Transcript January 2006

Welcome to
Starry Monday at Otterbein
Astronomy Lecture Series
-every first Monday of the monthJanuary 9, 2006
Dr. Uwe Trittmann
Today’s Topics
• Lifecycle of Stars
• The Night Sky in January
Feedback!
• Please write down suggestions/your interests on the
note pads provided
• If you would like to hear from us, please leave your
email / address
• To learn more about astronomy and physics at
Otterbein, please visit
– http://www.otterbein.edu/dept/PHYS/weitkamp.asp (Obs.)
– http://www.otterbein.edu/dept/PHYS/ (Physics Dept.)
The Lifecycle of the Stars
Reminder: Hertzsprung-Russell-Diagrams
• Hertzsprung-Russell diagram is luminosity vs.
spectral type (or temperature)
• To obtain a HR diagram:
– get the luminosity. This is your y-coordinate.
– Then take the spectral type as your x-coordinate. This
may look strange, e.g. K5III for Aldebaran. Ignore the
roman numbers ( III means a giant star, V means dwarf
star, etc). First letter is the spectral type: K (one of
OBAFGKM), the arab number (5) is like a second digit
to the spectral type, so K0 is very close to G, K9 is very
close to M.
Reminder: Spectral Classification of the Stars
Class
O
B
A
F
G
K
M
Temperature
30,000 K
20,000 K
10,000 K
8,000 K
6,000 K
4,000 K
3,000 K
Color
blue
bluish
white
white
yellow
orange
red
Examples
Rigel
Vega, Sirius
Canopus
Sun,  Centauri
Arcturus
Betelgeuse
Mnemotechnique: Oh, Be A Fine Girl/Guy, Kiss Me
Constructing a HR-Diagram
• Example: Aldebaran, spectral type K5III,
luminosity = 160 times that of the Sun
L
1000
160
100
Aldebaran
10
1
Sun (G2V)
O B A
F
G
K
M
Type
… 0123456789 0123456789 012345…
The
HertzprungRussell Diagram
• A plot of absolute
luminosity (vertical
scale) against
spectral type or
temperature
(horizontal scale)
• Most stars (90%) lie
in a band known as
the Main Sequence
Mass and the Main Sequence
• The position of a star
in the main sequence
is determined by its
mass
All we need to know
to predict luminosity
and temperature!
• Both radius and
luminosity increase
with mass
The Fundamental Problem in
studying the stellar lifecycle
• We study the subjects of our research for a
tiny fraction of its lifetime
• Sun’s life expectancy ~ 10 billion (1010)
years
• Careful study of the Sun ~ 370 years
• We have studied the Sun for only 1/27
millionth of its lifetime!
Suppose we study human beings…
• Human life
expectancy ~ 75
years
• 1/27 millionth of
this is about 74
seconds
• What can we learn
about people when
allowed to observe
them for no more
than 74 seconds?
Theory and Experiment
• Theory:
– Need a theory for star formation
– Need a theory to understand the energy production in
stars  make prediction how bight stars are when and
for how long in their lifetimes
• Experiment: observe how many stars are where
when and for how long in the Hertzsprung-Russell
diagram
•  Compare prediction and observation
Nuclear
Fusion is the
energy source
of the Stars
• Atoms: electrons orbiting
nuclei
• Chemistry deals only with
electron orbits (electron
exchange glues atoms together to
from molecules)
• Nuclear power comes from
the nucleus
• Nuclei are very small
– If electrons would orbit the
statehouse on I-270, the nucleus
would be a soccer ball in Gov.
Bob Taft’s office
– Nuclei: made out of protons (el.
positive) and neutrons (neutral)
Nuclear fusion reaction
–
–
–
4 hydrogen nuclei combine (fuse) to form a
helium nucleus, plus some byproducts
Mass of products is less than the original
mass
The missing mass is emitted in the form of
energy, according to Einstein’s famous
formulas:
E=
2
mc
(the speed of light is very large, so there
is a lot of energy in even a tiny mass)
Further Reactions – Heavier Elements
Start: 4 + 2 protons  End: Helium nucleus + neutrinos
Hydrogen
fuses to
Helium
Fusion is NOT fission!
• In nuclear fission one splits a large nucleus
into pieces to gain energy
• large nuclei Fission
• small nuclei Fusion
Check: Solar Neutrinos
• We can detect the neutrinos
coming from the fusion reaction
at the core of the Sun
• The results are 1/3 to 1/2 the
predicted value!
• Possible explanations:
1. Models of the solar interior are
incorrect
2. Our understanding of the physics
of neutrinos is incorrect
3. Something is horribly, horribly
wrong with the Sun
• #2 is the answer – neutrinos
“oscillate”
Theory of Star Formation
• A star’s existence is based on a competition
between gravity (inward) and pressure due to
energy production (outward)
Gravity
Heat
Gravity
Star Formation & Lifecycle
• Stage 1: Contraction of a cold interstellar cloud
• Stage 2: Cloud contracts/warms, begins radiating;
almost all radiated energy escapes
• Stage 3: Cloud becomes dense  opaque to radiation
 radiated energy trapped  core heats up
Example: Orion Nebula
• Orion Nebula is a place where stars are being born
Orion Nebula (M42)
Protostellar Evolution
• Stage 4: increasing
temperature at core slows
contraction
– Luminosity about 1000
times that of the sun
– Duration ~ 1 million years
– Temperature ~ 1 million K
at core, 3,000 K at surface
• Still too cool for nuclear
fusion!
