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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 T of the
star
– Stars are classified by T
• Decreasing T (bright to dim)
• O, B, A, F, G, K, M [Oh Be A Fine Girl, Kiss
Me ]
http://www.seasky.org/cosmic/sky7a01.html
Hertzsprung-Russell Diagram
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
Hertzsprung-Russell diagram
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
Life Cycle of Stars
Life Cycle of Stars
• Begin their lives as clouds of dust and gas called
nebula
• Gravity may cause the nebula to contract
• 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.
• Begins the main sequence phase of the star
• Most of its life is in this phase
Life Cycle of Stars
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 of Stars
• 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
Life Cycle of Stars
• 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
Life Cycle of Stars
• 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
Life Cycle of Stars
• 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