Star Formation Processes in Stellar Formation Sequence of Events Role of Mass in Stellar Formation Observational Evidence New Theories

Stellar Formation

Stages of Stellar Evolution There are 7 distinct stages of stellar development from interstellar cloud to main sequence star These stages are characterized by differing core and surface temperatures and radii of the prestellar object Gravitational attraction drives the evolutionary tract, leading ultimately to nuclear fusion, signaling the birth of a star

Stage 1—Interstellar Cloud Dense, dark, and cold interstellar cloud Large— parsecs across (10 14 – km) 1000X mass of our Sun Mainly atomic and molecular gas Gravitational instability in cloud-caused by some external event-triggers cloud collapse

Interstellar Cloud Collapse-Stage 2 Stars form inside relatively dense concentrations of interstellar gas known as molecular clouds. These regions are extremely cold, causing the gas to clump to high densities. Star formation begins when the denser parts of the cloud core collapse under gravity. These cores typically have masses around 10 4 solar masses. As the cores collapse they fragment into clumps around 0.1 parsecs in size and 10 to 50 solar masses in mass. These clumps then form into protostars and the whole process takes about 10 million years.

Stage 3 to 5

Protostar H-R Diagram-Stage 4 Evolutionary tract followed by contracting interstellar cloud fragment High luminosity results from large size of gas cloud Evolutionary track known as the Kelvin-Helmholtz contraction phase Internal heat gradually diffuses out and is radiated away

Evolutionary Time Scale

Route to Main Sequence The track from stage 4 to stage 6 is known as the Hayashi track Stars on this track are called T Tauri stars Luminosity drops dramatically as contraction occurs;core temperature rises to 5 million K Heat and gravity compete between stages 6 and 7 until core reaches about 10 million K; nuclear fusion begins.

Stars of Different Masses Features of the Hayashi Track similar for each mass star However, the time required to arrive on the main sequence differs considerably, decreasing rapidly as the mass increases Stars do not “evolve” along the main sequence; they arrive at some point on it depending on their mass and composition

Relative Sizes of Different Mass Stars

Conditions for Stellar Stability

Star Cluster Formation When stars are born they develop from large clouds of molecular gas. After the remnant gas is heated and blow away, the stars collect together by gravity. During the exchange of energy between the stars, some stars reach escape velocity from the protocluster and become runaway stars. The rest become gravitationally bound, meaning they will exist as collection orbiting each other forever.

Star Clusters Jewel Box-Young ClusterM80-Old Cluster

Brown Dwarfs—Failed Stars If a protostar forms with less than 0.08 solar masses, nuclear fusion never begins This failed star is called a brown dwarf, a planet sized object Brown dwarfs still emit energy, due to gravitational collapse Brown dwarfs are important to astronomy since they may be the most common type of star out there and solve the missing mass problem Brown dwarfs eventual fade and cool to become black dwarfs.

Evidence of Stellar Formation The region surrounding the nebula M20 shows evidence of contraction A huge, dark molecular cloud surrounds the visible nebula Density and temperature are low The glowing region of ionized gas results directly from a massive O-type star at stage 6 or 7 on its evolutionary track.

Evidence of Protostars Star forming regions known as "EGGs" are uncovered at the end of this giant pillar of gas and dust in the Eagle Nebula (M16) EGGs, short for evaporating gaseous globules, are dense regions of mostly molecular hydrogen gas that fragment and gravitationally collapse to form stars.

Shock Waves and Star Formation

Carbon Star