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Astronomy: Planets, moons, stars, galaxies
Our nearest galactic neighbor.
M31, a.k.a. the great galaxy in Andromeda
Our solar system in our galaxy
How big?
Pale Blue Dot
Don’t believe what you see on TV or in the
movies!
Kepler Spacecraft Discoveries
The Milky way as seen from Earth.
High res image of Milky Way
Progression
What’s in the “Universe”?
Star Systems - A star and it’s associated
planets is called a Star System. RememberOur sun is a star. All the stars in the sky at night
are suns, much like ours.
Galaxies - Star Systems cluster in groups
called galaxies.
The Universe - All the galaxies together
make the known universe.
The universe is not infinite.
The Nearest star system to
ours is Proxima Centauri.
It is 4 Light Years away
The Voyager Space craft left Earth
in 1977 to photograph the outer
planets.
It passed the orbit of Pluto in 1990
and is now on it’s way to Proxima
Centauri.
It is carrying a message from Earth.
The Golden Record
It is traveling 17 Km per Second.
(What I call “Space Normal Speed”.)
Which is about 40,000 miles per
hour.
At that speed it will arrive at
Proxima Centauri in 73,000 Years!
1 Light year equals 6,000,000,000,000 miles
How Many?
From Star system -> to Universe
There are about 200 billion star systems in a
galaxy.
And
There are about 200 billion galaxies in the
known universe.
Where does that leave you?
How big is a billion??
If you counted 1 number per second how long would it take
you to count to a billion?
To count to 10 takes 10 seconds
To count to 1000 takes 17 minutes
A million would take 12 days
A billion would take 32 years!
Let’s start small
Our star system is
composed of one star,
eight planets, dozens of
moons and countless,
asteroids, comets and
meteoroids.
We are located on the
outer edge of the Milky
Way Galaxy.
If we could leave our
galaxy and look back, it
would look like this.
The world has been known to be round for
thousands of years.
Why did Columbus
sail the ocean blue in 1492?
How it was done with Carl
Sagan "Cosmos"
ISON falls apart as it goes around the sun.
Geocentric Vs. Heliocentric
Early astronomers were Geocentric
which means:
“Earth Centered”
Aristotle (350 B.C.E.) and Ptolemy (100 C.E.)
were two well known Geocentric
astronomers.
But Geocentrism was wrong and later
astronomers became Heliocentric.
Which means:
“Sun Centered.”
Retrograde motion animation
Retrograde motion animation 2
A Little History:
These are the great names in astronomy. Practically
everything we know today is based on the accomplishments
of these people.
Nicolaus Copernicus
1473 – 1543
Tycho Brahe
1546 – 1601
Johannes Kepler
1571 – 1630
Galileo Galilei
1564 – 1642
Isaac Newton
1642 - 1727
Notice how their lives overlapped, allowing them
to hand off their knowledge and discoveries to the next one.
Phases of the moon
The lunar phases are a result of the sunlight reflected by the
moon as seen from Earth.
Half of the moon is always lit by the sun.
But, how much of that lit half we see depends on how much
of the lit half is facing the earth.
In this diagram imagine you are on the earth looking up at the moon. How much
of the moon would you see when the moon is in different locations around the
earth?
How much of the
lit half can you see
from Earth??
See next slide for larger image.
30 days of Lunar Phases
The planets are placed into Three groups based on
their location and physical characteristics.
Terrestrial
and
Jovian
Terrestrial Planets:
Small
Rocky
High density
Thin atmospheres
Close to the sun
Mercury, Venus, Earth,
and Mars
Mars
…and the Dwarf Planets
The planets are placed into two groups based on their
location and physical characteristics.
Jovian Planets:
Huge
Gaseous
Low density
Thick Atmospheres
Far from the sun
Jupiter, Saturn,
Uranus, and Neptune
Pluto is neither terrestrial nor Jovian.
Pluto, and it’s moon Charon, are actually
escaped bodies from the Kuiper Belt and are
now members of a new category called Dwarf
Planets.
Pressure Belts
and Wind Zones
Jupiter with Earth to scale
The sun and planets of our star system
drawn to scale.
This is the correct scale for the planet’s
sizes but not the distance between them.
