Lecture 33: Supernovae

Don't Panic

Sections

1. Introduction
2. History of Supernovae
3. Type Ia SNe
4. Type Ia SNe and Cosmology
5. Core Collapse SNe
6. The Old Gang

---

Introduction

---

Supernovae (SNe) is that are the powerful explosions of stars and are of considerable astrophysical importance.

They are among the brightest astrophysical objects and, unlike most observable astrophysical objects, they evolve rapidly in time.

The brightest type of SNe (at least on average), Type Ia SNe are currently of great importance in cosmology and, in particular, in the determination of the cosmological parameters.

All types of SNe eject into the interstellar medium (ISM) heavy elements (carbon and above) synthesized in the explosion or from pre-explosion evolution. This element yield from supernovae drives most of the heavy element evolution of the universe.

Rocky planets, like Earth, and life as we know it would probably not be possible without the heavy elements from supernovae.

The solar composition (by which we mean composition at the surface) by mass is:

         H (hydrogen)  70.7 +/- 1.8 %,

         He (helium)   27.4 +/- 2.1 %, and

         metals         1.89 +/- 0.17 % .

         Metals
         in astro-jargon are everything which is not H or
         He.

         Just accept it.

         The deep interior (i.e., the core)
         of the Sun and other stars is richer in
         He because of ongoing
         nuclear fusion
         which is discussed below
         section Nuclear Fusion in the Sun
         and in
         Lecture 22:  The Main Sequence Life of Stars.

         The H and He abundances are approximately right throughout the
         observable universe,
         except in those minor components (rocky bodies,
         icy bodies,
         asteroids, dust, etc.) which have
         relatively little H and He.
         Gas giant planets though have abundant
         H and He.

         The abundances of metals
         in stars
         (and this really means in the
         observable universe
         outside
         those minor components (rocky bodies,
         icy bodies,
         asteroids, dust, etc.)
         vary wildly from about 4 % down to 0.1 % or even much lower, but
         never 0 it seems
         (HI-414).

         The ratios of the metals among themselves often
         vary LESS wildly.


         The leading metals in decreasing order of solar surface abundance by mass fraction are
         oxygen (O),
         carbon (C),
         iron (Fe),
         neon (Ne),
         silicon (Si)
         nitrogen (N)
         magnesium (Mg),
         and
         sulfur (S)
         (Solar Composition).

---



A plot of is of the primordial nebula solar composition or, for short, the solar composition.

It's not exactly the composition of the Sun or any solar system astro-body, but rather the composition of the primordial nebula out of which the Solar System formed.

It is obtained from observations of the solar photosphere and from primitive meteorites: i.e., meteorites that seem to have undergone little chemical processing since the solar system formation.

The plot is a semi-log graph with the vertical axis being logarithmic and the horizontal axis being atomic number.

The plot is by mass fraction.

Many solar composition plots are by number fraction.

As one can see, hydrogen and helium are the dominant species.

Once you get to atomic numbers greater than copper, the abundances become small.

The data are from Asplund et al. (2005).

Credit: David Jeffery.

---

The impact of supernova ejecta, especially core-collapse SNe ejecta, on the ISM is also important in the creation and dispersal of star-forming regions.

---



Caption: "Twenty years ago (relative to 2007), astronomers witnessed one of the brightest stellar explosions in more than 400 years. The titanic supernova, called SN 1987A, blazed with the power of 100 million suns for several months following its discovery on 1987feb23. Observations of SN 1987A, made over the past 20 years by NASA's Hubble Space Telescope (HST) and many other major ground- and space-based telescopes, have significantly changed astronomers's views of how massive stars end their lives. Astronomers credit the HST's sharp vision with yielding important clues about the massive star's demise. This HST image shows the supernova's triple-ring system, including the bright spots along the inner ring of gas surrounding the exploded star. A shock wave of material unleashed by the stellar blast is slamming into regions along the inner ring, heating them up, and causing them to glow. The ring, about a light-year across, was probably shed by the star about 20,000 years before it exploded."

This image is, of course, of SN 1987A as a young supernova remnant.

The difference between supernova and supernova remnant may not be clearly defined: but a few years after explosion, the object is a remnant to most people.

SN 1987A was peculiar, subluminous core-collapse SN (a Type II SN, in fact).

It was the observationally brightest supernova since SN 1604 (AKA Kepler) because of its proximity: it is in the Large Magellanic Cloud (LMC).

Certainly, there have been closer SNe in Milky Way, but they were missed because, as bright as SNe are, they can be thoroughly extincted in the visible by interstellar dust in the Galactic disk.

The image may NOT true color---it's sometimes hard to know when astro images are true color since the image makers can make images with any color they like---and they use color often to bring out features of interest and not trueness often---and what is true color anyway---color is perception in the brain---a combination of out there and in here.

The stars with points are foreground stars in the Milky Way. They are very bright and are saturated, and thus one sees their diffraction pattern with its points.

Credit: NASA, ESA, Peter Challis (CfA), Bob Kirshner (1949--) (CfA).

Permission: Public domain at least in USA.

Image linked to Wikipedia.

