Week 5 Instructor Notes

Last week you learned how stars produce energy in an equilibrium mode, and that more massive stars exhaust their fuel tank more rapidly than the less massive stars. While on the main sequence, all stars are converting hydrogen to helium via nuclear fusion reactions. What happens when the hydrogen runs too low to continue to resist the crushing force of gravity? The answer depends on stellar mass. In the following discussion and in the book, it is CRUCIAL that you distinguish between INITIAL stellar mass and the mass of various end products such as planetary nebulae, white dwarfs, neutron stars, and black holes. In general, when a star ends life some to most of the initial mass of the star gets recycled as gas to the interstellar medium, and some fraction of the initial mass ends up a dense remnant (white dwarf, neutron star, black hole). Also, pay close attention to the ELEMENTAL COMPOSITION of the various end products.

One word that astronomers of the last century overused is "nebula," by which they meant any fuzzy patch in the sky. We now recognize many different kinds of nebula, and the terminology can be confusing. Last unit you learned about emission, reflection, and dark nebulae. This unit you will discover planetary nebulae and another type of nebula we now know as supernova remnants.

LOW-MASS STARS (0.08SM to 0.4SM, where SM = solar mass) convert most of their hydrogen to helium, live a very long time, and continually lose mass through solar wind. The longer lived stars (lowest mass stars) in this class may evaporate over their long life-times. Any remnants from this class will be helium white dwarf stars, we don’t expect planetary nebulae (end-of-life mass loss) from this class of stars, and the solar wind accounts for the material recycled to the interstellar medium – and it’s mostly hydrogen, with some helium.  In any case, the predicted lifetimes of these main sequence stars is far greater than the time the universe has existed, so there should be no observable remnants from death of these stars (and there are no such observed remnants).  There are observed main-sequence stars in this mass range - lots of them.

MEDIUM-MASS STARS (0.4SM to 8 SM) convert the hydrogen IN THE CORE to helium, then they swell up into red giants while they convert to shell-burning of hydrogen PLUS eventual helium fusion in the core (where helium had accumulated while on the main sequence). When helium fuses by the so-called "triple alpha process," three helium nuclei (alpha particles with 2 neutrons and 2 protons bound into the helium nucleus) combine to produce carbon-12 (6 neutrons and 6 protons) plus lots of energy. Ignition of helium fusion requires much higher temperature than that for hydrogen fusion - about 100 Million Kelvin. Therefore, gravitational compression must first supply the thermal energy through collapse of the core. The more massive stars in this category will also fuse some carbon with helium to produce oxygen-16 (8 neutrons and 8 protons), as well as some even heavier element fusion to produce neon-20 and magnesium-24. The core collapse stops when helium fusion begins in the core, and the star enjoys another period of hydrostatic equilibrium - but a brief one. During these giant stages, the star loses lots of mass in the form of what later appears as a planetary nebula. Our Sun may lose about 20-30% of its mass this way. More massive stars lose higher percentages, and vice versa. Most of this nebular material will be hydrogen (WHY?). The end result is a carbon-oxygen white dwarf plus the nebular cloud of expanding hydrogen/helium gas. Note also the text’s explanation of why gravity does not continue to reduce the size of the stuff in the core - NOT fusion reactions but a peculiar rule from quantum mechanics called electron degeneracy pressure - recall that the core always has electrons present. Also extremely important to note is the upper limit for the mass of a white dwarf, whatever its composition - 1.4 SM. Above that limit, electron degeneracy will not support the weight of the dwarf.

At end of life, STARS WITH ABOUT 8 SM TO ABOUT 30 SM generate high enough temperature in the core to continue fusing elements up to mass-56-nuclei in the core, with layers of other elements (sulfur 32, argon-36, calcium-40, scandium-44, titanium-48, chromium-52, and finally nickel-56).  In general, each successive elemental production time interval is shorter than the preceding one. When the mass 56 nucleus is produced, the production lasts only a day. Because the mass 56 nucleus is the most stable (tightly bound) nucleus we know of, further fusion can and does happen, but WITHOUT PRODUCTION OF ENERGY (oops - big oooooops!), and the nuclear energy production simply shuts down. With no nuclear energy to produce radiation pressure  to stop gravitational collapse, the real excitement begins. For stars in this mass category, the resulting explosion leaves a super-dense remnant called a neutron star. Again, ask what force stops complete gravitational collapse - the answer is now neutron degeneracy (as opposed to electron degeneracy in white dwarfs). During the collapse of the core, two processes (see below) cooperate to convert the iron nuclei to a ball of neutrons.  The following summarizes the steps and concepts involved in the explosion (which starts with implosion of the core) of a massive star - a viewgraph presented during class discussion