Touring our solar system
The Sun and all the planets are about 4.5 billion years old.
Mercury
Not much to say.
Small, hot on one side
and cold on the other.
Very high density
Surface very similar to
our moon.
Venus
Window to Venus click here
Nasty place
Extremely hot due to it’s
Greenhouse Effect.
An atmosphere which is 96%
CO2 results in a surface
temperature of 475 degrees C,
(900 degrees Fahrenheit!)
Completely surrounded by
clouds made of Sulfuric Acid.
Rotates backwards
Rotates so slowly that one day
on Venus is longer than one
year on Venus.
Earth
Window to earth click here
Nice place
Oxygen / Nitrogen
Atmosphere
Just enough Greenhouse
effect to allow liquid
water to exist.
75% Water 25% Land
The only planet where water is known to exist
in all three states of matter.
Mars
Window to Mars click here
The red planet
Surface covered with
iron oxide rock and dust.
Iron Oxide is rust.
It has a thin atmosphere.
It has lots of water but
it’s all frozen and
gaseous.
Jupiter
The giant gas planet.
It is not solid.
1300 earths would fit
inside.
It rotates fast, once every
10 hours.
The Great Red Spot is a
cyclonic storm.
Jupiter has about 20 -50
moons.
Saturn
Almost as big as Jupiter.
It has a beautiful and
elegant ring system.
Rings are composed of
material that should have
been a moon. But,
Saturn’s gravity
prevented the moons’
formation.
Uranus
The planet that was
knocked on it’s side.
One more of the giant gas
planets. There is no solid
surface.
Not much to look at in
visible light.
Planet has an unusual
sideways spin.
Uranus: cold and gassy
Neptune
Not much to look at.
It is just a big ball of gas
like Jupiter, Saturn and
Uranus.
It does have an unusual
feature in it’s clouds
known as the Great Dark
Spot.
Neptune and it’s Great Dark Spot
Pluto
Pluto is not considered
a planet.
It does not fit into
either category of
either Terrestrial or
Jovian
Pluto and it’s moon
Charon are probably
escaped bodies from
the Kuiper Belt.
Image of Pluto and it’s moon
Charon taken from the Hubble
Space Telescope
Best, and most recent, image of Pluto.
Minor members of the Solar System
Asteroids: Rocky fragments ranging in size from a
few meters up through a few kilometers. Most are
in a beltlike orbit in between Mars and Jupiter.
Comets
On
Iowa
Comets are frozen lumps
of ice and rock.
They are considered to be
some of the oldest things
in this star system.
They originate in the Oort
Cloud and the Kuiper Belt
millions of kilometers
away from the sun.
Note how a comet’s tail
always points away from
the sun, regardless of what
direction the comet is
traveling.
Our moon
Our moon
Nobody knows exactly where it came from.
There are three main ideas where our moon
came from.
Co-accretion: this model says the moon
formed at about the same time as the earth
and in the same fashion as the earth.
Fission model: this model says the moon
was flung off the earth while the earth was
still molten and spinning much more rapidly
than it is today.
Our moon
And the winner is . . .
The Impact Model: this model says that early in
this star system’s formation, while the earth was
still mostly molten, the earth was struck by a large
body. That object was destroyed along with a
large chunk of the earth. The debris from that
collision condensed, due to it’s own gravity, and
formed our moon.
This is the most widely accepted model today.
Giant Impact Model
Sometimes called “The Big Whack”
The internal structure of the
moon (a relatively small
iron poor core) along with
the presence of elemental
isotopes of oxygen found on
earth but no where else
currently support this model.
Lunar formation models
Our moon: cold, grey and dead
The moon’s surface
has three main
features.
Mountains
Mare’
Impact craters
Mountains on the moon
Unlike the earth, the
moon has no tectonic
activity. So, where do
the mountains come
from?
The mountains on the
moon are formed from
the debris piled up by
countless meteor
impacts.
Mountains on the moon are named after
mountain ranges on earth.
Impact craters
Divots on the moon’s surface.
Some are hundreds of
kilometers across. Some are
microscopic.
All of them are formed by the
impact of meteors.
Meteors are chunks of stone
and iron traveling through
space at tens of thousands of
miles per hour.
