Lecture 30 - Supernovae (4/1/96)

Seeds: Chapters 10, 11

1. Massive Stars
   • For massives stars, M > 3 solar masses, the carbon - nitrogen - oxygen core formed after helium fusion ends will become hot enough for carbon fusion to commence upon contraction.
   • For masses 3 to 9 Msun, this core is degenerate when fusion begins, and a runaway ignition occurs called carbon detonation, analogous to the helium flash.
   • Carbon detonation is more violent than the helium flash (hence its name) and may well destroy the star. It is likely, however, that the star manages to survive carbon detonation relatively unharmed.
   • Stars more massive than about 9 solar masses will have a non-degenerate C-N-O
   • After carbon fusion begins, the star once again makes a loop in the H-R diagram with core carbon fusion followed by collapse of the inert core and carbon shell burning.
   • The more massive of these stars are massive enough to pass through several stages of fusion of heavier and heavier elements at ever increasing densities and temperatures.
   • The behavior of these stars will be similar to the helium burning medium mass stars with mass loss and ejection of the envelope into planetary nebulae.
   • However, two things will disrupt this process: the formation of an iron core, and increase of core density beyond the electron degeneracy limit of 1.4 Msun.
   • When an inert iron core is formed after fusing carbon to oxygen - magnesium - neon, and neon - oxygen to silicon, and silicon to iron, you can no longer extract energy from fusion to heavier nuclei. The binding energy of the iron nucleus (per nucleon) is a minimum, and heavier elements are less tightly bound, so it would actually use up energy to fuse iron into a heavier element. Likewise, breaking apart iron into lighter fragments would also use up energy. Iron is the end of the line for thermonuclear energy generation!
   • In addition, as the iron core contracts in these stars generating gravitational energy and high temperatures, it passes the electron degeneracy limit of 1.4 Msun. The degeneracy cannot withstand the increasing core gravity and collapses to ever smaller radii, increasing in density and temperature.
   • At this point two things happen: the iron nuclei in the core begin to be broken apart by the high-energy photons (gamma rays) from the extremely high blackbody temperatures generated in the contracting core, and the electrons are squeezed into the nuclei themselves removing their pressure support from the core.
   • When the wavelength of the electrons becomes small enough that they merge with the protons in the iron nuclei, they are captured and the two particles become neutrons (this is basically the same reaction as the start of the proton-proton chain, with the outgoing positron replaced with an incoming electron): p + e -> n + v The neutrino v carries away the energy of this reaction, as it can stream straight out of the core of the star without being absorbed (it requires the weak interaction, which means that it is unlikely to happen). This effectively uses up energy from the core since the neutrinos escape with it, like little tiny thieves.
   • The fragmentation of the iron from the gamma ray photons into silicon and magnesium (for example) uses up energy also, supplied by the photons.
   • The result of these two processes is that energy is removed from the core, removing pressure support, causing it to contract further, increasing these reactions which in turn remove more energy. This is another runaway catastrophic process due to positive feedback.
   • Within a fraction of a second, the core collapses entirely releasing a flood of neutrinos. There is effectively no pressure support, and the collapse happens at free-fall. With the tremendous gravitational forces of the super-dense core, speeds of 70,000 km/s (about 0.25c) are achieved by the outermost layers of the core (note the core is only a few thousand kilometers in size at this point).
   • The iron core electrons and protons are quicky converted into neutrons. Thus the entire core is now made of neutrons.
   • The density in this neutron core rise during the collapse until the point of neutron degeneracy is reached. This is equivalent to electron degeneracy, but occurs at smaller distances because the neutron is about 2000 times more massive than the electron (see Lecture 29).
   • When the neutron degeneracy limit is reached, the core collapse is suddenly halted as the neutron degeneracy pressure takes over.
   • For the collapsing core, this is like hitting a brick wall. The outer parts of the core are collapsing at 70,000 km/s and run into the forming degenerate core, causing a "bounce" or rebound. This in turn runs into the material following the infalling core also at high velocity, causing a shock wave to propagate outward through the star.
   • This shock wave, like that caused by a supersonic airplane, or a large explosion, or an earthquake, carries tremendous energy, and as it blasts out through the stars envelope it blows the star completely apart. It is a vast tidal wave carrying all before it, hurling the material from around the core (rich in heavy elements) outward at speeds of around 10,000 km/s.
   • This titanic explosion (caused by implosion of the core) is called a supernova.
2. Supernovae
   • Supernovae release tremendous amounts of energy, and for a short while outshine everything else. At maximum brightness, supernove emit visible light with a luminosity of 10^10 Lsun! (Q: What absolute visual magnitude does this correspond to?)
   • The light emitted by the supernova is only a tiny fraction of the total energy (less than 0.001 of the total). The "luminosity" of the neutrinos produced in the first second is about 10^46 Watts!
   • Supernova suddenly appear a bright new stars in the sky. They grow in brightness until they reach a maximum luminosity, then as the explosion subsides they fade away over the course of a couple years.
   • Supernovae have been recorded throughout history. Bright new stars were obviously cause for some excitement, even though they would then disappear.
   • Arab astronomers reported a new star in 1006 AD.
   • Chinese astronomers reported a "guest star" in 1054 AD (as well as the 1006 supernova and one in 1181).
   • Tycho Brahe reported a new star in 1572 and Johannes Kepler one in 1604. The fact that new stars could appear in the sky help overturn the idea of the immutable heavens.
   • These were all visible with the naked eye and occured in our own galaxy.
   • Calculations show that we should expect a supernova somewhere in or very near our Milky Way galaxy every 25 to 100 years, though many of these will be hidden behind dust clouds or on the far side of the galaxy.
