trillion km), a fundamental unit for
astronomical distances.

Example: Proxima Centauri, the nearest star beyond the Sun, is 4.25 light-years away,
highlighting the vast scale of space.
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 Light Travel and Cosmic Tim

Due to light’s nite speed, observations of celestial objects reveal their past states.

Examples:
Light from a star 500 light-years away left the star 500 years ago.

Telescopes act as time machines, observing the universe’s distant history.

Bene t: This delay allows astronomers to piece together the evolution of the cosmos over
billions of years.
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 A Tour of the Universe

Earth:
Diameter: ~13,000 km.
Unique due to its abundant liquid water and the presence
of intelligent life.

Moon:
Distance: ~384,000 km from Earth.
Light travel time: ~1.3 seconds.
Re ects sunlight, in uencing Earth's tides and nocturnal
illumination.

Sun:
Diameter: ~1.5 million km.
Light travel time: ~8 minutes to Earth.
Provides energy essential for life and drives Earth's climate.
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The Milky Way:

A spiral galaxy containing hundreds of billions of stars, including our Sun.
Diameter: ~100,000 light-years.

Galactic Neighbors:
Andromeda Galaxy: 2 million light-years away
Local Group: A cluster of over 50 galaxies, including the Milky Way and Andromeda.

Clusters and Superclusters:

The Virgo Supercluster spans 110 million light-years and includes thousands of galaxies.
.

The Universe at Large and Small Scale

Large Scale:
Clusters of galaxies reveal the structure of the universe.
Quasars: Extremely bright centers of galaxies powered by supermassive black holes,
allowing us to observe billions of years into the past.

Small Scale:
Atoms, the building blocks of matter, are mostly empty space

Fundamental Forces: Gravity, electromagnetism, strong nuclear force, and weak nuclear
force govern all cosmic interactions.

smos. However, the geocentric view happens to be wrong. One of the great themes of our intellectual
y is the overthrow of the geocentric perspective. Let us, therefore, take a look at the steps by which we
uated the place of our world in the cosmic order.

Celestial Sphere
The Celestial Sphere
Gazing up, you get the impression that the sky is a great hollow dome with you at the center,
go on a camping trip or live far from city lights, your view of the sky on a clear night is pretty much
cal to that seen by people all over the world before the invention of the telescope. Gazing up, you get the
and
ssion that the all
sky isthe stars
a great hollow are
dome anwith equal
you at thedistance
center (Figurefrom
2.2), andyou on are
all the stars thean surface
equal of the dome. The top of that 2.1 • The Sky Above
ce from you on the surface of the dome. The top of that dome, the point directly above your head, is
dome,
the zenith, the the
and where point
domedirectly
meets Earth above your
is called the head,
horizon. is sea
From the called
or a flatthe zenith,
prairie, it is and where the dome meets Earth
that is turning around you, bringing different stars into view as it turns. The early Greeks regarded the sky as
o see the horizon as a circle around you, but from most places where people live today, the horizon is at
partiallyis called the horizon. just such a celestial sphere (Figure 2.3). Some thought of it as an actual sphere of transparent crystalline
hidden by mountains, Theorearly
trees, buildings, smog. Greeks regarded the sky as just such a celestial sphere
material, with the stars embedded in it like tiny jewels.
 Celestial Poles and Celestial Equator
Imagine a line going through Earth, connecting the North and South Poles. This is Earth’s axis,
and Earth rotates about this line. If we extend this imaginary line outward from Earth, the points
where this line intersects the celestial sphere are called the north celestial pole and the south
celestial pole. As Earth rotates about its axis, the sky appears to turn in the opposite direction
around those celestial poles. We also (in our imagination) throw Earth’s equator onto the sky
and call this the celestial equator.

For this observer, stars within 38° of the North Pole can never set. They are always above the
horizon, day and night. This part of the sky is called the north circumpolar zone.

A star very close to the north celestial pole called Polaris, the pole star, has the distinction of
being the star that moves the least amount as the northern sky turns each day.
between the celestial poles, just as Earth’s equator lies halfway between our planet’s poles.

