A Modern View of the Universe

Goals:
- Understand our place in the universe.
- Comprehend the size of the universe.

Hierarchy: Earth → Solar System → Milky Way Galaxy → Local Group → Local Supercluster → Universe
Universe size approx. $10^{26} km$ or 100 million light years
Local Supercluster size approx. $3 \times 10^{22} km$ or 3 million light years
Local Group size approx. $10^{22} km$ or 10 million light years
Milky Way Galaxy size approx. $10^{18} km$ or 100,000 light years
Solar System size approx. $10^{13} km$ or 60 AU

Star: A large, glowing ball of gas generating heat and light through nuclear fusion.
Planet: A moderately large object orbiting a star, shining by reflected light; can be rocky, icy, or gaseous.
Moon (Satellite): An object orbiting a planet.
Asteroid: A relatively small and rocky object orbiting a star.
Comet: A relatively small and icy object orbiting a star.
Solar (Star) System: A star and all the material orbiting it, including planets and moons.
Nebula: An interstellar cloud of gas and/or dust.
Galaxy: A great island of stars in space, held together by gravity and orbiting a common center.
Universe: The sum total of all matter and energy, everything within and between all galaxies.\n

Astronomical Unit (AU):
- Average distance between Earth and the Sun.
- $150$ million kilometers or $93$ million miles.
Light-year (ly):
- Distance light can travel in one year.
- A distance measurement, not time.
- About $10$ trillion kilometers or $6$ trillion miles.

Light travels at a finite speed of $300,000$ kilometers per second.
Examples:
- Moon: 1 second light travel time
- Sun: 8 minutes light travel time
- Sirius: 8 years light travel time
- Andromeda Galaxy: 2.5 million years light travel time
The farther away we look in distance, the further back we look in time.
Example: We see the Orion Nebula as it looked 1500 years ago; the Andromeda Galaxy as it looked 2.5 million years ago.

Due to the finite speed of light, we can only see as far as light has had time to reach us since the Big Bang.
We see a galaxy 7 billion light-years away as it was 7 billion years ago.
We see a galaxy 12 billion light-years away as it was 12 billion years ago.
Light from nearly 14 billion light-years away shows the universe shortly after the Big Bang.
We cannot see anything farther than 14 billion light-years away because its light has not had enough time to reach us.

If the Sun is the size of a large grapefruit (14 cm) on a 1-to-10-billion scale:
- Earth is the size of a ball point and is 15 meters away from the Sun.
- The Moon is in orbit only 4 cm from Earth.
- Pluto is just a few minutes' walk away.
- Alpha Centauri, the nearest star system, is the distance across the United States (2500 miles).

The Milky Way contains about $100$ billion stars.
Scaling down by a factor of a billion again (1:10^19) allows fitting the Milky Way on a football field.
On this scale, the distance to Alpha Centauri system is less than 5 millimeters.
More than a million stars would lie within an arm's reach.

Counting over 100 billion stars in our galaxy at a rate of one per second would take a few thousand years.

If 1 out of 1 million stars had a planet around it, there would be one-hundred thousand other planets with life in the Milky Way.

Goals:
- Understand how we came to be.
- Compare our lifetimes to the age of the universe.

The universe is expanding.
Galaxies are moving away from each other.
Tracing this motion backward leads to the Big Bang, the point where the expansion began.
Gravity drives the collapse of matter into galaxies and galaxy clusters.
Most galaxies formed within a few billion years after the Big Bang.
Gravity drives the collapse of clouds of gas and dust to form stars and planets.
Stars are born when gravity compresses material to a dense enough state for nuclear fusion.
Stars die after no more fuel is left for fusion.

Heavier elements like carbon, nitrogen, oxygen, and iron are manufactured by stars as they undergo nuclear fusion.

Compressing the history of the universe into one year:
- Humans don’t enter the picture until the evening of December 31st.

Goals:
- Understand how Earth is moving through space.
- Understand how galaxies move within the universe.

Earth is not sitting still; it's moving in several ways at fast speeds.
Earth orbits the Sun (revolves) once every year:
- At an average distance of 1 AU ≈ 150 million kilometers.
- At a speed of more than 100,000 kilometers per hour.
- With Earth's axis tilted by 23.5° (pointing to Polaris).
Earth rotates in the same direction it orbits, counterclockwise as viewed from above the North Pole.

Direction can be given by stating toward or away from the center of an object (e.g., Earth, Sun, Galaxy).

Our Sun moves randomly relative to other stars in the local solar neighborhood.
- Typical relative speeds of more than 70,000 kilometers per hour.
The Sun orbits the galaxy every 230 million years.

Collisions between star systems are extremely rare because the relative distance between stars is much larger than the size of stars.

Detailed study of the Milky Way's rotation reveals one of astronomy's greatest mysteries: most of the galaxy’s mass lies unseen in the spherical halo surrounding the entire disk.

Galaxies are carried along with the expansion of the universe.
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.