When they hit you . . . You
know it!
Lunar Mare’
Very large, smooth, circular
basins on the moon’s surface.
They appear dark when seen
from the earth.
They can be thought of as
supergiant impact craters.
The dark areas in this image are the Lunar Mare’
The lunar Mare’
Hundreds of years ago the Mare’ were thought to be oceans on the
moon. The word Mare’ is latin for ocean or sea.
The moon
Of course the moon has
no water and no
atmosphere whatsoever.
It is a grey, dead rock
hanging in space.
The dark areas in this image are the Lunar Mare’
Farside of Moon
Near and far side of Moon
Ocean tides
Ocean tides on the earth are
caused by the gravitational pull
of the moon and, to a lesser
extent, the sun.
Ocean tides
At any given time there are two high tides and two
low tides on the earth.
There will be one high tide on the side towards the
moon caused by the moon’s gravitational pull.
But, why is there a high tide on the side opposite the moon?
Because on the “Far Side of the earth” the moon’s gravity is
pulling the earth and leaving the water behind.
Spring Tides
The difference between high and low tide is called Tidal Range.
Twice a month tidal range is extreme. The high tides are extra high
and the low tides are extra low.
These tides are called Spring tides and they occur once during the
New Moon and once during the Full Moon.
Spring Tides Note how the Sun – Earth – Moon
are in line and enhance each others gravitational influence.
See next slide for larger image.
Neap Tides
The difference between high and low tide is called
Tidal Range.
Twice a month tidal range is not very extreme.
The high tides are not very high and the low tides
are not very low.
These tides are called Neap tides and they occur
once during the first quarter Moon and once
during the Last quarter Moon.
Neap Tides:
Note how the Sun - Earth - Moon are at right angles.
During Neap tides the
Sun and the moon are
at right angles to the
earth and therefore
their gravitational
forces are working
against each other.
See next slide for larger image.
Neap tides
Eclipses
There are two different types of eclipses.
Solar: the sun is being eclipsed
And
Lunar: the moon is being eclipsed
Solar Eclipse
The moon is blocking out the sun.
The moon’s shadow is small, therefore, the eclipse will be visible from only a
small fraction of the earth’s surface.
This is one reason why Solar eclipses are a rare event to view.
Click here for next total solar eclipse visible from the United States
Lunar Eclipse
The Earth is blocking out the sun.
The earth’s shadow is quite large. Therefore eclipses of the moon are
comparatively common and visible around much of the globe.
Lunar Eclipse
Doesn’t the moon get in between the earth and the sun, causing an
eclipse once every month??
Good question
Not exactly, the moon’s orbit is slightly tilted by about 5
degrees.
Because of this the Earth and moon’s shadow usually miss
each other.
See next slide for a diagram.
Because of this, lunar eclipses only happen during a Full Moon
And solar eclipses only occur when the moon is in it’s New Phase.
The Sun, Stars and Galaxies
The Pleiades star cluster
These are new baby stars
that just formed a few
hundred thousand years ago.
The blue haze around them
will condense into planets,
and everything that will ever
be on those planets.
Including, perhaps, life.
Stars are born in Nebulas
Nebulas are the left over
debris after a star explodes.
Exploding stars are called
Supernovas and they are
the most energetic events
known in the universe.
Nebulas are usually named
for what they look like
The Rosette Nebula
The Horsehead Nebula
So . . . Star systems recycle themselves.
The Ring Nebula
At the center is a white dwarf star.
But, not all stars go
supernova.
Only big, high mass stars
will blow up.
Our sun will not explode.
It is too small
Our star, the sun, will
simply shut down,
becoming a white dwarf.
Star shine
Stars produce energy through thermonuclear fusion
The universe is 99% Hydrogen and Helium.
So the stars, are primarily hydrogen
As the stars gain mass, the Hydrogen nuclei are
squeezed until they fuse and become one.
Hydrogen + Hydrogen = Helium
This is called the Proton – Proton Cycle.
Proton – Proton Cycle
The proton – proton cycle is almost perfectly
efficient
The fusion of 4 hydrogen nuclei generate
temperatures in excess of 20 million degrees C.
As a waste product it produces an atom of helium
and 2 atoms of hydrogen.
