Supernovae: The Death of Massive Stars

The Dependence on Mass - everything happens faster!

Low- vs High-Mass

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!

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

First, Some Observations 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, After A Millennium (the Crab Nebula)

It’s Still Expanding!

An Animated Version Follow this link: The Crab Nebula expanding The Crab Nebula expanding

Our Nearest Galaxy Neighbours

…as Seen from Chile

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

Supernova 1987A The most recent ‘nearby’ supernova

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 Fuel runs out, reactions stop, temperature and pressure fall, and the core contracts Fuel runs out, reactions stop, temperature and pressure fall, and the core contracts Potential energy is converted to thermal energy: the core gets even hotter than before. Potential energy is converted to thermal energy: the core gets even hotter than before. New fuel supply (the ‘ashes’ of the previous cycle) can now be fused New fuel supply (the ‘ashes’ of the previous cycle) can now be fused This fuel is of ‘lower quality’ (binding energy curve!) and less abundant  reduced potential lifetime This fuel is of ‘lower quality’ (binding energy curve!) and less abundant  reduced potential lifetime Eventually the available fuel no longer provides energy (peak of binding energy curve). The star is doomed to collapse Eventually the available fuel no longer provides energy (peak of binding energy curve). The star is doomed to collapse

Result: Stratification (‘Onion-Skin’)

Two Important Reminders 1. 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 is high enough to do so!

...and the Second 2. 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 Implication We can’t 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 about to explode! There is no “fuse” we can watch, gradually counting down the seconds.

Consider the Sizes!

Consider the Temperatures!

Representative Reactions as the Star Progresses

…and Some More You can see why the temperature needs to be extremely high! The intra-nuclear repulsion is very strong.

Short Reaction Timescales (here, for a star of 8 solar masses) Hydrogen10 million years Hydrogen10 million years Helium1 million years Helium1 million years Carbon300 years Carbon300 years Oxygen200 days Oxygen200 days Silicon2 days (!!!) Silicon2 days (!!!)

…and Then

One Consequence: Enrichment of the ISM Remember that low-mass stars (like the Sun) ‘puff off’ a 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!

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.

How Much Material? How Many Atoms?

How Do We Count? An aquarium pool containing 1 whale and 1 person is 50:50 (whales vs people) by number but 99.9% whales by mass

The Overall Cosmic Abundance

The Proportions For every 90 Hydrogens (total mass = 90), there are 9 Heliums (total mass = 9x4 = 36) Hydrogen is about 70% of the total mass, the rest being mostly Helium. the rest being mostly Helium.

One Detail: Note the “Odd-Even” Effect

Two Important Questions 1. Why do we see the striking odd-even pattern of abundances? (Why is there more Carbon and Oxygen than Nitrogen, for example?) 2. 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.)

Pair Them Up

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

Second Question: Where Do the Heaviest Elements Come From?

It’s a Puzzle! The binding energy curve tells us that the heaviest elements can’t be simply built up by progressive phases of thermonuclear fusion in the core of the star.

To Answer that Question… …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!

So: The Supernova Event We have argued that the pressure support will vanish, and gravity will win. But an outward explosion seems to be the consequence (at least sometimes) Why?