Figure 2.4 Circling the South Celestial Pole. This long-exposure photo shows trails left by stars as a result of the apparent rotation
of the celestial sphere around the south celestial pole. (In reality, it is Earth that rotates.) (Credit: ESO/Iztok Bončina)

Now let’s imagine how riding on different parts of our spinning Earth affects our view of the sky. The apparent
 Rising and Setting of the Sun
For thousands of years, astronomers have been aware that the Sun does more than just rise and
Figure 2.5 Star Circles at Different Latitudes. The turning of the sky looks different depending on your latitude on Earth. The red
set. It changes position gradually on the celestial sphere, moving each day about 1° to the east
circle in each case is your horizon. Your zenith is the point above your head. (a) At the North Pole, the stars circle the zenith and do
not rise and set. (b) At the equator, the celestial poles are on the horizon, and the stars rise straight up and set straight down. (c) At
intermediate latitudes, the north celestial pole is at some position between overhead and the horizon. Its angle above the horizon

relative to the stars, taking a period of time we call 1 year to make a full circle.
turns out to be equal to the observer’s latitude. Stars rise and set at an angle to the horizon.

As the months go by and we look at the Sun from different places in our orbit, we see it projected against
different places in our orbit, and thus against different stars in the background (Figure 2.6 and Table 2.1)—or
The path the Sun appears to take around the celestial sphere each year is called the ecliptic.
we would, at least, if we could see the stars in the daytime. In practice, we must deduce which stars lie behind
and beyond the Sun by observing the stars visible in the opposite direction at night. After a year, when Earth

Because of its motion on the ecliptic, the Sun rises about 4 minutes later each day with respect
has completed one trip around the Sun, the Sun will appear to have completed one circuit of the sky along the
ecliptic.

to the stars.

Figure 2.6 Constellations on the Ecliptic. As Earth revolves around the Sun, we sit on “platform Earth” and see the Sun moving
ST. PIUS X COLLEGE, RAJAPURAM
DEPARTMENT OF PHYSICS

Figure 2.5 Star Circles at Different Latitudes. The turning of the sky looks different depending on your latitude on Earth. The red
circle in each case is your horizon. Your zenith is the point above your head. (a) At the North Pole, the stars circle the zenith and do
not rise and set. (b) At the equator, the celestial poles are on the horizon, and the stars rise straight up and set straight down. (c) At
intermediate latitudes, the north celestial pole is at some position between overhead and the horizon. Its angle above the horizon
turns out to be equal to the observer’s latitude. Stars rise and set at an angle to the horizon.

As the months go by and we look at the Sun from different places in our orbit, we see it projected against
The ecliptic does not lie along the celestial equator but is inclined to it at an angle of about
23.5°. In other words, the Sun’s annual path in the sky is not linked with Earth’s equator. This
40 2 • Observing the Sky: The Birth of Astronomy
is because our planet’s axis of rotation is tilted by about 23.5° from a vertical line sticking out
of the plane of the ecliptic.

The inclination of the ecliptic is the

reason the Sun moves north and south
in the sky as the seasons change.

Figure 2.7 The Celestial Tilt. The celestial equator is tilted by 23.5° to the ecliptic. As a result, North Americans and Europeans see
the Sun north of the celestial equator and high in our sky in June, and south of the celestial equator and low in the sky in December.
 Constellations on the Ecliptic

Constellation on the Ecliptic Dates When the Sun Crosses It

Capricornus January 21–February 16

Aquarius February 16–March 11

Pisces March 11–April 18

Aries April 18–May 13

Taurus May 13–June 22

Gemini June 22–July 21

Cancer July 21–August 10

Leo August 10–September 16

Virgo September 16–October 31

Libra October 31–November 23

Scorpius November 23–November 29

Ophiuchus November 29–December 18

Sagittarius December 18–January 21

Table 2.1
 Fixed and Wandering Stars
The seven classical ‘planets’ (all are not) are those easily seen with the naked eye, and were
thus known to ancient astrologers. They are the Moon, Mercury, Venus, Sun, Mars, Jupiter,
and Saturn
Greeks of 2000 years ago distinguished between what they called the fixed stars - those that
maintain fixed patterns among themselves through many generations - and the wandering stars,
or planets. The word “planet,” in fact, means “wanderer” in ancient Greek.
They dedicated a unit of time, the week, to the seven objects that move on their own; that’s why
there are 7 days in a week.
The planets, the Sun, and the Moon are thus always found in the sky within a narrow 18-degree-
wide belt, centered on the ecliptic, called the zodiac. (The root of the term “zodiac” is the same
as that of the word “zoo” and means a collection of animals; many of the patterns of stars within
the zodiac belt reminded the ancients of animals, such as a fish or a goat.)
 Constellations
We could see about 3000 stars with the unaided eye.

