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Stars
What is a star?
• The objects that heat and light the
planets in a system
• A star is a ball of plasma held
together by its own gravity
– Nuclear reactions occur in stars (H 
He)
– Energy from the nuclear reactions is
released as electromagnetic radiation
Characteristics of Stars
• DISTANCE
–Measured in light-years
• The distance which a ray of light
would travel in one year
• About 6,000,000,000,000 (6
trillion) miles
• 186,000 miles per second
Characteristics of Stars
• Magnitude (brightness)
– A measure of brightness of celestial objects
• Smaller values represent brighter objects than
larger values
– Apparent magnitude
• How bright a star appears to be from Earth
– Absolute magnitude (luminosity)
• How bright a star actually is
Characteristics of Stars
• Temperature & Color
– The color of a star indicates the
Temperature of the star
– Stars are classified by Temperature
• Decreasing Temperature (bright to dim)
• O, B, A, F, G, K, M [Oh Be A Fine Girl, Kiss
Me ]
http://www.seasky.org/cosmic/sky7a01.html
Types of Stars
Classification
Class
Temperature
Color
O
20,000- 60,000 K
Blue
B
10,000 – 30,000 K
Blue-white
A
7,500 – 10,000 K
White
F
6,000 – 7,500 K
Yellow-white
G
5,000 – 6,000 K
Yellow
K
3,500 – 5,000 K
Orange
M
2,000 – 3,500 K
Red
Main Sequence Stars
• A major grouping of stars that
forms a narrow band from the
upper left to the lower right
when plotted according to
luminosity and surface
temperature on the HertzsprungRussell diagram
Hertzsprung-Russell Diagram
http://www.answrs.com/topic/stellar-classification
Size Video
• http://www.youtube.com/watch?v=LsgKp_
Vf6wI&list=PL9D64A9B670629B26&index
=12
Life Cycle of
A Star
Life Cycle of Stars
Life Cycle
• Stars begin their lives as
clouds of dust and gas called
Nebulae (Greek for cloud)
• Gravity may cause the nebula
to begin to contract
Eagle
Nebula
Life Cycle
• Matter in the gas cloud will begin
to condense into a dense region
called a Protostar
• The Protostar continues to
condense, it heats up. Eventually,
it reaches a critical mass and
nuclear fusion begins.
Star Nurseries
Life Cycle
After nuclear fusion
begins, stars begin the
main sequence phase.
• Most of its life is in this
phase
•
The Sun: the nearest Main Sequence star to the Earth. This
is an X-ray image from the Yohkoh satelite.
Pleiades or the 7 Sisters
Logo for the Subaru car company.
Life Cycle
Life span of a star depends on its size.
– Very large, massive stars burn their fuel
much faster than smaller stars
– Their main sequence may last only a few
hundred thousand years
– Smaller stars will live on for billions of
years because they burn their fuel much
more slowly
• Eventually, the star's fuel will begin to
run out.
Life Cycle
• It will expand into what is known as
a red giant
• Massive stars will become red
supergiants
• This phase will last until the star
exhausts its remaining fuel
• At this point the star will collapse
BETELGEUSE –RED GIANT
• Only a few million
years old, Betelgeuse is
already dying.
Astronomers predict
that it's doomed to
explode as a soon,
within 1,000 years or
so, an event that will be
spectacular for Earth's
future inhabitants.
(Conceivably, it's
already happened as
Betelgeuse is 640 lightyears away!)
Hubble image of Betelgeuse a Red Supergiant
Life Cycle
• Most average stars will blow away
their outer atmospheres to form a
planetary nebula
• Their cores will remain behind and
burn as a white dwarf until they
cool down
• What will be left is a dark ball of
matter known as a black dwarf
Image of Sirius A and Sirius B taken by the
Hubble Space Telescope
Ant Planetary Nebula
Expelled gas streaming away at 1,000
kilometers per second create a strange ant
shape
Cats Eye Nebula
Life Cycle
• If the star is massive enough, the
collapse will trigger a violent
explosion known as a supernova
• If the remaining mass of the star is
about 1.4 times that of our Sun, the
core is unable to support itself and it
will collapse further to become a
neutron star
Just one teaspoon of a neutron star would have the mass of over 5 x 1012
kilograms.
When neutron stars form, they maintain the
momentum of the entire star, but now
they’re just a few kilometers across. This
causes them to spin at tremendous rates,
sometimes as fast as hundreds of times a
second.
Life Cycle
• The matter inside the star will be
compressed so tightly that its atoms are
compacted into a dense shell of neutrons.
If the remaining mass of the star is more
than about three times that of the Sun, it
will collapse so completely that it will
literally disappear from the universe.
What is left behind is an intense region
of gravity called a black hole
Life Cycle of Stars