Stellar Evolution: The Live and Death of a Star Star ch. 20

Standards Understand the scale and contents of the universe, including stars Describe how stars are powered by fusion, how luminosity and temperature indicate their age, and how stellar processes create heavier and stable elements that are found throughout the universe.

As a star begins to run out of fuel & die, its properties change greatly, and it moves along an evolutionary path on the H-R diagram that takes it off the main sequence. A star’s ultimate fate depends on its mass.

Leaving the Main Sequence Most stars spend most of their life on the main sequence. The coolest M – type stars burn so slowly not one has yet left the main sequence. The most massive O & B – type stars evolve from main sequence after only a few tens of millions of years Most high mass stars that ever existed perished long ago

Structural Change As hydrogen is consumed, balance between gravity and pressure begins to shift, both internal structure and outward appearance begin to change, and the star leaves the main sequence. The end of a star’s life depends critically on its mass. Low mass stars die relatively gently High mass stars die catastrophically The dividing line between the two is about 8 times the mass of the sun

Evolution of a Sun-like Star A solar mass star does not experience sudden, large-scale changes in properties. Its average surface temperature remains constant, while luminosity increases very slowly over time After about 10 billion years of steady core fusion, a sun-like star begins to run out of fuel (like a car cruising down the highway at a constant 70 mph for many hours, only to have engine suddenly cough & sputter as the gas gauge reaches empty).

The Sub-Giant Branch Composition of the star’s interior changes: It has increased helium and decreased hydrogen. The helium content increases fastest in the center When hydrogen becomes depleted in the center fusion moves to higher layers in the core

The Sub-Giant Branch An inner core of non-burning helium starts to grow The gas pressure weakens in the helium core and gravity causes the inner core to begin to contract

Hydrogen Shell-Burning Stage Hydrogen burns at a furious rate in a shell surrounding the non-burning inner core of helium “ash”

Hydrogen Shell-Burning Stage The hydrogen shell generates energy faster than the original main sequence fusion, & energy production continues to increase as the helium core continues to shrink The star’s response is to get brighter

Hydrogen Shell-Burning Stage After a lengthy stay on the main sequence, the star’s temperature and luminosity begin to change The star evolves to the right on the H-R diagram to the subgiant branch

The Red Giant Branch The star is now off the main sequence and no longer in stable equilibrium The helium core is unbalanced and shrinking The rest of the core is unbalanced & fusing at an increased rate

The Red Giant Branch Gas pressure exerted by enhanced fusion forces star’s non-burning outer layers to increase in radius, and the overlying layers to expand and cool Star is on its way to becoming a red giant This change takes around 100 million years A red giant’s luminosity is many hundreds of times the luminosity of the sun and its radius is around 100 solar radii

Helium Fusion A few hundred million years after a solar-mass star leaves the main sequence helium begins to burn in the core The helium fuses into carbon and the central fires reignite

Helium Flash At highest densities in the core, gas enters a new state of matter governed by the laws of quantum mechanics In this state, the Pauli exclusion principle prohibits electrons in the core from being squeezed too close together This is called electron degeneracy and the pressure associated with it is called electron degeneracy pressure

Helium Flash In the core’s degenerate state, helium burning becomes unstable with explosive consequences When burning starts and temperature increases, there is no corresponding rise in pressure, no expansion of gas & no stabilization of core The rapid temperature rise results in runaway explosion called the helium flash The helium burns ferociously for a few hours, until equilibrium is reached and the stable core then fuses helium into carbon

Back to the Giant Branch Whatever helium exists in the core is rapidly consumed (lasts a few tens of millions of years after helium flash) As helium fuses to form carbon, a new carbon-rich inner core forms, surrounded by helium burning, hydrogen burning and non-burning shells The non-burning layer expands and star becomes red giant or red supergiant

Core of Carbon Ash

Death of a Low-Mass Star The inner carbon core becomes too cool for further fusion and continues to contract The fires go out Before the core attains the temperature necessary to fuse carbon, its density reaches a point where core can no longer be compressed At this density, a cubic centimeter of core matter would weigh 1000 kg on Earth: a ton of matter compressed into a volume the size of a grape

Planetary Nebulae Driven by increasing radiation and instabilities in the core and outer layers, all of the star’s outer envelope is ejected into space in less than a few million years at a speed of a few 10’s of km/s

Planetary Nebulae The star now has two distinct parts: a core of carbon ash (a.k.a. white dwarf) and an expanding cloud of dust and cool gas spread over a volume roughly the size of our solar system This is a planetary nebula (they have no association with planets)

Planetary Nebulae It continues to spread out over time, and eventually disperses into interstellar space, enriching it with atoms of helium, carbon, oxygen & heavier elements These elements eventually get swept up into nebulae (see ch. 18) and formed into new stars and planets

White and Black Dwarfs The carbon core at the center of the planetary nebula continues to evolve The core is very small, size of Earth or smaller Its mass is about half the mass of the sun It shines by stored heat, not nuclear reactions The core’s temperature & size give it the name of white dwarf

White and Black Dwarfs Once a star becomes a white dwarf, its evolution is over It eventually becomes a black dwarf – a cold, dense, burned-out ember in space that remains about the size of Earth

Evolution of Stars More Massive than the Sun High-mass stars evolve much faster than low-mass stars. Its ravenous fuel consumption shortens its main sequence lifetime. A solar mass star spends 10 billion years on the main sequence A 5 solar mass B-type star is on main sequence for about a 100 million years A 10 solar mass O-type star is on main sequence for about 20 million years

Evolution of Stars More Massive than the Sun At 8 solar masses and larger, stars can fuse carbon, oxygen and even heavier elements. These stars die in violent explosions soon after leaving main sequence (next chapter!!!)