AST 111

 The Sun is the STAR at the center of the Solar System  Anything else belonging to the Solar System is NOT a star!

What is the difference between a star and a planet?

 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

 The Sun (and all the other stars) spend their lifetimes fusing H into He  Depletion of H signals 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

What is the Sun made of and why?

Mostly H, some He, and a small amount of heavier elements (nitrogen, oxygen, iron, etc.)

 Diameter: 865,000 miles (108 D Earth )  Mass: 330,000 M Earth  Determined from Kepler’s 3 rd Law  Composition: 70% Hydrogen, 28% Helium, 2% other elements  From spectroscopy

 Contains over 99.9% of the mass of the Solar System  Keep in mind the size of Earth, Jupiter, etc.  The surface is 10,000 o F  It is gaseous H and He, not solid  The core is 15,000,000 o F  5 billion years old  Will shine for another 5 billion years

 From P. 497 of the book: “If we could somehow capture and store just 1 second’s worth of the Sun’s luminosity, it would be enough to meet current human energy demands for roughly the next 500,000 years.”

 PLASMA is the phase of matter in the Sun  Tenuous (gas-like) at outer surface, much more dense toward core  The high temperatures do not allow electrons to stay bound to nuclei

 The Sun shines.  “Creates its own light”  The planets reflect light from the Sun  They do glow, but well below visible light frequencies

 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

 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

E = mc 2

 Nuclear reactions fuse H into He  Mass is converted to energy in this process  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

 Doesn’t gravity still try to collapse the star?  Don’t the nuclear reactions try to blow it apart?

 They balance!  Gravity pulls matter inward  Pressure from hotter layers and nuclear reactions pushes matter outward

 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 keeps the Sun’s size stable  What happens when the Sun has fused all of its hydrogen into helium?