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Stars and Galaxies
Earth Science 11
Chapter 21
Constellations
Constellation:
A groups of stars that appear to form patterns in the sky.
88 different constellations can be seen from the Northern and
Southern hemispheres.
Best known is the Big Dipper:
Part of a larger constellation called Ursa Major, or the Big Bear
Constellations can be used for:
Navigational aids
Find other constellations including POLARIS and “North Star”
Constellations
Circumpolar constellations:
Never set below the horizon
In the north appear to rotate around the north star
How many you see depends on your latitude
EX. URSA Major, URSA Minor and Cassiopeia (northern)
The apparent movement of these circumpolar constellations is
due to the earth’s rotation.
Earth rotates from WEST to EAST
Therefore stars, the moon, and the sun all RISE in the EAST and
SET in the WEST
Seasonal Changes in
Constellations
Circumpolar constellations
change in the sky with the
change in seasons
Fall:
Big dipper is near northern
horizon
Cassiopeia is nearly
straight overhead
Spring:
Big dipper is high overhead
Cassiopeia is near northern
horizon
Seasonal Changes in
Constellations
Each star is moving on its own, sometimes at very high
velocities.
In time the constellations we are familiar with today will no
longer be there
Each star will have moved and new constellations will be made
Some constellations only appear at certain times of the
year.
The most famous winter constellation is Orion the Hunter
Orion contains the red supergiant Betelgeuse AND the blue
supergiant Rigel
Distances to Stars
The closest star to the earth is the SUN
Distance is measured 2 ways:
1. Astronomical unit (AU):
The average distance between earth and the sun is 150 million km
Sun – 1AU
Jupiter = 4AU (at closest encounter)
Pluto = 38AU (at closest encounter)
How far is the nearest star?
If the earth was a dot 1 cm away from the sun then using the same scale
the next star would be 2.5 km away (Alpha Centauri)
In km Alpha Centauri is 40 trillion km away. Using km is not practical in
astronomy because the scale is too small.
2. Light year (LY)
Distance light travels in 1 year (9.5 trillion km in 1 year)
Ex. Earth to:
Moon = 2 sec
Sun = 8 mins
Alpha Centaury = 4.3 LY
North Star = 680 LY
Physical Properties of Stars
Our sun is
classified as a
Yellow Dwarf
with a diameter
of 1,380,000 km
Elements in Stars
Most stars are 70% Hydrogen and 28% Helium
1-2% of a star’s mass may be heavier elements
such as oxygen, carbon, nitrogen, calcium,
sodium
The spectrum radiated by a star depends on
both it’s composition and it’s temperature
No two stars have exactly the same spectrum.
A star’s spectrum is like its fingerprint
Star Brightness
1. Luminosity = true
brightness of a star
Depends only upon
the size and
temperature of a star
If two stars had the
same temperature,
the larger star would
be more luminous
than a cool star
Star Brightness
2. Apparent magnitude or brightness:
How bright a star appears on earth.
Dependent on star’s luminosity and distance from us
The brightest stars are “first-magnitude” stars
The faintest stars that can be seen with the unaided eye are
“sixth magnitude”
Each star’s magnitude differs from the next be a factor of 2.5
Ex. First magnitude star is 2.5 brighter than a second magnitude star
Some stars are even brighter than first magnitude stars (1.0)
Ex. Sirius the brightest star in our sky, has an apparent magnitude of
-1.43
Ex. A 100 watt light bulb is much brighter than a flashlight bulb
The 100 watt bulb has GREATER luminosity
However, if held up close, the flashlight bulb would look brighter
Flash light has a greater apparent magnitude
Star Brightness
3. Absolute Magnitude:
How bright the star would be at 32.6 light years
Used to express the luminosity of stars as if all stars were
the same distance from Earth
Ex. Sun = 4.8 (average); Rigel = 6.4 (very bright)
Giants, Supergiants, & Dwarfs
Giant:
Larger in diameter than the Sun
Luminosity: 10-1000 times the Sun
Stars more luminous than giants are called supergiants
Supergiant:
Mass: 8-12 times the Sun
Luminosity: 10,000-1,000,000 times the Sun
Red supergiants are the largest of all the stars
Dwarf:
Less luminous
Absolute magnitude (brightness) no more than 1
Most are red, orange, yellow or white
White dwarfs are very faint, small, and dense (same size as earth
but 100,000 times more dense
Origin of a Star
According to the Proto-star Theory, stars form wherever dense clouds of gas
and dust exist
These are called stellar nurseries and have a average diameter of 25 light years
HUGE clouds of gas and dust occur (nebulae) in parts of space between the
stars
Contain as much material as the stars themselves
These clouds are about 99% gas (hydrogen)
Remaining 1% is a mixture of very fine particles of silicon carbide, graphite,
diamonds, nitrogen and other elements
It is believed this gas and dust comes from remains of exploding stars and
supernovas
Sometimes these great clouds of dust and gas start to come together under their
own gravity
Nebulae:
Areas in space where such cloud formations can be found
Most are invisible
When these clouds are lit up by a star we can see them and they are immense
Nebulae that are not near stars may show up as dark patches in space.
