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Supernovae Padi Boyd Key Idea: Supernova explosions are responsible for providing nearly all of the heavy elements seen in nature. The universe starts out with only Hydrogen (75%), Helium (~25%), and a smattering of Lithium. All other elements are created by nuclear reactions taking place inside stars, or in the final moments of supernova explosions. Elements 2002 Workshop
Violent lives and deaths of stars You’ve heard about the relatively calm phases in stars’ lives Mass of the star is key to its fate Mass is also key factor in its death, Explosions that happen in the cosmic wink of an eye have had a profound effect on the universe as we know it. “To make an apple pie from scratch, you must first invent the universe.” Carl Sagan, COSMOS What is a Supernova?
You’ve seen how stars create atoms with increasing numbers of protons and neutrons in their nuclei by fusion at the core of the star, up to and including iron. How can that material, which is gravitationally bound to the star, and deep in the interior, ever make it out? Where do the elements heavier than iron come from? Answering these questions requires fitting lots of pieces of the puzzle together! Get Away from that Star!
Reading Break—Arno Penzias, “The Origin of the Elements,” 1978 With Robert Wilson 1965, Homdel, NJ In his Nobel lecture from December 8, 1978, Penzias beautifully weaves together history, math, science and technology into a coherent tapestry displaying our current understanding of how everything around us came into being. Today
Now that we know how stars create elements in their cores, we need to understand the final moments of stellar evolution to see how this material can be spread through space. Collecting the pieces of the puzzle to answer a question. Starting at the Beginning
Historical Supernova Observations The Guest Star of 1054 (Seen by astronomers in China, Arizona (Chaco Canyon), Guam, etc. From the Sung-shih [Annals, of the Sung Dynasty, China] (Astronomical Treatise, chapter 56). "On the 1st year of the Chi-ho reign period, 5th month, chi-chou (day) [July 4, 1054], a guest star appeared approximately several inches to the south-east of Tian-kuan [Aldebaran]. After a year and more it gradually vanished."
Historical Supernova Observations Tycho Brahe reports (from Burnham's Celestial Handbook): (year is 1572) ``On the 11th day of November in the evening after sunset, I was contemplating the stars in a clear sky. I noticed that a new and unusual star, surpassing the other stars in brilliancy, was shining almost directly above my head; and since I had, from boyhood, known all the stars of the heavens perfectly, it was quite evident to me that there had never been any star in that place of the sky, even the smallest, to say nothing of a star so conspicuous and bright as this. I was so astonished of this sight that I was not ashamed to doubt the trustworthiness of my own eyes. But when I observed that others, on having the place pointed out to them, could see that there was really a star there, I had no further doubts. A miracle indeed, one that has never been previously seen before our time, in any age since the beginning of the world.''
Energy released from nuclear fusion counter-acts inward force of gravity. The balance of these two forces determine the stages of a star’s life. As one fuel is expended, a new fuel is consumed. Recall the Stellar Interior Balancing Act
Massive stars burn a succession of elements. Iron is the most stable element and cannot be fused further. Instead of releasing energy, it uses energy. The End of the Line for Massive Stars This is a nice story which certainly explains a lot about stars, but how can we possibly know what’s going on deep inside the core of a massive star, which we can’t peer into??? 1.Hydrogen burning: 10 Myr 2.Helium burning: 1 Myr 3.Carbon burning: 1000 years 4.Neon burning: ~10 years 5.Oxygen burning: ~1 year 6.Silicon burning: ~1 day
Core Collapse Occurs for Mcore > a few Msun During the Red Giant phase, iron is produced in the core. Because iron cannot "burn", the core contracts. The temperature rises to billions of degrees. When the iron core becomes very dense, the electrons attain high enough energy to penetrate atomic nuclei. Protons and electrons combine into neutrons and neutrinos "Disappearance" of the electrons => no more electron degeneracy pressure The core collapses catastrophically. Neutrinos escape and carry away the energy. The neutrons fall toward the center reaching speeds ~0.1-0.2 c. The collapse occurs over ~1 second. The Pauli Exclusion Principle for neutrons eventually takes effect => the falling matter stops instantly Shocking!
Supernova Today! (astronomically speaking!) February 23, 1987 Before February 23, 1987
The rate of Supernova occurrence is: ~ 1 SN / Galaxy / 50 years But there hasn't been one seen in our galaxy in over 390 years!
