This Week in Astronomy Internet hoaxers were at it again, predicting that an asteroid would crash near Puerto Rico between September 15 and September 28. No Asteroid Doom! Question: How do they know where it will hit if they don’t know to within two weeks when it will hit?

Homework  The access code for Mastering Astronomy is bundled with your textbook, or can be bought online.  From Blackboard, click on "MasteringAstronomy.com instructions") to receive instructions on setting up an account on Mastering Astronomy.  The course ID for this class is: IRWINAY101F2015  Do this as soon as you can, as a practice (ungraded but mandatory) homework assignment designed to get you comfortable with the MasteringAstronomy.com interface will be assigned today.  The access code for Mastering Astronomy is bundled with your textbook, or can be bought online.  From Blackboard, click on "MasteringAstronomy.com instructions") to receive instructions on setting up an account on Mastering Astronomy.  The course ID for this class is: IRWINAY101F2015  Do this as soon as you can, as a practice (ungraded but mandatory) homework assignment designed to get you comfortable with the MasteringAstronomy.com interface will be assigned today.

Homework #1  Due Monday, August 31, 6PM  Designed to get your familiar with the MasteringAstronomy.com interface  Mandatory, but ungraded (as long as you complete it on time, you get full credit). You will receive 100% even if the program says you did not receive a 100%.  Lose 10% per day for every day late, for 5 days – after that you can still complete the assignment until the day before the final exam for 50% credit (this will be true for all homeworks).  Due Monday, August 31, 6PM  Designed to get your familiar with the MasteringAstronomy.com interface  Mandatory, but ungraded (as long as you complete it on time, you get full credit). You will receive 100% even if the program says you did not receive a 100%.  Lose 10% per day for every day late, for 5 days – after that you can still complete the assignment until the day before the final exam for 50% credit (this will be true for all homeworks).

The Length Scale of the Universe

How large is the Solar System?

How Large is the Solar System? Wrong! Children’s books often give us the wrong impression of how large planets are relative to their distances. If the Sun is a grapefruit, Neptune is a small pebble located somewhere 3 blocks away! Our Solar System is almost entirely empty space !

How Far Away is the Nearest Star on This Same Spatial Scale?

Clickers: Step 1: Press the Channel button Step 2: Type in “12” (will always be 12 for this course) Step 3: Hit “Enter” Step 4: When question is asked, hit “A”, “B”, “C”, etc. button

How far away is the nearest star (if the Sun is the size of a grapefruit in Washington DC)? A) 20 blocks B) 5 miles C) 50 miles D) in California E) current Earth-Moon distance Full credit (4/4) for all who answer.

How far away is the nearest star (if the Sun is the size of a grapefruit in Washington DC)? A) 20 blocks B) 5 miles C) 50 miles D) in California E) current Earth-Moon distance Full credit (4/4) for all who answer.

Nearest Star at Distance of San Francisco! Actual distance: 4.3 light years

Galaxy If stars are so sparsely spaced, then why do galaxies look like this?

Galaxy Answer: Our telescopes are unable to resolve the actual size of all the stars – we see a blur of many, many faint stars. Galaxies are mostly empty space (neglecting diffuse gas they contain)!

How large is the Galaxy? Venus 11m Earth 15m Uranus 287m Neptune 450m Pluto 590m Mercury 6m Let’s shrink the Universe by another factor of 1 billion 1:10 19 (10 quintillion!) scale – equivalent to the Milky Way Galaxy being as big as a football field. Sun is now the size of an atom. Sun How far from Sun to nearest star?

How far away is the nearest star (if the Milky Way Galaxy is the size of a football stadium and the Sun is at the 20 yard line)? A) 4 millimeters away B) 1 foot away C) 17 yard line D) the nearest end zone Full credit (4/4) for all who answer.

How far away is the nearest star (if the Milky Way Galaxy is the size of a football stadium and the Sun is at the 20 yard line)? A) 4 millimeters away B) 1 foot away C) 17 yard line D) the nearest end zone Full credit (4/4) for all who answer.

How large is the Galaxy? Venus 11m Earth 15m Uranus 287m Neptune 450m Pluto 590m Mercury 6m 1:10 19 scale – equivalent to the Galaxy being as big as a football field. Sun How far from Sun to nearest star? 4 mm! (half the width of your pinky)

How large is the observable Universe (if the Milky Way Galaxy is the size of a football stadium)? A) the size of Tuscaloosa B) the size of Texas C) the size of the Pacific Ocean D) the size of our current Solar System Full credit (4/4) for all who answer.

