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The Sun
SESAME Astronomy
Week 4
Our star
• Not special: typical mass, typical
temperature, typical size, typical planetary
system
• about halfway through its 10 billion year
lifespan
Vital statistics
•
Radius: 7 x 105 km = R⊙
• Mass: 2 x 1030 kg = 1 M⊙
•
109 R
332,000 M
Luminosity: 3 x 1026 Watts = 1 L⊙
• Temperature: 5800* K (surface)
•
“day” 25 earth days at equator, 33 earth
days at poles
•
surface gravity: 28 times stronger than
Earth’s
* Depending on which source you choose, you will find a
range of values from 5500-6000
a little info about light
light is a wave
blackbody radiation
•
a hot object glows
•
the hotter the object, the
brighter it glows
•
the hotter the object, the bluer it
glows
Structure
• core - where the
fusion takes place
• not really differentiated
like the planets
(although core is
slowly going from
mostly Hydrogen to
mostly Helium)
fusion
proton-proton chain (pp chain)
•
2 protons get close
•
if one happens to turn into a neutron
at the moment they are close, the
neutron and other proton stick
together forming Deuterium (when
proton turns into neutron, it emits a
positron)
•
Deuterium gets hit by another proton
forming Helium-3 and emitting
energy
•
2 Helium-3 nuclei collide forming a
Helium-4 nucleus and emitting 2
protons
convection
•
heat from bottom makes material just above it expand
and rise
•
this material leaves empty space that is filled from the
sides
•
this material leaves empty space that is filled from above
•
heated material cools when it reaches the top
•
previously heated material is pushed aside by rising
newly heated material from below
•
previously heated, now cooled material that has been
pushed aside sinks back to bottom
•
and the whole thing starts again...
convection
• it happens in the sun
• result is called “granules”
Sun’s magnetic field
•
rotating faster at equator than at poles
•
magnetic field gets pulled along with charged
particles
•
field gets tangled up and pops out in places
prominence - where the magnetic
field sticks out of surface of sun
charged particles
spiral around field
lines
flare - when the field lines break
• particles that had been
trapped are released
into space at high
speeds
• sometimes flare is in
direction of Earth
Sunspots
•
magnetic field prevents hot gas
from rising to surface where it
sticks out of surface
•
places where hot gas can’t rise to
surface cool off a little (still VERY
hot)
•
sunspots only look dark compared
to even brighter surface of sun
•
always come in pairs (why?)
sunspots
sunspot cycle
• know WHAT happens but not WHY
• every 11 years@ or so magnetic field of sun
flips
• just after a flip, few sunspots, sunspots at
middle latitudes
• at solar maximum, lots of spots and close to
equator, lots of prominences and flares*
• then field flips and sunspots few and at
middle latitudes
* we’ll talk about flares on a later slide
@ranges
from 7-15, average is 11 years
The sunspot cycle
solar maximum
•
more sunspots
•
more prominences
•
more flares
solar minimum
• fewer sunspots
• fewer prominences
• fewer flares
• number of sunspots varies from cycle to
cycle
How does the energy get
out?
• random walk - photons (the packets of
energy released during fusion) bounce
around bumping into atoms and nuclei for
30,000-1,000,000* years until they finally
wander out
* depends on who you ask
and what assumptions he/she
made
Summary
•
•
•
•
the sun is an average star
the sun is about halfway through its life
•
Sun converts Hydrogen to Helium in its core (and only in its core) via the
proton-proton chain (fusion)
light takes up to a million years to make its way from the core to the
surface
fun fact: Helium was first discovered in the sun
magnetic field:
•
•
•
light is a wave
objects glow when they’re hot, and how bright and what color depends on
temperature: hotter = brighter and bluer, cooler = dimmer and redder
✴sun’s magnetic field gets tangled up and flips every 11 or so years
✴magnetic field sticks out of surface (in prominences) and causes prominences
and cooler places called sunspots
✦
flares happen when field lines in a prominence breaks