Chapter One: Our Place in the Universe
1.1 Our Modern View of the Universe
What is our place in the universe?
o Earth is a planet orbiting the Sun
o Our Sun is one of more than 100 billion stars in the Milky Way Galaxy
o Our galaxy is one of about 40 galaxies in the Local Group
o The Local Group is one small part of the Local Supercluster, which is one small part of
o Ancestors -> heavens revolved around stationary Earth
Earth centered = geocentric
o Earth orbits sun (large star)
o Cosmic Address = our place in the universe
Earth (planet) in Solar System (sun + all orbiting objects/planets)
Sun + stars = Milky Way Galaxy (galaxy = island of stars)
Billions of galaxy in space, some isolated, many in groups
Local Group = galaxies in a group
Galaxy cluster = more than a few dozen galaxies
Arranged in chains; huge voids between them
Supercluster = cluster of galaxy clusters; tightly packed
o Our Local Group is on outskirts of Local Supercluster
o Universe = sum of everything (all matter, energy, superclusters, voids, etc.)
How did we come to be?
o The universe began in the Big Bang and has been expanding ever since, except in
localized regions where gravity has caused matter to collapse into galaxies and stars.
o The Big Bang essentially produced only two chemical elements: hydrogen and helium.
o The rest have been produced by stars, which is why we are “star stuff.”
o Universe is expanding = avg. distance btn galaxies is increasing with time
Galaxies in past were closer together, ultimately as one
o Big Bang = beginning of the Universe
Rate of expansion implies14 billion years ago
o Universe continues to expand on smaller scales
o Galaxies & clusters lie where gravity beat expansion
Universe expands, but galaxies/clusters DON’T
Matter clumped into galaxies/clusters as expansion happened
o Gravity causes gas/dust to collapse & form stars/planets
Stars are NOT living, but have life cycles
Born -> gas/dust compressed by gravity
Dense/hot enough -> nuclear fusion -> atomic nuclei fuse together
Lives -> long as generates enough energy from fusion
Dies -> uses up all its fuel Content goes back into space
o Supernovae -> massive star explosion
Matter becomes new gas/dust; cycle restarts
Yields “generations” of stars/planets
Our solar system = many generations of recycling
Universe beginning -> hydrogen & helium
Earth -> carbon, nitrogen, oxygen, iron
o Must have come from past star cycles
Solar System = 4 billion years old
How can we know what the universe was like in the past?
o Light takes time to travel through space, so the farther away we look in distance, the
further back we look in time.
o When we look billions of light-years away, we see pieces of the universe as they were
billions of years ago.
o Determine history of Universe -> study light from distant stars/galaxies
o Speed of light = 300,000 km/s
Circle Earth 8 times in 1 sec.
1 sec. to reach Earth from Moon
8 mins to reach Earth from Sun
o Light Years = distance light travels in 1 year (10 trillion km) -> DISTANCE, not TIME
o Farther you look in distance = farther you look into past
8 light years away = 8 years for light to reach us (viewing it 8 years in the past)
Can we see the entire universe?
o No. The age of the universe limits the extent of our observable universe.
o Because the universe is about 14 billion years old, our observable universe extends to a
distance of about 14 billion light-years.
o If we tried to look beyond that distance, we’d be trying to look to a time before the
o (Universe = 14 billion years old) + (distance in space = distance in time) = limit on
o If try to look more than 14 billion light years -> looking beyond 14 billion years ago
(Universe DIDN’T exist yet = nothing to see)
Doesn’t limit size of ENTIRE universe, just what we can see
o Observable Universe = 14 billion light years; portion of universe we can see
1.2 The Scale of the Universe
How big is Earth compared to our solar system?
o On a scale of 1 to 10 billion, the Sun is about the size of a grapefruit.
o Planets are much smaller, with Earth the size of a ball point and Jupiter the size of a
marble on this scale. o The distances between planets are huge compared to their sizes, with Earth orbiting 15
meters from the Sun on this scale.
o Voyage Scale Model (Wash. DC) -> “walk” the Solar System
Sun = large grapefruit, Jupiter = marble, Earth = pin tip
Sun is larger than any planets & outweighs all combined by 1,000x
Has everything in straight line; if spread out for orbits, = 300 football fields
Moon -> 4 cm from Earth (Mars is 200x farther)
How far away are the stars?
o On the 1-to-10-billion scale, it is possible to walk from the Sun to Pluto in just a few
o On the same scale, the nearest stars besides the Sun are thousands of kilometers away.
o Voyage Scale Model (Wash. DC)
600 meter distance from Sun to Pluto
Would need to walk to California to reach next star (on that scale)
Nearest star system = 4.4 light years away (4,400 km on scale)
o Blackness of sky allows naked eye to see the star light
Telescopes make it brighter (still can’t see star surface features)
o Voyager 2 -> spacecraft; flown to Jupiter, Saturn, Uranus, and Neptune
50,000 km/hr (take 100,000 yrs to reach closest star system @ speed)
How big is the Milky Way Galaxy?
o Using a scale on which the Milky Way galaxy is the size of a football field, the distance to
the nearest star would be only about 4 millimeters.
o There are so many stars in our galaxy that it would take thousands of years just to count
o Reduced scale: 1 light year = 1 mm
100,000 LR diameter of Milky Way = 100 meters (length of football field)
Galaxy centered mid-field
Solar System = microscopic dot on 20 yard line
Solar System to nearest star system = 4.4 mm
o More than 100 billion stars = 100 billion seconds to count them all (3,000 years!)
How big is the universe?
o The observable universe contains roughly 100 billion galaxies, and the total number of
stars is comparable to the number of grains of dry sand on all the beaches on Earth.
o Milky Way is 1/100 billion galaxies of observable universe
o 100 billion stars/galaxy (estimate), 100 bil. Galaxies x 100 bil. Stars =