Formation of a Red Giant
When a star has used up its “stable” fuel, the force
of fusion no longer balances with the force of
gravity and the star loses its stability
1. when this occurs the core contracts in upon
itself and becomes very hot causing the outer
layers of the star to expand away from the core.
2. now this radiation and heat starts fusion in the
star’s outer layers causing even greater expansion
3. as the expansion continues the star becomes a
red-giant or SUPERGIANT
Formation of White Dwarfs
Finally we come to a stage in the stars life
where most of the fuel for fusion is used up
1. the temperature and pressure of the core
can no longer support the weight of its
outer layers
2. the Giant then collapses
3. with most of its fuel gone the white
dwarf cannot maintain its high temperature
and in a billion years it will eventually glow
fainter until it becomes cold and dark
Supernovas
When fusion has stopped it leaves an iron core
As the star cools this core collapses in upon itself
With this collapse, the pressure and temperature within the core
rises dramatically
The iron core starts to fuse into even heavier elements
Now the core wants to collapse even further
In this rush to collapse the star EXPLODES so violently that
half its mass is blown out into space
This explosion has a very intense flare and bright light we call a
Super nova
For just a few weeks or months this one star can outshine an
entire galaxy
Supernovas
The best recorded
supernova was recorded
by the Chinese in the
year 1054.
This brilliant star
faded after a year and
its outer shell was
changed into a great
expanding cloud of
gas we now know as
the crab nebulla.
Neutron Stars & Black Holes
Supernovas eject half of their mass during the explosion. So
what happens to the other half ?
The mass that remains is what astronomers call a neutron star
In the core of a supernova the forces are so great that every
atom’s electrons are crushed into its nucleus.
The collapsed electrons combine with the protons to form
neutrons
A neutron star is only about 10 km in diameter and trillions
of times more dense than the sun.
Neutron Stars & Black Holes
What would happen if an even more massive star would
explode into a supernova leaving behind a core that is even
more dense than a neutron star?
Such gravitational forces would be so great that not even light
could escape
We call these black holes
We cannot see these Black Holes
Must determine their location by the effect they have on other
objects nearby
By the energy (X-rays) given off by the matter that is falling into
them
What are Galaxies?
With the Hubble Telescope we now recognize that space has
BILLIONS of Galaxies and each galaxy has BILLIONS of
Stars
The galaxy to which our Sun belongs is the Milky Way
galaxy
Our sun is one of 100 Billion stars in the Milky Way
The diameter of the Milky way is about 140,000 light years
The Milky Way belongs to a small cluster of 17 galaxies
called the Local Group
Types of Galaxies
1. Spiral Galaxies
These have a lens shape, and a bright nucleus made of millions
of stars
Stars are arranged in spiral arms around the nucleus
Each arm contains millions of stars.
Ex. Milky Way
2. Elliptical Galaxies
Range from spherical to lens shaped
Most stars are close to the center and they have no arms
3. Irregular Galaxies
These are smaller and fainter, their stars are spread unevenely
Origin of the Universe
Big-Bang Hypothesis: (No not the TV show)
The whole universe was originally packed into one dense sphere
of hydrogen
About 15 Billion years ago this mass of hydrogen exploded
forming a gigantic expanding cloud
Some parts of the cloud moved faster than others, but all parts
moved outward, away from the center and are still doing so
today
Eventually the clouds cooled and condensed into galaxies
Billions of galaxies were formed (all moving outward)
What is the support for this theory?
1. DOPPLER Shift
2. Background Radiation