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Stars &
Constellations
Astronomy 2
What is a Star?
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Star: A body of gas that
gives off a tremendous
amount of radiant energy
in the form of light and
heat
Stars close to home…
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The nearest star to Earth is our
Sun
 It takes approximately 8
minutes for the light from the
sun to reach Earth
The next nearest star is called
Proxima Centauri
 It takes approximately 4.2 years
for the light to reach Earth
(traveling at light speed)
The sun and Proxima Centuri are
only two of the billions of stars in
the Milky Way galaxy
What is Light?
A form of electromagnetic radiation which is
energy that travels in waves
 Electromagnetic radiation is arranged in a
continuum called the electromagnetic spectrum
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Light and the Universe
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Light Year—used as a
measurement of the great
distances in space
A light year is the distance
that a ray of light can travel
in a year, or:
A light year is
5,865,696,000,000 miles
(9,460,800,000,000
kilometers).
A star or a star system’s
distance from earth is
measured in light years.
Parallax

A change in an object’s direction due
to a change in the observer’s position.
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Astronomers also use parallax to
calculate the distance to the stars.

Parsec—short for “parallax second”—
is a special unit of distance for a star’s
distance from earth.
> 1 parsec = 3.258 light years
Why Do Stars Twinkle??

The scientific name for the
twinkling of stars is stellar
scintillation (or astronomical
scintillation)
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Stars (except for the Sun) appear as
tiny dots in the sky
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As their light travels through the
many layers of the Earth's
atmosphere, the light of the star is
bent (refracted) many times and in
random directions

This random refraction results in
the star winking out (it looks as
though the star moves a bit, and
our eye interprets this as twinkling)
Characteristics of Stars
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Size: anywhere from 20km to
1 trillion km in diameter
Mass: the amount of matter
Color: stars can be red, blue,
white, orange or yellow
Composition: contain
different elements
determined using spectra
Characteristics of Stars
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Temperature:
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Blue is the hottest
(35000°C)
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Red is the coolest
(3000°C)

Yellow (our sun)
(5500°C)
Characteristics of Stars
Absolute Magnitude—A measure
of how bright a star would be if all
Stars were at the same distance—
ten parsecs—from Earth
• Apparent Magnitude—A measure
of how bright a star appears to be
on earth
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•
• Does not measure how bright a star
actually is

Luminosity—the actual brightness
of a star
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Depends on size and temperature
The lower the number, higher the
brightness
Magnitudes
-26.5
-30
We can not see dimmer
than +6 with naked eye
-2
-20
-10
3 6
0
10
Brighter
20
30
Dimmer
Our
Sun
Sirius
Brightest in the
sky from Earth
Polaris/North
Star
Apparent Magnitude
v.
Luminosity
Star 1
Star 2
High luminosity
Low luminosity
Far from earth
Close to earth
= lower
apparent
magnitude
= higher
apparent
magnitude
Hertzsprung-Russel Diagram