But what about stuff beyond Iron? The pre-supernova star has an onion-skin structure -- an iron core surrounded by various layers of material. the core collapse generates a shock wave which moves out through the outer layers of the star. Initially, the shock travels at high energy and therefore heats the inner layers of the star to high temperatures. These high temperatures lead to nuclear re-processing of the inner outer layers of material into elements ranging from magnesium to iron. As the shock moves outward (losing energy all of the time), eventually a point is reached where the temperatures generated by the shock cannot ignite nuclear reactions. The shock then just pushes the outer layers away and so these layers reflect the normal evolution of the star. The transition occurs around the neon-oxygen layer. What about elements heavier than iron? Well, the SN outburst is a strong source of neutrons. This is a key point since there is no electrical barrier for the addition of neutrons to nuclei. This means that one can build up very massive elements if there are sufficient neutrons. SN are good sites for high neutron fluxes.
What Happens After the Explosion? 1: Most of the energy is released as neutrinos 2: There is a lot of energy available so that many endothermic nuclear reactions can take place. Elements heavier than iron are produced as nuclei smash into each other. The primary products are cobalt and nickel. But everything up to (and perhaps beyond) uranium are produced. This is the only process in the universe we know of that can produce the heavy elements. 3: Much of the cobalt and nickel radioactively decays back down to iron, producing a continuing energy source for the supernova for several days or weeks. 4: The neutron star is sometimes left behind and can often be observed. However, there is a mass limit for neutron stars, just as there is for white dwarfs, and if that is exceeded: black hole. 5: The area around the supernova is completely disrupted—a bubble of low density, hot gas with a dense, wall of gas on the boundary is produced: a supernova remnant.
Supernova Remnants: SN1987A • a) Optical - Feb 2000 • Illuminating material ejected from the star thousands of years before the SN • b) Radio - Sep 1999 • c) X-ray - Oct 1999 • d) X-ray - Jan 2000 • The shock wave from the SN heating the gas a b c d
Chandra’s view of SN Remnamts Crab Nebula 011.2-003 Kepler’s SN 0509-68.7 0525-69.6 0540-69.3
Supernova Remnants: Cas A Optical X-ray
All X-ray Energies Silicon Calcium Iron Elements from Supernovae Aha! Data for testing models of element creation in the cores of massive stars! That non-smooth (clumpy) appearance implies that there are some significant asymmetries in the pre-supernova object. In fact, only an asymmetric collapse can explain the high velocities that many radio pulsars are found to possess. NSs are kicked out of the womb upon their birth!
Forefront of Modern Astrophysics! 3D supercomputer simulations show an impressive amount of turbulence just below the supernova shock wave. This may be partially repsonsible for the mixing seen in the remnants’ spectra.
Heavy fluids, light fluids The Rayleigh-Taylor instability may play a major role in jumbling up the elements from the interior of the star just before the collapse of the core. Here is a simulation showing the different layers of the core at four times just before NS formation. The first shot is the upper right and time moves counter-clockwise. Time 2 Time 1 Time 3 Time 4
SN interaction with ISM Supernovae compress gas and dust which lie between the stars. This gas is also enriched by the expelled material. This compression starts the collapse of gas and dust to form new stars. The death of one old star leads to the birth of new stars!
What is a Type 1a Supernova? The SN Ia progenitor is a white dwarf accreting matter from a companion star (OLD!) The WD gets near the Chandrasekhar mass 1.4 M, core density gets high enough to ignite Carbon WD is degenerate, so increased temperature does not increase pressure; star cannot expand to reach hydrostatic equilibrium (Big problem!) Thermonuclear runaway; everything consumed!
Little Object—Big Insight! Since all type 1a SNe presumably arise from remarkably similar progenitors, they give off about the same absolute brightness and can be used as standard candles. SN 1994D (Ia) And also, since the entire core is obliterated, ALL that iron is returned to space!
TWO VERY DIFFERENT TYPES OF SUPERNOVAE Supernova Type Thermonuclear (Type Ia*) Core Collapse (Type II) Maximum Luminosity3 x 10E9 Suns 3 x 10E8 Suns Spectrum No hydrogen lines Hydrogen lines many heavy elements Continuum Where foundAmong old star systems Among young star systems (globular clusters, galactic (young star clusters, star-forming bulge, elliptical galaxies) regions in disk galaxies) Parent Star White dwarf in binary Massive star (usually a red supergiant) Trigger MechanismMass transfer Collapse of iron core Explosion Thermonuclear explosion of Rebound shock from neutron star Mechanism carbon/oxygen core --> iron surface; neutrino pressure Left behindNothing Neutron star Debris Mostly iron All kinds of elements