The ancients grouped stars that made some familiar

geometric pattern or (more rarely) resembled
something they knew. Each civilization found its
own patterns in the stars. The ancient Chinese,
Egyptians, and Greeks, among others, found their
own groupings - or constellations - of stars. These
were helpful in navigating among the stars and in
passing their star lore on to their children.
Therefore, during the early decades of the 20th century, astronomers from many countries decided to
establish a more formal system for organizing the sky.

Figure 2.8 Orion. (a) The winter constellation of Orion, the hunter, is surrounded by neighboring constellations, as illustrated in the
seventeenth-century atlas by Hevelius. (b) A photograph shows the Orion region in the sky. Note the three blue stars that make up
the belt of the hunter. The bright red star above the belt denotes his armpit and is called Betelgeuse (pronounced “Beetel-juice”). The
bright blue star below the belt is his foot and is called Rigel. (credit a: modification of work by Johannes Hevelius; b: modification of
work by Matthew Spinelli)

Today, we use the term constellation to mean one of 88 sectors into which we divide the sky, much as the
United States is divided into 50 states. The modern boundaries between the constellations are imaginary lines
Today, we use the term constellation to mean one of 88 sectors into which we divide the
sky, Not all constellations are the same size
The modern constellation of Orion is a kind of box on the sky, which includes, among many
other objects, the stars that made up the ancient picture of the hunter.
Some people use the term asterism to denote an especially noticeable star pattern within a
constellation (or sometimes spanning parts of several constellations). For example, the Big
Dipper (Saptarshi) is an asterism within the constellation of Ursa Major, the Big Bear.

Big Dipper
 Ancient Astronomy
Ancient Babylonian, Assyrian, and Egyptian astronomers knew the approximate length of the
year. The Egyptians of 3000 years ago, for example, adopted a calendar based on a 365-day
year.
The Chinese also had a working calendar; they determined the length of the year at about the
same time as the Egyptians.
The Chinese also recorded comets, bright meteors, and dark spots on the Sun.
The Mayan culture in Mexico and Central America developed a sophisticated calendar based
on the planet Venus
The Polynesians learned to navigate by the stars over hundreds of kilometers of open ocean
In Britain, before the widespread use of writing, ancient people used stones to keep track of
the motions of the Sun and Moon. We still find some of the great stone circles they built for
this purpose, dating from as far back as 2800 BCE.
 Early Greek and Roman Cosmology

Aristotle (384–322 BCE), described how the progression of the Moon’s phases.
Aristotle also knew that the Sun has to be farther away from Earth than is the Moon because
occasionally the Moon passed exactly between Earth and the Sun and hid the Sun temporarily
from view. We call this a solar eclipse.

The Aristotelian Universe: Spherical and Earth-

centered, with celestial bodies moving in perfect
circular orbits.
Earth-centered model dominated cosmological
thinking for centuries.
day. They describe how the progression of the Moon’s phases—its apparent changing shape—results from our
seeing different portions of the Moon’s sunlit hemisphere as the month goes by (see Earth, Moon, and Sky).
Aristotle also knew that the Sun has to be farther away from Earth than is the Moon because occasionally the
Moon passed exactly between Earth and the Sun and hid the Sun temporarily from view. We call this a solar
eclipse.
Aristotle cited convincing arguments that Earth must be round. First is the fact that as the
Aristotle cited convincing arguments that Earth must be round. First is the fact that as the Moon enters or
Moon enters or emerges from Earth’s shadow during an eclipse of
emerges from Earth’s shadow during an eclipse of the Moon, the shape of the shadow seen on the Moon is
the Moon, the shape of
always round (Figure 2.9). Only a spherical object always produces a round shadow. If Earth were a disk, for
the shadow seen on the Moon is always round.
example, there would be some occasions when the sunlight would strike it edge-on and its shadow on the
Moon would be a line.