Relationship between the absolute magnitude
(luminosity) and the temperature of stars.
Spectral Class—designates the
color of stars
>depends on temperature
>Hottest to the coolest
>Blue to White to
yellow to orange to Red
Kinds of Stars
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Super giants- largest of all
stars, very luminous
Red giants- cooler, large,
very luminous
Dwarf stars- less luminous,
red, orange or yellow
White dwarf- very faint,
small and dense
Variable Stars- vary in
brightness over regular
periods or cycles
Kinds of Stars
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2 types of variable stars:
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A.
pulsating change in
brightness as they contract
(brighter)expand (dimmer)
Pulsars
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star that releases light and radio
waves in pulses
may be the neutron star
formed in a supernova
it looks like it pulses because it
is rotating, like a searchlight
Kinds of Stars
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2 types of variable stars:
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B. non-pulsating
Eclipsing Binary
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2 stars of unequal
brightness that revolve
around each other
brightness depends on
which one is in front of
the other
Life Stages of Stars &
Constellations
Astronomy 2
Nebulae
Seen only in infrared
 Huge clouds of dust (1%) & gas (99%)
 This is where most stars are born.
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Stellar nursery is a nebula ( a
large cloud of hydrogen gas
in space) in which star
formation is occurring
Life Cycle of a Star
 A star begins it’s life in a cloud of cold
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gas and tiny-grained dust called a
NEBULA.
Parts of the Nebula begin to condense
due to some outside force—a
shockwave—which acts upon it.
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Due to gravity, the gas and dust
pockets continue to condense and their
temperature increases.
Eventually, parts of the Nebula begins
to glow—PROTOSTARS are formed.
Gravitational contraction of the
Protostars continues causing them to
become hotter and brighter.
Finally, fusion takes place in the center
of a protostar, halting gravitational
condensation, and a star is born.
Life Cycle of a Star
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Main Sequence Star (Our
Sun)—
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90% of all stars
The star is stable
Burns Hydrogen gas to
Helium in its core
through nuclear fusion
The energy released
causes the star to shine.
Stars spend about 90% of
their active lifetime as
main sequence stars.
Life Cycle of a Star
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Red Giant—
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The star begins to run out
of fuel and the core
begins to shrink
Helium turns into Carbon
Rapid burning of helium
causes outer layer to puff
out, cooling the star
The star turns red
Life Cycle of a Star
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Planetary Nebula
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Outer layers are ejected as
core continues to shrink
Shell of hot gas
Core is exposed
White Dwarf—
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Low mass core continues
to shrink creating a white
dwarf
Surrounded by the
Planetary Nebula
Stars Evolution
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Hydrogen fuses more quickly and
when a star starts to die, iron nuclei
are formed
Star swells to 100 times diameter of
the sun—Super Giant
Iron nuclei absorbs energy and
core quickly and suddenly collapses
If large enough it explodes into a
brilliant burst of light—Super
Nova then
Either a neutron star forms - dense
mass of neutrons
Or a Black Hole (depending on the
size) - a concentration of mass
great enough that the force of
gravity will not allow anything to
escape
Novas & Supernovas
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Believed to only happen
in binary-stars systems.
Gases from one star hit
the surface of another
and cause a nuclear type
explosion.
Supernova is a brilliant
burst of light that follows
the collapse of the iron
core of a massive star.
Super Nova
Quasars
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Quasars: give off radio & X-waves.
They are the most distant objects
in space. Give off tremendous
amounts of energy.
Quasars give off enormous
amounts of energy - they can be a
trillion times brighter than the Sun!
Quasars are believed to produce
their energy from massive black
holes in the center of the galaxies
in which the quasars are located.
Because quasars are so bright, they
drown out the light from all the
other stars in the same galaxy.
Where did the Constellations come
from?
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Farmers invented the
constellations
It is known that crops are
planted in the spring and
harvest in the fall. But in some
regions, there is not much
differentiation between the
seasons
Since different constellations
are visible at different times of
the year, you can use them to
tell what month it is.
For example, Scorpius is only
visible in the northern
hemisphere's evening sky in
the summer.
Where did the Constellations come
from?
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Some historians suspect
that many of the myths
associated with the
constellations were
invented to help the
farmers remember them
- when they saw certain
constellations, they
would know it was time
to begin the planting or
the reaping
Looking at the Constellations
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Constellations are not real – they
are only as a way to remember
where stars are in the sky
Constellations are a group of stars
that appears to form a pattern as
seen from Earth.
The constellations help by breaking
up the sky into more manageable
bits
Example: if you spot three bright
stars in a row in the winter evening,
that's part of Orion – the star
Betelgeuse is Orion's left shoulder
and Rigel is his foot
Looking at the Constellations
Constellations
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There are 88
constellations or sky
divisions
The constellations
change position with
each season
Some constellations only
come up during certain
seasons, some not at all
Some never change
position due to their
position to the poles—
circumpolar
Constellation of Orion is
seen during winter
Constellations
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Polaris (North Star)
Found at the end of the handle of
the Little Dipper (asterism)-which
makes up the constellation Ursa
Minor (Little Bear)
Also can be found for using the
pointer stars in the Big Dipper
(asterism)-which makes up the
constellation Ursa Major (Big Bear)
Also used for navigational purposes
The Constellation, Orion