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Chapter 1
Our Place in the Universe
1.1 A Modern View of the
Universe
- 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?
Our goals for learning:
What is our place in the universe?
A large glowing ball of gas that generates heat
and light through nuclear fusion
Star
Planet
A moderately large object that orbits a star; it
shines by reflected light. Planets may be rocky,
icy, or gaseous in composition.
Mars Neptune
Moon (or satellite)
An object that orbits
a planet.
Ganymede (orbits Jupiter)
Nebula
An interstellar cloud
of gas and/or dust
Galaxy
A great island of stars in space, all held
together by gravity and orbiting a
common center
M31, The Great Galaxy in Andromeda
Universe
The sum total of all matter and energy;
that is, everything within and between
all galaxies
How did we come to be?
How can we know what the universe was
like in the past?
- Light travels at a finite speed (300,000 km/s).
- Thus, we see objects as they were in the past: The farther away we look in distance, the further back we look in time.
Andromeda Galaxy 2.5 million years
Sirius 8 years
Sun 8 minutes
Moon 1 second
Destination Light travel time
Example:
We see the Orion Nebula as it looked 1, years ago.
M31, The Great Galaxy in Andromeda
How far is a light-year?
1 light - year = (speed of light) × (1 year)
= 300,000 km s
× 365 days 1 yr
× 24 hr 1 day
× 60 min 1 hr
× 60 s 1 min
Can we see the entire universe?
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.
Thought Question Why can’t we see a galaxy 15 billion light-years away? (Assume universe is 14 billion years old.)
What have we learned?
- What is our physical place in the universe?
- Earth is part of the Solar System, which is a member of the Milky Way galaxy, which is a member of the Local Group of galaxies in the Local Supercluster
- How did we come to be?
- 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 stars and then recycled into new star systems, 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
1.2 The Scale of the Universe
- How big is Earth compared to our solar system?
- How far away are the stars?
- How big is the Milky Way Galaxy?
- How big is the universe?
- How do our lifetimes compare to the age of the universe?
Our goals for learning:
How big is the Milky Way Galaxy?
The Milky Way has about 100 billion stars.
On the same ten billion-to- one scale….
Thought 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
How big is the Universe?
- The Milky Way is one of about 100 billion galaxies.
- 10^11 stars/galaxy x 1011 galaxies = 10^22 stars
As many stars as grains of (dry) sand on all Earth’s beaches…
- Now let’s step through the Universe in powers of 10:
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.
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.
How is Earth moving in our solar system?
- Contrary to our perception, we are not “sitting still.”
- We are moving with the Earth in several ways, and at surprisingly fast speeds…
The Earth rotates around its axis once every day.
Earth orbits the Sun (revolves) once every year:
- at an average distance of 1 AU ≈ 150 million km.
- with Earth’s axis tilted by 23.5º (pointing to Polaris)
- and rotating in the same direction it orbits, counter- clockwise as viewed from above the North Pole.
Our Sun moves randomly relative to the other stars in the local Solar neighborhood…
- typical relative speeds of more than 70,000 km/hr
- but stars are so far away that we cannot easily notice their motion … And orbits the galaxy every 230 million years.
More detailed study of the Milky Way’s rotation reveals one of the greatest mysteries in astronomy:
Most of Milky Way’s light comes from disk and bulge …
…. but most of the mass is in its halo
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?
Hubble discovered that:
- All galaxies outside our Local Group are
moving away from us.
- The more distant the galaxy, the faster it is
racing away.
Conclusion: We live in an expanding universe.
1.4 The Human Adventure of Astronomy
- How has the study of astronomy affected human
history?
Our goals for learning:
How has the study of astronomy affected
human history?
- Copernican Revolution showed that Earth was not the center of the universe (Chapter 3)
- Study of planetary motion led to Newton’s Laws of motion and gravity (Chapter 4)
- Newton’s laws laid the foundation of the industrial revolution
- Modern discoveries are continuing to expand our “cosmic perspective”
What have we learned?
- How has the study of astronomy affected
human history?
- Throughout history, astronomy has
provided an expanded perspective on
Earth that has grown hand in hand with
social and technological developments
What have we learned?
- How has the study of astronomy affected human
history?
- Throughout history, astronomy has provided
an expanded perspective on Earth that has
grown hand in hand with social and
technological developments
