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Stars
The Suns of Other Worlds
What are Stars?
• Giant
• Luminous
• Plasma.
– Energized Gas
• Powered by fusion of hydrogen or heavier elements
• Stars are NOT burning
– No oxygen
How do we learn about Stars?
• Studying the electromagnetic spectrum.
• We can only see a tiny bit of the light stars
produce.
• Visible light
• Stars emit lots of different kinds of ‘light’
How did we learn about the EMS?
• Sir Isaac Newton
• If you pass sunlight through a prism it
separated out into a spectrum of all the
colors
Infrared Light
• William Herschel discovered IR by accident.
• Do colors have temperatures?
• He found that an area just beyond the red part of
the spectrum was hotter.
• He named that invisible light infra-red meaning
beyond red
Ultraviolet (UV) Light
• Johann Ritter
• Found chemicals that darkened when
exposed to sunlight had a greater reaction
just above the violet end of the spectrum
• Ultra-violet, meaning above violet.
What can we learn about Stars?
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Mass
Temperature
Stage of Development
Chemical Composition
Star Light, Star Bright…
• Absolute magnitude
– How much light a star produces
• Apparent magnitude
– How much light actually makes to Earth
• Absolute magnitude requires we know the
distance to the star.
How Far?
• Parallax
– The apparent movement of an object
seen at different viewpoints.
– The more the object moves (the
greater the parallax), the closer it is.
– The Hipparcos satellite
Color
• Color
• Different Temperature
– Mass
– Temperature
• Missing Color
– Shows presence of other chemicals
• Spectra
– A view of the color produced by a star
Hertzsprung-Russell Diagram
• If you make a graph of stars
brightness and color stars fall into
certain areas
• Stars move through these areas as
they age
Types of Stars
• Two different methods
– Temperature
• Color alone
– Age
• Color and brightness
• HR Diagram
Temperature
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O - 33,000K+
B - 10,500 - 30,000K
A - 7,500 - 10,000K
F - 6,000 - 7,200K
G - 5,500 - 6,000K
K - 4,000 - 5,250
M - 2,600 - 3,850K
-bluest
-bluish
-blue-white
-white
-yellow white
-orange
-red
HR Diagram
• Four major groups
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Super Giants
Giants
Main Sequence
White Dwarfs
• Some ‘stars’ didn’t get graphed
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Neutron Stars
Black Holes
Black Dwarfs
Brown Dwarfs
Main Sequence Stars
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Dwarf stars
Beginning of stars life
Bigger, hotter stars spend less time here
Fusing hydrogen as their main fuel.
Giants
• Larger(10x) and brighter than main
sequence stars
• Average main sequence stars that have run
out of hydrogen for fusion.
• Helium fusion is more explosive
– outward pressure >inward
Super Giants
• Largest and brightest of all stars
– 10-70 times size of the sun
– 30,000+ times as bright as the sun
• Large MSS that ran out of hydrogen
• Short life spans
White Dwarfs
• The smallest and faintest of all stars
– Stopped fusion process
– Glow only due to stored energy
• Mass of sun, size of earth
• The final stage of most stars lives
• Eventually cool to form Black Dwarfs
Star Life Cycles
• Nebula – birth place of stars
• Protostars – baby stars
– Gravity and fusion pressure not balanced yet
– Brown Dwarf – failed star
• MSS – first stage of a stars life
• What happens next depends on mass of star
Sun-like Stars
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Run out of hydrogen
Outer layers expand
Red giant
Outer layers eventually drift off into space
– Planetary nebula
• Hot core remains as a White Dwarf
• Hypothetically if heat is gone becomes a Black
Dwarf
Big Stars
• Stars 1.5-3 times mass of sun
• Become Red Super Giants
• Fuse heavier elements until core is iron
– Collapses, causes explosion
• Super Nova
– Create all other elements
– Planets, people
Neutron Stars
• Neutrons.
• 2x times mass of sun.
• 20-40km
– Extremely dense
– Teaspoon - 5,000,000,000,000kg
• Gravity makes surface perfectly smooth
Giant Stars
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Also become red super giants.
Same onion structure
Same Super Nova
If remaining core is >3 times the mass of the sun it
forms a Black Hole
• All the matter is squeezed into a space smaller
than an atom.
• Light can’t escape
Black Hole
• Any matter falling into a black hole produces
deadly gamma rays.
• Event Horizon
– Distance from the center of the black hole beyond
which nothing can escape
• Spaghettification
– Youtube video
Star Structure
• Stars are made of several layers that each
have differing properties.
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Core
Radiation Zone
Convection Zone
Photosphere
Chromosphere
Corona
Core
• Site of Nuclear Fusion
• Extreme amounts of pressure
• 10,000,000 K
Radiation Zone
• Pressure from the core balance pressure
from above layers, particles don’t move.
• Energy bounces around inside this layer for
an average of 170,000 years.
• 7-2,000,000 K
Convection Zone
• Energy moves with motion of plasma.
• In main sequence stars, CZ is near the
surface.
• In giant stars it is right next to the core.
• ~1,000,000 K
Photosphere
• Coolest Layer (6,000K)
• Sun spots
– Disruptions in magnetic field
• Solar Flares
– Violent explosions that send sub atomic
particles into space.
– Affect satellites, astronauts, power grids.
Chromosphere
• Atmosphere of sun
• Hotter than photosphere due to magnetic
activity.
– 6-20,000 K
Corona
• 1,000,000 K
• Extends out several million km.
• Visible during solar eclipse