# lecture2

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Announcements
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Class’s web site no live!
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www.astro.umass.edu/~mauro/astro100
Syllabus is on line, check it!
Lectures #1 and 2 are on line
Scales of the Universe
Course Web Site:
www.astro.umass.edu/~mauro/astro100
Today’s Assigned Reading
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Chapter 1, all
Chapter 2, all
Appendix A
Now… on to scales in the Cosmos…
Let’s explore the universe in steps of
increasing magnification, from small
to big
At each step, let’s change scale by
TWO orders of magnitude at a time
and see what happens.
52 feet across
1 mile across
= 6000 feet
=1.6 km
160 kilometers across
1 mile = 1.61 kilometers
Infrared photo
12,756 km
1,600,000 km
=1.6 x 106 km
Moon
1.6x108 km =1.6x1011m
1AU = 1.5x1011 m
AU is a distance unit
1.6 x 1010 km, or a trillion (1012) times wider than the first picture!
32 AU
100 AU
10,000 AU
Stars are so far apart from each other!
If the Sun is a golf ball in NYC, the nearest star is a golf ball in Chicago!!!
106 AU = 17 ly
1 ly =63,000 AU = 1013 km
1700 ly
170,000 ly
1.7 x 107 ly
The local group of galaxies
1.7 x 109 ly
Announcements

Class’s web site is live!



