Supernovae

Lab 9

Let’s go Supernova!

• Stars which are ≥ 8x massive than our Sun end their lives in a most spectacular way; they go supernova!

• First, the star swells into a red supergiant

• Then core yields to gravity and begins shrinking

• As it shrinks, it grows hotter and denser

Sequence of events

• Then a new series of nuclear reactions begin to occur....temporarily halting the collapse of the core

• When the core contains just iron, there is nothing left to fuse and fusion ceases

• In less than a second, the star begins the final phase of gravitational collapse

• The core temperature rises to over 100 billion degrees as the iron atoms are crushed together

Core Collapse!

• The repulsive force between the nuclei overcomes the force of gravity

• This combination, a process called "electron capture", creates a neutron and releases a neutrino

• The neutrinos escape from the core, carrying away energy and further accelerating the collapse, which proceeds in milliseconds as the core detaches from the outer layers of the star

• This is called core collapse!• So the core compresses, but then recoils

Ka-boom!!!

• The energy of the recoil is transferred to the envelope of the star, which then explodes and produces a shock wave

• As the shock wave travels to the star's outer layers, the material is heated, fusing to form new elements and radioactive isotopes

• The shock then propels the matter out into space• The material that is exploded away from the star is now

known as a supernova remnant• All that remains of the original star is a small, super-dense

core composed almost entirely of neutrons -- a neutron star• If the original star was very massive (≥15 or more times the

mass of our Sun), even the neutrons cannot survive the core collapse...and a black hole forms

Implosion to Explosion!

• Even neutrons sometimes fail depending on the mass of the star's core. When the collapse is abruptly stopped by the neutrons, matter bounces off the hard iron core, thus turning the implosion into an explosion: ka-BOOM!!!

Remnant of Kepler's Supernova, SN 1604

Temps of Fusion

• Min temp required for the fusion of Hydrogen is 5 million degrees

• More protons in nuclei require higher temperatures - to fuse Carbon requires a temperature of about 1 billion degrees

• Lighter elements release energy when they fuse and heavier elements release energy when they split

• As iron "ash" begins to accumulate in the core of the star, gravity pulls more and more mass into the area of fusion, which, in turn, goes through all of the steps of fusion: Hydrogen → helium by the proton chain, He→ carbon by the triple α process, C and He combine into oxygen, O fuses into neon, Ne into magnesium, Mg into silicon and Si into iron

The Crab Nebula is an expanding cloud of gas created by the 1054 supernova

Animation of a supernova

• http://heasarc.gsfc.nasa.gov/docs/snr.html

Types of Supernovae

• Two distinct types of supernovae -- those which occur for a single massive star and those which occur because of mass transfer onto a white dwarf in a binary system

• Difference between the two types lies only in what gets the process started toward the explosion

Supernovae Classification based on H

• The presence or absence of a line from hydrogen

• If a supernova's spectrum contains a hydrogen line, it is classified Type II, otherwise it is Type I

A White Dwarf Goes Thermonuclear – Type I

• A white dwarf star in a binary star system will draw material off its companion star if they are close to each other

• Once the in-falling matter from the companion star cause the white dwarf to approach a mass of 1.4 times that of the Sun (the Chandrasekhar limit), the pressure at the center increases so C and O nuclei to start to fuse uncontrollably

• This results in a thermonuclear detonation of the entire star

• Nothing is left behind, except whatever elements were left over from the white dwarf or forged in the supernova blast

• Among the new elements is radioactive nickel, which liberates huge amounts of energy, including visible light

Type II

• A much larger star, however, has enough gravity needed to create T and P to cause the C in the core to fuse once the star contracts

• The cores of these massive stars become layered like onions as progressively heavier atomic nuclei build up at the center

• An outermost layer of H gas, sinks down on a layer of H fusing into He, the He sinks down into a layer of He fusing into C, and the C sinks down to fuse into heavier elements

Type II contd

• These stars go through progressive stages where the core will shrink, atomic nuclei which were previously unfusable start fusing then the core springs back into equilibrium with gravity

• This causes them to be irregular variables - each new burst of fusion pushes elements out of the fusing core into the "stellar envelope“

• This dims the star, and causes gravity to pull mass back into the fusing core and begin the cycle over again

Type II subdivisions

• Type II supernovae can be further classified based on the shape of their light curves into Type II-P and Type II-L

• Type II-P reach a "plateau" in their light curve while II-Ls have a "linear" decrease in their light curve ("linear" in magnitude versus time, or exponential in luminosity versus time)

• This is due to differences in the envelope of the stars. • II-Ps have a large H envelope that traps energy released

in the form of gamma rays and releases it slowly, while II-Ls are believed to have much smaller envelopes converting less of the γ ray energy into visible light

Subdivisions• Subdivisions according to the presence of other lines and the shape of its

light curve• Type I

- No hydrogen Balmer lines – Type Ia

- Si II line at 615.0 nm – Type Ib

- He I line at 587.6 nm – Type Ic– Weak or no He lines

• Type II- Has hydrogen Balmer lines

– Type II-P -Plateau

– Type II-L - Linear

Type Ia

• Observations of Type Ia supernovae reveal a picture of the cosmic environment in the way that the width of tree ring growth indicates the Earth's climatic environment over time

• Unlike the other types of supernovae, Type Ia supernovae are generally found in all types of galaxies, including ellipticals.

Hypernovae

• There has been some speculation that some exceptionally large stars may instead produce a "hypernova" when they die

• Here, the core of a very massive star collapses directly into a black hole and 2 extremely energetic jets of plasma are emitted from its rotational poles at nearly light speed.

• These jets emit intense γ rays

Are We Made of Stardust?

• Yes! (recycled, that is)• The common elements are made through

nuclear fusion in the stable cores of stars• Supernovae are not stable, so they can

make heavy elements beyond iron that require more energy to form.

• These are the elements that make up stars, planets and everything on Earth -- including ourselves