Introduction to Stars
Our Sun is a star and its closeness to
Earth has allowed astronomers to
study and analyze its properties and
characteristics and apply this
knowledge to the study of other stars.
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Composition
Through spectroscopy, the dark lines
(or bright lines) in the spectra show
scientists what elements are present in
the gases of stars.
Hydrogen and helium and smaller
amounts of other elements are found in
stars.
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What are a stars characteristics?
Stars differ in…
Mass
Size
Temperature &
Energy
Color
Luminosity
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Mass and Composition of Stars
Mass and composition determines
most of the properties of a star.
The more massive a star is, the greater
the gravity, the hotter and denser a star
must be.
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Star Size
The diameters of stars range from as
little as .1 the sun’s diameter to
hundreds of times larger.
The mass of stars can be from less
than .01 to 20 or more times that of our
sun. Extremely rare are stars that are
over 50 times the Sun’s mass.
Our Sun is a medium sized star.
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Star Energy
The enormous pressure and heat in a
star’s core convert matter into energy.
Stars consist of controlled atomic
reactions called nuclear fusion in which
hydrogen (nuclei) atoms fuse to form
helium (nuclei) atoms.
During each step of the process, mass
is lost and energy is released.
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Star Energy cont.
The energy released causes the star
(Sun) to shine and gives the star its
high temperature.
Star stability – this energy stabilizes a
star by producing the pressure needed
to counteract gravity.
Fusion>Gravity = expansion
Fusion <Gravity = contraction
Fusion = Gravity = stable
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Star Energy cont.
If a star’s temperature is hot enough,
carbon can react with helium to form
oxygen, then neon, then magnesium,
and then silicon.
Other types of reactions can produce
even heavier elements which we learn
about later in the life cycle of a star.
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Temperature of stars
Stars in the sky show tinges of different
colors which reveal the star’s
temperatures.
Blue stars shine with the hottest
temperatures and red stars shine with
the coolest
Our Sun is a yellow star having a
surface temperature of about 5,500ºC
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Temperature of Stars
The cooler red stars have longer
wavelengths and may be only detected
with infrared telescopes
The hot blue stars have shorter
wavelengths may be detected with
ultraviolet or x-ray telescopes
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Spectral Type Classification
Astronomers place stars in
spectral (color) class categories
based on their surface
temperature.
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Classification of Stars
Spectral Class
Color
Surface
temp (ºC)
Examples
Blue
Above 30,000
10 Lacertae
O
Blue-white
10,000 – 30,000 Rigel, Spica
B
White
7,500 – 10,000
Vegas, Sirius
A
Yellow-blue
6,000 – 7,500
Canopus,
Procyon
F
Yellow
5,000 – 6,000
Sun, Capella
G
Orange
3,500 – 5,000
Arcturus,
K
Aldebaran
Red
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Less than 3,500
Betelgeuse,
Antares
M
The Spectral Class is OBAFGKM
or
Oh Be a Fine Guy/Girl Kiss Me
From Hottest to Coolest:
Violet, Indigo, Blue, White, Yellow, Orange,
Red
O or violet is the hottest (>25,000oC) and
M or red is the coolest (3,500-2,000oC)
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Absolute magnitude
The true brightness of a star if all
stars were at a uniform distance
from Earth
is in comparison to the Sun ( 1
being the Sun’s AM ) and multiplied either ( X a
whole number) for an amount greater than or (X a
decimal number) for an amount less than the AM of
the Sun.
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Apparent Magnitude
The brightness of a star as it
appears from Earth with the
naked eye.
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Luminosity
The energy output from the surface of a star
per second measured in watts.
How bright a star is relative to the Sun
The brightness of a star depends upon the
distance and its luminosity.
Think it over
The star Rigel in Orion is about 60,000 times
larger than our sun.
Why does our sun appear brighter than Rigel?
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The H-R Diagram
The properties of a star are closely related
and can be demonstrated on a graph called
the Hertzsprung - Russell diagram (H-R
diagram)
An H-R diagram plots stars according to their
luminosity and temperature
(spectral
class)
About 90% of stars including the Sun fall
along a broad strip of the H-R Diagram called
the Main Sequence.
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Included in the H-R Diagram will be:
Temperature noted in ºC
Upper left hand corner – hot, luminous stars
Lower right corner – cool, dim stars
Star size and color
Spectral Classification noted also because of
its temperature relationship
Absolute Magnitude is in comparison to the
Sun (
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HR Diagram cont
Stars that are part of the Main
Sequence indicate that all these stars
have similar internal structure and
functions.
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Stars are grouped into types by
temperature and luminosity
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#1. Main Sequence Stars,
•90% of stars
•A middle aged stable star
•Range is upper left to lower
right
Upper left stars are
Large,Hot,Luminous
Blue Giants
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Lower right stars are
Small, Cool, Dim
Red Dwarfs
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Locate Main Sequence Stars
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#2. Upper Right Stars
Red Giants & Super
Giants, 1% of stars
Large
Cool
Luminous
Our sun in 5 billion years
will be 2000 times
brighter and 100 times
larger (large enough to
expand past Mars orbit)
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Locate Red Giants & Super Giants
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#3. Lower Left Stars
White Dwarfs, 9% of stars
Small
Hot
Dim
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Locate White Dwarfs
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Position on H-R Depends on Mass
What is the connection between mass and
luminosity?
The larger the amount of matter or mass in a
star the more luminous
Giants: more mass - more luminous
Dwarfs: less mass - less luminous
During its lifetime a star will evolve on the H-R
diagram depending on its mass.
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Bibliography
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