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Tsokos Lesson E-5 Stellar evolution

IB Assessment Statements Topic E-5, Stellar Processes and Stellar Evolution Nucleosynthesis E.5.1. Describe the conditions that initiate fusion in a star. E.5.2. State the effect of a star’s mass on the end product of nuclear fusion. E.5.3. Outline the changes that take place in nucleosynthesis when a star leaves the main sequence and becomes a red giant.

IB Assessment Statements Topic E-5, Stellar Processes and Stellar Evolution Evolutionary Paths of Stars and Stellar Processes E.5.4. Apply the mass-luminosity relation. E.5.5. Explain how the Chandrasekhar and Oppenheimer-Volkoff limits are used to predict the fate of stars of different masses. E.5.6. Compare the fate of a red giant and a red supergiant. E.5.7. Draw evolutionary paths of stars on an HR diagram. E.5.8. Outline the characteristics of pulsars.

Objectives Describe how a star is formed State the main sequences of nuclear reactions taking place in a star and say how the mass of the star determines which sequences actually take place Describe the main stages in the evolution of a star and say how the mass plays a determining role

Objectives Describe the end stages of stellar evolution and appreciate the significance of the Chandrasekhar limit State the main properties of a black hole State the main properties of pulsars State the main characteristics of quasars State the meaning of the term gravitational lensing

Nucleosynthesis Interstellar space consists of gas and dust at a density of about 10-21 kg/m2 One atom of hydrogen per cubic meter of space 74% hydrogen 25% helium 1% other

Nucleosynthesis When the gravitational energy of a given mass exceeds the average kinetic energy of the thermal random motion of its molecules, the gas becomes unstable and tends to collapse Jeans criterion

Nucleosynthesis Stars form when relatively cool gas clouds (10 – 100 K) collapse under their own gravitation When they contract, they heat up Typical for the cloud to break up into smaller clouds and form several stars

Nucleosynthesis When the collapsing gas is hot enough to produce visible light, the star is called a protostar

Nucleosynthesis As the gas continues to compress, its temperature and pressure continue to increase When the pressure in the core of the star equals the gravitational force, the star becomes fixed in shape

Nucleosynthesis However, the combination of temperature (T = 106 to 107 K) and pressure in the core is so great that nuclear fusion (proton-proton cycle in E-2) reactions take place releasing large amounts of energy

Nucleosynthesis The star maintains its shape so long as the number of fusion reactions creates enough heat and pressure to balance the gravitational force of contraction.

Mass – Luminosity Relation The mass and age of a star will determine its luminosity Applies to main sequence stars α is between 3 and 4 Used to estimate the lifetime of a star on the main sequence (Note that T = time, not temperature or period)

Mass – Luminosity Relation Take α to be equal to 4 A star with one solar mass will spend 1010 years on the main sequence A star with mass equal to 10 solar masses will spend 1/103 or 10-3 times less than that

Mass – Luminosity Relation Main sequence stars on the upper left of the HR diagram have very high luminosity and are thus very massive

Mass – Luminosity Relation After a star has used 12% of its hydrogen (Schönberg – Chandrasekhar limit) its core will contract but its outside will expand substantially making it a very large star. It then begins to leave the main sequence and move over to the red giant area

Mass – Luminosity Relation Example Q3 We know the power output of our sun We know the energy released per fusion reaction We can determine the number of reactions per unit time to maintain this power If we know the mass of the sun we can then find time to consume 12% of its hydrogen

Nuclear Reactions Beyond Main Sequence When hydrogen in the core is depleted, fusion stops and the core contracts Contraction heats up the core and that energy creates hydrogen fusion reactions on the surface of the core causing the outer layers to expand The outer layers cool because of expansion The star becomes bigger and cooler on the outside, but the core continues to heat up This is a red giant

Mass and a Star’s Future Low mass stars (less than 0.25 solar masses No further nuclear reactions take place Core stays a core of helium

Mass and a Star’s Future Mass Between 0.25 and 4 solar masses Temperature of the core reaches 108 K Adequate for nuclear reactions involving helium to take place No further reactions take place and the core consists of carbon and oxygen

Mass and a Star’s Future Mass Between 0.25 and 4 solar masses

Mass and a Star’s Future Mass Between 4 and 8 solar masses Core temperature continues to rise Nuclear reactions involving carbon and oxygen take place Produces a core of oxygen, neon and magnesium

Mass and a Star’s Future Mass Over 8 solar masses The core of oxygen, neon and magnesium continues to contract reaching temperatures high enough for these elements to fuse and create heavier elements In the outer layers, hydrogen continues to produce helium and helium produces carbon and oxygen Processes continue with heavier elements settling in the core Eventually, iron is produced – the heaviest element that can be produced through fusion

Mass and a Star’s Future Mass Over 8 solar masses The star ends its cycle of reactions with iron at its core, lighter elements surround it The outer layers have expanded to a very large size It is now a red supergiant

Mass and a Star’s Future Mass Over 8 solar masses

Mass and a Star’s Future Temperatures required for fusion reactions Mass determines gravitational force which drives core temperature and pressure