---

The physics of the explosions of all supernova types is of intrinsic interest and is also of interest in understanding material properties under extreme conditions.

So SNe are important in evolution of the universe.

But they are a bit complicated too.

A first reason is that the two main classes of SNe---Type Ia SNe and core-collapse supernovae---are distinct phenomena.

Yes they have many similar aspects and yes the same researchers study both, but their essential explosion mechanisms are different. So any general talk on SNe must be a talk on two things.

A second reason is the growth of knowledge---the more we know about some field, the more complex it becomes.

Climbing the hill of any specialized field is hard these days---especially if you are rolling downhill like me.

---

History of Supernovae

---

Strange sights have been seen in the sky since forever---and we still have UFOs today---though, of course, they are NOT extraterrestrials---I've seen two myself---one turned out to be breaking up Russian satellite and the other a flock of Canada geese---who seemed in a dark sky to be a flying wing executing a nifty U-turn.

Among these are new stars or novae---using both terms in their historical sense, not in a modern sense. The term nova follows from the use by Tycho Brahe (1546--1601) in 1573.

Novae were star-like objects that appear where no star was before and then disappear within months or years. They showed no motion relative to the fixed stars and no stellar parallax---and so they could to be assumed to be in the realm of the fixed stars---but not eternal.

Aristotelian cosmology---which became a philosophical dogma in Greco-Roman antiquity and later in Medieval Islamic society, Medieval European Society, and Renaissance Europe---posited that there was no change in the heavens above the Moon.

---

Aristotle (384--322 BCE), the supreme authority---with appropriate smugness.

He was the Wikipedia of the Middle Ages.

---

Novae, in Aristotelian cosmology, were thus anomalies to be explained away as somehow sublunary or to be just ignored. Astronomers in the Aristotelian cosmology tradition do seem to have missed seeing a lot of novae.

But the ancient Chinese astronomers---perhaps because they had no Aristotelian prejudices---or maybe they were just better observers---did observe a fair number of novae starting from the time of the Han dynasty (206 BCE--220 CE). They called these objects guest stars---just visitors in the celestial realm. What were the novae/guest stars?

In some cases, it's hard to tell from ancient records. Many were probably cataclysmic variable stars or novae in the modern sense. These are both cases where accretion from a close binary companion onto a white dwarf star leads to a surface nuclear explosion---titanic events, but much less so than SNe.

A few were SNe.

The earliest nova (in a historical sense) that is likely to have been a supernova was SN 185---which occurred in 185 CE in constellation Circinus and was recorded by Chinese astronomers (i.e., it was a guest stars).

Other retroactively recognized important SNe are SN 1006 (in Lupus), SN 1054 (in Taurus), SN 1572 (in Cassiopeia), SN 1604 (in Ophiuchus), and SN 19885A (in the Andromeda Galaxy (M31)).

SN 1054 is famous for having given birth to the Crab nebula supernova remnant and the Crab pulsar.

Supernova videos:
1. Birth of the Tycho Brahe's 1572 supernova remnant An animation good for illustrating how remnant the expands in interstellar medium (ISM). The remnant eventually breaks up and merges with the ISM enriching it with metals including radioactive ones. From Max Planck Institute for Astronomy---but star motions still look phoney and where is the companion star to the exploding white dwarfs after the explosion. Short enough for the classroom.
2. What's Up For Nov. 2009? Crab Nebula Pretty good. Longish, but probably just short enough for the classroom.

A supernova remnant is the expanding ejecta of a supernova explosion. It carries the heavy elements synthesized in the supernova or pre-supernova stellar evolution out into interstellar space. Supernova remnants are often roundish shells, but they can also be messy and have filaments like the Crab nebula. They exist for thousands of years before being broken up and dispersed in the ISM out of which new stars form.

Pulsars are radio-emitting young neutron stars. Neutron star are the compact remnants core-collapse supernovae---they are the collapsed core. Neutron stars are super-dense objects with masses typically in the range 1.35--2 solar masses and radii of order 10 kilometers. As their name suggests, they are mainly made of neutrons

SN 1572 (AKA Tycho) was observed and reported on by Tycho Brahe (1546--1601)---that report made him famous and proved---although it took decades and a lot of other evidence for full acceptance---that Aristotelian cosmology was wrong and the heavens were not changeless.

---



Caption: "Tycho's Wall Quadrant. An engraving of Tycho Brahe (1546--1601) in his Uraniborg observatory on the Swedish island of Hven, probably from the 1598 printing of his Astronomiae Instauratae Mechanica, hand coloured.".

Tycho---he's one of those historic people known by their first names (like Galileo (1564--1642)) was the greatest pre-telescopic observer.

He's lucky that Johannes Kepler (1571--1630) managed to get a hold of his data---Tycho's unastronomical heirs would have hidden it away---like the dog in the manger---until it was worthless.

Tycho first became famous in his lifetime by his observations of and report on SN 1572 (AKA Tycho) was

The report proved---although it took decades and a lot of other evidecne for full acceptance---that Aristotelian cosmology was wrong and the heavens were not changeless.

Credit: Unknown 16th century artist, unknown colorer.

Permission: Public domain at least in USA.

