irn bru from the stars (or, the stellar creation of the heavy elements) dr. lyndsay fletcher...
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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!