oursunthestar Flashcards
Chemical burning
cannot account for the Sun’s luminosity
Gravitational contraction
contradicts Earth’s age
The Sun is not (just) a ball of gas
it is made of plasma
The Sun generates energy via
nuclear fusion reactions
nuclear fusion reactions SOL
Hydrogen (H) is converted into Helium (He) in the core
Mass difference between He & H transformed into energy—>Einstein’s theory: E = mc2
Solar mass will provide enough energy to
shine for 10 billion (b) years
About 5.5b more years to go!
The Sun was born ~4.5b years ago from a
a collapsing gas cloud.
The core became hot & dense enough for nuclear fusion
get very annoying and mathy about sun energy
Ignited fusion produces energy (EFusion) in the Sun’s interior.
Ultimately, most of it is radiated away (EOutputRadiation) & the resulting heat (Efusion–EOutputRadiation) exerts an outward pressure.
This pressure perfectly balances
inward force of gravity. This balance is called gravitational equilibrium (or
hydrostatic equilibrium)
Pressure is largest deep inside the Sun
get very annoying and mathy about sun energy part 2 you nerd
In conclusion, the Sun’s steady behaviour is due tothe SOLAR THERMOSTAT = the complex equilibrium
between:
Outward pressure (due to the difference
between energy generated via fusion and that
radiated away)
The inward push of the gravitational force
Striking a Balance (cont’d)
Composition (weight%): 70% H2, 28% He
+ 2% heavier elements, of which O, C & Fe are the most important
Surface temp.: 5800 K (average) but 4000 K in sunspots
Core temp.: 15×10^6 K = 15mK
Composition of the Sun (and stars in general) is deduced from
spectroscopic measurements.
Notice the presence of black lines in Sun’s spectrum:
absorption lines due the quantized energy levels of the component atoms.
faculae
slightly brighter and hotter areas T ~ 5900K
talk about the layers of the sun, lets go from out to in
photosphere, chromosphere, convection zone, radiation zone, core
Solar Core
Energy generation through nuclear fusion takes place ONLY in the core (just ~10% of total Sun’s mass!)
T = 15 m K, Depth = from centre to 0.25RSun
Density: >100 times of water.
Pressure: 200b times that on Earth’s surface.
Energy produced takes 100,000s years to reach surface!
The solar core is a cosmic crucible in which one chemical element is created from another
.
What is nuclear fusion?
Process by which heavier nuclei are created from combining lighter nuclei. Electromagnetic (EM) force causes nuclei to repel each other. To fuse, nuclei must move very fast to overcome EM force--- > Fusion needs VERY high temperatures & pressures When nuclei touch, nuclear force binds them together.
Hydrogen fusion in the Sun’s core 1
Step 1: 2 protons fuse to form a deuterium nucleus (1 proton + 1 neutron) and emit 1 neutrino (ve) and 1 positron (e+)
- Since a proton is converted into a neutron, total nuclear charge is reduced
- The lost positive charge (for overall balance of charge) is carried off by a positron (antielectron = antimatter particle similar but opposite to a electron)
- Neutrinos are emitted for conservation of lepton
number (nr. of emitted e+ & ve must be the same)
- This step is required twice in the overall reaction
Hydrogen fusion in the Sun’s core 2
A deuterium nucleus fuses with a proton to form the nucleus of a 3He isotope and emit a γ ray photon - This step is also required twice in the overall reaction
Hydrogen fusion in the Sun’s core 3
2 3He nuclei fuse to form a
normal 4He nucleus and release 2 protons
Hydrogen fusion in the Sun’s core - Summary
IN: 6 H
OUT: He, 2 H, 2e+, 2νe , 2γ
Hydrogen fusion in the Sun’s core - Summary 2
The overall reaction needs 6 protons to enter the reaction and
produces 1 He nucleus, neutrinos, positrons and γ radiation, and releases back 2 excess protons.
Effectively 4 H nuclei are converted into 1 He nucleus & energy is released.
Where does the solar energy come from? FUCK
Note that
Mass of He = 99.3% of (Mass of 4 H)
0.7% of mass is lost!
Lost mass converted into energy according to E = mc2.
98% → kinetic energy of particles &
γ-rays → sunlight
2% → neutrinos
600b kg (600
m tons) of H converted to 596m tons of
He every second.
4b kg (4m tons) of matter converted into energy
Negligible since Sun’s mass is 1030 kg (1027 tons = billion billion billion !)
Fusion can also produce other elements if conditions are right (T, p)
The Gradually Brightening Sun
Solar luminosity is stable over the short-term.
But luminosity slowly increases in time over the long-term.
4 H nuclei→ 1 He nucleus
Number of particles in core decreases with time
Core will contract, causing it to heat up
Fusion rate increase to balance higher gravity
New equilibrium reached at higher energy output
Models indicate solar luminosity has increased 30% since itformed 4.5b years ago.
Output increased
(↑) from 2.9×1026 W to 3.8×1026W
Models indicate solar luminosity has increased 30% since itformed 4.5b years ago.
Output increased
(↑) from 2.9×1026 W to 3.8×1026W
But luminosity slowly increases in time over the long-term.
4 H nuclei→ 1 He nucleus
Number of particles in core decreases with time
Core will contract, causing it to heat up
Fusion rate increase to balance higher gravity
New equilibrium reached at higher energy output
Interior Zones D’SOL
T < 8m K, Depth = 0.25 … 0.999 RSun
Energy transported through the interior.
Two Zones d’sol’d’interior
Radiation zone:
Convection zone:
Radiation zone:
energy moves outward primarily due to
radiative diffusion of photons
Convection zone:
energy travels upward by the rising of
hot fluid & falling of cool fluid (convection)
Boundary: yeah i dont know what this is either
T = 2mK Depth = 0.7 RSun
Methods of Energy Transport
Fusion releases most energy in the form of photons.
Deep in the Sun’s interior, plasma is very dense.
A photon moves less than 1 mm before bumping into a particle
‘Deflected’into a new random direction (actually: absorbed by a particle
and immediately re-emitted)
Bounces around in a haphazard way (random walk)—-> VERY slow outward migration known as radiative diffusion ————–> it takes hundreds of thousands of years!
Very high temperature in radiation zone
(~ 10m K)
Very high temperature in radiation zone dit moi plus
It “cools” towards the surface, reaching ~2m K at the boundary with the convection zone Photons move out primarily by randomly bouncing off particles
Plasma in the convection zone has a
has a lower temperature(<2m K)——————->absorbs photons more readily rather than bouncing them around
Bottom of zone heated
Hot gas rises to top (less dense!), cooler gas (denser) sinks to the bottom
Plasma convection transports energy to photosphere
Photosphere
T = 5,800 K, Depth = 400 km
