Supernovae: The Death of Massive Stars

Low- vs High-Mass

The Dependence on Mass - everything happens faster!

Counter-Intuitive Behaviour! When the most massive stars run out of fuel, you would expect gravity to win. (They are too massive for Chandrasekhar to save them!) They should – and indeed do – collapse inwards. But the immediate consequence is a spectacular outward explosion – a supernova! Why??

A Colossal Explosion! Material comes off at very high velocities (measured by Doppler shifts), and involves a large fraction of the mass of the star. It can be ejected at speeds of ~10,000 km/sec (but it slows down when it encounters the Interstellar Medium [ISM]) A fantastically vigorous explosion! The dying star becomes as bright as a whole galaxy of many billions of stars.

Supernovae! These super-bright stars were first recognized as a special class by Fritz Zwicky, in the 1930s. (He also foresaw neutron stars and dark matter.) He was quite a character.

Observations of Supernovae A typical big galaxy has about one bright supernova a century The Milky Way is a little ‘overdue’ – the most recent couple were seen in the time of Tycho (1572) and Kepler (1604). There was a spectacular one in 1054 AD.

SN 1054 AD -- A Millennium Later! (the Crab Nebula)

An Animated Version https://vimeo.com/71117055

Our Nearest Galaxy Neighbours

The LMC a small nearby galaxy (only 150,000 l.y. away!)

Supernova 1987A The most recent ‘nearby’ supernova

In a More Remote Galaxy (about 55 million light years away)

…and here a star dies in a very distant galaxy

The Lead-Up to the Explosion The fuel runs out, reactions stop, the temperature and pressure fall, and the core contracts. Potential energy is converted to thermal energy: the core gets even hotter than before. A new fuel supply (the ‘ashes’ of the previous cycle) can now be fused. This continues for fuels beyond Carbon (unlike the sun-like stars that are supported by electron degeneracy and never get hot enough). Each successive fuel is of ‘lower quality’ (binding energy curve!) and less abundant  reduced potential lifetime Eventually the available fuel no longer provides energy (i.e. we are at the peak of binding energy curve). The star is doomed to collapse.

The Binding Energy Curve Again

Along the Way: Stratification (‘Onion-Skin’ Structure)

Two Important Reminders The stratification is NOT because heavy elements settle to the centre. It is merely a consequence of the fact that the progressively heavier elements are created near the centre, where the temperature progressively becomes high enough to do so! The outer envelope of the star is still the pristine material from which it was made (mainly Hydrogen and Helium). This means that before the explosion, the star’s spectrum does not reveal any of the enormous and frantic compositional changes which are occurring deep within it.

One Important Implication… We cannot tell, from its outward appearance, that a particular star is on the verge of using up its last bit of fuel in the core and is about to explode! There is no obvious “fuse” we can watch, gradually counting down the seconds.

The Internal Changes Occur Deep Within a Hugely Expanded Outer Envelope

Consider the Temperatures!

Consider Representative Reactions, as the Star Evolves You can see why the temperature needs to be extremely high to force these nuclei together! The inter-nuclear repulsion is very strong.

Potential Lifetimes (here, for a star of 8 solar masses) Hydrogen 10 million years Helium 1 million years Carbon 300 years Oxygen 200 days Silicon 2 days (!!!)

One Result: Enrichment of the ISM Remember that low-mass stars (like the Sun) gently ‘puff off’a planetary nebula shell that is still largely H and He. By contrast, the massive stars throw out a great deal of newly-generated heavy elements, from the deep interior! That’s where your atoms were created! – the iron in your haemoglobin, the calcium in your bones, the sodium and potassium in your blood,…

Joni Mitchell Says It All (Woodstock) I came upon a child of God, walking down the road I asked him, where are you going? And this he told me He said I'm going down to Yasgur's Farm, Just join in a rock and roll band. Get back to the land and set my soul free. (He said) we are stardust, we are golden, And we got to get ourselves back to the garden.

Can We Understand the Details? What elements will be produced, and in what proportions? How do those calculations agree with the observed amounts in the universe as a whole? (not just in a special localized enmvironment, like the Earth...)

The ‘Cosmic Abundance’ of the Various Elements First, how do we do the calculation? An aquarium pool containing 1 whale and 1 person is 50:50 by number (whales vs people) but 99.9% whales by mass

The Overall Cosmic Abundance For every 90 Hydrogen atoms (a total mass of 90), there are 9 Heliums (with a total mass of 9x4 = 36); and so on for the other elements. Thus Hydrogen is about 70% of the total mass, the rest (~25% or so) being mostly Helium. This justifies my earlier statements that the universe is ~2/3-3/4 Hydrogen, with most of the rest being Helium and just a few percent of ‘everything else.’

One Surprise! There’s a Remarkable “Odd-Even” Effect

Two Important Questions Why do we see the striking odd-even pattern of abundances? (Why is there more Carbon and Oxygen than Nitrogen, for example?) Where do the ‘super-heavy’ elements (like Uranium and Gold) come from? (The binding energy curve suggests that nuclear fusion can routinely make heavy elements up to Iron, but no farther.)

The Binding Energy Curve Again

1. The Odd-Even Effect Explained In all stars, the first round of thermonuclear fusion converts Hydrogen to Helium. By doing so, it pairs up the protons. Subsequent reactions dominantly involve species with even numbers of protons.

2. Where Do the Super-Heavy Elements Come From? The binding energy curve tells us that the heaviest elements (those beyond iron) can’t be simply built up by progressive phases of thermonuclear fusion in the core of the star. To answer the question of their origin, we need to consider the actual mechanism of a supernova. Only then will we know how the very heaviest elements are created (in trace quantities) – and how they get out!