Stellar Evolution for high mass stars

The Death of Stars How a star dies depends on a star’s 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 fuse 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!)

Example: Supernova 1987A Luminosity= 100 billion times brighter than the sun What is produced in a supernova? Light and radiation wave of neutrinos (99% of energy) Elements heavier than IRON

Remnant of a supernova observed in a.d. 1054 Supernova Remnants X-rays The Crab Nebula: Remnant of a supernova observed in a.d. 1054 Cassiopeia A Optical The Veil Nebula The Cygnus Loop