set the controls for the heart of the sunfaraday.uwyo.edu/~admyers/astr1050/handouts/set the...
Post on 17-Feb-2021
4 Views
Preview:
TRANSCRIPT
-
Set the Controls for the Heart of the Sun
-
! What is “one solar luminosity”?! What powers the Sun? Why couldn’t the Sun be powered
by combustion (e.g. burning gasoline) or by contraction?! Is fusion the process used in nuclear power plants?! What temperature does the p-p chain require? What are
the main inputs to, and products of, the p-p chain?! From the p-p chain, and the Sun’s luminosity (Energy
output per second) and mass, how can we find the Sun’s age? What is the Sun’s current age and total lifetime?
! How hot is the Sun’s core? How does energy move from the core to the photosphere? How long does this take?
! How do neutrinos prove that nuclear fusion powers the Sun? How long do neutrinos take to escape the Sun?
Learning Objectives
-
All of the world’s nuclear weapons: 3 x 104 megatonsThe Sun releases over 3 million times this every second
The Sun’s Energy Output3.85 x 1026 Watts, but how much is that?
We’ll generally refer to an energy release of 3.85 x 1026 Joules each second (Watts) as: 1L⊙ (1 Solar Luminosity)
-
What Powers the Sun?! The Sun’s total available energy is:
!The Sun’s total Mass (in kg) x The energy that can be released per kg (Energy per kg)
! The Sun’s lifetime in years is then:! total available energy / energy output per year
! We know the Sun’s energy output per year from how much energy hits the Earth (3.85 x 1026 Joules per second) and we know the Sun’s mass from its gravitational influence (1.99 x 1030 kg)
! The Sun’s lifetime in years turns out to be about:
1 x 10-4 x Energy (in Joules) released per kg
-
What Powers the Sun?
-
What Powers the Sun?! If the Sun were powered by combustion (e.g.,
gasoline) it would last a few thousand years! Up until the early 20th Century it was thought that
the Sun might shine due to energy released from gravitational contraction
! This gives a maximum life of about 20 million years!The Sun would contract ~20 meters per year!Some of the Earth’s rocks are older than 100
million years old (some rocks are radiometrically dated to over 4.4 billion years old)
!A new energy source was needed to explain the Sun’s incredible power output
-
Matter-Energy Equivalence ! The key to solar energy was
provided by Einstein’s Theory of Relativity
! It said mass could be converted into energy (and vice-versa)!E = mc2!Tiny amounts of matter can be
converted into tremendous amounts of energy!A single gram of matter could provide enough
energy to serve your household needs for a million years
-
Fusion!A helium atom has two protons and two neutrons!Hydrogen has only one proton
!But, helium is 0.7% less massive than four hydrogen atoms
! If you create 1 helium atom from 4 atoms of hydrogen the left over mass becomes energy!Based on the number of kilograms of mass the
Sun must burn to provide its observed energy output, and 0.7% (0.007) of the hydrogen mass being converted to energy, the Sun burns 600 billion kg of hydrogen each second
-
Fusion!Conclusion...the Sun burns 600 billion kg of
hydrogen each second! this is 2 x 1019 kg
of hydrogen mass burning each year
!We know the mass and composition of the Sun (so we know exactly how much of the mass is currently hydrogen)! The Sun’s total mass is about 2 x 1030 kg! More than a half of this mass is still hydrogen
-
Note: Fusion Not Fission! Merging light nuclei:
fusion!Happens in the Sun!The hydrogen bomb
! Breaking up heavy nuclei: fission!Used in nuclear
power plants!The atom bomb
Fusion
Fission
-
!The Proton-Proton Chain (called the “p-p chain”)!4 hydrogen
atoms fuse!Requires high
temperatures (at least 7 million K)
!The products are a helium atom, energy (in the form of photons) and a particle called the neutrino (more about neutrinos later)
The Proton-Proton (p-p) Chain
Nuclear Fusion in the Sun
-
Solar Fusion and the p-p Chain
-
! The Sun is currently stable
! But it’s made from gas, it’s not solid like the Earth
! So, why doesn’t the Sun collapse under its own gravity?
! Outward pressure due to radiation created by fusion balances the inwards force of gravity
! The Sun is in hydrostatic equilibrium
Why Doesn’t the Sun Collapse?
-
! It is simple (honest) to estimate the force of the Sun’s gravity on itself
! Also easy to estimate the temperature of gases at any pressure
! Using these estimates we find that inside the Sun’s core the temperature is about 15 million K!This is easily high enough to allow the p-p chain
The Solar Interior
-
The Solar Interior! The Sun’s core contains
~30% of the Sun’s mass !about 6 x 1029 kg...but
not quite all of this mass started out as hydrogen
! Burning 2 x 1019 kg of hydrogen each year, this is enough fuel to last about 10 billion years!The Sun is currently about halfway through its
fuel supply (it’s about 5 billion years old)
-
! The layer of the Sun that lies just outside the fusion-driven core
! In the radiative zone, the temperature is too low for nuclear fusion
! Insulates the core, keeps it hot
! Quiet, stable! Energy from fusion in the
core moves out slowly through the radiative zone
The Radiative Zone
-
Inside the Radiative Zone! Photons are created by fusion in the core! They are absorbed by atoms and re-
emitted as lower-energy photons in different directions
! This absorption and emission process is repeated over and over again
! Because of the high density in the radiative zone, it takes hundreds of thousands of years for energy to get from the core of the Sun to the outer layers of the Sun
-
The Convective Zone! Outside the
radiative zone! Transports
energy by convection (i.e. currents of heat) rather than by radiation
! Hot gas rises, cool gas falls! The photosphere is at the very top of the
convective zone; granules are convective gas reaching the top layer of the Sun
! For our purposes convection = boiling
-
Neutrinos! Nuclear fusion in the core of the Sun
also produces a particle called a neutrino! Matter is almost transparent to neutrinos! On average, it would take a block of lead
about one light-year in size to stop one half of a stream of neutrinos
! Roughly 100 billion pass through every square centimeter of you every second!
! They escape the Sun immediately, not in hundreds of thousands of years
-
The Sun in Neutrinos! A long exposure
picture of neutrinos emitted by the Sun
! Another confirmation that nuclear fusion is happening in the Sun’s core
! 500 days of data Credit: R. Svoboda and K. Gordan (LSU)
mailto:svoboda@phlash.phys.lsu.eduhttp://www.phys.lsu.edu/
-
Next Time
The Electromagnetic Spectrum
top related