Presentation on theme: "Chapter 20 Stellar Evolution. Important concepts Luminosity depending on temperature and radius. Behavior of gases when they expand or contract. Gravitational."— Presentation transcript:
Chapter 20 Stellar Evolution
Important concepts Luminosity depending on temperature and radius. Behavior of gases when they expand or contract. Gravitational energy transformed into heat. Nuclear fusion processes
The seven stages so far:
Stars Must Change Stars are constantly radiating energy. The energy is replaced by nuclear fusion. The energy available from fusion is very large, but finite. Eventually the fusion sources change, then halt. The star’s luminosity or temperature will change.
What was fighting gravity when the star was on the main sequence?
A star remains stable as long as it remains in hydrostatic equilibrium 1.Small increase in temperature results in 2.Increase in pressure, which results in 3.Expansion, which results in 4.Cooling, which results in 5.Recovering its equilibrium and vice versa 1.Small decrease in temperature, results in 2.Decrease in pressure, which results in 3.Gravitational contraction, which leads to 4.Heating up 5.Recovering its equilibrium
1) as a protostar. 2) as a red giant. 3) as a main-sequence star. 4) as a white dwarf. 5) evolving from type O to type M. Question 1 A star will spend most of its “shining” lifetime
1) as a protostar. 2) as a red giant. 3) as a main-sequence star. 4) as a white dwarf. 5) evolving from type O to type M. Question 1 A star will spend most of its “shining” lifetime In the main-sequence stage, hydrogen fuses to helium. Pressure from light and heat pushing out balances gravitational pressure pushing inward.
Low-Mass Stars The rates and types of fusion depend on the star’s mass. Therefore all stars are unique. Generally, stars with M < 3 M share many characteristics. These are called low-mass stars.
Higher Mass = Shorter Lifetime We can figure out main sequence lifetimes: (lifetime) = (energy available) / (rate energy used). More mass = more fuel available. Rate energy used = rate energy emitted (luminosity). More massive stars have much higher luminosity. They use their fuel up more quickly and leave the MS faster.
Main Sequence Lifetimes Mass (M )Luminosity (L ) Lifetime in billion years 17.552,5000.01 2.0141.1 1.0 10 0.670.1553 0.210.011290
Lifetime and Mass
The End of the Main Sequence Main sequence (MS) stars fuse hydrogen to helium in their cores (creating heat and pressure). This is the definition of the MS. More massive stars have higher central temperatures, greater luminosity. Eventually, much of the core H is converted to He. The star will then expand and cool and become a subgiant.
1) they gradually become cooler and dimmer (spectral type O to type M). 2) they gradually become hotter and brighter (spectral type M to type O). 3) they don’t change their spectral type. Question 2 As stars evolve during their main- sequence lifetime
1) they gradually become cooler and dimmer (spectral type O to type M). 2) they gradually become hotter and brighter (spectral type M to type O). 3) they don’t change their spectral type. Question 2 As stars evolve during their main- sequence lifetime A star’s main sequence characteristics of surface temperature and brightness are based on its mass. Stars of different initial mass become different spectral types on the main sequence. CLL: HR diagram and lifetimeHR diagram and lifetime
Stage 8, Subgiant branch: Composition changes Hydrogen burns fastest at the center
What happens to the pressure as nuclear fusion runs out fuel?
Without pressure fighting gravity, what happens to the core?
As the fuel in the core is used up, the core contracts; when it is used up the core begins to collapse.
What happens to core’s temperature as it collapses? As gases compress, they heat up. Gravitational kinetic energy is converted into kinetic energy (temperature) of the gas.
Where does that heat go, if it cannot be used to stimulate fusion? Half of the heat increases the temperature. The other half radiates away into the outer shell.
Hydrogen begins to fuse outside the core and star expands. What happens to the envelope as it heats up? Density drops. Temperature remains same. What happens to the density of the star’s outer layer as it expands?
The stages on the HR diagram Nuclear fusion inside slows down. Star expands and cools down. Hydrogen burning in the shell: Star expands and becomes more luminous. Core collapsesShell expands
What inevitably forces a star like the Sun to evolve away from being a main-sequence star? Question 1) The core begins fusing iron. 2) The star uses up all of its supply of hydrogen. 3) The carbon detonation explodes it as a type I supernova. 4) Helium builds up in the core, while the hydrogen-burning shell expands. 5) The core loses all of its neutrinos, so all fusion ceases.
