Chapter 1 Our Place in the Universe - Astronomy

Chapter 1
Our Place in the Universe

1.1 Our Modern View of the
Universe
Topics we will explore:
• What is our place in the universe?
• How did we come to be?
• How can we know what the universe was like in the
past?
• Can we see the entire universe?
• First, let’s take a look to some basic astronomical
objects

Solar System
(in general planetary system)
A star, in our case the Sun and all the material that orbits
it, including its planets, moons, asteroids, comets, small
bodies and gas and dust.

Star (Sun)
A large, plasma sphere that generates heat and light through
nuclear fusion in its core.
In the case of the sun, it fuses hydrogen into helium
Plasma: Is a state of matter in which the matter is in ionized form. Composed
of particles electrically charged. Normally composed of electrons (negative) and
protons and ions (positive)

Planet
Mars
Neptune
A moderately large object that orbits a star; it shines by
reflecting light from the star. The constitution of
planets may be rocky, icy, or gaseous in composition.
We will discuss the formal definition given by the IAU (International
Astronomical Union) later in the semester

Moon (or Satellite)
An object that orbits
a planet
Ganymede (orbits Jupiter)

Asteroid
A relatively small and rocky object that
orbits a star

Comet
A relatively
small and icy
object that
orbits a star

Nebula
What is outside the solar system?
An interstellar cloud of
gas and/or dust.
Parts of a nebula may
reflect light from a star
(s), some part may
emit light from exited
gas atom by UV
radiation from a star
(s). Part may absorb
light .
(In the picture, the Orion
nebula)
The Orion nebula belongs to the
Milky way (our Galaxy)

Galaxy
What is outside the Milky Way?
A huge collection of stars in space, all held together by gravity and
orbiting a common center. The Milky Way and the Andromeda galaxy
are examples of galaxies
M31, the great galaxy
in Andromeda

Clusters of galaxies and
superclusters
• Groups of galaxies with more than a few
dozen members are called galaxy clusters
• Regions where galaxies and galaxy clusters are
most tightly packed are called superclusters
• Galaxy clusters and superclusters form giant
chains and sheets with large voids between
them

Universe
The sum total of all matter and energy;
that is, superclusters of galaxies, voids
and everything within and between all
galaxies

The observable Universe
• The portion of the universe than can be seen
from Earth. The observable Universe is
probably only a portion of the entire Universe

What is our place in the universe?

The Big Bang and the expanding of the Universe
• Observations of galaxies show that the entire universe is expanding, the average
distance between galaxies is increasing with time.
• This means that galaxies ( or at least matter) must have been close together in the
past.
• If we go back far enough, all the matter was concentrated in a small radius from
which the expansion began.
• That is called the Big Bang. From the rate of expansion it is estimated that it
occurred about 14 billions year ago.
• The universe has continued to expand and evolve since then.
• On a small scale, the force of gravity has drawn matter together forming galaxies,
stars formed inside the galaxies and planets formed around some of those stars.
• The only two chemical elements created during the Big Bang were hydrogen and
helium
• The rest of the elements heavier than H and He are generated inside the stars
• As Carl Sagan said: We are “star stuff”, all the chemical elements on our body came
from stars

How did we come to be?

Let’s address the question: How can we know what
the universe was like in the past?
• Light travels at a finite speed (300,000 km/s).
Destination
Moon
Sun
Sirius
Andromeda Galaxy
Light travel time
1.2 second
8 minutes
8 years
2.5 million years

How can we know what the universe was like
in the past?
• Light travels at a finite speed (300,000 km/s).
Destination
Moon
Sun
Sirius
Andromeda Galaxy
Light travel time
1.2 second
8 minutes
8 years
2.5 million years
Thus, we see objects as they were in the
past:
The farther away we look in distance,
the further back we look in time.

The meaning of a Light-Year
• The distance light can travel in 1 year
• About 10 trillion kilometers (10x10^12 km)
or (6 trillion miles)

How far is a light-year?
Distance = Speed x Time
1light-year = (speed of light)
(1 year)
km 365 days 24 hr 60 min 60 s
= 300,000 s 1 yr 1 day 1 hr 1 min

How far is a light-year?
1light-year = (speed of light)
(1 year)
km 365 days 24 hr 60 min 60 s
= 300,000 s 1 yr 1 day 1 hr 1 min
=9,460,000,000,000 km

Example:
The distance to
the Orion
Nebula is about
1500 light years.
We see the Orion
Nebula as it
looked 1500
years ago.

