Stellar Evolution
Stellar Evolution... Refers to the life cycle of stars is based on the H-R diagram. Theoretical plots of the H-R diagram are compared to real star data. (Computer simulations of the reactions inside a star match up with what we observe! )
How long does all of this take?
Protostar to Main Sequence Star A. protostar formed by gravitational contraction of dust/gas B. contraction causes 1. Pressure to rise 2. Temp. to rise C. heat radiates from core and may cause interstellar gases to glow
Birth of Stars
Protostar to Main Sequence Star D. At temps. Around 10 million K, nuclear fusion takes place. E. diagram F. hydrostatic equilibrium--a balance between pressure of gases and gravity
G. How does mass affect the time of star formation? --larger masses contract faster and burn brighter H. How does the mass of main sequence star affect its lifetime? --larger masses die faster (run out of hydrogen faster)
Red Giants A. hydrogen is used up and core is primarily helium B. Gravity causes temp. to rise and fusion at Helium core to speed up (“Helium Flash”)
Red Giants C. energy released causes 1. Star to be brighter 2. Size to increase D. Expansion causes the overall temp. to decrease E. Further contraction at the core causes 1. Heavier elements to form
Red Giants F. Examples are 1. Betelgeuse 2. Arcturus (appear red/orange in the sky)
The Death of Stars A. Depends greatly on a stars MASS ! 1. Small stars form white dwarf then fade out. 2. 1 to 5 solar masses form white dwarf then fade out. 3. 5 to 15 solar masses have fast core collapse and form neutron star or pulsar. 4. Greater than 15 solar masses will form a black hole.
Mass Predetermines what a star will become on the main sequence
The Main Sequence Lifetime for Stars of Different Masses
NEW STUFF! What about stars that are bigger than our sun????
Variable Stars A. Some larger stars move off the Red Giant phase into the AGB group (variable star group) B. pulsating variable star--fluctuate in size and brightness C. Cepheid variables--large, luminous stars whose output oscillates
Cepheid variables D. Importance of cepheids 1. The longer the period, the greater the luminosity. 2. cepheids serve as distance markers when objects are too far away to see parallax
Evolution of High-Mass Stars Same as before… 1. intersteller cloud of dust 2. protostar 3. main-sequence star but as a BLUE GIANT 6. When a high-mass star exhausts the hydrogen fuel in its core the star leaves the main sequence and begins to burn helium.
7. The star becomes a Red Supergiant after millions of years of helium fusion. 8. When helium is depleted, fusion of heavier elements begins. This process is called nucleosynthesis. H -> He -> C -> O -> Si -> Fe (eventually goes to IRON)
Why is iron (mass number = 56) the last stage? Atoms will naturally fuse into more stable nuclei You can’t get more energy out of fusing iron
9. A star with an iron core is out of fuel. (Iron atoms cannot fuse and release energy.) 10. The core collapses due to reduced pressure converting the iron core into mostly neutrons. 11. The core pressure then surges and lifts the outer layers from the star in a titanic explosion - a supernova! (KABOOM!)