What inevitably forces a star like the Sun to evolve away from being a main-sequence star? 1) The core begins fusing iron. 2) The star uses up all of its supply of hydrogen. 3) The carbon detonation explodes it as a type I supernova. 4) Helium builds up in the core, while the hydrogen-burning shell expands. 5) The core loses all of its neutrinos, so all fusion ceases. When the Sun’s core becomes unstable and contracts, additional H fusion generates extra pressure, and the star will swell into a red giant.
Stage 9:Red Giants Later, the star expands and its luminosity rises. The star becomes a red giant. H fusion takes place in a shell around the core. The core becomes more dense, and becomes electron degenerate. This means pressure is not from moving atoms, but from a quantum mechanical effect: there’s a limit to how tightly electrons can be packed together (Pauli exclusion principle).
Why the Luminosity Increases When the core shrinks, the pressure increases. Nuclear fusion in the shell goes faster. Faster nuclear reactions release more energy. This energy leaves the star’s surface at a higher rate.
Stage 10:Helium fusion Temperatures 100,000,000K Core is small, dense, electron degenerate. Outer envelope is greatly expanded, cooler. Fusion of He begins in the degenerate core. Helium fuses to carbon via the triple-alpha process. This starts suddenly in the helium flash. Star shrinks and heats up.
Temperature and helium fusion The average kinetic energy in a gas of atoms is The kinetic energy must supply the work to push against the electrostatic forces The electromagnetic force is proportional to the product of the charges Q1 and Q2. Therefore T ~ Q1 x Q2
fusion Once the core temperature has risen to 100,000,000 K, the helium in the core starts to fuse, through a three-alpha process: 4 He + 4 He → 8 Be + energy 8 Be + 4 He → 12 C + energy The 8 Be nucleus is highly unstable and will decay in about 10 –12 s unless an alpha particle fuses with it first. This is why high temperatures and densities are necessary. Stage 10: Helium Fusion
The pressure within the helium core is almost totally due to “electron degeneracy”—two electrons cannot be in the same quantum state, so the core cannot contract beyond a certain point. This pressure is almost independent of temperature—when the helium starts fusing, the pressure cannot adjust and expand the gas, which would result in cooling. For hours the temperature increases rapidly until final the normal pressure increases and expansion cools the star again. The helium flash:
Question What type of atomic nuclei heavier than helium are most common, and why? 1) those heavier than iron, because of supernovae 2) iron, formed just before massive stars explode 3) odd-numbered nuclei, built with hydrogen fusion 4) even-numbered nuclei, built with helium fusion
1) those heavier than iron, because of supernovae 2) iron, formed just before massive stars explode 3) odd-numbered nuclei, built with hydrogen fusion 4) even-numbered nuclei, built with helium fusion What type of atomic nuclei heavier than helium are most common, and why? Helium nuclei have an atomic mass of 4; they act as building blocks in high- temperature fusion within supergiants.
The Horizontal Branch & the AGB After the helium flash, the stars are on the horizontal branch of the H-R diagram. At first, He C in the core, H He in a shell around the core. Helium is then used up in the core. He fusion in an inner shell and H fusion in an outer shell. Star gets more luminous and cool, enters the asymptotic giant branch (AGB).
1) when T-Tauri bipolar jets shoot out. 2) in the middle of the main sequence stage. 3) in the red giant stage. 4) during the formation of a neutron star. 5) in the planetary nebula stage. Question The “helium flash” occurs
1) when T-Tauri bipolar jets shoot out. 2) in the middle of the main sequence stage. 3) in the red giant stage. 4) during the formation of a neutron star. 5) in the planetary nebula stage. The “helium flash” occurs When the collapsing core of a red giant reaches high enough temperatures and densities, helium can fuse into carbon quickly – a “helium flash”.
Stage 11: The AGB Stage 11: The AGB Double shell burning
After the AGB Electrons become degenerate again, stopping increase of temperature necessary to ignite massive carbon fusion. The star expands, cools. The outer layers are ejected into space. The ejected material creates a planetary nebula. The core shrinks and gets very hot, eventually to cool into a compact white dwarf.
This graphic shows the entire evolution of a Sun-like star. Such stars never become hot enough for fusion past carbon to take place. The Death of a Low-Mass Star
There is no more outward fusion pressure being generated in the core, which continues to contract. The outer layers become unstable and are eventually ejected. The Death of a Low-Mass Star Red giant instability due to helium flashes
Stage 12: Planetary Nebula Formation UV radiation ionizes the inner parts of the cloud that was formed from ejected star materials.