SEQUENCE OF CORE EVENTS – TYPE II SUPERNOVA

1. At start, core composition is mass-56 nuclei (nickel/iron) (made of neutrons & protons), plus about the same number of electrons as there are protons in nuclei.  At start, the size of the core is comparable with the size of Earth, and the core mass is around 1.4 solar mass (the white dwarf limit) or more.

2. As fusion in core quits, it shrinks and temperature rises to around 10 billion Kelvin, and high energy (black-body) gamma radiation is energetic enough to break up the nuclei into free neutrons & protons.

3. At these temperatures, the electrons combine with protons to make neutrons and neutrinos.   After this process, what's left is entirely neutrons.  As usual, the neutrinos rapidly leave the scene and stream into space, taking most (about 99%) of the released energy with them.

4. The core collapse, which started in step 1 and ended with step 3, is complete in less than a tenth of a second - faster than an eye blink!

5. What stops the core collapse is neutron degeneracy pressure (analogous to electron degeneracy pressure), provided the core mass is under 3 solar mass. Most model calculations I've seen indicate that a 20-30 solar mass star will have about this 3 solar mass core. The outer layers of the star free-fall toward the 'neutronized ' core, impact the core as the core is rebounding, and the outer layers get blasted via shock away from the core.   In principal, you'd later observe a neutron star remnant and several solar masses worth of ejected outer star layers in the form of a gaseous nebula.

{If the star's core mass is over 3 solar mass, no known mechanism can halt the collapse, and a black hole forms.  What happens to the outer layer material is not clear - some may fall into the black hole and some may be blasted to space in the form of jets (hypernova).}

6. The flood of escaping neutrinos helps sustain the destruction of the outer parts of the star. These neutrinos carry away about 99% of the energy released. The other 1% shows up as light output from the star’s destruction. Recent 3-dimensional computer simulations (2007) show that not even the combination of core rebound and neutrino energy will disrupt the outer part of the star, however.  Another source of energy that may complete the destruction is sound wave energy - apparently sound around the 300 hertz range (F above middle C, the middle of  a male tenor singer's range) will do the trick.  In any case, most of the mass of the star ends up blasted to space, and the remnant (the former core) is a "neutron star" - not a true "star" of course since there is no fusion happening anywhere inside it.

Following destruction of the 8-30 solar mass star, all that is left is almost entirely neutrons packed together at nuclear density, and only about 10-20 km diameter.  Neutron stars are associated with some of the weirdest (an poorly understood) physical behavior in the universe.  It is important to note that this TYPE II Supernova explosion (or core-collapse supernova) is a shock wave, neutrino, and soundwave-driven explosion, NOT a nuclear explosion, although the neutrinos that result from nuclear reactions provide a much-needed assist. One final important point - for reasons similar to the fact that white dwarf stars cannot be more massive than 1.4 SM, NEUTRON STARS CANNOT BE MORE MASSIVE THAN ABOUT 3 SM. It is also obvious that lots of material gets recycled to the interstellar medium - but this material is the stuff of the onion-like layers overlying the core and includes elements up to (but not including much) iron; the ejected material is also rich in hydrogen/helium from the outermost layers.

STARS MORE THAN ABOUT 20-30 SM follow much the same process outlined in the previous category, but the core is massive enough to go over the 3 SM limit for neutron stars and THERE IS NOTHING ELSE (meaning no physical process) TO SUPPORT THE CORE AGAINST ULTIMATE COLLAPSE. The remnant is therefore something with zero radius and more than 3 SM - a black hole. THERE IS MAJOR DEBATE ABOUT HOW THIS EXPLOSION HAPPENS, WHAT GETS RECYCLED, AND HOW MUCH MASS ENDS UP IN THE BLACK HOLE. For at least some explosions resulting in black hole formation, there must have been expulsion of a lot of mass, because most of the known stellar black hole candidates are under 15 SM, down to about 5 SM (recall the original star mass range above 20-30 SM).  Explosions of this sort are a leading possibility for explaining some (but not all) gamma ray bursts (the most energetic events in the universe, which we witness about once a day somewhere in the observable universe).

