End of Ch. 1

1.1 Our Modern View of the Universe Our goals for learning: What is our place in the universe? We are on a planet, orbiting a star, in a galaxy (which is a member of the Local Group of galaxies in the Local Supercluster) How did we come to be? How can we know what the universe was like in the past? Can we see the entire universe?

STAR A large, glowing ball of gas that generates heat and light through nuclear fusion

PLANET A moderately large object which orbits a star; it shines by reflected light. Planets may be rocky, icy, or gaseous in composition.

The definition of a planet is not so clear What about Pluto? What about objects larger than Pluto that have been discovered? What about asteroids? The definition of a planet is not so clear (it was much easier for the ancient Greeks….for them a planet was a star that moved and there were 5 of them)

MOON An object which orbits a planet.

ASTEROID A relatively small and rocky object which orbits a star.

COMET A relatively small and icy object which orbits a star.

SOLAR (STAR) SYSTEM A star and all the material which orbits it, including its planets and moons

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

THE UNIVERSE The sum total of all matter and energy; that is, everything within and between all galaxies

Cosmic Calendar: see details in book AGE OF UNIVERSE Age of Universe: Age of Solar System: Cosmic Calendar: see details in book

1.1 Our Modern View of the Universe Our goals for learning: What is our place in the universe? We are on a planet, orbiting a star, in a 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. How can we know what the universe was like in the past? Can we see the entire universe?

Where do we come from? The first (and simplest) atoms were created during the Big Bang. More complex atoms were created in stars. When the star dies, they are expelled into space…. to form new stars and planets! Most of the atoms in our bodies were created in the core of a star!

SPEED OF LIGHT The speed of light in the vacuum of space is constant! All light travels the same speed! c = speed of light = 290,000,000 m/sec = 2.9 x 108 m/sec = 290,000 km/sec

Looking back in time Light, although fast, travels at a finite speed. It takes: 8 minutes to reach us from the Sun 8 years to reach us from Sirius (8 light-years away) 1,500 years to reach us from the Orion Nebula The farther out we look into the Universe, the farther back in time we see!

1.1 Our Modern View of the Universe Our goals for learning: What is our place in the universe? We are on a planet, orbiting a star, in a 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. 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?

1.1 Our Modern View of the Universe Our goals for learning: What is our place in the universe? We are on a planet, orbiting a star, in a 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. 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? Nope!

1.1 Our Modern View of the Universe Our goals for learning: What is our place in the universe? We are on a planet, orbiting a star, in a 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. 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. ALSO (not in Ch. 1 of the book), we can “see” only about 4% of the universe, 96% is made of “dark matter” and “dark energy”.

What have we learned? How is Earth moving in our solar system? It rotates on its axis once a day and orbits the Sun at a distance of 1 AU = 150 million km 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 expansion of the universe: the more distant they are, the faster they’re moving Are we ever sitting still? No!

1.2 The Scale of the Universe Our goals for learning: How big is Earth compared with 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?

The scale of the solar system On a 1-to-10 billion scale: Sun is the size of a large grapefruit (14 cm) Earth is the size of a tip of a ball point pen, 15 meters away. Use this slide/tool to introduce the Voyage scale (Voyage is the name of the model solar system using this scale on the National Mall in Washington, DC). At this point,we suggest that you stick to the solar system in using this tool; we’ll move out to the stars in the next couple of slides. Other points to emphasize: This tool taken from the Scale of the Universe tutorial on the web site; encourage students to try it for themselves. Voyage shows a straight line, but remember that planets orbit. In class, put the grapefruit for the Sun on a table, then walk 15 meters to Earth position, and ask students to imagine it going around Sun once a year. A major lesson from all this is that the solar system is almost entirely made of nearly empty space (that’s why they call it “space”!).

the distance across the United States (2500 miles) 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 the nearest star, Alpha Centauri? 1 mile 10 miles 100 miles the distance across the United States (2500 miles) Now we continue outward to the stars. Try asking this question of your students; most are quite surprised at how far away the stars are…

How do our lifetimes compare to the age of the Universe? The Cosmic Calendar (p.14 and 15): A scale on which we compress the history of the universe into 1 year. Our favorite way to present the scale of time: a modified version of Carl Sagan’s Cosmic Calendar. Worth noting: Since we are compressing the 14 billion-year history of the universe into one calendar year, 1 month represents about 1.2 billion real years, 1 day represents about 40 million years; 1 second represents about 440 years. the universe already 2/3 of the way through its history before our solar system even formed. dinosaurs arose the day after Christmas, died yesterday. All of (recorded) human history is in the last 30 seconds. You and I were born about 0.05 seconds before midnight, Dec. 31.

What have we learned? How big is the Earth compared to our solar system? On a scale of 1-to-10 billion, the Sun is about the size of a grapefruit. The Earth is the size of a tip of a ball point pen about 15 m away. The distances between planets are very large compared with their sizes. How far away are the stars? On the same scale, the stars are thousands of kilometers away. How big is the Milky Way Galaxy? 100 billion stars; it would take more than 3,000 years to count the stars in the Milky Way Galaxy at a rate of one per second. The Milky Way Galaxy is about 100,000 light-years across.

What have we learned? How big is the universe? 100 billion galaxies in the observable universe 14 billion light-years As many stars as grains of sand on 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 one year, human civilization is just a few seconds old, and a human lifetime is a fraction of a second.

