Stellar Evolution: After the main Sequence Beyond hydrogen: The making of the elements

Leaving the main Sequence Slowly fuses hydrogen into helium(hydrogen burning) at the core. In a state of hydrostatic equilibrium –Inward gravity is balanced by outward pressure due to Hydrogen burning  Size does not vary over time In a state of Thermal equilibrium  Temperature remains constant over time Main Sequence Stars

Changes in the Solar composition

In main sequence stars, the core temp. is not high enough to “burn” helium. Eventually, the hydrogen becomes depleted at the core –the nuclear fire there ceases, and the location of principle burning moves higher layers of the core - shell burning hydrogen. –In inner core - the temp. still not high enough to ignite helium. Leaving the main Sequence

Without the nuclear reactions to maintain, the outward pressure weakens in the helium core, and the core begins to contract under gravity. This collapse causes the core temperature to rise from about 15 million kelvin to about 100 million kelvin. Rising heat in the contracting core creates pressure that causes outer layers to expand. Leaving the main Sequence

During this post-main-sequence phase, the star’s outer layers expand to many time its original size while the core contracts. The expansion of the outer layers, causes the these layers to cool down –This will give the star a red color The star is then referred to as a red giant Red giants are former main sequence stars, now at a different stage of its evolution. Leaving the main Sequence

Red Giant stars

Life of a Sun-like star 1. Protosun forms - cloud collapses and heats (energy from gravitational collapse) –characterized by bipolar flow and jets from star’s poles 2. Main sequence star (Sun today) –Nuclear fusion starts. Hydrogen burns to helium in core and star is in hydrostatic equilibrium, AS ARE ALL MAIN SEQUENCE STARS!

Life of a Sun-like star (cont.) 3. Red giant - –Core begins to run out of hydrogen fuel, begins to contract and heats. Remaining hydrogen burns faster in the shell around core and generates extra energy, disrupting hydrostatic equilibrium and causing outer regions to expand and cool. Star turns red. –Core (helium) becomes a degenerate gas (quantum mechanical state) - conducts heat easily and is incompressible due to degenerate electron pressure. –Helium flash - eventually core is heated enough for helium to ignite all at once, explosively. Degeneracy ends - Helium burning starts.

Helium Burning? Helium Burning - triple alpha process: 4 He + 4 He  8 Be 8 Be + 4 He  12 C +  (photons) Some of the carbon nuclei can fuse with a Helium nuclei to form oxygen 12 C + 4 He  16 O +  (photons)

Stages in stellar evolution

Life of a Sun-like star (cont.) The gases inside a star behaves like an ideal gas under most circumstances –Pressure  (density), (Temperature)

What is a degenerate gas? In a star with mass less than 3M  the gas in the core behaves differently - degenerate gas. The free electrons in this highly compressed core is prohibited from getting any closer to each other by the Pauli exclusion principle. A gas in such a state is referred to as a degenerate gas and the pressure that exist due to this resistance to compression is called degenerate-electron pressure.

What is a degenerate gas? Temperature rises to a required level He burning. Degenerate-electron pressure is independent of temperature. He heats up, He burning happens faster. Without having a “ pressure safety valve”, temperature becomes too high to make the electrons no longer degenerate. Star’s core ends up in helium flash.

Life of a Sun-like star (cont.) 4. Yellow giant - Helium is burned into Carbon in the core. Star shrinks, turns yellow, and pulsates Analogy: pot with lid on it. –Pulsating variable stars Helium Burning - triple alpha process: 4 He + 4 He  8 Be 8 Be + 4 He  12 C +  (photons) 12 C + 4 He  12 O

Life of a Sun-like star (cont.) 5. Red giant (again) - core runs out of helium fuel, helium burns in shell, core contracts, heats up, outer regions expand. Star is more luminous than during previous red giant stage.