Just what material gets recycled into the interstellar medium in these explosive events? After the nickel-56 core collapses, the nickel disappears from the universe in either of the two preceding stellar mass categories (i.e., formation of a neutron star or black hole); it is mainly the elements LIGHTER than nickel which end up recycled to interstellar material. So if elements making up our world come from stellar explosions, where do nickel and elements heavier than nickel come from? The answer is: mostly from TYPE 1a SUPERNOVAE, and that discussion entails understanding how binary star pairs can evolve if they are close enough together. Astronomers see about as many of the Type Ia supernovae as they do Type II core-collapse supernovae in galaxies like our Milky Way (spiral galaxies).

The type Ia supernova begins with a dwarf star (made of carbon/oxygen) in a binary pair with a companion star that evolves to the giant stage. If the stars are close enough, material from the giant star’s envelope can fall on the dwarf. The in-falling material (mostly hydrogen) gets hot enough to fuse at the surface of the dwarf and can lead to quasi-periodic outbursts of activity known as novae (surface explosions). If enough material accumulates on the dwarf to send it over the 1.4 SM limit, then rampant fusion can break out once more inside the dwarf and result in a NUCLEAR FUSION explosion. There is still considerable debate and research into understanding the details of how this explosion progresses in the white dwarf, through 3-dimensional computer modeling on super-computers.  The latest thinking (2007) is that the explosion originates at a spot on the surface, progresses around the entire surface, and subsequently involves the entire white dwarf.  In a very short time, material inside the dwarf fuses up to nickel-56 (and releases energy to drive the explosion) and beyond (which takes up energy). Elements beyond nickel-56 are produced by successive neutron absorption reactions. Starting with mass 56, it would take, for example, 182 neutron absorptions to produce uranium-238, the most massive naturally occurring nucleus on Earth. In the type Ia supernova explosion, the entire white dwarf blows up and ALL the resulting material returns to the interstellar medium. It is during this blowing up period that most of the heavy element production occurs. The material has very little hydrogen (WHY? - the white dwarf started with no hydrogen, and the hydrogen falling from the giant star's envelope fuses to heavier elements in the explosion) and lots of heavier elements (the upper part of the periodic table as well as elements up through nickel/iron).  Computer simulations, as well as observations, indicate that around half the white dwarf mass will convert to mass-56 nickel/iron during the explosion. 
NOTE:  confusion about mass-56 nickel/iron:  the fusion process actually produces Nickel-56, which is radioactive.  It decays to Cobalt-56 with a 6 day half-life; the Cobalt-56 is also radioactive, and decays with a 78 day half-life to Iron-56.  Iron-56 is a stable (non-radioactive) isotope.  Most astronomy texts simply ignore this distinction or appear unaware of it, and at our introductory level of astronomy it is not important.

The combination of the two supernova types has led to the observed heavy element composition of the universe as we see it today. This begs the question of where the hydrogen and helium came from - a subject you will encounter in unit 7. The bottom line is that the big bang beginning of the universe included manufacture of the hydrogen and helium, and stellar evolution (including supernovae) is slowly changing that primordial mix and making elements that lead to the possibility of planets like Earth and beings like us.  We are the products of nuclear reactions in stars, which means we are one form of nuclear waste. 

One final note - the accretion disk model described in this week's reading is well worth spending a little extra time studying. You will see it again and again in later chapters. Another point worth considering is this: although almost all the discussion about stellar evolution to date concerns individual stars, remember that most stars are in binary systems, and binary system stars can evolve in strange ways if they are close enough to interact at some point of their evolution. Stellar interactions of this sort continue to intrigue astronomers and can explain some otherwise disturbing observations that would appear to defy the standard single-star evolution picture.