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How are elements created?
What physical processes lead to the evolution of stars?
Can observations of the present state of the universe predict the future outcome of the universe?
Understandings
Guidance:
How can we know how far away stars are?
How can we know how far away stars are?
Luminosity - total power radiated (W)
Apparent brightness - power received per unit area of the detector (B2)
The solar constant (i.e. amount of energy falling on the Earth) is 1.37x103 Wm-2 and the sun’s luminosity is 3.9x1026W.
Calculate the distance between the sun and the Earth.
1 AU 1.5x1011 m
We need to know the Luminosity.
Which star is hotter?
Shape of the curve
If we can measure the wavelength at which the emitted radiation has its maximum intensity, we can calculate the surface temperature of a star.
See what the effect of the atmosphere is
Finding the surface temperature of a star
If we can measure the wavelength at which the emitted radiation has its maximum intensity, we can calculate the surface temperature of a star.
Area under the graph = total Energy radiated
Absorption spectra: light absorbed by a cool gas
Emission spectra: given out by a hot gas
Here is a reminder of the difference between absorption and emission spectra
Three Views Spectrum Demonstrator
Wien’s law tells us the relationship between the surface area of the star, the temperature, and its luminosity
Parallax
Astronomical distances
An arc second is 1/3600 of a degree
1 degree = 3600 arc-seconds > not in data booklet
Limitations
How many metres are there in a pc?
What happens to the parallax angle when a star is far away?
Parallax - limited to stars that are 100pc away (within our galaxy)
Deneb
Vega
Doppler effect for light
Start at 19:25!
source
Speed of light is the same in all frames of reference!
RECAP!
Redshift > moving away
Blueshift > moving towards
RECAP!
How are elements created?
What physical processes lead to the evolution of stars?
Can observations of the present state of the universe predict the future outcome of the universe?
Look at the size tab: At which stage in its life cycle will the Sun be at its largest? |
Look at the light bulb tab: At which stage in its life cycle will the Sun be at its brightest? |
Look at the thermometer tab: At which stage in its lifecycle will the Sun be at its hottest? |
Look at the pie chart tab: In which stage of its life will the Sun spend most of its time? |
Look at the mass tab: What happens to the mass of the Sun as it gets older? |
Red Giant� |
Red Giant� |
Beginning of white dwarf� |
Main sequence, 9000 Myr� |
Stars lose mass gradually by converting hydrogen into helium and heavier elements (mass of parts > mass of product) |
If we know the luminosity and temperature, we can also find the radius!
1.4 solar masses
1.4 -3 solar masses
3 solar masses
Oppenheimer–Volkoff limit
Chandrasekhar limit
“It can take thousands of years for a photon to reach the surface of the sun from its core”
For more detail see https://www.askamathematician.com/2013/08/q-why-does-it-take-thousands-of-years-for-light-to-escape-the-sun/
The p-p cycle
Neutrinos are important because they allow scientists to peek into the interior of the sun and learn about the processes there.
Nuclear fusion in the main sequence
What is a main sequence star?
What nuclear equations describe the fusion processes in a main sequence star?
Fusion in main sequence stars
Main sequence stars like our Sun
Fusion in stars
Fill in the missing bits in the nuclear equations
Fusion in stars
Fill in the missing bits in the nuclear equations
Fusion in stars
Fill in the missing bits in the nuclear equations
This is another way to produce Helium.
All elements in the intermediate stages are used up
This happens in more massive stars.
It is called the CNO cycle.
More massive main sequence stars
CNO cycle
The carbon nucleus has a charge of +6, so the barrier that must be overcome for carbon to fuse is much higher. This requires higher temperatures
This is another way to produce Helium.
Time that stars will spend on main sequence
After about 12 % of the total mass of Hydrogen in the star has been fused the star will no longer be in equilibrium
The ‘lifetime’ of the star as a main sequence star depends on
More massive stars have more concentrated cores at higher temperatures so they have a higher fusion rate.
The HR diagram
Red giant phase
Helium flash
The first element to be produced as it enters the red giant stage is carbon
For stars up to 8 M
Triple alpha process:
Helium fusion in core.
Since the hydrogen shell was the primary contributor to the star's high luminosity, the reduction in its energy output causes the total luminosity to drop
What is the subsequent evolution for these stars?
Stars with an initial mass less than 8 solar masses
Outer layers are ejected (planetary nebula)
Dense core remains (white dwarf)
a white dwarf shines because of the residual thermal energy trapped in its dense core
White dwarf
Thus, a white dwarf shines because of the residual thermal energy trapped in its dense core, which it slowly radiates away over billions of years.
Planetary nebula
Stars more massive than 8M
Formation of Oxygen
Neon
Sodium
Magnesium
Silicon
The instability strip
A star more massive than the sun may go back and forth on the HR diagram several times as shown in the diagram. The star is now in the instability region of the HR diagram.
The delicate balance between radiation and
gravitational pressures has been disturbed; the star
tries to find a new equilibrium state but instead the star
pulsates, i.e. it grows and then shrinks in size.
Iron cannot undergo fusion to release energy, so no outward radiation pressure exists to counteract gravity.
The core collapses under its own gravity, compressing protons and electrons into neutrons.
This creates a dense core. The collapsing gas suddenly rebounds, generating a powerful shockwave.
The outer layers of the star are blasted into space, releasing enormous energy
Fusion reactions
The Sun is powered by the fusion of Hydrogen into Helium. This happens in a series of reactions called the p-p cycle. Write an equation for each step and hence one overall equation for the whole pp cycle
RECAP
Energy released in fusion
Calculate the energy released in these reactions
Δm=5.3 x 10-3 u
E=5 MeV
Mass of proton 1.007276 u
Mass of deuterium 2.014102 u
Mass of tritium 3.016049 u
Mass of He-4 4.002602 u
Mass of He-3 3.016029 u
Mass of neutron 1.008665 u
40 million Kelvin temperature to overcome the “Coulomb barrier”.
RECAP
Calculate the time for which the sun would take to burn all of its hydrogen.
Summary
Nebula→ Main sequence star
A star with an initial mass < 8M ⊙
Red giant → Planetary nebula→ white dwarf
A star with an initial mass > 8M ⊙
Red supergiant→ Supernova→ neutron star or black hole
< 1.4 M☉ | White Dwarfs (gradually cool and fade) |
1.4 - 3 M☉ | Neutron Stars (some become pulsars) |
> 3M☉ | Black Hole from which no light escapes |
Core Mass
All of the hydrogen and most of the helium in the universe were produced at the very earliest moments in the life of the universe.
Elements up to iron are produced in stellar cores.
Other elements are produced in supernovae explosions
Higher Level