(for low mass stars– like the Sun) Stellar Evolution (for low mass stars– like the Sun)

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 and observations. (Computer simulations of the nuclear reactions inside a star match up with what we observe. )

I. First stage of a star’s life: Protostar A. protostar formed by gravitational contraction (accretion) of dust/gas B. contraction causes 1. Pressure to rise 2. Temp. to rise C. Infrared and microwave radiation given off; radiation sweeps away gases and may cause interstellar gases to glow

Star-disk systems in Orion's Trapezium

II. Protostar to Main Sequence Star D. At temps. around 10 million K, nuclear fusion (hydrogen to helium) takes place. E. diagram

F. hydrostatic equilibrium--a balance between pressure of gases and gravity Hydrostatic equilibrium: the star is now stable and remains main sequence for the majority of its life…our Sun is a class G2V star

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)

The Main Sequence Lifetime for Stars of Different Masses

III. Red Giants A. Eventually, hydrogen is used up and core is primarily helium B. Gravity causes temperatures to rise to ~100 million K and a hydrogen shell around the helium core starts to burn

Red Giants C. energy released causes 1. Star to be brighter 2. Size to increase D. Expansion causes the overall surface temperature to decrease E. Further contraction at the core causes Heavier elements to form (carbon & oxygen)

Red Giants F. Examples are 1. Betelgeuse (actually a red supergiant) 2. Arcturus (appear red/orange in the sky)

IV. Planetary nebula stage – Red Giant star loses mass, forming a planetary nebula

V. White dwarf stars…these are Ancient, White Dwarf Stars in the Milky Way Galaxy A. White dwarfs radiate their energy and will fade out over time (over few million years). Their collapse is prevented by degenerate electrons.

For our sun…all this takes ~10 billion years from beginning to end.

Many have been detected in star forming regions like the Orion Nebula. What’s a brown dwarf? A “failed star” too small to start H fusion to He Hard to find because they are very faint and cool; emit mostly in the infrared. Many have been detected in star forming regions like the Orion Nebula.