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.

Gravity versus Pressure

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 4 H = He + ENERGY 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.  Over a very long time (about 13.8 billion years), a white dwarf will cool and its material will begin to crystallize (starting with the core).  an indicative measure of the timescale of cooling of a white dwarf is given by t = E / L where L is the luminosity of the white dwarf (energy emitted per unit time) and E is the energy content of the white dwarf, which is given by N k T (N = number of atoms, k = Boltzmann's constant, T = temperature of the white dwarf). With this formula, you can try to work out yourself the cooling timescale for some typical white dwarf (M = 0.6 solar masses, T = 10000 K).

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. The event horizon of a 3 MSun black hole is also about as big as a small city

If we can’t see black holes, how do we know they are there?

Summary of Stages