Figure 2.9 Earth’s Round Shadow. A lunar eclipse occurs when the Moon moves into and out of Earth’s shadow. Note the curved
shape of the shadow—evidence for a spherical Earth that has been recognized since antiquity. (credit: modification of work by Brian
As a second argument, Aristotle explained that travellers who go south a significant distance
Paczkowski)

As a second argument, Aristotle explained that travelers who go south a significant distance are able to
are able
observe starsto
thatobserve stars
are not visible that
farther are
north. Andnot visible
the height of thefarther north.
North Star—the starAnd
nearestthe height of the North Star -
the north
celestial pole—decreases as a traveler moves south. On a flat Earth, everyone would see the same stars
the star nearest the north celestial pole - decreases as a traveler moves
overhead. The only possible explanation is that the traveler must have moved over a curved surface on Earth,
south. On a flat Earth,
everyone would
showing stars from see the
a different angle.same
(See thestars
How Dooverhead.
We Know Earth The onlyfeature
Is Round? possible
for moreexplanation
ideas on is that the traveler
proving Earth is round.)
must have moved over a curved surface on Earth
One Greek thinker, Aristarchus of Samos (310–230 BCE), even suggested that Earth was moving around the
Sun, but Aristotle and most of the ancient Greek scholars rejected this idea. One of the reasons for their
conclusion was the thought that if Earth moved about the Sun, they would be observing the stars from
different places along Earth’s orbit. As Earth moved along, nearby stars should shift their positions in the sky
relative to more distant stars. In a similar way, we see foreground objects appear to move against a more
One Greek thinker, Aristarchus of Samos (310–230 BCE),
even suggested that Earth was moving around the Sun, but
Aristotle and most of the ancient Greek scholars rejected
this idea. One of the reasons for their conclusion was the
thought that if Earth moved about the Sun, they would be
observing the stars from different places along Earth’s orbit.
As Earth moved along, nearby stars should shift their
positions in the sky relative to more distant stars.
 overhead, but was slightly south of the zenith, so that its rays made an angle with the vertical equal to ab
1/50 of a circle (7°). Because the Sun’s rays striking the two cities are parallel to one another, why would t
two rays not make the same angle with Earth’s surface? Eratosthenes reasoned that the curvature of the
Earth meant that “straight up” was not the same in the two cities. And the measurement of the angle in
Measurement of Earth by Eratosthenes
Alexandria, he realized, allowed him to figure out the size of Earth. Alexandria, he saw, must be 1/50 of E
circumference north of Syene (Figure 2.11). Alexandria had been measured to be 5000 stadia north of Sy
(The stadium was a Greek unit of length, derived from the length of the racetrack in a stadium.) Eratosth
thus found that Earth’s circumference must be , or 250,000 stadia.

The Greeks were able to measure the size of earth.

The first fairly accurate determination of Earth’s
diameter was made in about 200 BCE by
Eratosthenes (276–194 BCE). The observed
circumference was 250,000 stadia.
Figure 2.11 How Eratosthenes Measured the Size of Earth. Eratosthenes measured the size of Earth by observing the angle
 Hipparchus and Precession
Hipparchus (190 BCE- 120 BCE)
Erected an observatory on the island of Rhodes around 150 BCE.

Measured the positions of objects in the sky, compiling a

star catalog with about 850 entries.

Designated celestial coordinates for each star, specifying its

position in the sky

Divided the stars into apparent magnitudes according to their

apparent brightness.

