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The Sun: Part 3
and
Measuring the Stars: Part 1
Nuclear Fusion of H -> He in the Sun
Net result:
4 protons
→
4He
+ 2 neutrinos + energy
Mass of end products is less than mass of 4
protons by 0.7%. Mass converted to energy.
600 millions of tons per second fused. Takes
billions of years to convert p's to 4He in Sun's
core. Process sets lifetime of stars.
Hydrostatic Equilibrium: pressure from fusion reactions balances
gravity. Sun is stable.
Fusion as an Energy Source
Can we build fusion reactors on Earth to generate clean (no carbon
dioxide) energy?
Maybe.
2H
JET tokomak
+ 3H → 4He + neutron + energy
Trouble is
1) Confinement of the reaction
2) safely stopping the neutrons
Methods:
1) Magnetic confinement (tokomaks) JET => ITER =>
DEMO
2) Inertial confinement (lasers)
Question 8
The solar neutrino
problem refers to the
fact that astronomers
a) cannot explain how the Sun is stable.
b) detect only one-third the number of
neutrinos expected by theory.
c) cannot detect neutrinos easily.
d) are unable to explain how neutrinos
oscillate between other types.
e) cannot create controlled fusion reactions
on Earth.
Question 8
The solar neutrino
problem refers to the
fact that astronomers
a) cannot explain how the Sun is stable.
b) detect only one-third the number of
neutrinos expected by theory.
c) cannot detect neutrinos easily.
d) are unable to explain how neutrinos
oscillate between other types.
e) cannot create controlled fusion reactions
on Earth.
Further experiments have shown that
solar neutrinos can change into other
types that were not initially detected.
Sunspots
Roughly Earth-sized
Last ~2 months
Usually in pairs
Follow solar rotation
Sunspots
They are darker because they are cooler (4500 K vs. 5800 K).
Related to loops of the Sun's magnetic field.
radiation from hot gas flowing
along magnetic field loop at
limb of Sun.
Filament Ejection Movie
Sunspot numbers vary on a 11 year cycle.
0.1% variation from maximum to minimum
Sun's magnetic field changes direction every 11 years.
Maximum sunspot activity occurs about halfway between
reversals.
Clicker Question:
Besides being darker relative to other parts of
the photosphere, sunspots are characterized by
what quality?
A: They rotate faster than adjacent regions
B: They have stronger magnetic fields than adjacent regions
C: They have much greater density than adjacent regions
D: They have much higher temperature than adjacent regions
Above the photosphere, there is the chromosphere and...
The Corona
Best viewed during eclipses.
T = 106 K
Density = 10-15 g/cm3 only!
We expect X-rays from gas at this temperature.
Yohkoh X-ray satellite
X-ray brightness varies over 11-year Solar Cycle: coronal activity
and sunspot activity go together.
The Solar Wind
At top of corona, typical gas speeds are close to escape speed => Sun
losing gas in a solar wind.
Wind escapes from "coronal holes", seen in X-ray images.
Wind speed 500 km/sec (takes a few days to reach Earth).
106 tons/s lost. But Sun has lost only 0.1% of its mass from solar wind.
Space Weather
Today’s forecast: solar wind velocity = 331.5 km/s
density = 4.3 protons/cm3
Sunspot number: 1
days without a sunspot since:
For update see www.spaceweather.com
List of recent and upcoming Near-miss
encounters and space related news.
Active Regions
Prominences: Loops of gas ejected from surface. Anchored in
sunspot pairs. Last for hours to weeks.
Flares: A more energetic eruption. Lasts for minutes. Less well understood.
Prominences and flares occur most often at maximum of Solar Cycle.
Space weather and solar science
●
Coronal Mass Ejections: solar
science and ultimately
predicting space weather
Solar Probe in 2018
Clicker Question:
What is the source of energy in the sun?
A: fusion of protons into heavier nuclei
B: burning of coal and other hydrocarbons
C: the slow gravitational collapse of the sun
D: nuclear fission of heavy nuclei into lighter elements
Spectral Classes
Strange lettering scheme is a historical accident.
Spectral Class
Surface Temperature
O
B
A
F
G
K
M
30,000 K
20,000 K
10,000 K
7000 K
6000 K
4000 K
3000 K
Examples
Rigel
Vega, Sirius
Sun
Betelgeuse
Further subdivision: BO - B9, GO - G9, etc. GO hotter than G9.
Sun is a G2.
Classification of Stars Through Spectroscopy
Ionized helium. Requires extreme UV
photons. Only hottest stars produce many of these.
Remember: stellar spectra show
black-body radiation and absorption
lines.
Pattern of absorption lines depends on
temperature (mainly) and chemical
composition.
Spectra give most accurate info on
these as well as:
density in atmosphere
gravity at surface
velocity of star towards or from us
Stellar Sizes - Direct Measurement
For a few nearby giant stars we can image them directly using HST or the
VLA. Almost all other stars are too far away
Stellar Sizes - Indirect Method
Almost all stars too far away to measure their radii directly. Need
indirect method. For blackbodies, use Stefan's Law:
Energy radiated per cm2 of area on surface every second  T 4
(T = temperature at surface)
And:
Luminosity = (energy radiated per cm2 per sec) x (area of surface in cm2)
So:
Luminosity  (temperature) 4 x (surface area)
Determine luminosity from apparent brightness and distance, determine
temperature from spectrum (black-body curve or spectral lines), then
find surface area, then find radius (sphere surface area is 4 p R2)
The Wide Range of Stellar Sizes
Clicker Question:
If the temperature of the Sun (at the
photosphere) suddenly doubled from 6000 K
to 12000 K, but the size stayed the same, the
luminosity would:
A: decrease by a factor of 4
B: increase by a factor of 2
C: increase by a factor of 4
D: increase by a factor of 16
Clicker Question:
Suppose two stars (star A and star B) appeared
equally bright but we knew that star A was 10
times further away, what do we know about the
luminosity of star A?
A: The two stars have equal luminosity.
B: Star A is 10 times more luminous than star B.
C: Star A is 100 times more luminous than star B.
D: Star B is 10 times more luminous than star A.
How Massive are Stars?
1. Binary Stars. Orbital period depends on masses of two stars and
their separation.
2. Theory of stellar structure and evolution. Tells how spectrum and
color of star depend on mass.