Our goals for learning: 1.3 Spaceship Earth Our goals for learning: 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?

Are we ever sitting still? Earth rotates on axis: > 1,000 km/hr Earth orbits Sun: > 100,000 km/hr Solar system moves among stars: ~ 70,000 km/hr Milky Way rotates: ~ 800,000 km/hr Milky Way moves in Local Group This slide summarizes our motion with spaceship Earth… Universe expands

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? Describe the raisin cake analogy, and have students work through the numbers with you to make the table. (E.g., “How far away is Raisin 1 at the beginning of the hour? [1 cm] How far is it at the end of the hour? [3 cm] So how far would you have seen it move during the hour? [2 cm] So how fast is it moving away from you? [2 cm/hr]”

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. Now relate the raisin cake analogy to the real universe…

Outline of lecture 2 (Ch 2) Patterns in The Sky: Stars and constellations Celestial coordinates: Celestial sphere, poles, equator, ecliptic, right ascension*, declination* (*not in book) Seasons: Tilt in Earth’s axis (23.5 degrees), Equinoxes and soltices, precession The Moon and Eclipses Lunar and Solar Eclipses Ancient Mystery of the Planets: Apparent Retrograde motion of planets

2.1 Patterns in the Night Sky Our goals for learning: What does the universe look like from Earth? Why do stars rise and set? Why do the constellations we see depend on latitude and time of year?

What is a constellation? A constellation is a region of the sky. The sky is divided into 88 official constellations.

Constellations Most official constellation names come from antiquity. The patterns of stars have no physical significance! Stars that appear close together may lie at very different distances. Modern astronomers use them as landmarks.

Constellations: Orion

What is the celestial sphere? An imaginary sphere surrounding the Earth upon which the stars, Sun, Moon, and planets appear to reside.

North & South celestial poles The Celestial Sphere North & South celestial poles the points in the sky directly above the Earth’s North and South poles celestial equator the extension of the Earth’s equator onto the celestial sphere ecliptic the annual path of the Sun through the celestial sphere, which is a projection of ecliptic plane

The Celestial Sphere If you do not have a model of the celestial sphere to bring to class, you might wish to use this slide; you will probably want to skip it if you have a model that you can discuss instead…

The Milky Way You’ve probably seen this band of light across the sky. What are we actually seeing?

The Milky Way Our Galaxy is shaped like a disk. Our solar system is in that disk. When we look at the Milky Way in the sky, we are looking along that disk.

Measuring the Sky We measure the sky in angles, not distances. Full circle = 360º 1º = 60 arcmin 1 arcmin = 60 arcsec

Measuring Angles in the Sky

the point directly above you The Local Sky zenith the point directly above you horizon all points 90° from the zenith altitude the angle above the horizon meridian due north horizon zenith due south horizon

Review: Coordinates on the Earth Latitude: position north or south of equator Longitude: position east or west of prime meridian (runs through Greenwich, England)

The Daily Motion As the Earth rotates, the sky appears to us to rotate in the opposite direction. The sky appears to rotate around the N (or S) celestial poles. If you are standing at the poles, nothing rises or sets. If you are standing at the equator, everything rises & sets 90 to the horizon.

Time Exposure Photograph: Star Trails

The Daily Motion daily circles --- CCW looking north, CW looking south

Annual Motion As the Earth orbits the Sun, the Sun appears to move eastward with respect to the stars. The Sun circles the celestial sphere once every year.

2.3 Seasons What is the cause of the seasons on Earth?

Annual Motion The Earth’s axis is tilted 23.5° from being perpendicular to the ecliptic plane. Therefore, the celestial equator is tilted 23.5° to the ecliptic. As seen from Earth, the Sun spends 6 months north of the celestial equator and 6 months south of the celestial equator. Seasons are caused by the Earth’s axis tilt, not the distance from the Earth to the Sun!

Annual Motion ecliptic equinox solstice zodiac the apparent path of the Sun through the sky equinox where the ecliptic (the Sun) intersects the celestial equator solstice where the ecliptic (the Sun) is farthest from the celestial equator zodiac the constellations which lie along the ecliptic

The Cause of the Seasons

Coordinates on the Celestial Sphere (not in book) Latitude: position north or south of equator Longitude: position east or west of prime meridian (runs through Greenwich, England) Declination: position north or south of celestial equator (in degrees) Right Ascension: distance (in hours, 0 to 23h 59 min.) East of vernal equinox (where the sun crosses the celestial equator going North)

Question 1 What point on the celestial sphere is defined by a Right Ascension of zero hours and a Declination of zero degrees?

Question 2 What point on the celestial sphere is defined by a Right Ascension of 12 hours and a Declination of zero degrees?

Question 3 What is an equinox?

Question 4 What point on the celestial sphere is defined by being on the ecliptic at a Declination of +23.5 degrees?

Question 5 What point on the celestial sphere is defined by being on the ecliptic at a Declination of minus 23.5 degrees (i.e. 23.5 degrees South)?

Axis tilt causes uneven heating by sunlight throughout the year.

When is summer? (in the Northern Hemisphere) The solstice which occurs around June 21 is considered the first day of summer. However, it takes time for the more direct sunlight to heat up the land and water. Therefore, July & August are typically hotter than June.

2.4 Precession What is the Earth’s cycle of precession?