Life of a Sun-like star (cont.) 6. Planetary nebula- Outer region of star is so cool that helium, carbon and oxygen flakes condense. High luminosity (photons) push these flakes off, which drag gas as they go, stripping Star down to a carbon core. 7. White dwarf - this is all that is left of the Star. Core of carbon is very hot, but is no longer burning anything, so will eventually cool to a cinder.

Structure of Sun-like stars after the main sequence Interiors of Sun-like stars layered like onions. Each layer further down consists heavier element, created as the layer above burns.

Life of a high mass star 1. Protostar: Interstellar cloud collapses gravitationally, heats and glows. –Characterized by bipolar flow and jets of matter from star’s poles 2. Main sequence massive star - burns hydrogen to helium in core and is in hydrostatic equilibrium, AS ARE ALL MAIN SEQUENCE STARS! Burns very luminously (quickly).

Life of a high mass star (cont.) 3. Yellow giant (pulsating) - core begins to run out of hydrogen and contracts, heats up. Helium begins to burn (no helium flash necessary - temperature is high enough -100 million K for He burning - in massive star’s cores), then carbon, oxygen, etc. The most massive stars will have cores of iron. Nothing past iron is created. (To create iron requires 2 billion K.)

Life of a high mass star (cont.) 4. Red giant - Later stages of burning – hotter, core burning heavier elements, means more heat produced, which means greater luminosity and expands outer regions of star, which cools.

Life of a high mass star (cont.) 5. Supernova explosion - Core of iron grows until it can not support itself under its own weight. So compressed that protons and electrons join to form neutrons. Core shrinks instantaneously. Rest of star falls in, then rebounds off of neutron star or black hole created in core collapse. Rebound is outward explosion. 6. Neutron star or black hole - after explosion, this is all that is left.

Nucleosynthesis: making heavy elements from light ones Helium: created as hydrogen is burned. Light elements (carbon, oxygen): created as helium is burned in low mass stars. Heavier elements up to iron (Fe): created by burning of carbon, oxygen, etc. in more massive stars. Iron can not be fused and release energy. Elements heavier than Fe: Created mostly in supernova! We are stardust!

Structure of massive stars after the main sequence Interiors of massive stars layered like onions. Each layer further down consists heavier element, created as the layer above burns.

Main sequence lifetime of stars and the importance of gravity The more massive a star, the faster it burns hydrogen in its core, and the shorter its life (despite there being so much more fuel available). –More gravity (more massive star) means star must create more energy (outward pressure) to support its weight. So massive stars burn fuel much faster than Sun-like stars, and have much hotter cores.

Main sequence Lifetimes 0.5 M   0.03L  billion years on main sequence 1 M   1L  (Sun) - 10 billion years on main sequence 3 M   60L  - 1/2 billion years 25 M   80,000L  - 3 million years

Evidence of these processes We have observed –planetary nebula (the end of one Sun-like star, the beginning of new interstellar clouds) –supernova remnants (the end of one massive star, the beginning of new interstellar clouds)

Stellar evolution after the main sequence Zero-age main sequence (ZAMS) - newborn stars lie on this line.

Star clusters on the HR diagram and stellar evolution

How do we know? These evolutionary tracks are fine, but how do we know they are right? –The physics works –all the observed types of stars are explained Star clusters –A cluster of stars forms when a large gas cloud collapses into many stars of many different masses. Each cluster is a snapshot of stellar evolution.

Young Star Cluster Open Cluster: The Hyades cluster. About 600 million yrs. old

Old Star Cluster Globular Cluster M80: About 12 billion yrs. old

Star Populations Population I stars:. –Relatively young stars –Metal rich Population II stars: –Ancient stars –Metal poor During the last stages of population II stars, they produce “metals”, and when they die, these metals are expelled into space. Population I stars are second generation stars formed from these metal rich nebulae.

Check questions What determines a main sequence star? What makes a star move off the main sequence? How does stellar mass influence location on the main sequence? Subsequent evolution? Name the steps in the evolution of a low and high mass star, and the energy source at each step.