# Presentation: 127673

## Supernovae

***Padi Boyd***

- Elements 2002 Workshop

**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.

**Notes:**
We’re seeing what goes on inside a star: lighter elements are fused into heavier elements in order for the star to maintain hydrostatic equilibrium against the inward pressure of gravity.  But all that happens in relatively small, isolated regions of space (the stars themselves).  How is that material propagated to other regions of space?  How is it injected in such a way that it can play a role in the birth of new star systems?  And how do we get anything heavier than iron anyway?

## What is a Supernova?	

**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**

**Notes:**
The time scales discussed up to now are incredibly long compared to a human lifetime.  Even in the most massive stars, the hydrogen-burning main sequence phase of its life lasts for millions of years.  But once iron has reached significant densities in a massive star’s core, all leisurely timescales are thrown out the window, and the next, most significant events occur in fractions of a second.  They impact the local universe around them for the rest of the duration of the universe!

## Get Away from that Star!

- 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!

**Notes:**
First thing we need is a way to get elements out of the cores of massive stars, to which they are gravitationally bound.

We also need to understand observational facts like “guest stars” neutron stars, etc.

Would be nice to have a connection to star formation as well, so that seeding the local ISM with elements can be tied directly to the formation of SSs.

We’ve only got a snapshot of the universe now, and we have to put together a coherent model of its evolution.  We’ve only really been looking scientifically for a short period of time compared to the age of the universe.  And as our methods of looking get more sophisticated, we end up with results that take a great deal of cooperation (and a little frustration!) to sort out.

## Reading Break—Arno Penzias, “The Origin of the Elements,” 1978

**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**

**With Robert Wilson**
**1965, Homdel, NJ**

**Notes:**
Since new observations, interpretations, and understandings of astronomical events are continuously woven together to form our modern view of the universe, let’s look at a snapshot in time: the Nobel lecture given by Arno Penzias in 1978.  He talks about some aspects of modern physics at a high level that I haven’t introduced yet, and some that I won’t get to (but other speakers might).  But reading his comments on the composition of the universe give an accurate (and beautiful!) description of the interplay between understanding the early universe, what stars do in their cores, and what new data from an unexplored region of the EM spectrum can tell us about both!

Discuss this if the teachers have had time.

Now we’ll delve into one area that Arno P. discussed in his talk: nucleosynthesis.  We’ve got the benefit of insights and observations that hadn’t happened when he gave his speech: Supernova 1987A and the Type 1a supernova cosmology survey.

## Starting at the Beginning

- 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.

**Notes:**
Fitting supernovae into the puzzle.  Lucky for us, there were some very observant astronomers around about a thousand or so years ago!

## 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."

**Notes:**
There is enough information left over from this time that the records of the Stung dynasty can be used to identify a region in the sky where any leftover record of their “Guest Star” might still reside.

Many believe the anazasi indians created cave drawings to record the same astronomical event.  The phase of the moon and relative location is reasonably close to how the SN would have appeared in 1054 on the night it exploded.

## The Crab Nebula Today

**Notes:**
The Crab Nebula is located 2000 parsecs (6500 light years) from Earth, and is 1.35 parsecs (4.4 light years) in radius. It consists of an expanding filamentary shell of gas. The currently measured expansion velocity of 1400 km/second is sufficient to have carried the shell outward by 1.35 parsecs in the past 943 years, since the ``guest star'' was first noted. 

Plausible hypothesis: What the Chinese astronomers saw 943 years ago was a giant explosion. What we see today is the expanding debris from that explosion. 

## Look at it go!

**Notes:**
Using two images of the Crab nebula separated in time (1978 and ??) we can make accurate measurements of its expansion velocity, and run the movie back in time to determine if it is consistent with having started in 1054.  IT is.

## 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.''

**Notes:**
Tycho Brahe, whose observations are so often mentioned as being a cornerstone of Kepler’s work to develop his laws of planetary motion, was also lucky enough to spot a supernova.  He was 26 years old.  Always a careful recorder of what he saw, his early notes on his new finding are used in much the same way as the Chinese notes on the Crab SN.

## How Did that Happen!?!?!

## Recall the Stellar Interior Balancing Act

- 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.

**Notes:**
This is an important principle which governs the life stages of a star.

This will just be a review of what we have already seen, in case anyone missed its significance the first time.  I won’t spend much time on this slide.