Example:
This photo shows the Andromeda Galaxy as it looked
about 2.5 million years ago.
Question: When will
we be able to see what
it looks like now?

• At great distances, we see objects as they were
when the universe was much younger.
We see a galaxy 7 billions light-years away as it was 7 billion
years ago.
Light from nearly 14 billion light years away shows the
universe as it looked shortly after the Big Bang, before galaxies
existed

Can we see the entire universe?

Question
Why can’t we see a galaxy 15 billion light-years away?
(Assume the universe is 14 billion years old.)
A. Because no galaxies exist at such a great
distance.
B. Galaxies may exist at that distance, but their
light would be too faint for our telescopes to
see.
C. Because looking 15 billion light-years away
means looking to a time before the universe
existed.

Question
Why can’t we see a galaxy 15 billion light-years away?
(Assume the universe is 14 billion years old.)
A. Because no galaxies exist at such a great
distance.
B. Galaxies may exist at that distance, but their
light would be too faint for our telescopes to
see.
C. Because looking 15 billion light-years away
means looking to a time before the universe
existed.

What have we learned?
• What is our physical place in the universe?
– The Earth is a small planet in the solar system.
– The solar system is one of many planetary systems in the Milky
Way Galaxy
– The Milky Way galaxy is one of about 40 galaxies in the Local
Group of galaxies.
– The Local Group is one a many groups in the Local Supercluster
of galaxies
• How did we come to be?
– Part of the matter in our bodies came from the Big Bang, which
produced hydrogen and helium.
– All other elements were constructed from H and He in the core of
the stars and then recycled into the interstellar space from which
new star systems were formed, including our solar system.

What have we learned?
• How can we know what the universe was
like in the past?
– When we look to great distances, we are
seeing events that happened long ago because
light travels at a finite speed.
• Can we see the entire universe?
– No. The observable portion of the universe is
about 14 billion light-years in radius because
the universe is about 14 billion years old.

How big is Earth compared to our solar system?
Let’s reduce the size of the solar system by a factor of 10
billion; the Sun is now the size of a large grapefruit (14
cm diameter).
How big is Earth on this scale?
A. an atom
B. a ball point
C. a marble
D. a golf ball

Let’s reduce the size of the solar system by a factor of
10 billion; the Sun is now the size of a large grapefruit
(14 cm diameter).
How big is Earth on this scale?
A. an atom
B. a ball point
C. a marble
D. a golf ball

Our Solar System - Sizes
1 mile = 1.6 km
Earth diameter =12,756 km = 1.28 x 10 4 km
Jupiter diameter = 142,984 km = 1.43 x 10 5 km
Sun diameter = 1,292,000 km = 1.292 x 10 6 km
Diameter of Sun ~ 109 times diameter of Earth
Diameter of Jupiter ~ 12 times diameter of Earth

The scale of the solar system
• On a 1-to-10- billion
scale:
– The Sun is the size
of a large
grapefruit (14
cm).
– Earth is the size of
a ball point, 15
meters away.
– The Earth is 8
light-minutes
away from the
Sun

How far away are the
stars?
On our 1-to-10-billion scale, it’s just a few minutes’
walk to Pluto.
How far would you have to walk to reach Alpha
Centauri (one of the closes start to the solar system)?
(Alpha Centauri is 4.3 light-years away)
A. 1 mile
B. 10 miles
C. 100 miles
D. the distance across the United States
(2500 miles)

Answer: D, the distance across the
United States

How big is the Milky Way Galaxy?
The Milky Way has
about 100 billion
stars. Its diameter is
about 100,000 light
years
On the same 1-to-10-
billion scale, how big
is the Milky Way?

Question
Suppose you tried to count the more than 100 billion
stars in our galaxy, at a rate of one per second.
How long would it take you?
A. a few weeks
B. a few months
C. a few years
D. a few thousand years

Suppose you tried to count the more than 100 billion
stars in our galaxy, at a rate of one per second.
How long would it take you?
A. a few weeks
B. a few months
C. a few years
D. a few thousand years

How big is the universe?
• The Milky Way is one of about 100 billion galaxies.
• 10 11 stars/galaxy x 10 11 galaxies = 10 22 stars
There are as many stars as grains of (dry) sand on all Earth’s beaches.

How do our lifetimes compare to the age
of the universe?
• The cosmic calendar: a scale on which we compress the
history of the universe into 1 year.