Evolutionary Paths and Stellar Processes The path taken in the HR diagram and the ultimate fate of a star are dependent on mass

Evolutionary Paths and Stellar Processes

Evolutionary Paths and Stellar Processes Star paths based on mass

Stars Under 8 Solar Masses Energy from core contraction and fusion reactions in outer layers may force ejection of outer layers – planetary nebula Causes a reduction in the mass of a star though it still continues to contract – white dwarf Without fusion reactions in the outer layers, the star eventually cools to practically zero – black dwarf

Stars Under 8 Solar Masses Electron pressure generated by Pauli Exclusion Principle (no two electrons can occupy the same quantum state) can stop star contraction only if the mass of the core is less than 1.4 solar masses – Chandrasekhar limit Result is white dwarf If core mass is greater than 1.4 solar masses, result is neutron star or black hole

Stars Under 8 Solar Masses During transition from white to black dwarf, mass from other stars can be absorbed by the white dwarf This mass heats up and emits light, increasing the star’s luminosity for a short period -- nova

Stars Over 8 Solar Masses The photons produced by fusion reactions have enough energy to break iron nuclei in the core apart into protons and neutrons Electrons are forced into protons turning them into neutrons (Pauli principle) and releasing neutrinos

Stars Over 8 Solar Masses When the neutrons in the core are compressed to a certain point, they rebound to expand the core to a larger equilibrium size This rebounding cause a shock wave that rips apart the outer layers – supernova Core that remains is called a neutron star Neutron pressure keeps star stable provided the mass is not more than 3 solar masses – Oppenheimer-Volkoff limit

Star Evolution

Star Evolution

Star Evolution – Ultimate Demise If the mass of the core of a star is less than the Chandrasekhar limit of about 1.4 solar masses, the star will become a stable white dwarf in which electron pressure keeps the star from collapsing further.

Star Evolution – Ultimate Demise If the core is more massive than the Chandrasekhar limit, the core will collapse further until electrons are driven into protons, turning them into neutrons. Neutron pressure now keeps the star from collapsing further and the star has become a neutron star.

Star Evolution – Ultimate Demise If the core is substantially more massive than the Oppenheimer-Volkoff limit of about 2-3 solar masses, neutron pressure will not be enough to oppose the gravitational collapse and the star will become a black hole.

Black Holes (can you see it?)

Black Holes If the mass of a collapsing star is greater than a few tens of solar masses, gravitational collapse is unstoppable Reaches a density such that the escape velocity from the surface equals the speed of light, so nothing can escape (Hotel California – ‘You can check out any time you like, but you can never leave.’) Light cannot escape.

Black Holes We can calculate the radius that this occurs -- Gravitational Radius (Rg), Swarzchild Radius (Rs), or Event Radius

Evidence for Black Holes Motion of nearby stars affected by its gravitational attraction Watch for binary stars that emit X-rays where one star is invisible Nine known of so far Gas forced into black holes accelerate, become hot, then emit X-rays

Pulsars Neutron star with a magnetic field on the order of 108 T May also rotate as well with periods from 30ms to 0.3s Emit electromagnetic waves in the radio and X-ray ranges of the spectrum Detected by radiotelescopes Rotation causes radiation

Pulsars Radiation is along a narrow cone around the axis of the magnetic field direction As the star rotates, the axis of radiation precesses Pulsing comes from passage of the axis

Quasars Stars exhibiting bookoo red-shifting Corresponds to 95% of speed of light Believed to be very active centers of very young galaxies Theory is that they are powered by black holes

Gravitational Lensing Radio signals from distant quasars are bent around a massive galaxy Appear to be two different images Provides information about the lensing body.

Σary Review Can you describe how a star is formed? Can you state the main sequences of nuclear reactions taking place in a star and say how the mass of the star determines which sequences actually take place? Can you describe the main stages in the evolution of a star and say how the mass plays a determining role?

Σary Review Can you describe the end stages of stellar evolution and appreciate the significance of the Chandrasekhar limit? Can you state the main properties of a black hole? Can you state the main properties of pulsars? Can you state the main characteristics of quasars? Can you state the meaning of the term gravitational lensing?

IB Assessment Statements Topic E-5, Stellar Processes and Stellar Evolution Nucleosynthesis E.5.1. Describe the conditions that initiate fusion in a star. E.5.2. State the effect of a star’s mass on the end product of nuclear fusion. E.5.3. Outline the changes that take place in nucleosynthesis when a star leaves the main sequence and becomes a red giant.

IB Assessment Statements Topic E-5, Stellar Processes and Stellar Evolution Evolutionary Paths of Stars and Stellar Processes E.5.4. Apply the mass-luminosity relation. E.5.5. Explain how the Chandrasekhar and Oppenheimer-Volkoff limits are used to predict the fate of stars of different masses. E.5.6. Compare the fate of a red giant and a red supergiant. E.5.7. Draw evolutionary paths of stars on an HR diagram. E.5.8. Outline the characteristics of pulsars.

Questions?

Homework #1-26 (6 Points)