He called the brightest ones “stars of the first magnitude”; the next
brightest group, “stars of the second magnitude”; and so forth.
tudes according to their apparent brightness. He called the
e next brightest group, “stars of the second magnitude”; and so
form, still remains in use today (although it is less and less

By observing the stars and comparing his data with older observations, Hipparchus made one
with older observations, Hipparchus made one of his most

of his most remarkable

of the north celestial pole had altered over the previous century
his had happened not only during the period covered by his
discoveries: the position in the sky of the north celestial pole had
time: the direction around which the sky appears to rotate
he section onaltered
celestial polesover the previous
and the celestial equator that the century and a half.
h’s North Pole into the sky. If the north celestial pole is

Today, we understand that the direction in which Earth’s axis points does indeed change
ng the wobbling. Today, we understand that the direction in
owly but regularly—a motion we call precession. If you have
ved a similar kind of motion. The top’s axis describes a path in
slowly
opple it (Figure 2.12). but regularly—a motion we call precession.

It takes about 26,000 years for Earth’s axis to complete

one circle of precession. As a result of this motion, the point
where our axis points in the sky changes as time goes on. While
Polaris is the star closest to the north celestial pole today (it will
reach its closest point around the year 2100), the star Vega in
the constellation of Lyra will be the North Star in 14,000 years.
ning top wobbles slowly in a circle, so the axis of Earth wobbles in a
he star Polaris, but about 5000 years ago it was close to a star called
 Ptolemy’s Model of the Solar System
Claudius Ptolemy (or Ptolemaeus) (AD 100 -170)

He wrote a mammoth compilation of astronomical knowledge,

which today is called by its Arabic name, Almagest (meaning
“The Greatest”). Almagest does not deal exclusively with
Ptolemy’s own work; it includes a discussion of the astronomical
achievements of the past, principally those of Hipparchus. Today,
it is our main source of information about the work of Hipparchus
and other Greek astronomers.
He geometrically represented the solar system and predicted the
positions of the planets for any desired date and time.
 results from the combination of their own motions with Earth’s orbital revolution. As we watch the pl
from our vantage point on the moving Earth, it is a little like watching a car race while you are compe
Sometimes opponents’ cars pass you, but at other times you pass them, making them appear to mov

Retrograde motion backward for a while with respect to you.

Figure 2.13 shows the motion of Earth and a planet farther from the Sun—in this case, Mars. Earth tr
Normally, planets move eastward in the sky aroundoverthe Sun in the same direction as the other planet and in nearly the same plane, but its orbita
faster. As a result, it overtakes the planet periodically, like a faster race car on the inside track. The fig
the weeks and months as they orbit the Sun,where we but see the planet in the sky at different times. The path of the planet among the stars is illustr
the star field on the right side of the figure.

from positions B to D, as Earth passes the

planets in our example, it appears to drift
backward, moving west in the sky. Even though
it is actually moving to the east, the faster-
moving Earth has overtaken it and seems, from
our perspective, to be leaving it behind. As Earth
rounds its orbit toward position E, the planet
again takes up its apparent eastward motion in
the sky. The temporary apparent westward
motion of a planet as Earth swings between it By following the lines from each Earth position through each corresponding Mars position, you can see h
Figure 2.13 Retrograde Motion of a Planet beyond Earth’s Orbit. The letters on the diagram show where Earth and Ma
different times.
retrograde path of MarsPtolemy
looks against thetried explain
background stars. the complex problems like
and the Sun is called retrograde motion retrograde motion while assuming a stationary Earth.
 direction as the center of the epicycle; from Earth, the planet appears to be moving ea
is at y, however, its motion is in the direction opposite to the motion of the epicycle’s c
Ptolemy solved the problem of explaining
choosingthe observed
the right combination of speeds and distances, Ptolemy succeeded in having
motions of planets by having each westward at the correct
planet revolve in speed
a and for the correct interval of time, thus replicating ret
model.
small orbit called an epicycle. The center of the
epicycle then revolved about Earth on a circle called a
deferent. When the planet is at position x in on the
epicycle orbit, it is moving in the same direction as the
center of the epicycle; from Earth, the planet appears to
be moving eastward. When the planet is at y, however,
its motion is in the direction opposite to the motion of
the epicycle’s center around Earth. By choosing the
right combination of speeds and distances, Ptolemy
succeeded in having the planet moving westward at the
Figure 2.14 Ptolemy’s Complicated Cosmological System. Each planet orbits around a small circle ca
correct speed and for the correct interval
orbits on aof time,
larger thusthe deferent. This system is not centered exactly on Earth but on an offse
circle called

replicating retrograde motion with his model.