What have we learned?
• How big is Earth compared to our solar system?
– The distances between planets are huge compared to
their sizes—on a scale of 1-to-10-billion, Earth is the
size of a ball point and the Sun is 15 meters away.
• How far away are the stars?
– On the same scale, the stars are thousands of
kilometers away.
• How big is the Milky Way Galaxy?
– It would take more than 3000 years to count the 100
billion stars in the Milky Way Galaxy at a rate of one
per second, and they are spread across 100,000 lightyears.

What have we learned?
• How big is the universe?
– The observable universe is 14 billion lightyears
in radius and contains over 100 billion
galaxies with a total number of stars
comparable to the number of grains of sand
on all of Earth’s beaches.
• How do our lifetimes compare to the age of the
universe?
– On a cosmic calendar that compresses the
history of the universe into 1 year, human
civilization is just a few seconds old, and a
human lifetime is a fraction of a second.

1.3 Spaceship Earth
Let’s explore these topics:
• How is Earth moving in our solar system?
• How is our solar system moving in the Milky Way
Galaxy?
• How do galaxies move within the universe?
• Are we ever sitting still?

How is Earth moving in our solar system?
• Contrary to our perception, we are not “sitting still.”
• We are moving with Earth in several ways, and at
surprisingly fast speeds.
The Earth rotates
around its axis once
every day.

How can we calculate the speed of a person
located at the equator due to earth rotation?
• Rotation speed at equator = equatorial circumference/length of day
(Remember that Speed = Distance/Time)
The Earth’s equatorial radius is 6378 km
The circumference is 2 x Pi x radius = 2 x 3.1416 x 6378 = 40,074 km
One day is equal to 24 hours
Rotation speed at equator = 40,074/24 = 1670 km/hr

Earth orbits the Sun (revolves) once every year
• The time it takes the Earth to complete one orbit around the Sun is called the orbital
period ( ~365 days)
•The average distance between the Earth and the Sun is 1 AU ≈ 150 million kilometers.
(AU:Astronomical Unit, the mean distance between the Earth and the Sun)
• The Earth orbit the Sun at an average speed of 107,000 km/hr
• With Earth’s axis tilted by 23.5º (pointing to close to Polaris)
It rotates in the same direction it orbits, counterclockwise as viewed from above the
North Pole.
The Earth orbital path defines a flat plane called the Ecliptic. The ecliptic is also the
apparent path of the Sun in the celestial sphere

How is our Sun moving in the Milky Way Galaxy?
Our Sun moves relative to nearby stars in the local solar
neighborhood…
•At typical relative speeds of more than 70,000 km/hr
• but stars are so far away that we cannot easily notice
their motion
The Sun is located at about
28,000 light-years from the
center of the Milky Way
… and the Sun orbits the
galaxy every 230 million
years.
The speed at which our solar
system rotates around the
center of the galaxy is about
800,000 km/hr

More detailed study of the Milky Way’s rotation reveals
one of the greatest mysteries in astronomy:
Most of the galaxy’s light comes from stars and gas in the galactic
disk and central bulge
But measurements suggest that most of the mass lies unseen in the
spherical halo that surrounds the entire disk
This reveals the presence of a mass that we cannot see. It is called dark matter

How do galaxies move within the universe?
Galaxies are carried along with the expansion of the universe. But
how did Hubble figure out that the universe is expanding?
The analogy of the raisin cake:

The analogy of the raising cake
• A raisin that was at 1 cm, after baking for one hour is at 3 cm
It moves at a speed of 2 cm/hr
• Another raising that was at 2 cm, after baking for one hour is
at 6 cm
It moves at a speed of 4 cm/hr
The more distant the raisins are, they move at larger speed.

Edwin Hubble discovered in the 1920’s by measuring
the velocity and distance of galaxies that:
• All galaxies outside our Local Group are
moving away from us.
• The more distant the galaxy, the faster it is
racing away.

Hubble plotted the velocity and the distance of the galaxies.
It gives a linear relationship between the two parameters.
It is called the Hubble law
Conclusion: We live in an expanding universe.

Are we ever sitting still?

What have we learned?
• How is Earth moving in our solar system?
– It rotates on its axis once a day (24 hours) and
orbits the Sun in ~365 days at a distance of 1
AU = 150 million kilometers.
• How is our solar system moving in the Milky
Way Galaxy?
– Stars in the Local Neighborhood move
randomly relative to one another and orbit
the center of the Milky Way in about 230
million years.

What have we learned?
• How do galaxies move within the universe?
– All galaxies beyond the Local Group are
moving away from us with the expansion of
the universe: the more distant they are, the
faster they’re moving.
• Are we ever sitting still?
– The answer is No! Earth is constantly in
motion, even though we don’t notice it.