How large is the observable Universe (if the Milky Way Galaxy is the size of a football stadium)? A) the size of Tuscaloosa B) the size of Texas C) the size of the Pacific Ocean D) the size of our current Solar System Full credit (4/4) for all who answer.

How large is the Observable Universe? Venus 11m Earth 15m Uranus 287m Neptune 450m Pluto 590m Mercury 6m 1:10 19 scale Sun If the Milky Way is the size of a football stadium how big is the observable Universe? Slightly larger than the Pacific Ocean!

 Now let’s step through the universe in powers of 10:

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

The Time Scale of the Universe

 Current Age ~ 13.7 billion years  Sun's Age ~ 4.6 billion years  Current Age ~ 13.7 billion years  Sun's Age ~ 4.6 billion years

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 Big Bang occurred on January 1 on this calendar, ~14 billion years ago.  The Big Bang occurred on January 1 on this calendar, ~14 billion years ago. 1 month = ~1 billion years 1 day = ~40 million years

How do our lifetimes compare to the age of the universe?  Matter condenses, and galaxies form about a billion years later – late January/early February.

How do our lifetimes compare to the age of the universe?  Sun, Earth, solar system form about 4.6 billion years ago – early September.

How do our lifetimes compare to the age of the universe?  Simple single cell organisms appear on Earth in late September.

How do our lifetimes compare to the age of the universe?  Life on Earth greatly increases in variety during the Cambrian Period in mid-December (including trilobites).

How do our lifetimes compare to the age of the universe?  Dinosaurs dominate Earth shortly after Christmas.

How do our lifetimes compare to the age of the universe?  Dinosaurs go extinct 4 days later.

How do our lifetimes compare to the age of the universe?  Early human ancestors evolve when the New Year’s party gets started.

How do our lifetimes compare to the age of the universe?  Modern humans (Homo Sapiens) arrive as the Ball begins to drop in Times Square.

How do our lifetimes compare to the age of the universe?  Humans begin farming 25 seconds before midnight.

How do our lifetimes compare to the age of the universe?  Egyptians built the pyramids 11 seconds before midnight.

How do our lifetimes compare to the age of the universe?  Our modern view of the solar system was established by Kepler and Galileo 1 second before midnight.

How do our lifetimes compare to the age of the universe?  Typical college student born 0.05 (1/20 th ) seconds before midnight.

A Universe in motion  We are not “sitting still.”  We are moving with the Earth.  and not just in one direction  We are not “sitting still.”  We are moving with the Earth.  and not just in one direction The Earth rotates around its axis once every day. ~1500 km/hr in Alabama (0.4 km/s)

Rotation vs. Revolution Rotation - spinning of an object about its own axis (e.g., a spinning top) - Earth spins on its axis once per day Revolution - movement of an object around another object (e.g., a ball tied to a string) - orbit of the Earth around the Sun every 365 days

The Earth orbits (revolves) around the Sun once every year The Earth’s axis is tilted by 23.5°. 108,000 km/hr, or 30 km/s…

Our Sun moves relative to the other stars … Our Sun and local stars orbit around the center of the Milky Way Galaxy every 230 million years. 800,000 km/hr, or 220 km/s… ~70,000 km/hr, or 19 km/s

Galaxies have orbital motions in clusters HST Several hundred to few thousand km/s

All of this together with the expansion of the Universe

Edwin 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.  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.

The expansion of the Universe Galaxies are moving away from us. The farther they are from us, the faster they go. They all move apart from each other: space itself expands. No galaxy can be thought of as the “central” galaxy

Are we ever sitting still? NO! No place in the Universe is “at rest”. We can’t even define what “at rest” means.

Chapter 1 Study Guide 1)Planets, moons, asteroids, comets, stars, nebula, star clusters, galaxies, groups/clusters of galaxies, Universe - know their context and relative sizes 2) Finite speed of light means we see objects as they were in the past, when the light was emitted - looking at more distant objects means we are looking farther back in time 3) Our cosmic address: Earth  Solar System  Milky Way Galaxy  Local Group of galaxies  Universe

4) Kilometers, AUs, light years – know when each distance unit is appropriate 5) Space is mostly empty: think of two grapefruit Suns separated by the length of the United States 6) Cosmic Calendar: Earth formed in September; all of modern human history occurred in last 2 minutes before New Year’s. 7) Nothing is at rest: Earth spins  Earth revolves around Sun  Sun moves among other stars, around Milky Way  Galaxies moves  Universe expanding Chapter 1 Study Guide