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Stellar Evolution. Chapters 16, 17 & 18. Protostars. Protostars form in cold, dark nebulae. Interstellar gas and dust are the raw materials from which stars form. Example of a star forming nebula. Star Cluster N81. Collapse may be triggered
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Stellar Evolution Chapters 16, 17 & 18
Protostars • Protostars form in cold, dark nebulae. • Interstellar gas and dust are the raw materials from which stars form.
Example of a star forming nebula Star Cluster N81
Collapse may be triggered • Gas condenses due to gravity, pressure and temperature increase • Cloud flattens and spins faster. • Protostar gives off heat (infrared, but no light) • Planets may form in disk.
Growth of a Protostar • They continue to accrete matter until temperature and pressure in core are high enough for fusion.
Main Sequence • It takes small red stars over 50,000,000 years to reach MS. • Large blue stars take only 60,000 years to reach MS. • Nuclear fusion begins when hydrogen starts to burn.
Main Sequence Stars • These stars generate energy by hydrogen fusion. • 4 Hydrogen molecules smash together to form Helium and energy. • Star begins to shine. 4 H = He + ENERGY
Main Sequence Stars Blue Giants Sun Class 20,000 K + Red Dwarfs 3,000 K • The type of Main Sequence star depends on initial mass. Bigger = Higher Temperature = bluer color.
Lower Limit for stars • Brown dwarfs, not massive enough to start fusion M<0.08MSun
Upper limit for stars • Very massive stars are giving off so much energy the pressure of photons drives their matter into space • Observations show the limit is 100MSun
Red Giant • Hydrogen fuel is running out. • Core shrinks, begins helium fusion. • Radiation pressure pushes atmosphere out and it expands.
What’s happening: • As the core contracts, it gets hotter, heating the layer of gas around it. • Hydrogen fusion starts in this shell causing the atmosphere to expand. • As it expands, it cools and becomes redder.
During this phase, dredge ups occur when the star has a small mass.
Low Mass Star - White Dwarf • White dwarfs, are the carbon and oxygen cores of dead stars. • WD are about the size of earth. • The more massive a WD is, the smaller it is in size. • Chandrasekhar limit: 1.4 Msun
Electron degeneracy pressure supports them against gravity. • Eventually they will cool down, and end up as a black dwarf. • WDs are surrounded by planetary nebula, the remains of the star’s atmosphere.
Nova • Nova- Occurs in binary system, white dwarf + other aging star. • Gases from companion fall on white dwarf surface. • Outer layer of WD burns hydrogen. • Can happen repeatedly.
High Mass Stars- Red Supergiant • A high mass star, can have a diameter of 778 Million km, which is almost the size of Jupiter’s orbit.
Once helium in the core is consumed. • The core contracts & heats up. A new element begins to burn. • The surrounding layers heat, they also undergo fusion. • The last stage is when iron is formed in the core!
High Mass: Supernova • When the most massive stars run out of fuel gravity quickly crushes the core. • The atmosphere is ripped apart by shock waves in a cataclysmic explosion.
A supernova explosion can create a neutron stars or black holes.
Neutron Stars • Formed by core collapse of very massive star. • The core is so compressed that electron and protons combine to form neutrons. • Neutron degeneracy pressure of neutrons supports star against gravity.
Most Massive Stars: Black Hole • Collapsed core of most massive stars. Infinitely small & dense. • Its gravity stops even light. • The spherical surface is known as the event horizon. • Astronomers believe black holes exist because they bend the fabric of space.