## The End of the Line for Massive Stars

- 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.

**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

**Notes:**
Fusion stops at Iron, and star collapses under its own weight.
The star contains products of the fusion processes.

What happens next?  Gravitational contraction can resume unabetted because there exists no reaction which liberates the energy to counteract the collapse.

## Core Collapse 

- 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!

**Core Collapse**

## Supernova Today! (astronomically speaking!)

***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!

**Notes:**
Again, this is a statistic developed based on the rate we see supernovae in other galaxies, and like all statistics, it does not have much meaning when you are talking about a sample of one.  Just because the average time between Sne is 50 years, doesn’t mean you’re bound to see a supernova in a typical lifetime.

:-(

## 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.

**Notes:**
where the heck did all those elements heavier than iron come from? There are two puzzles about these elements. One is
that the fusion reactions which created them are ENDOTHERMIC, i.e. they soak up energy rather than release it. The second is that these elements have
a large positive charge from all the protons they contain, so it's difficult to get them to stick together and fuse in the first place. The first puzzle is resolved
by having the fusion reactions occur as an energetically unimportant byproduct of something else that's going on. That something else is the release of
free neutrons, which resolves the second problem.

Basically there are times in a star's life when there are reactions which release free neutrons. Because the neutron carries no positive charge, it's easy for it
to get close enough to a nucleus to fuse with it. One place where this happens is in that supernova explosion! Gravity has squeezed protons and electrons
together to make neutrons in the newborn neutron star, and SOME of those neutrons fuse with other nuclei to make very heavy elements which are then
ejected into space in the explosion.

## 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.

**Notes:**
So even though we don’t have fusion creating heavier elements, during the supernova shock expansion, the conditions are right for neutrons to be captured and to create heavier elements up to and probably beyond uranium.

## Cartoon of SN Remnant

**Notes:**
Looking at the system just before and just after the SN.  The remnant is what remains after the explosion has had some time to disperse.

## Supernova Remnants: SN1987A

**a**

**b**

**c**

**d**

**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**

## Chandra’s view of SN Remnamts

**0509-68.7**

**0525-69.6**

**0540-69.3**

**Crab Nebula**

**Kepler’s SN**

**011.2-003**

**Notes:**
Here’s where we really get to test our models of the interiors of stars and the physics of the SN explosion and expansion.  The observational consequences must be decoupled though.
We’re seeing a lot of asymetry as these images from Chandra show.

## Supernova Remnants: Cas A

**Optical**

**X-ray**

## Spectrum of Supernova

## 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!

## Supernova Spectroscopy

**Notes:**
Chandra spectra of SNRs showing why it is so important to be able to do spectroscopy at these high energies.  We’re probing the structure of these stars just before and after the SN explosion and finding a jumble.

## 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 1**

**Time 2**

**Time 3**

**Time 4**

**Notes:**
This instability is important in many areas of physics, and may drive the iron out near the other elements as we saw in the CasA spectral images.

## 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!

## But there is more than one type

## Spectra have a major difference

## 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!

**SN 1994D (Ia)**

- 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**.

- And also, since the **entire** core is obliterated, **ALL** that iron is returned to space!

**Notes:**
At first glimpse an old white dwarf in a binary system may seem like one of the more nondescript objects in the universe, but their role as a reliable standard candle has led to one of the most intriguing very recent results in cosmology—the accelerating universe, and the need for dark energy!

In Type 2 Sne much of the Fe core remains in the compact object and is not recycled as efficiently as the iron from Tyle 1a SN.

## TWO VERY DIFFERENT TYPES OF SUPERNOVAE

- Maximum
- Luminosity	3 x 10E9 Suns   		3 x 10E8 Suns
- Spectrum 	No hydrogen lines 	Hydrogen lines
- 		many heavy elements	Continuum
- Where found        Among 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
- Mechanism	Mass transfer    			Collapse of iron core
- Explosion             Thermonuclear explosion of   Rebound shock from neutron star Mechanism           carbon/oxygen core --> iron    surface; neutrino pressure
- Left behind  	Nothing                                    Neutron star
- Debris                  Mostly iron		      All kinds of elements

## Which Brings us Back to ...

**Notes:**
This brings us back to our life cycle !