Chapter 13 Star Stuff

Evolution of Low-Mass Stars 1. The Sun began its life like all stars as an intersteller cloud. 2. This cloud collapses due to gravity into a dense core. 3. In about a million years a small, hot, dense core called a protostar forms.

Rotating Cloud Fragment Jets of high speed gas Proto-stellar disk Protostellar Wind Artist’s Conception of Star Birth

Schematic Illustration of protostellar disk-jet structure

4. When the temperature reaches 10 million Kelvin in the core, fusion begins transforming the protostar into a main-sequence star. 5. Low mass stars like the Sun remain on the main-sequence for about 10 billion years. Massive stars stay on the main-sequence for about 1 billion years.

Life track of a 1M star from protostar to main-sequence star

Life tracks from protostar to main sequence for stars of different masses.

6. After all of the Hydrogen is depleted in the core, fusion begins in a shell around the core and the star expands into a Red Giant. 7. After most of the hydrogen is fused into helium, helium fusion begins in the hotter core in an event called the Helium Flash. 8. Stars can then become unstable and turn into pulsating stars like RR Lyrae Variables or Cephied Variables.

After a star ends its main-sequence life, its inert helium core contracts while a hydrogen shell begins fusion at a higher rate. This forces the star’s outer layers to expand outward.

Core Structure of helium burning star.

The onset of helium fusion The onset of helium fusion. When helium fusion begins, the star’s surface shrinks and heats. The star’s life track therefore moves downward and left on the H-R diagram

9. As a star burns helium into carbon the radiation pressure pushes the star's outer atmosphere away from the core creating a second (double shell) red giant stage and then a Planetary Nebula. Electron degeneracy pressure of the Carbon core halts any further collapse. Fusion process in the core stops. 10. This leaves an exposed core called a White Dwarf. These have about the same diameter as the Earth.

The life track of a 1M star from main-sequence to white dwarf. Core structure at key stages

Gaseous shells surround the remnant carbon core

Evolution of High-Mass Stars 1 to 5. Same as before… intersteller cloud  dense core   protostar  zero-age main-sequence star  main-sequence star 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

Stellar Nucleosynthesis Evolutionary Time Scales for a 15 M Star

The Multiple Layers Of Nuclear Burning In The Core Of A High Mass Star During Its Final Days

Life Tracks On The H-R Diagram From Main-sequence Star To Red Supergiant For Selected High Mass Stars

9. Fusion stops with iron (Fe) and a star with an iron core is out of fuel. Reason: 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!

Iron Core degenerates into a neutron core (neutron degeneracy). Electrons and protons combine to form neutrons with the release of neutrinos Average mass per nuclear particle from hydrogen to iron decreases and then increases for atomic masses greater than iron.

Origin Of The Elements- Stellar Nucleosynthesis Observed Relative Abundances Of Elements In The Galaxy In Comparison To The Abundance Of Hydrogen.

Changing H-R diagram of a hypothetical star cluster.

The Double Cluster “h and  Persei Only 10 million years old

Glodular cluster 47 Tucanae. ~ 11 billion years old

Evolution of a Binary Star System Each star can be pictured as being surrounded by a “zone of influence” or Roche lobe.

What can happen?

What can happen?

The sun’s core will eventually become Iron/Nickel Carbon Neutrons only Electrons only

In about 5 billion years the Sun will become A Black Hole A Neutron Star A Red Giant A Red Dwarf

A High Mass Star Will Eventually Become A White Dwarf Nothing, as it has exploded. A Red Dwarf A Black Hole or Neutron Star

End of Chapter