SUPERNOVAE

The Brightest Lights in the Universe

By Michael Davis

So, exactly what is a supernova anyway?

Main Entry: su·per·no·va Pronunciation: "sü-p&r-'nO-v&Function: nounEtymology: New Latin1 : the explosion of a very large star in which the star may reach a maximum intrinsic luminosity one billion times that of the sun2 : one that explodes into prominence or popularity; also : SUPERSTAR

Merriam-Webster Online Dictionary

Supernovae come in two main types. Astronomers have cleverly named them Type I and Type II supernovae.

What observable difference caused astronomers to divide supernovae into two groups?

•Type I: supernovae DON’T HAVE hydrogen absorption lines in their spectrum.

•Type II: supernovae HAVE hydrogen absorption lines in their spectrum.

This is a small but important difference. More on this later.

Type I supernovae are further subdivided into three types that astronomers have cleverly named types Ia, Ib, and Ic.

What observable differences led astronomers to subdivide type I supernovae into three types?

•Type Ia: no hydrogen lines, no helium lines, strong silicon lines.

•Type Ib: no hydrogen lines, strong helium lines.

•Type Ic: no hydrogen lines, no helium lines, no silicon lines.

So including Type II, there a total of four types of supernovae recognized by astronomers.

But Wait! There’s more. It turns out that Types Ib and Ic are actually just special cases of Type II supernovae. So in reality, there are really only two types of supernovae after all. More on this later.

Supernovae were first classified into four types based on their spectra alone before astronomers actually understood the mechanisms that produce them.

Technology and theory have evolved since then, and we now (think) we know what causes all supernovae, and we now know that there are really only two types.

The reason that there are two different types of supernovae is because there are two different mechanisms that lead to supernovae explosions.

Before we can understand the two different mechanisms that lead to supernova explosions, we first need to understand how stars work. We need to know how different mass stars live, and how they die in order to understand supernovae. So lets review how stars work.

A star is just a big ball of Hydrogen gas. All stars start out made of essentially the same stuff. Mostly just Hydrogen gas with a little Helium and traces of other elements thrown in.

The only thing that separates one star from another and determines how long it will live and how it will die is its mass.

•Low mass stars live a very long time and die quietly without producing supernovae.

•Middle mass stars live shorter lives and may, or may not end in Type Ia supernovae.

•High mass stars live very short lives that almost always end in Type II Supernovae.

No matter their mass, all stars initially produce their energy through the same mechanism; Nuclear Fusion of Hydrogen into Helium.

Low mass stars, < 0.3 Sol, never make it beyond the Hydrogen burning phase and never produce supernovae.

Heavier stars, 0.3 – 3 Sol, move on to the next stages of nuclear fusion. Helium “ash” builds up until sufficient heat and pressure exists to initiate Helium burning which produces Carbon and Oxygen.

Once Helium burning begins, the star swells into a red giant and begins pulsating. Over time, much of the star’s atmosphere is blown off into space by these pulsations forming a planetary nebula and leaving behind the naked Carbon/Oxygen core of the star as a white dwarf.

The white dwarf left behind at the end of the planetary nebula phase has a mass somewhere between 1/3 and 1.4 solar masses and is made almost entirely of Carbon and Oxygen. A newly born white dwarf is quite hot. So hot that it radiates most of its energy in the x-ray and ultra-violet regions of the spectrum. Over hundreds of billions of years the white dwarf cools and will eventually become a black dwarf.

White dwarves contain the mass of an entire star in a sphere only about the size of the Earth. The density and surface gravity are extremely high. The density is so high that white dwarves are made of degenerate matter.

Degenerate matter can’t support itself above 1.4 solar masses.

White dwarves are quite common. The universe is full of them. Globular clusters are full of them, as are galactic halos.

Probably the best known and probably closest white dwarf to Earth is Sirius B, only 8.6 LY away.

Since white dwarves are so common, it’s obvious that some can be found in close binary systems. When the second star in the system enters the red giant phase, it’s possible that the white dwarf can skim off material and increase in mass.

Now we have the potential for a supernova explosion. The degenerate matter of the white dwarf can’t support itself if its mass exceeds 1.4 solar masses. As the white dwarf skims mass away from its companion it gets closer and closer to this limit. Once the limit is exceeded, run-away nuclear fusion of Carbon and Oxygen into Iron begins. The reaction is so violent that the white dwarf is totally destroyed in a colossal explosion. This is a type Ia supernova explosion.

Type Ia supernovae are the most powerful type of supernova explosion. Also, since all type Ia supernovae begin with a 1.4 solar mass Carbon/Oxygen white dwarf, they are all remarkably uniform in spectrum and absolute brightness.

Since all type Ia supernovae are essentially identical, they are used by astronomers as “standard candles” for measuring the distance to distant galaxies, and for measuring the expansion rate of the universe.

Type Ia supernova 1994D in NGC4526 as seen by HST.

Sn 2006 X in M100 a type Ia supernova

The origin of Type II supernovae is different. They originate in the death throws of massive stars.

