The Death of Stars

• Evolution of Stars on the Main Sequence
  • Smaller Stars
  • Medium Stars
  • Neutron Stars

The Life Cycle of a Dying Star

Smaller Stars

As we have seen, very low mass stars (under 0.4 Solar Masses) called red dwarfs mix their internal gases through convection, so there is no well defined core area. Hydrogen burns slowly throughout the entire star. In theory, since the star lacks the mass to continue to the next stage of thermonuclear synthesis and fuse helium to beryllium, it should simply fade out, but the lifetime of these stars is greater than the age of the universe. We have not observed the "death" of red dwarfs.

Medium Stars

Stars with masses between 0.4 M_☉ and 4.0 M_☉ pass through two distinct red giant phases.

Phase I

In a moderate-mass star on the main sequence, energy from hydrogen fusion in the core is transmitted by radiation to outer layers, where heat drives convection currents. The star can remain in this state burning hydrogen in the core for several billion years. When the core runs out of hydrogen, the star goes through a series of steps to become a red giant.

1. Supply of core hydrogen available for thermonuclear fusion to helium, carbon, nitrogen, and oxygen drops as hydrogen is converted to helium.
2. Volume of core where thermonuclear fusion occurs shrinks and heats up., but there is no hydrogen left to fuse in the core.
3. Hydrogen in shell surrounding shrinking core ignites and begins thermonuclear fusion to helium, which "rains" into core, increasing its mass and density.
4. Gases in outer layers heat up from below and expand outward, cooling as they expand. While the star becomes more luminous as a giant and moves up in position on the HR diagram, its decreasing temperature also moves it to the right. The star is now a red giant. Convection dredges up elements produced in the core and circulates them through the upper layers. Some mass is lost by evaporation of gases from the expanded outer layers of the star.
5. Core continues shrinking and absorbing helium from hydrogen burning shells.
6. Core helium ignites with a chain reaction (helium flash), expands and cools slightly.
7. Both in the helium-fusing core and surrounding hydrogen-fusing layers, the rate of thermonuclear activity slows with the lower temperatures.
8. The outer layers of the star contract and heat up. While the star's luminosity doesn't change at this stage, its temperature increases, and in moves to the left on the chart. The star is now on the horizontal branch.
9. Helium fusion to carbon and oxygen ceases as helium runs out in the core, causing it to contract until electron pressure prevents further contraction. A second dredge-up occurs.
10. Helium fusion begins in a shell around the core. Hydrogen fusion continues in a shell above the helium fusion shell.

Phase II

Helium undergoes thermonuclear fusion in the core, and hydrogen fusion occurs in the shell above the core. Energy from both is transmitted as radiation to the outer gas layers, where heat is circulated by convection in the expanded gas envelop of the red giant.

1. Heat from the thermonuclear shells causes the outer layers to expand again, and stars go through a second red giant phase. The star is now an AGB star, and on the HR diagram, its position shifts to the asymptotic giant branch, above its former horizontal branch because the expanded star is more luminous than in its earlier phases.
2. Expansion causes the star to cool down (relatively speaking), and thermonuclear fusion in the hydrogen shell ceases. A third dredge-up process may occur if the stellar mass is over 2 M_☉ .
3. The large size of the star reduces gravitational hold on its outer layers, and these evaporate at a very high rate. Carbon stars may become surrounded by a "soot" cocoon.
4. Helium fusion in the helium-burning shell ceases as the helium is used up, and the shell cools down and shrinks.
5. As the hydrogen-containing layer above the now dormant helium layer also shrinks and condenses, it heats up enough to begin fusing hydrogen again, and the resulting helium rains down into the helium shell, which itself shrinks and reignites its own thermonuclear activity. The helium flash creates a burst of energy or thermal pulse that causes the outer layers of the star to expand rapidly and even explosively. This cycle can repeat itself until only the core is left.

Phase III

The star is now in its second and larger red giant phase. It has a core where carbon fusion occurs. This core is surrounded by a helium-fusing layer "raining" carbon into the core, and that layer is surrounded by a hydrogen fusing-layer raining helium into the helium fusing layer.

Phase IV

The outer layers of the star have blown off to form the nebula, and the core of the star collapses.

1. The expanding shell of gas around the star is excited by the energy still pouring out of the star, and the gas shell begins to glow: a planetary nebula. Eventually, the expanding shell is so far from its central star that the gases no longer receive enough energy to react, and the planetary nebula stops glowing.
2. The remaining core of a star originally of 4 M_☉ or less now contains mostly oxygen and carbon. Its mass is not great enough to sustain thermonuclear fusion of carbon into heavier elements, so thermonuclear activity ceases. The stars luminosity drops, but because of the great pressure due to its high density, the core remains hot. The star becomes a white dwarf to the lower left of the main sequence on the HR diagram.
3. The size of the white dwarf is inversely related t its mass, with stability determined by an upper mass of 1.4 M_☉. Stars with mass less than this can be kept from further collapse by degenerate electron pressure. Ions in the nucleus form rigid lattice structures

Neutron Stars

In 1967, Jocelyn Bell discovered a new class of stars that produced regular fluctuations in high intensity lightwaves in the radio range, but with periods variations mere seconds long that could not be explained by eclipsing binary stars or any known variable star mechanisms. No model of main sequence or giant star predicted periodic contraction and expansion that could occur within seconds, so astronomers had to come up with new models to explain pulsars.