Image linked to Wikipedia.

---









SN 1604 (AKA Kepler) is the last supernova observed in the Milky Way.

---



Caption: Johannes Kepler (1571--1630).

Kepler's greatest claim to fame is his discovery Kepler's 3 laws of planetary motion.

But he also wrote an account of SN 1604 (AKA Kepler) which is the last supernova observed in the Milky Way.

Credit: Unknown artist in 1610.

Permission: Public domain at least in USA.

Image linked to Wikipedia.

---











---

Type Ia SNe

---

---



Caption: The single-degenerate scenario for the formation of Type Ia SNe.

Credit: NASA, ESA, A. Feild (STScI), User:Chrkl.

Image linked to Wikipedia.

Permission: Licensed under the Creative Commons Attribution ShareAlike 2.5.

---















---

Type Ia SNe and Cosmology

---

---



Caption: Timeline of the observable universe.

Maybe TOE can explain the observable universe, but maybe not---it might depend on what one counts as an explanation.

In the diagram, the 2-dimensional time slices represent 3-dimensional space at each epoch.

The slices grow from THE BEGINNING, very rapidly at first and then more slowly.

The growth is the expansion of the universe---which means the growth of space itself which is explainable in the theory of general relativity of Albert Einstein (1879--1955).

You and me, the planets, the stars, and the galaxies are NOT expanding---all of these are gravitationally bound---it's the space between the galaxies and/or galaxy cluster that is growing.

The rapid phase is cosmic inflation from a minute early universe and the slow phase is ordinary cosmology from a hot dense phase (the Big Bang) to the present.

In the slow phase, a hot gas of mainly hydrogen and helium forms.

The gas cools with the expansion of the universe.

Gravity causes the gas to clump into galaxies and stars.

Initial density perturbations after the inflation seed the growth of clumps.

Denser regions grow denser and less dense regions grow less dense.

The Big Bang and evolution since then form a well established theory: it would be surprising if that theory were plain wrong.

Before the Big Bang, everything is much more speculative---one of those deep mysteries of modern physics.

It wouldn't be surprising---well not very surprising anyway---if inflation were just plain wrong.

Theories have different statuses as mentioned above.

Image linked to Wikipedia.

Permission: Public domain at least in USA.

---

---



Caption: "Raisin bread model of the The animation illustrates expansion of the universe.

The basic idea is that all length scales between gravitationally bound systems grow with time by the cosmic scale factor a(t).

Gravitationally bound systems do not expand.

What is the universe is hyperspherical, nothing. In this case, the hypersphere. But there is nothing off the surface.

On the other hand, the observable universe could be a small part of a universe domain which is expanding in and embedded in a multiverse. This scenario is part of the theory of eternal inflation---which is nothing like infernal inflation---but isn't a million light-years removed from the the cosmology of the atomist philosophers Leucippus (first half of 5th century BCE) and Democritus (ca. 460--ca. 370 BCE)

Credit: NASA.

Permission: Public domain at least in USA.

Image linked to Wikipedia.

---

A cartoon Hubble diagram.

Accelerating universe based on CM-455.

---

Core Collapse SNe

---

---



Caption: "This diagram shows a simplified (not to scale) cross-section of a massive, evolved star (with a mass greater than about 8 solar masses) Where the pressure and temperature permit, concentric shells of hydrogen (H), helium (He), carbon (C), neon (Ne), magnesium (Mg), oxygen (O), silicon (Si), plasma are burning inside the star. (The star has developed an onion-layered structure.) The resulting fusion by-products rain down upon the next lower layer, building up the shell below. As a result of silicon burning, an inert plasma iron core is steadily building up at the center. Once this core reaches about the Chandrasekhar mass (about 1.4 solar masses) (see Type II supernova: Core collapse), the iron core can no longer sustain its own mass and it undergoes a collapse. This can result in a core-collapse supernova explosion."

Credit: User:Rursus.

Image linked to Wikipedia.

Permission: Licensed under the Creative Commons Attribution ShareAlike 2.5.

---

---

The Old Gang

---

Just a few images of the old gang of supernova research.

1. David Branch (1942--) as of today.

2. Stirling Colgate (1925--) when youthful.

3. ---

   

   Caption: Bob Kirshner (1949--) in Chile circa 2005.

   Credit: User:Puzhok.

   Permission: Use under GNU Free Documentation License.

   Image linked to Wikipedia.

   ---

4. And some of the new gang and me in Cefalu, 2006.

5. And if it was disproven would they have to take away the 2011 Nobel Prize in Physics for discovering the accelerating universe from my old pals?

   ---

   

   Caption: "Shaw Prize Astronomy 2006."

   Here we see left to right, Saul Perlmutter (1959--), Adam Riess (1969--), and Brian Schmidt (1967--) receiving,NOT the Nobel Prize, but another prize for discovering the accelerating universe. They were, of course, leading members of large groups of people who worked on the discovery.

   They did eventually get 2011 Nobel Prize in Physics.

   Credit: User:Ariess (AKA Adam Riess.

   Permission: Public domain at least in USA.

   Image linked to Wikipedia.

   ---