Planetary Nebulae Spirograph nebula: Dr. Raghvendra Sahai (JPL) and Dr. Arsen R. Hajian (USNO), NASA and The Hubble Heritage Team (STScI/AURA). Eskimo nebula: NASA/ A. Fruchter and the ERO team (STScI). NGC 6751: A. Hajian (USNO) et al., Hubble Heritage Team (STScI/AURA), NASA. Helix nebula: Anglo- Australian Telescope, photograph by David Malin. Cat's eye nebula: J.P. Harrington and K.J. Borkowski (University of Maryland), HST, NASA. M2-9: Bruce Balick (University of Washington), Vincent Icke (Leiden University, The Netherlands), Garrelt Mellema (Stockholm University), and NASA.
White Dwarfs They start hot and cool with time (important). Not very luminous - small. Masses 0.6 – 1.4 M , radius like Earth. Core is mainly carbon and is degenerate. Final state depends on how much mass is lost in the planetary nebula phase. There is also mass lost in the red giant phase. All low-mass stars become white dwarfs.
Summary CLL: Death sequence, Low mass star: Death sequence, Low mass star
Review Stage 1 through 7: Formation of a protostar. What allowed the protostar to contract without reaching equilibrium between pressure and gravity? a) Increasing density b) Decreasing pressure c) Cooling through radiation
Low density star contracts as energy is radiated away preventing pressure to oppose contraction.
Stage 8: Subgiant branch What is the reason that a star leaves the mainsequence? a) Increase of pressure due to enhanced nuclear fusion inside the core. b) Ceasing of hydrogen burning inside the core. c) Hydrogen burning inside the shell
Hydrogen burning ceases inside the core
The HR diagram shows a decrease in Temperature and increase in size on the subgiant branch What are the reasons for the increase in size? a) Core collapses but shell expands due to hydrogen burning b) Pressure inside core increases, star expands c) Size doesn’t matter.
Collapsing core produces heat (gravitational energy transformed into heat), that radiates into the shell surrounding the core. Hydrogen begins to fuse inside the shell and star expands. Nuclear fusion in the shell goes faster. Faster nuclear reactions release more energy. This energy leaves the star’s surface at a higher rate.
Stage 9: Red Giant What keeps the core from collapsing further? a) Pressure regained due to increase in heat. b) Pauli’s principle. c) Rudy’s principle
H-burning shell and electron degenerate core
Stage 10:Helium fusion What happens during stage 10? a) Shell is burning helium into carbon, core contracts. b) Core is burning helium into carbon and shell contracts.
The pressure within the helium core is almost totally due to electron degeneracy, so the core cannot contract beyond a certain point. This pressure is almost independent of temperature—when the helium starts fusing, the pressure cannot adjust.
Helium begins to fuse extremely rapidly; within hours the enormous energy output is over, and the star once again reaches equilibrium
Stage 11: Asymptotic Giant Branch Why does the star again develop into a Red Giant? a) Hydrogen burning in the outer shell and Helium burning in the inner shell. b) Increased hydrogen burning in the core c) Carbon burning in the core.
Back to the giant branch As the helium in the core fuses to carbon, the core becomes hotter and hotter, and the helium burns faster and faster. The star is now similar to its condition just as it left the Main Sequence, except now there are two shells:
The star has become a red giant for the second time
This graphic shows the entire evolution of a Sun-like star. Such stars never become hot enough for fusion past carbon to take place. The Death of a Low-Mass Star
There is no more outward fusion pressure being generated in the core, which continues to contract. The outer layers become unstable and are eventually ejected.
The ejected envelope expands into interstellar space, forming a planetary nebula.
The star now has two parts: A small, extremely dense carbon core An envelope about the size of our solar system. The envelope is called a planetary nebula, even though it has nothing to do with planets—early astronomers viewing the fuzzy envelope thought it resembled a planetary system.
Planetary nebulae can have many shapes: As the dead core of the star cools, the nebula continues to expand and dissipates into the surroundings.
Stages 13 and 14: White and black dwarfs Once the nebula has gone, the remaining core is extremely dense and extremely hot, but quite small. It is luminous only due to its high temperature.