   • There has not been a galactic supernova seen for nearly 400 years. However, in 1987, a bright supernova in a small neighbor galaxy gave modern astronomers their first glimpse of a supernova in action.
   • In addition to seeing supernovae themselves, we can also study the disrupted star-stuff they leave behind: a supernova remnant.
   • In the center of the supernova remnant will be the leftover core of the star, now compacted down to neutron degenerate densities comparable to that of the atomic nucleus itself. This is a neutron star.
   • The neutron star will initally be very hot from the collapse (billions of degrees K), but tiny in size. It will therfore be very faint because of its tiny surface area (remember L=R^2T^4) and hard to see.
3. The Crab Nebula
   • Supernova remnants tend to be shell-like nebulae, not to be confused with planetary nebulae. There are shell-like remnants from Tycho's and Kepler's supernovae, and the supernova of 1006 (SN1006).
   • These shells are the blasted off envelopes of the star. Q: At a speed of 10,000 km/s, how far would the shell of SN1006 be from the central star now?
   • However, not all supernova remnants look like this.
   • In 1781, the French astronomer Charles Messier compiled a catalogue of celestial objects likely to be confused with comets. These included nebulae, star clusters, and other such objects. This catalogue is known as the Messier catalogue and the objects in it, the "Messier" objects, are often targets of telescopic observations even today.
   • The first object in Messier's catalogue, designated M1, is in the constellation Taurus, and appears to be a diffuse nebula. It is also known as the "Crab Nebula" for its vague crablike shape.
   • The "guest star" reported by the Chinese in 1054 AD. was also in the constellation Taurus, in the location now occupied by the Crab nebula.
   • Astronomical measurements show that the Crab Nebula is about 1.35 pc in radius, and is expanding at a velocity of 1400 km/sec. At this expansion rate, the Crab Nebula was created about 900 years ago, consistent with its being the remnant of SN1054.
   • However, the Crab Nebula does not appear like most other supernova remnants. It is not a shell, or fragments of shells. It appears to be a filled volume, called a plerion. It also has numerous filaments and tendrils.
   • The Crab Nebula is also relatively bright, emitting a considerable amount of radiation from radio waves up to X-rays.
   • The luminosity of the Crab Nebula is about 3 x 10^31 J/s (Watts), or about 10^5 Lsun! Where does this energy come from?
   • It could be left over energy from the 10^44 Joules liberated in the supernova, though it appears that there is something in the nebula actively supplying the power.
4. Supernova 1987A
   • A supernova was discovered in the southern sky on February 24, 1987
   • It was located in the Large Magellanic Cloud (LMC), about 53,000 parsecs from the Sun. Light takes about 160,000 years to get here from there.
   • The official designation of the supernova was SN1987A, the first (A) supernova (SN) discovered in 1987.
   • At discovery, the apparent visual magnitude of SN1987A was around 5, over the following weeks it brigtened to about 3rd magnitude.
   • At the point of discovery, SN1987A was about 20 hours "old", that is since the core collapse itself (plus the 160,000 years for the light to get here!).
   • Progenitor star was about 20 solar masses (spectral type O) when on the main sequence, formed around 10 million years ago, with main sequence luminosity of about 60000 Lsun.
   • Just before the explosion, it was a blue supergiant, on the horizontal branch (after becoming a red giant, then supergiant) with a luminosity of 10^5 Lsun. In the core it was burning silicon, oxygen, neon, carbon, helium, and hydrogen in concentric shells about the forming iron core.
   • At the time of core collapse, the core reached a temperature of about 200 billion K and a collapse velocity of 70,000 km/s. The collapse took a fraction of a second.
   • The total luminosity liberated was around 10^44 Joules. For about a month, SN1987 put out a luminosity of over 10^8 Suns.
   • Almost all of the energy of the supernova came out in light weakly-interacting neutrinos. About 10^58 neutrinos were produced in the core collapse. On Feb 24, 1987, about 10^13 neutrinos passed through your body from the supernova! About a million people on the Earth had an "interaction" with a neutrino, of course with no noticable effect.
   • During this period, there were several neutrino detection experiments running on the Earth. These were designed to measure neutrinos coming from nuclear fusion in the Sun's core. Two of these experiments, one in Japan, and one in a mine near Cleveland under Lake Erie, detected neutrinos from the supernova!
   • Astrophysicists at Penn were involved in the Japanese Kamiokande experiment.
   • These experiments detected about a dozen neutrinos each from SN1987A. They used tons of water as a detector with phototubes to detect the light emitted by particles produced after the interaction. Since the supernova was in the southern sky, the neutrinos detected had already passed through the Earth!
   • These precious neutrinos confirmed the theories of the core collapse trigger for supernovae. Furthermore, the energy of the neutrinos gave the core temperature at collapse - about 2x10^11 K!
   • SN1987A is still being monitored to study the earliest stages of supernova remnant formation and to look for the appearance of a pulsar in the remnant.
5. What's Left Over
   • If the core mass is less than 1.4 Msun, we are left with a white dwarf, supported against gravity by electron degeneracy.
   • A star up to 7 - 8 Msun can lose enough mass through winds and planetary nebula formation to be left with a 1.4 Msun core. Stars from 8 to 10 Msun may be able to also form a O-Mg-Ne white dwarf.
   • If it has a mass greater than this, then the core left over has a mass greater than 1.4 Msun, and it undergoes core collapse in a supernova explosion.
   • The remnant is supported by neutron degeneracy, as the electrons were captured on the protons and converted to neutrons (emitting copious neutrinos). This is called a neutron star.
   • The 1.4 Msun limit for a white dwarf versus a neutron star is called the Chandrasekhar limit after the astrophysicist who first calculated it.
   • A neutron star has over 1.4 Msun of mass compressed to densities of 10^11 kg per cm^3! The radius of a neutron star is about 10-15 km (the size of downtown Philadelphia).

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