www.astro.umass.edu/~mauro/astro100
Syllabus is on line, check it!
Lectures #1 and 2 are on line
Basic classes of celestial objects:
Comets and Asteriods
Planets
Earth as one
Stars
Sun as one
Galaxies
Milky Way as one
Their sizes
Comets and Asteroids: 10-100 km
Earth:
104 km
Sun:
106 km
Milky Way
1018 km
dust
poppy seed
grape fruit
Earth-Sun distance
A Sense of Space
1. The Sun would hold 1.3 million Earths.
i.e. the radius of the Sun is about 100 times
that of the Earth.
2. There are ~100 billion &quot;Suns&quot; in a galaxy like our
own Milky Way Galaxy.
3.Astronomers can see billions of galaxies.
Your local address:
What is our address in the
Universe?
•UMass, Amherst, Massachusetts, USA
•Earth,
•Solar System
•Milky Way Galaxy,
•Local Group,
•Local Supercluster
•Our Universe
•Are there other universes? Most likely yes!
•Multi-verses as opposed to Uni-verse
So…How Big is the Observable
Universe anyway?
…about 14 billion-billion-billion centimeters in diameter
or
14,000,000,000,000,000,000,000,000,000 cm
or
1.4x1028 cm
or
14 billion ly
or
6000 Mpc
The finite speed of light lets us “look” into the past!
Supernova
Earth
100,000 l-years
now
97,997 BC
Consider this:
If you had friends throughout the galaxy, how could you
create a “live” TV show?
If the Sun were a grapefruit on one side of the
room, what would be Earth?
1) A poppy seed on the other side.
2) Another grapefruit on the other side.
3) A poppy seed in my office.
4) An apple on the other side.
If the Sun were a grapefruit on one side of the
room, what would be Earth?
1)
A poppy seed on the other side.
(Earth-Sun distance~100x radius of Sun~ 100 radius of Earth)
2) Another grapefruit on the other side.
3) A poppy seed in my office.
4) An apple on the other side.
What would then be
the nearest star?
1) A poppy seed at the Amherst downtown.
2) A poppy seed on the west coast
3) A grapefruit at the Amherst downtown.
4) A grapefruit on the west coast.
What would then be
the nearest star?
1) A poppy seed at the Amherst downtown.
2) A poppy seed on the west coast
3) A grapefruit at the Amherst downtown.
4) A grapefruit on the west coast.
(Nearest star is 4Ly ~ 300,000 AU)
A Sense of Time
If we were to compress the time since the Big Bang
into one year, and make the time of the Big Bang
January 1,
 The Earth was formed in mid-September.
 The mammals appeared on December 26.
 All human prehistory (from the first known stone
tools) and history have occurred in the last &frac12; hour of
New Year's Eve.
All of human history is but a fleeting instant on the
cosmic timescale.
So MANY objects….How Do We
Make Sense of it ALL??
Our investigation of the universe has become quite sophisticated…
Here is the NASA/ESA Hubble Space Telescope (HST)
The Sky and its Cycles
Goals:
To describe and locate objects in the sky
To understand the apparent motions of celestial
objects
To introduce the seasons
How Do We Locate Objects on the
Sky?
The Celestial Sphere (C.S.)
Stars, planets, the Sun , external galaxies, are at different distances from
us, yet for studying patterns in the sky, we represent them in projection
onto the C.S.
Angular Size
The Sun is physically 400 times larger than the Moon.
Why is their angular size is the same?
S
a
arad = S/D
Angle diameter (in radians) = diameter/distance
Full circle = 2 (in radians)=360 degrees=360o
1o=60 arminutes = 60’
1’ = 60 arcseconds = 60” ; 1 rad = 206,264.81”
Angular Size
Some Examples:
Horizon to zenith (point overhead)
90o
Your fist at arm's length
10o
Your fingernail at arm's length
1o = 60'
Sun or Moon seen from Earth
0.5o = 30'
Smallest detail visible to naked eye
1' = 60''
Smallest detail visible by a single telescope from
Earth's surface:
1''
Angular size of Pluto: 0.15”
Betelgeuse (largest star) seen from Earth 0.004''
The Orion
Constellation
Constellations
88 official constellations divide the sky into
areas with clearly boundaries.
The names of constellations are in Latin. But
most bright star names derived from ancient
Arabic.
The original constellations were invented by
farmers over 5000 years ago.
Remember that the constellations are not real!
They are just imaginative figures “invented by
ancient cultures
Projection
Big Dipper ---- an asterism
Humans have logarithmic senses
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Our senses are logarithmic, i.e. respond logarithmically to
stimuli.
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If a stimulus I is exerted on our senses (vision, hearing,
touch, all work the similarly)…
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… our body DOES NOT sense I, but sense Log(I) instead
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For example, if one doubles I (e.g. turn the volume of the
sound from 3 to 6), we only perceive an increment of sound
of Log(2)=0.3, i.e. 30% louder
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Same thing for vision
Luminosity and Magnitude
Log(I) is what we sense we our eyes
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mV = -2.5*Log(I) + C
I is the true intensity of the light
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m1 – m2 = -2.5*Log(I1/I2)
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If I1=2xI2, then m1 – m2 = -2.5*Log2 = -0.75
If I1=10xI2, then m1 – m2 = -2.5*Log10 = -2.5
If I1=100xI2, then m1 – m2 = -2.5*Log100 = -5
I1/I2 = 10-0.4*(m1 – m2) = 2.512(m1 – m2)
The Motions of Earth
1. The Earth rotates – this leads to day and night.
2. The Earth revolves around (orbits) the Sun – this
leads to the seasons.
3. The Earth and Sun revolve around (orbit) the
center of the Galaxy.
4. The Galaxy moves through the Universe.
Latitude and
longitude make a
convenient coordinate
system for locating
objects on the Earth.
Altitude and Azimuth
give us the direction
to look on the celestial
sphere.
Time Zones and Universal Time
Time
We live on a rotating sphere (Earth) that receives lights
only from one source (the Sun).
If here is day, on the other side of Earth it is night.
So, how do we synch the time across the planet?
The time for many astronomical events is given in
Universal Time (UT), which is (approximately) the
local time for Greenwich, England --- the Greenwich
Mean Time or GMT.
Cycles of the Sky
Earth's rotation causes the sun and stars to rise in the
east and set in the west daily.
But stars and the Sun do not set and rise daily
everywhere!
Near the poles, the sun rises and sets every half year,
and circumpolar constellations such as Ursa Major, never
set.
What other cycles do you know?
There are two ways
to think about the
motion of the skey:
- the C.S. rotates
- or the Earth
rotates while the
sky sits still.
Sometimes one
works better than
the other …
The altitude of Polaris
above the horizon is
approximately the
same as the observer's
latitude in the
Northern Hemisphere.
Are there stars in the sky during the daytime?
How long to go all the
way around?
1 day
b.
1 week
c.
1 month
d.
1 year
Earth also orbits around the Sun. Takes 1 year for one round.
Note the tilt between the ecliptic and Earth’s rotation axis
Ecliptic
The constellations along the ecliptic are called the zodiac
Everything in the Universe is
Moving! … but …
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the further away from us something is, the more it has
to move to be noticeable.
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the motion of the Earth rotating on itself (day/night)
is the easiest to see
Followed by the motion of the Moon around the
Earth (~29 days)
followed by the motion of the Earth around the Sun
and finally the motion (over tens or hundreds of
years) of close stars
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What causes the seasons?
What are the differences between days in
summer and winter?
Longer days
stronger Sun
in summer than in winter
Seasons are due to the tilt of the earth.
Are the seasons due to the changing
distance from the sun? NO!
(Not to scale)
The Earth-Sun distance does NOT matter with the seasons!