Download Public Lecture - Size of the Universe

SIZE OF THE UNIVERSE
James Martin
Facebook.com/groups/AstroLSSC
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“Somewhere, something incredible is waiting
to be known.”
-Carl Sagan
Measuring Distance
• Base unit = meters (m) and kilometer (km)
– Used to describe “smaller” distances (ie: Earth to Moon)
• Astronomical Unit (AU) = 1.496 x1011 m
– AU is the average Earth-Sun distance
– Used to measure solar system distances
• Light year (ly) = 63240 AU
– The distance light travels in one year!
– Used to measure distances between nearby stars
• Parsec (pc) = 3.26 ly
– Used to measure galactic distances
– Kiloparsec (1000 pc) and Megaparsec
(1,000,000 pc) used for extragalactic distances
Distance is a Measure of Time
• All information comes in the form of light
– Max light speed = 300,000 km/s
• Farther away an object is (spatial distance), the more time
light has taken to reach us
• Celestial objects are not observed as they are, but rather
as they were
16 x 16 m
1 mile x 1 mile
100 miles x 100 miles
Diameter of the Earth 12,756 km
Earth-Moon Distance: 384,000 km
Sun-Earth Dist.: ~150,000,000km (or 1 AU)
Solar System (approx. 100 AU)
(Almost) Empty Space Around Our Solar
System: 10,000 AU
Solar Neighborhood (17 ly)
Nearest star to the Sun:
Proxima Centauri, ~4.2 light years
Traveling constant at the fastest recorded manned
rocket speed, it would take more than 114,000 years to
reach Proxima Centauri!
Extended Solar Neighborhood (1,700ly)
Diameter of Milky Way ~100,000 ly
Local Group of Galaxies
Distance to the nearest large galaxies:
several million light years
The Universe on Very Large Scales
Clusters of galaxies are grouped into superclusters.
Superclusters form filaments and walls around voids.
How big is the Universe?
• The Milky Way is one of about 100-500 billion galaxies
• Each galaxy is estimated to contain approximately 100-400
billion stars
• Estimated number of stars
– Low: 10 billion, billion = 10 sextillion
– High: 200 billion, billion = 200 sextillion
• That’s a lot of stars!
Scientific Notation
• Compact manner in which very large numbers are made
manageable
– For example: 5.9742 x 1024 kg = 597,420,000,000,000,000,000 kg
Name (U.S.) Number
Scientific
Notation
How long would it take to count to
this number from 0
One
1
100
1 second
Thousand
1,000
103
17 minutes
Million
1,000,000
106
12 days
Billion
1,000,000,000
109
32 years
Trillion
1,000,000,000,000
1012
32,000 years (longer than any civilization
on Earth)
Quadrillion
1,000,000,000,000,000 1015
32 million years (longer than humans on
Earth)
How big is the Universe?
• Postulate: The Universe has as
many stars as grains of dry sand
on all of Earth’s beaches.
• How many grains of dry sand are
there on all of Earth’s beaches?
𝑆𝑡𝑎𝑟𝑠 ≥ 𝑆𝑎𝑛𝑑?
Calculating Grains of Sand
• One grain of dry sand measures approximately 0.5 mm
– 20 mm of side-by-side sand would create 1 centimeter (< 0.5 in)
• To estimate the amount of sand on all of Earth’s beaches
– Must estimate the average volume of a sandy beach and the
average amount of coastlines which are beaches
– Estimated to be 700 trillion cubic meters of beach
• or about 2.5-10 sextillion grains of sand
At the low end, there are just as many, if not more stars in
the universe than grains of sand on all of Earth’s beaches!
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