Irn Bru from the Stars

(or, the stellar creation of the heavy elements)

Dr. Lyndsay Fletcher

University of Glasgow

Formation of the light elements- primordial nucleosynthesis

The hot Big BangT

ime

After nucleosynthesis, the universe contains 1 neutron for every 10 protons (ie hydrogen nuclei).

Neutrons and some of the protons collide at high energy forming deuterium, helium, and a little lithium.

But the universe is cooling rapidly, so collision energy decreases and no heavier elements can be formed.

Formation of the heavy elements-stellar nucleosynthesis

How were the first stars formed?

We don’t know, as we can’t look back in time that far.

However, we think that they formed about 400 million years after the big bang, and later clustered into the first galaxies

image: Robert Hurt, Caltech

Part of the Hubble ‘Deep Field’:

Galaxies in the distant universe ~ 4 billion yrs after the big bang

image: R. Williams, STScI

a globular cluster

The “Sombrero” Galaxy (M104)(somewhat bigger than the Milky Way)

Molecular cloud in Orion - a star-forming region

protostellar cloud

A cloud of gas and dust in

space…

…may be perturbed by external pressures..

e.g. by a shock wave from a supernova

M81 M82

Or by a collision with another galaxy

Visible light image

Visible plus infrared light – showing star formation regions

Star’s own self-gravity takes over, making it contract

It breaks up into smaller clouds

“Thackeray’s Globules”

Each smaller blob continues to shrink

and is probably rotating slowly

Star forming region in Orion

The Sun:

Surface temperature

6000oC

Core temperature

15,000,000 K

nuclear reactor

Core nuclear Fusion

E = mc2

Hot, massive stars

Cool, less massive stars

The Hertzsprung-Russell Diagram

brig

htne

ss

temperature

What happens when the hydrogen fuel runs out?

Star core contracts, and outer layers swell to become a red giant

If the star is massive enough, helium burning might start in the core, producing carbon

- Core He burning- Shell H burningouter layers swell up and drift off into space

The fate of a solar-mass star

This stage is called a planetary nebula

The nebula is mostly hydrogen, helium, plus some carbon and oxygen

White dwarfs: earth-sized stellar relics

2,000,000,000 km 1000 km

Shell burning in a > 4 solar mass star

supergiant phase

After iron, no more energy is available from fusion

Fusion stops, and the star’s core collapses – until the density is so high that protons and electrons are forced together into neutrons

300 km

• Stellar core ‘solidifies’ into neutron lattice.

• Enormous quantities of neutrinos stream outwards.

Both of these cause the collapsing layers above the core to ‘bounce’ outwards, forming a shock front.

The whole process takes about 1s

Formation of elements heavier than iron

In the colossal densities and temperatures in the shock, free neutrons can get close enough to heavy nuclei to be captured.

But too many extra neutrons produces an unstable nucleus. Beta decay changes a neutron into a proton.

Supernova 1987a

The Crab Nebula in Taurus

The blast wave from a star which exploded as a supernova

950 years ago

Stellar remains – a neutron star the diameter of the West End, spinning 33 times per second

The Cosmic Cycle:

Supernova remnants return gas, dust, and both light and heavy elements to the interstellar medium

So, the next round of stellar formation can take place - there have been at least 2 stellar generations here before us!