Galaxies were known to produce radio "jets" from their poles. If a star could produce similar jets and rotate rapidly, so that its beams swept past Earth as a lighthouse beam sweeps across the ocean, the periodic flashing could be explained. But such a star would need to be small to rotate so rapidly, and massive to create a magnetic field capable of produce radio beams with the observed intensity.

Astronomers had one other clue. Pulsars are found near the center of some but not all supernovae remnants. If a giant star collapsed rapidly but retained a mass in a narrow range between 1.4 and 2.0 solar masses, it could form a very small, dense core with enough matter to generate the beam. The 1.4 solar mass limit is sometimes called the Chandrasekhar limit, and defines the mass of stars able to support their structure by degenerate electron pressure. Stars above this mass overcome degenerate electron pressure to collapse further, forming either neutron stars or black holes.

Conservation of angular momentum would force this new star to spin rapidly. But the density of the star would be greater than degenerate electron pressure would support, and atomic structure would give way entirely, fusing electrons and protons in the nucleus together. The remnant star core would be made primarily of neutrons, and perhaps even stranger forms of matter, quarks, at the core. It would be denser than any other object in the universe, except for a black hole, kept from a final ultimate collapse by degenerate neutron pressure. Stars over three solar masses will overcome even degenerate neutron pressure, and collapse to black holes.

The collapse and condensation of the star would concentrate the original relatively weak magnetic field of the star in a very small volume, creating an enormously powerful, local magnetic field. Charged particles in such fields accelerate at relativistic speeds, about 10% of the speed of light. The radio beams we observe are energy in the form of synchrotron radiation, which is light emitted by relativistic electrons (those traveling at significant speeds of light) spiraling, and therefore changing direction under the force of the magnetic field. Over time, the loss of energy would slow the rotation. Pulsar rates indicate the age of the pulsar: the older the pulsar, the longer the period between flashes. This accounts for the observed slowing down of pulsar beats over time that has been observed for individual pulsars.

This model could also account for why we don't see pulsar behavior from all neutron stars: some neutrons stars are oriented so their radio beam never points toward Earth. It also explained occasional "skips" in the pulsar beat. Neutron stars in binary systems are influenced by the gravitational field of their partner, and "wobble" as they rotate, so that sometimes the beam doesn't point directly at Earth on a single pass.

So the "lighthouse theory" can explain why not all neutron stars we see from Earth have pulsating light, why those that do pulse so rapidly, why the pulse rate slows over time (as the star loses energy and its rotation slows), and glitches in the pulse rate due to wobble.

Models of the structure of the neutron star propose five main layers:

• An atmosphere of hydrogen and helium.
• A rigid outer crust of ions and electrons.
• An inner crust of ions and superfluid neutrons.
• An outer core of superconducting protons.
• An inner core whos structure is unknown, but which may include quarks from neutrons that have decomposed under extreme pressure.

A neutron star technically might not be considered a star at all, because thermonuclear activity occurs on its surface, and not in its core, as with main sequence and giant stars.

The atmosphere of hydrogen would from from gases left from the primary explosion collapsing back under gravitational pull, or in binary systems, matter siphoned from the nearby star. As matter collects on the surface of the neutron star, it experiences high pressures from the gravitational pull of the dense star below it, and it heats up enough to begin thermonuclear fusion, even if only sporadically. A similar phenomenon occurs on white dwarf stars in binary systems, but these stars are less dense. Hydrogen gas heating up on the surface of a white dwarf eventually explodes, creating a nova event. But hydrogen gas falling on a neutron star eventually ignites thermonuclear fusion, and through all available hydrogen in a moment, creating a high-energy flash of light and becoming -- for a moment -- an X-ray burster.

Neutron stars are marked by layers of superfluidity and superconductivity. Superfluidity occurs when conditions in a liquid foster low viscosity (lack of friction), where cohesion/adhesion forces are greater than gravitational forces. Cohesion between the superfluid interior and the hard crust creates slippage that causes glitches or abrupt changes in rotation. Superconductivity occurs when charge can flow continuously without resistance. Both conditions apply to layers in in neutron star interiors. Most of what we know about superfluid and superconductive materials come from matter at normal pressure but near absolute zero temperatures. Just how this kind of material would behave in high-density neutron stars at high temperatures is a matter of intense debate among astronomers.

The outermost layer of electrons and ions is another source of "glitches" in the pulse patterns of neutrons stars in binary systems. Like white dwarfs, which have cooled and solidified, neutron stars are not gas balls but have solid crusts, like planets. Under tidal pressure from its partner, the rigid crust of a neutron star in a binary system cracks and sends shock waves through the star, a phenomenon called a "star quake". The contraction causes the star to momentarily speed up, but fluid neutron material expands through the cracks and slows the spin again in a few revolutions.

Discussion Questions

• How does mass affect the life-cycle of a star?
• What key characteristics of a star change as mass increases? as mass decreases?

Optional Web Reading

• Check out the NASA page on the The Life and Death of Stars for the latest information on end-of-star-life situations that have been carefully studied.

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