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AST 112. Stellar Structure I. Our Star. The Sun is the STAR at the center of the Solar System. What is the difference between a star and a planet?. Stars vs. Planets. Stars have enough gravity to initiate and sustain nuclear fusion reactions at their cores Planets do not - PowerPoint PPT Presentation
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STELLAR STRUCTURE IAST 112
OUR STAR
The Sun is the STAR at the center of the Solar System
STARS VS. PLANETS
What is the difference between a star and a planet?
STARS VS. PLANETS
Stars have enough gravity to initiate and sustain nuclear fusion reactions at their cores
Planets do not Cores of planets are hot, but are not
producing nuclear reactions
STARS DON’T LAST FOREVER
The Sun (and all the other stars) spend their lifetimes fusing H into He
Depletion of H signals the beginning of the end of a star’s life
The initial amount of H and the rate at which the star fuses H into He determine a star’s lifetime
COMPOSITION OF THE SUN
What is the Sun made of and why?
COMPOSITION OF THE SUN
Mostly H, some He, and a small amountof heavier elements (nitrogen, oxygen,
iron, etc.)
THE SUN
Yellow Main Sequence Star
Diameter: 864,000 miles (109 DEarth)
Mass: 330,000 MEarth
Density: (water is 1000 kg/m3) Surface: 5.0 x 10-6 kg/m3
Core: 162,200 kg/m3
The surface is 10,000 oF
Lifespan: 10 billion years
SIRIUS
Blue Main Sequence Star
Diameter: 1,480,000 miles (1.77 DSun)
Mass: 2.02 MSun
Luminosity: 25.4 LSun
The surface is 17,500 oF
Lifespan: 1 billion years
(FAKE PICTURE)
BETELGEUSE
Red Supergiant
Diameter: 1,000,000,000 miles (1180 DSun)
Mass: 19 MSun
Density: (water is 1000 kg/m3) Surface: 1/100000 that of air at sea
level on Earth
Luminosity: 140,000 LSun
The surface is 5840 oF
100,000,000 years old and maybe on its way out
PLASMA
PLASMA is the phase of matter in a star
Tenuous (gas-like) at outer surface, much more dense toward core
The high temperatures do not allow electrons to stay bound to nuclei
(NOT) WHY STARS SHINE
Ancients believed that the Sun was a type of fire
Wood burning, coal burning, and other chemical processes insufficient to generate the Sun’s energy
(NOT) WHY STARS SHINE
Late 1800’s, an incorrect idea: The Sun slowly contracts gravitationally, generating
large amounts of thermal energy Recall conservation of energy
Potential energy Thermal energy
Calculations gave 25 million years worth of shining
Fossils and rocks on Earth were known to be way more than 25 million years old
WHY STARS SHINE
Nuclear reactions convert H to He.
Some of the mass is lost to pure energy:
E = mc2
WHY STARS SHINE
When H fuses into He:
Two smaller nuclei mash into one larger one The larger one weighs less than the sum of
the two smaller ones The mass deficit is converted to energy
STRONG VS. COULOMB
Nuclei have + charge Should repel, right?
The Coulomb force does cause them to repel, but:
At extremely short distances, nuclear strong force takes over and binds nucleons together High pressure and high temperature are required
HYDROSTATIC EQUILIBRIUM
Isn’t there a “contradiction” going on in a star?
Doesn’t gravity still try to collapse the star?
Don’t the nuclear reactions try to blow it apart?
HYDROSTATIC EQUILIBRIUM
They balance!
Gravity pulls matter inward
Pressure from hotter layers and nuclear reactions pushes matter outward
HYDROSTATIC EQUILIBRIUM
A stack of acrobats: Bottom one is
pressed the hardest, and pushes the hardest
Middle one presses less, is pushed less
Top one isn’t pressed from above, doesn’t push
HYDROSTATIC EQUILIBRIUM
Hydrostatic equilibrium keeps the Sun’s size stable
What happens when the Sun has fused all of its hydrogen into helium?
