IAL 32: Supernovae

Don't Panic

Sections

Introduction
History of Supernovae
Type Ia SNe
Type Ia SNe and Cosmology
Core Collapse SNe
The Old Gang

Introduction

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.

solar composition

         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
         IAL 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,
         rocky-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,
         rocky-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)
         (Table:  Solar Composition).

See the solar composition displayed in the figure below (local link / general link: solar_composition.html).

The impact of supernova ejecta, especially

core-collapse SNe

ISM

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 1987 feb23. 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/Permission: NASA, ESA, Peter Challis (CfA), Bob Kirshner (1949--) (CfA), 2007 / Public domain.
Image link: 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

NOT

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 Classical 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.

Novae

Aristotelian cosmology

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.

See Supernova videos below (local link / general link: supernova_videos.html):

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.