What do you think it does with those 2 left over
atoms of Hydrogen?
Proton – Proton Cycle
What do you think it
does with those 2 left
over atoms of
Hydrogen?
It goes back into the
cycle to be fused
again!
Absolutely beautiful
in it’s efficiency.
Proton – Proton Cycle
Because a star makes it’s own hydrogen fuel. It
can go on for a very long time.
Our Sun is a typical star.
It has been producing energy like this for almost
5 billion years.
And it has 5 billion years left to go before all of
it’s hydrogen fuel has been converted to Helium.
Main Sequence
While a star is producing
energy by fusing
hydrogen it is a “normal”
star.
In astronomy “Normal”
stars are called
“Main Sequence” stars
The Hertzsprung-Russell Diagram
Our sun is a typical Main
Sequence star
The Hertzsprung-Russell Diagram
The H-R
Diagram
is a graph which
plots a star’s
Brightness Vs.
Temperature
The Hertzsprung-Russell Diagram
However,
most astronomers
use the “Spectral
Class” across the
top of the
diagram.
Spectral Classification of stars
O
B
A F G K
M
The Spectral Class of stars is
O,B,A,F,G,K,M
With O & B stars being hot and
bright and getting progressively
cooler and dimmer as you
move right through to the K &
M class stars
Spectral Classification of stars
O
B
A F G K
M
Yes, this sequence is
strangely random.
However you can
remember it by using the
following Mnemonic
Memory Device.
Oh Be A Fine Girl Kiss Me
Honestly, every astronomer
in the world uses this
mnemonic 
Temperature Vs. Color
A star’s color is a direct
result of it’s surface
temperature.
Hot stars are Blue
Cool stars are red
Our Sun is medium
temperature @ 6000
degrees C.
That makes it yellow
Everything dies
Stars grow old and die.
How a star dies is predestined by it’s initial mass.
A star of 1 to 5 solar masses will quietly use up the
last of it’s hydrogen fuel, it will briefly become a Red
Giant as it begins to fuse helium. Then just die,
ending it’s life as a White Dwarf.
Everything dies
Stars grow old and die.
How a star dies is predestined by it’s initial mass.
A star greater than 5 solar masses will quietly use up
the last of it’s hydrogen fuel, it will briefly become a
Red Giant as it begins to fuse helium. Then go
Supernova, possibly ending as a Pulsar or Black Hole
A Pulsar is a type of Neutron star
Sometimes a neutron star
begins
to rotate like a light house
emitting X-Rays.
This is called a Pulsar.
When a star blows up it
crushes it’s core making
the core very dense.
Sometimes the core
becomes a Neutron star.
A teaspoon of neutron
star would weigh a billion
tons.
Yes, I said billion.
A Black Hole
Only extremely massive
stars become Black Holes.
(Greater than 5 solar masses.)
The gravitational pull of a
Black Hole is so great. It’s
escape velocity is greater
than the speed of light.
Since nothing can go faster
than light, it is impossible
to escape a Black Hole
Galaxies
Stars are bound, gravitationally, in groups called
galaxies.
There are about 200 to 400 billion stars per galaxy.
And there are about 300 to 400 billion galaxies in
the universe.
The universe is not infinite. There is an end to it.
The edge of the universe is known as “Cosmic
three degree background radiation”.
Galaxies are classified according to their shape
Two galaxies passing through each other.
Irregular galaxies
Irregular Galaxies
Elliptical Galaxies
An elliptical
galaxy with a smaller
irregular galaxy in
orbit around it.
Spiral Galaxies
A spiral
galaxy with a smaller
spiral galaxy in
orbit around it.
Stars are probably moving between them.
Spiral Galaxies
Spiral Galaxy M81
Barred Spiral Galaxies
Note the Bar running across the middle.
Barred Spiral Galaxies
It is a collection of
hundreds of billions of
star systems with
trillions of planets.
How many of those
planets have things
looking back down at
you?
Remember what you are looking at young Jedi.
Barred Spiral Galaxies
Barred Spirals are
my favorite. 
Field of Galaxies
Every point of light
in this image is a
Galaxy
Not a star, a galaxy
So, where do you fit in?