Our Cosmic Address:
Earth, Sun/Solar, System,Milky Way, Galaxy,Local Group,Local, Supercluster
Scale Of the Universe:
1:1010 (1:10,000,000,000) -- Sun = grapefruit • Earth = pinhead, 15 meters • Mars = pinhead, 23 meters
• Jupiter = marble, 78 meters • Pluto = tiny grain, 1/4mile away
New Scale for the Galaxy: 1:1019
: Each light year is about 1 mm; Stars are microscopic located 2 mm apart; Way galaxy is
~100 meters in diameter, contains 1011
(100’s of billions) of stars. On this scale, the nearest stars would be a system formed by a
cantaloupe, a small apple and a kiwi fruit, located in the middle of Alaska. There is essentially nothing (Nothing = solar/stellar
winds, magnetic fields, atoms, molecules, cosmic rays, neutrinos, photons (light), dust, rocks, comets, dark matter, dark energy.
Yet Another Scale for Everything Else:
Galaxies are 10” paper plates, Milky Way and nearest neighbor Andromeda are 5
meters apart, Galaxy groups and clusters contain 10’s to 1000’s of galaxies, Superclusters 50 meters across (size of buildings in
our scale model) are the largest structures we see, Observable universe is about size of Boulder county on this scale.
How to grasp astronomical numbers?
Powers of 10:
count the number of zeros behind the digit; 1000= 1 thousand;
1,000,000 = 1 million; 1,000,000,000 = 1 billion .
.. 10,000,000,000,000,000,000,000 = approximately the number of stars in the
observable universe - more than the grains of sand on all the beaches on Earth
Measuring cosmic distances:
Most useful measure is based on the speed of light = 300,000 km/sec = 3 x 1010 cm/sec ; Like
saying “I live 30 min from Boulder” ; Constant speed for light traveling in space; Nothing travels faster through space.
which of the following sentences makes sense? A. We’ll wait light years before Mars is as close as it is tonight (Not
A: light years measure distance, not time. 1 ly ~ 10 trillion km = 1016 m) B. The Galileo spacecraft has traveled 22 light years
since its launch in 1989 (Not B: Light travels 1 light year in 1 year. Nothing travels faster than light!)
C. The globular cluster
M13 is located 16,000 light years away from Earth.
D. We’re light years ahead of the competition.
Measuring distances with light
: Earth-Moon = 1.5 light-seconds; Earth-Sun (a.k.a. astronomical unit, or AU)
= 8 light minutes;
Solar system = light hours; Nearest stars = several light-years; Milky Way= 100,000 light years = 105 ly; Local group = several
million light years =106 ly; Observable universe = 14 billion ly =
1.4 x 1010 l.
A message from outer space arrived today which was sent on the day you were born. The friendly aliens sending you
the birthday message live:
In a galaxy in the local group B.
In the solar system
In the local arm of the Milky Way
In a galaxy far, far away E.