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Stars
A self-luminous
celestial body
consisting of a mass of
gas held together by
its own gravity in
which the energy
generated by nuclear
reactions in the
interior is balanced by
the outflow of energy
to the surface, and the
inward-directed
gravitational forces are
balanced by the
outward-directed gas
and radiation
pressures
Stars
Massive
luminous
balls of
plasma
held
together by
gravity
Characteristics of Stars
a. Color
b. Temperature
c. Mass
We use these
characteristics to
understand and
examine stars
a. Color
• Color is a clue to a star’s
temperature.
• Blue stars are hotter/
newer/closer
• Red stars are
cooler/older/more
distant
b. Temperature
• Very hot stars emit shortwavelength light
BLUE
• Cooler stars emit longwavelength light
RED
c. Mass - A unified body of matter
with no specific shape
• Mass is how much “stuff” is squeezed into a
space
Japan subway car without much “stuff” inside =
Less Mass
Crowded Japan
subway car with
lots of “stuff” =
More mass
Lots of “stuff”
squeezed into a
space, place or
thing
Binary Stars – pairs of stars pulled
toward each other by gravity
• Many stars orbit
each other
• More than 50% of
stars occur in pairs
or multiples.
• Binary stars are
used to determine
the star property
most difficult to
calculate – It’s mass
If the sizes of
the orbits are
known
Then the
stars mass
can be
determined
Hertzsprung-Russell
Diagram
• Shows the
relationship between
the absolute
magnitude and the
temperature of stars
• 90 % main-sequence
stars
• Giants
• Supergiants
• White dwarfs
Light Year – the distance light travels in
1 year
9.5 x 10 to the 12th power
Or
9.5 trillion kilometers
Or
186000 miles per second
• Distances to stars are so large
that units like miles or kilometers
are too hard to use
• The numbers get really, really
confusing and big
Measuring distance to stars – it’s
difficult
The most basic way to measure
distance to stars is parallax.
Parallax – the slight shifting of a nearby star due to the
orbital motion of Earth
uses photographs of stars to compare to distant
stars in the background
shifting angles are compared
the angles compared are very small
Close stars =
large parallax
angles
Distant stars
= smaller
parallax
angles
Apparent Magnitude a stars brightness as it
appears from Earth
3 factors control the
brightness from Earth
1. How big
2. How hot
3. How far away
Absolute Magnitude – how bright a
star actually is
Variable Stars – some stars fluctuate in
brightness
1. Cepheid – gets
brighter in a
variable pattern
2. Nova – sudden
brightening of a star
due to a flare up
Interstellar Matter - between existing
stars is “the vacuum of space”
Except for
Nebulae…
Nebulae – clouds of dust, gas, and thinly
scattered matter
• Stars and planets form
from this interstellar
matter
1. Nebulae begin to
contract
– Gravity squeezes
particles in the nebula
towards the center
– Nebula shrinks
– Gravitational energy is
converted into heat
energy
Protostar Stage – a developing star not hot enough
to begin nuclear fusion
1. Contraction lasting
1 million years
2. Collapse causes the core
to heat more than the
outer layer
–
Causes gas to increase
it’s motion
3. When the core reaches
10 million K, nuclear
fusion of hydrogen
begins
4. A star is born
Main-Sequence Star – a star balanced between 2
forces, gravity and gas pressure
• Gravity(external force)
• Gas (internal force)
• Hydrogen fusion lasts a few
billion years
• 90% of an average stars life is
in this hydrogen burning stage
– When a star’s hydrogen fuel in
the core is depleted, it
evolved rapidly and dies.
– Some stars delay death by
burning heavier elements and
become giants
Gravity (external force)
Gas (internal force)
Red Giant Stage
• Inner Core consumes all
hydrogen fuel energy
and begins to contract
– Helium core is left
behind
– Core contracts and heat
is radiated outward.
– This energy heats the
outer layer and causes
expansion
– Results in giant body size
of star 100 to 1000 X
size of it’s original mainsequence size
Death and Burnout of Stars
• All stars run out of fuel and collapse because of
gravity
Death of Low Mass Stars
•
•
•
•
Small
Cool
Red
Consume hydrogen fuel
slowly
• Not hot enough to fuse
helium
• Remain on the main
sequence for up to 100
billion years
• Collapse into White
Dwarfs
Death of Medium Mass
Stars
• Masses similar to
our Sun
• Evolve into Giants
• consume
hydrogen and
helium at fast
rate
• Collapse into White
Dwarfs
• During collapse
from Red Giant
into White
Dwarf they cast
off their outer
shell and leave a
cloud of gas
called planetary
nebulae
Death of Massive Stars
• Massive stars have short
lives
• End star life in brilliant
explosions called
supernova
• Rare
1. Death is triggered when
nuclear fuel is consumed
2. Star collapses
3. Implodes
4. Sends shock wave out
from the stars interior, this
destroys the star blasting
the shell into space
• None have been observed
since the invention of the
telescope
Nucleosynthesis – the process that produces
chemical elements inside stars
• Occurs in dying
stars
• Stars produce all
naturally
occurring
elements beyond
helium in the
periodic table
• Mass of the star
determines the
highest atomic
number of the
elements it can
produce
• More massive
stars produce
heavier elements
Study of Light
Information about
the universe is
obtained from the
study of the light
emitted from stars
and other bodies
in space
Electromagnetic Radiation
1.
2.
3.
4.
5.
6.
7.
All energy travels through the vacuum of space at the
speed of light
Gamma Rays
X-Rays
Ultraviolet Rays
Visible Light
Infrared Radiation
Microwaves
Radio waves
Electromagnetic Spectrum – arrangement of
waves according to their wavelengths and frequencies
1. Wavelengths
2. Photons – a
stream of
particles
When the spectrum of a star is studied, the
spectral lines act as “fingerprints”. Spectral
lines identify the elements present in a star
and tell of the stars chemical composition