The Lives and Deaths of Stars Astrophysics of Life 2: Stellar Evolution The Lives and Deaths of Stars

A stars’ mass determines its luminosity Protostars. These objects radiate energy away in the form of light. That energy comes from gravity – released by contraction. On the Main Sequence, the contraction stops because gravity and internal pressure exactly balance each other. Nuclear reactions occur at exactly the right rate to balance gravity. A stars’ mass determines its luminosity

Nuclear reactions slowly convert H to He in the core. Newly formed stars are typically: ~91% Hydrogen (H) ~9% Helium (He) In the Sun’s core, the conversion of H to He will take ~10 billion years (its Main Sequence lifetime). Composition of a Sun-like star

What happens when the core Hydrogen is used up? Nuclear reactions stop. Core pressure decreases. Core contracts and gets hotter - heating overlaying layers. 4H  1He burning moves to a hot “shell.”

Ascension up the red giant branch takes ~100 million years Post-Main Sequence evolution Ascension up the red giant branch takes ~100 million years 4H  1He reactions occur faster than before. The star gets brighter (more luminous)! The hot shell causes the outer layers to expand and cool! The star moves off the Main Sequence, …up the “Red Giant” branch.

What happens in the core as it continues to contract and get hotter? Helium begins to fuse into Carbon at >108 K. This reaction is called “triple alpha” = 3He  C. Remember why Hydrogen burning requires 107 K? (Protons repel each other.) Helium nuclei (2 protons) repel each other even more…

The Helium Flash: Post-Main Sequence evolution After the core reaches 108 K, Helium “ignites” to make Carbon. The onset of this burning causes the temperature to rise sharply in a runaway explosion - Helium Flash (stage 9) Eventually the core expands, density drops and equilibrium is re-established Core structure is now readjusted during Helium core burning and total luminosity is actually decreased. During core Helium burning, the star is on the Horizontal Branch.

Relative sizes of main-sequence, red giant, and horizontal branch stars. Stage 7 Stage 9 Stage 10

…until the core Helium runs out…. in just 20-50 million years… Carbon Helium Stage 10 - Helium-to-Carbon burning occurs stably in core, with Hydrogen-to- Helium burning in shell… …until the core Helium runs out…. in just 20-50 million years…

becoming a red supergiant (stage 11) The increased shell burning causes the outer layers to expand and cool (again). The star moves up the asymptotic giant branch (in only ~10,000 years!). becoming a red supergiant (stage 11)

During this phase the Carbon core contracts and heats up. Helium is burning to Carbon in a shell around the Carbon core while H-to-He burning occurs in an outer shell. What do you suppose happens next in the core? For a Sun-like star: nothing…. Why? Solar mass stars cannot squeeze and heat the core enough to ignite Carbon.

The Carbon core continues to contract and heat. So what does happen? The Carbon core continues to contract and heat. Shell He burning grows more intense. He flashes occur in the shell. Surface layers pulsate and are finally ejected (slowly, at ~10s of km/s). The hot, tiny core (White Dwarf) is revealed. And a Planetary Nebula appears! (expanding emission line nebula heated by intense radiation from the hot white dwarf) White Dwarf

Planetary Nebulae have nothing to do with planets. They emit line radiation (hot gas) but are much smaller than the emission nebulae (HII regions) we discussed in Chapter 11. They are important sources of heavy elements (C, N and O), which will go into the next generation of stars.

Very low luminosities, (L = 4R2 T4) White dwarfs have about ½ the Sun’s mass (the rest was expelled). They are about the size of the Earth! (~0.01 solar radii) Density: ~6600 lbs/cm3 !! Very low luminosities, (L = 4R2 T4) It gets cooler and fainter (at the same radius). What eventually happens to a white dwarf?

This is the End: White Dwarf fades away. Planetary Nebula dissipates into interstellar space. End of story for stars like the Sun.

Summary:

What happens to higher mass stars? Gravity squeezes and heats the core enough to ignite Carbon. Then Oxygen, then Neon…as each fuel gets exhausted in the core, its burning moves to a shell. Concentric fusion shells form an “onion skin” structure. Formation of an Iron core is the last stage…

Why is Iron formation the end of the line? Creating elements heavier than Iron requires energy! With no more sources of energy, and Fe fusion taking energy from the gas, pressure support in the star’s core is lost… The core quickly collapses under its own weight….

Protons and electrons are crushed together in the collapsing core, making neutrons. Eventually, the neutrons are so close together they “touch” neutron degeneracy pressure (which is very stiff). The densities reach 100 billion kg/cm3 (at those densities the whole Earth would fit in our football stadium!!) The collapsing neutron core then bounces! An explosive shock wave propagates outward, expelling all outer layers. Supernova

This supernova was recorded by Chinese astronomers in 1054 AD. Supernova ejecta like this spread heavy elements throughout the Galaxy. Crab Nebula