Lecture 9
6/18/07
Astro 1001
The Sun
• The explanation of how the
Sun is giving the Earth so
much energy from 150
million kilometers away is
very tricky
– Chemical burning, gravitational
contraction not possible
• Nuclear Fusion is the answer
– Sun turns mass into thermal
energy
– Need extreme pressure and
temperatures to begin fusion
The Stable Sun
• Sun is in gravitational
equilibrium
– Outward pressure balanced by
inward gravity
• Gravitational contraction
allowed the Sun to get
started
• The Sun has enough fuel to
burn for about 10 billion
years
– Goes through 4 million tons
of mass per second
Solar Basics
• Sun is made out of
plasma
• Radius is 100x larger
than Earth, and its
300,000x the mass of
Earth
• Sun released 4 x 1026
watts in all directions
– Only a small fraction
reaches Earth
Atmosphere
• The Solar Wind is a stream of
charged particles blown out from
the Sun
• Corona extends millions of
kilometers away from the visible
surface of the Sun
• Chromosphere is where temp
drops to about 10,000K
• Photosphere is the visible surface
of the Sun
– At about 6,000K
– Contains the Sunspots
Solar Interior
• Convection zone is where
convection is occurring
– Convection is the reason that the Sun’s surface
appears granulated
• Radiation zone is where photons
carry the energy
– Millions of Kelvins
• Core has temps of about 15 million
Kelvins
– Pressure is about 200 billion bars
– Only place where nuclear fusion is happening
– Energy takes hundreds of thousands of years
to reach the surface
Alchemy
• Changing the number of protons in the
nucleus of an atom changes the element
• Can split big atoms
– Nuclear fission
– Powers atomic bombs, nuclear power plants
• Can stick together small atoms
– Nuclear fusion
– Powers hydrogen bombs, Stars
Group Work
• The Sun generates energy by fusing hydrogen into
helium. But, some stars shine by turning heavier
elements (like helium) into even heavier elements
(possibly oxygen, carbon). Do temperatures need
to be higher or lower for the fusion of other
elements to occur? Why?
• Challenge: Estimate how much higher or lower
the temps have to be.
Proton-Proton Chain
• Two protons fuse to form a
nucleus consisting of 1
proton and 1 neutron
• Deuterium nuclei collides
with a proton to form
helium-3
• Another neutron must be
added
– Probably due to helium-3
collision
• Helium-4 nucleus is .7% less
massive than 4 hydrogen
nuclei
Feedback
• If nuclear fusion rates varied, so would the Solar
temperature
– Need a feedback mechanism to keep this in check
– Remember the Carbon cycle here on Earth
• Rate of nuclear fusion is very sensitive to
temperature
– Suppose the core temp rose: fusion rate would increase,
pressure would push the core apart, make it larger,
cooling it down
How the Energy Gets Out
• Journey of solar energy
from core to surface takes
hundreds of thousands of
years
• Photons constantly collide
with electrons, get thrown
off in a random direction
• Eventually, solar temp is
low enough so that the
plasma absorbs photons, so
convection takes over
How Do We Know All This?
• Mathematical models provide insight
• Solar vibrations
– Use our friend the Doppler Shift again
– Called helioseismology
• Solar neutrinos
– Lots of them
– Inch of lead stops an X-Ray, but lead would
have to be a lightyear thick to stop a neutrino
Solar Weather
• Sunspots are easy to see
– Slightly cooler than surface
(4000K vs 6000K)
– Due to magnetic fields
– Sunspots occur where the
field lines are knotted
– Prevents convection, so not
as much energy reach that
point
– Solar prominences often
occur near sunspots
More Solar Weather
• Magnetic fields are
believed to sometimes get
so knotted that they snap
and rearrange themselves
– Leads to solar flares
– Magnetic fields are also
wiggled by convection, so
they give energy to corona
– Cause solar wind
The Sun and Humans
• Coronal Mass Ejections
can affect Humans
– A large number of charged
particles are ejected from
the Sun
– Reach the Earth in several
days
– Can cause 100s of
millions of dollars of
damages
Solar Variability
• Sunspot Cycle
– Number of sunspots gradually rises and falls
over 11 years
• Reasons for cycle are not understood fully
– Thought to occur because the Sun rotates
differently at different latitudes
– Causes magnetic fields to gradually twist
Stellar Luminosities
• How bright a star appears depends on its
intrinsic brightness and how far away it is
– Betelgeuse and Procyon appear equally bright,
but Betelgeuse emits 5000x as much energy
• Apparent Brightness is how bright a star
appears to us here on Earth
• Luminosity refers to how much power a star
emits into space
Parallax (again)
• More distant stars have
smaller parallax angles
• A parsec is a parallax
arcsecond
– Leads to a simple
distance formula
Luminosity Range
• Lsun represents one solar luminosity
• Dimmest stars have luminosities about
1/10,000 that of the Sun
• Brightest stars have luminosities 1 million
times that of the Sun
• Dim stars are a lot more common than
bright stars
The Magnitude System
• Devised by Hipparchus
– Brightest stars were first rank, dimmest
were 6th rank
• Each difference of five magnitudes
represents a factor of 100 in
brightness
• A few objects have magnitudes less
than 1
• Absolute magnitude describes stellar
luminosities
– Its what the stars apparent magnitude
would be if it were 10 parsecs away
Stellar Temperatures
• We measure surface temperatures by
looking at the star’s color
• Remember that the peak color that
something emits depends on its
temperature
– Red objects are cooler than blue objects
• Stars have a spectral type based on
their temperature
– From hottest to coolest: OBAFGKM
– Each category is subdivided into
categories (for example, B0, B1, B2…)
Stellar Masses
• Need to use binary systems so that we can
use Newton’s version of Kepler’s 3rd Law
• Use Doppler shift to determine the orbital
velocity
• Use telescopes to determine the greatest
separation
• Stars have masses from about 150x that of
the Sun to about .08x that of the Sun
Group Work
• Run through the mathematical exercise on
page 512, but now Doppler measurements
indicate that the orbital speed of the stars
are v = 50,000 m/s. What is the mass of the
stars?
The Hertzsprung-Russell
Diagram
• Plot surface
temperature
(backwards!) on the
x-axis
• Plot luminosity on
the y axis
Patterns in the HR Diagram
•
•
•
•
Most stars fall on the main sequence
Stars in the upper right are called Supergiants
Stars just below that are called Giants
Stars near the lower left are small and hot, and are
called White Dwarfs
• Luminosity classes are used to further clarify
things
– I are supergiants, III are giants etc
The Main Sequence
• The Main Sequence is when stars fuse
hydrogen into helium in their core
• Stellar masses decrease downward along the
main sequence
• Mass is the most important attribute of a
hydrogen burning star
The Other Stars
• Giants and Supergiants are near the end of
their lives
– Staving off energy crisis
• White dwarfs have run out of fuel
– Earth sized
– Extremely dense
Star Clusters
• A cluster of stars is a
bunch of stars that formed
at the same time and
place
• Globular clusters
– Very dense
– Contains millions of stars
• Open Clusters
– Smaller, closer
Age of Clusters
• Look for the
main sequence
turn off