A star with 10 or more times the mass of the sun will go beyond burning Helium into Carbon and Oxygen. The heat and pressure in the core of a massive star will ignite the Carbon and Oxygen “ash” in the core before it builds up to the critical 1.4 solar mass point. The Carbon and Oxygen will quietly fuse into Neon and Magnesium. Then the Neon and Magnesium will fuse in to Silicon and Sulfur. Eventually the Silicon and Sulfur will build up to the point where it ignites and begins fusing into Iron.

The core of a massive star begins to look like the layers of an onion with all the different shells of fusion.

The “onion” layers in a massive star. (Not to Scale.)

Iron is the end of the road for nuclear fusion within the star. Iron sits at the very top of the Curve of Binding Energy. Elements lighter than Iron can be fused into heavier elements with a release of energy. Elements heavier than Iron can be fissioned into lighter elements with a release of energy. However, there is no nuclear reaction that can get energy out of Iron.

The Iron is truly inert, and simply builds up in the core of the star.

Since the Iron is not producing any energy to support itself against gravity, once the critical 1.4 solar masses of Iron have built up, electron degeneracy can no longer support the core. The core collapses into a neutron star.

In a fraction of a second, the roughly 1000 mile diameter Iron core collapses into a 10 mile diameter neutron star. Protons and electrons in the atoms are squeezed together until they merge to form neutrons and neutrinos. The neutrinos escape the core collapse taking away most of the energy of the collapse with them. The neutrons left behind aren’t compressible, and so the collapse stops.

The newly formed neutron star is actually over-compressed. The inertia of the collapse has compressed the ball of neutrons to a higher density than it wants to be, so it rebounds in a tiny fraction of a second. The rest of the massive star is trying to fall into the void left by collapsing Iron core. The rebounding neutron star slams into in-falling material and drives it back outward with tremendous force.

The core rebound creates a shock wave that begins moving outward through the star tearing it apart. Neutrinos boiling out of the collapsed core (about 10^58 in only a few seconds) impart extra energy to the shock wave, accelerating it outward with even more energy.

When the shock wave reaches the surface of the star it explodes as a Type II supernova. The outer layers of the star are blown into space at a large fraction of the speed of light.

So much energy is available that it drives many nuclear fusion reactions that are not normally possible. Heavy elements beyond Iron on the Periodic Table are created in massive quantities and blown out into the interstellar medium by the force of the explosion, thus enriching the universe in heavy elements.

Sn 1987 A in the LMC a type II supernova. Before

and after pictures.

“One day in 1987, an astronomer by the name of Ian Shelton was making observations of the Tarantula Nebula in the Large Magellanic cloud at Las Campanas Observatory, and he noticed something strange on his image. After seeing this, he did something that astronomers haven't done for some time, he went outside and looked up.” – rochesterastronomy.org

Sn 1987 A peaked at magnitude 2.7, easily visible to the naked eye.

Sn 1987 A was the closest and brightest supernova in 400 years. It rapidly became the most studied object in the sky.

The neutron star left over at the center of a type II supernova explosion is amazingly dense. It packs at least 1.4 times the mass of our sun into a sphere only a few miles across. Everything about neutron stars is extreme.

•The surface gravity is in the Billions of G’s

•The magnetic field is in the Billions of Gauss

•The spin rate can be up to 38000 RPM

•The temperature is 100 Million degrees K.

Being made almost entirely of neutrons, a neutron star is essentially one giant atomic nucleus.

As mentioned earlier, types Ib and Ic supernovae are believed to really be just special cases of type II supernovae. To review:

•Type Ib: no hydrogen lines, strong helium lines.

•Type Ic: no hydrogen lines, no helium lines, no silicon lines.

In type Ib supernovae the outer Hydrogen envelope of the star has been blown away by strong stellar winds before the explosion

In type Ic supernovae, both the Hydrogen and Helium layers have been blown away before the explosion.

Otherwise, types Ib and Ic supernovae are the same as a classic type II supernova.

The debris from supernovae explosions continue expanding at high speed into space for millions of years, creating a large, glowing nebula. The initial glare of the Supernovae will fade, but then often brighten again months or years later as debris moving at high speed away from the center of the explosion slams into clouds of material shed by the progenitor star earlier in its life, or dense clumps of gas in the interstellar medium. These collisions energize the gas and make it glow.

Supernovae also re-brighten due to light echoes. Light from the initial explosion illuminates clouds of dust in an expanding sphere as the light of the explosion moves outward into the universe.

The Veil Nebula is an older supernova remnant.

Over billions of years, the debris from supernovae explosions enriches the universe in heavy elements. All elements heavier than Lithium were created inside stars and spread throughout the universe by supernovae explosions.

The shock waves from supernovae explosions may also provide the catalyst needed to cause the collapse of galactic gas clouds to form new stars. Those new stars will also be enriched in heavy elements by the passage shock wave allowing solid planets like Earth to form in their orbits.

So, as Carl Sagan used to say, “We are all made of star stuff.”

We wouldn’t exist without supernova.