The small star Sirius B is a white-dwarf companion of the much larger and brighter Sirius A:
As the white dwarf cools, its size does not change significantly; it simply gets dimmer and dimmer, and finally ceases to glow.
This outline of stellar formation and extinction can be compared to observations of star clusters. Here a globular cluster:
It can be seen from this H-R diagram that stars more massive than the Sun follow very different paths when leaving the Main Sequence Evolution of Stars More Massive than the Sun
High-mass stars, like all stars, leave the Main Sequence when there is no more hydrogen fuel in their cores. The first few events are similar to those in lower-mass stars—first a hydrogen shell, then a core burning helium to carbon, surrounded by helium- and hydrogen-burning shells.
Stars with masses more than 2.5 solar masses do not experience a helium flash—helium burning starts gradually. A 4-solar-mass star makes no sharp moves on the H-R diagram—it moves smoothly back and forth.
The main difference between low and high mass star are fusion processes into heavier elements. Why can high mass stars create conditions for fusion into heavier elements? More mass means more gravitational energy can be transformed into heat as star contracts.
A star of more than 8 solar masses can fuse elements far beyond carbon in its core, leading to a very different fate. Its path across the H-R diagram is essentially a straight line—it stays at just about the same luminosity as it cools off. Eventually the star dies in a violent explosion called a supernova.
Massive Post-MS Evolution So far, pretty similar to lower mass stars studied already, but Now we feel the big difference: higher M means gravity can crush the C core until it reaches T > 7 x 10 8 K so Carbon CAN ALSO FUSE 12 C + 4 He 16 O + Some: 16 O + 4 He 20 Ne + Also some: 12 C + 12 C 24 Mg + This fuel produces less energy per mass so C is burnt quickly. Loops in the H-R diagram.
Massive Post MS Evolution on H-R Diagram
Evolution of Stars More Massive than the Sun
Mass Loss from Giant Stars All stars lose mass via some form of stellar wind. The most massive stars have the strongest winds; O- and B-type stars can lose a tenth of their total mass this way in only a million years. These stellar winds hollow out cavities in the interstellar medium surrounding giant stars.
The sequence below, of actual Hubble images, shows a very unstable red giant star as it emits a burst of light, illuminating the dust around it:
The following series of H-R diagrams shows how stars of the same age, but different masses, appear as the whole cluster ages. After 10 million years, the most massive stars have already left the Main Sequence, while many of the least massive have not even reached it yet. Observing Stellar Evolution in Star Clusters
After 100 million years, a distinct main-sequence turnoff begins to develop. This shows the highest- mass stars that are still on the Main Sequence. After 1 billion years, the main-sequence turnoff is much clearer.
After 10 billion years, a number of features are evident: The red-giant, subgiant, asymptotic giant, and horizontal branches are all clearly populated. White dwarfs, indicating that solar-mass stars are in their last phases, also appear.
This double cluster, h and chi Persei, must be quite young—its H-R diagram is that of a newborn cluster. Its age cannot be more than about 10 million years.
The Hyades cluster, shown here, is also rather young; its main-sequence turnoff indicates an age of about 600 million years.
Something odd with Algol Algol is a 3.8-solar mass main sequence star of spectral type B8 with a red sub-giant binary companion of 0.8-solar mass in a nearly circular orbit. Both are assumed to have formed at the same time. What’s odd about this? The larger star should have evolved faster than the smaller, that is already on the subgiant branch.
Each star has its zone of influence, called Roche lobe Inside a star’s Roche lobe, its gravitational pull dominates the effects of rotational motion of the binary system and the gravitational pull of the companion star. Any matter inside the RL belongs to the star and cannot flow away. Outside the RL, matter can flow to either star. The Lagragian point is where the gravitation pull from both stars balance.
Binary Star Evolution Most stars are in binary or multiple systems If the binary is close enough, evolution is affected More massive stars still can be on MS while less massive has evolved off (like Algol) Only possible if there is mass transfer through Lagrangian point (L 1 ) between Roche lobes
Binary Evolution Depends on Separation Detached, evolve separately Semi-detached, one fills Roche lobe, dumping on other Contact or common-envelope, both overflow: single star with two fusion cores Depending on orbital size and evolutionary stages, different scenarios are possible
Binary Evolution: Algol Type Start detached More massive leaves MS, blows up and overflows Roche lobe, dumping mass into less massive star. Now 2nd star is more massive but still on MS