– Size ~ orbit of Mercury
The T Tauri Stage
Stage 5 (T Tauri):
• Violent surface activity
• high solar wind blows out
the remaining stellar nebula
– Duration ~ 10 million years
– Temperature ~ 5106 K at
core, 4000 K at surface
• Still too low for nuclear fusion
– Luminosity drops to about 10
 the Sun
– Size ~ 10  the Sun
Path in the Hertzsprung-Russell
Diagram
Stages 1-5
Observational Confirmation
• Preceding the result of
theory and computer
modeling
• Can observe objects in
various stages of
development, but not
the development itself
A Newborn Star
• Stage 6: Temperature and
density at core high
enough to sustain nuclear
fusion
• Stage 7: Main-sequence
star; pressure from nuclear
fusion and gravity are in
balance
– Duration ~ 10 billion years
(much longer than all other
stages combined)
– Temperature ~ 15 million K
at core, 6000 K at surface
– Size ~ Sun
Mass Matters
• Larger masses
– higher surface
temperatures
– higher luminosities
– take less time to form
– have shorter main
sequence lifetimes
• Smaller masses
– lower surface
temperatures
– lower luminosities
– take longer to form
– have longer main
sequence lifetimes
Failed Stars: Brown Dwarfs
• Too small for nuclear fusion to ever begin
– Less than about 0.08 solar masses
• Give off heat from gravitational collapse
• Luminosity ~ a few millionths that of the Sun
Main Sequence Lifetimes
Mass (in solar masses)
Lifetime
10 Suns
10 Million yrs
4 Suns
2 Billion yrs
1 Sun
10 Billion yrs
½ Sun
500 Billion yrs
Luminosity
10,000 Suns
100 Suns
1 Sun
0.01 Sun
Why Do Stars Leave
the Main Sequence?
• Running out of fuel
Stage 8: Hydrogen Shell Burning
• Cooler core  imbalance
between pressure and
gravity  core shrinks
• hydrogen shell generates
energy too fast  outer
layers heat up  star
expands
• Luminosity increases
• Duration ~ 100 million
years
• Size ~ several Suns
Stage 9: The Red Giant Stage
• Luminosity huge (~ 100
Suns)
• Surface Temperature lower
• Core Temperature higher
• Size ~ 70 Suns (orbit of
Mercury)
Lifecycle
• Lifecycle of a
main sequence G
star
• Most time is
spent on the
main-sequence
(normal star)
The Helium Flash and Stage 10
• The core becomes hot and
dense enough to overcome
the barrier to fusing
helium into carbon
• Initial explosion followed
by steady (but rapid)
fusion of helium into
carbon
• Lasts: 50 million years
• Temperature: 200 million
K (core) to 5000 K
(surface)
• Size ~ 10  the Sun
Stage 11
• Helium burning continues
• Carbon “ash” at the core
forms, and the star becomes
a Red Supergiant
•Duration: 10 thousand years
•Central Temperature: 250
million K
•Size > orbit of Mars
Stage 12
• Inner carbon core becomes
“dead” – it is out of fuel
• Some helium and carbon
burning continues in outer
shells
• The outer envelope of the
star becomes cool and
opaque
• solar radiation pushes it
outward from the star
Duration: 100,000 years
Central Temperature: 300  106 K • A planetary nebula is formed
Surface Temperature: 100,000 K
Size: 0.1  Sun
Planetary Nebulae
“Eye of God”
Nebula
“Cat’s Eye”
Nebula
“Wings of the Butterfly” Nebula
The Ring
Nebula
(M57)
“Eskimo”
Nebula
“Stingray”
Nebula
“Ant” Nebula
Stage 13: White Dwarf
• Core radiates only by
stored heat, not by
nuclear reactions
• core continues to cool
and contract
• Size ~ Earth
• Density: a million
times that of Earth – 1
cubic cm has 1000 kg
of mass!
Stage 14: Black Dwarf
• Impossible to see in a telescope
• About the size of Earth
• Temperature very low
 almost no radiation
 black!
Evolution of More Massive Stars
• Gravity is strong enough to
overcome the electron
pressure (Pauli Exclusion
Principle) at the end of the
helium-burning stage
• The core contracts until its
temperature is high enough
to fuse carbon into oxygen
• Elements consumed in core
• new elements form while
previous elements continue
to burn in outer layers
Evolution of More Massive Stars
• At each stage the
temperature increases
 reaction gets faster
• Last stage: fusion of
iron does not release
energy, it absorbs
energy
 cools the core
 “fire extinguisher”
Neutron Core
• The core cools and shrinks
• nuclei and electrons are crushed
together
• protons combine with electrons to
form neutrons
• Ultimately the collapse is halted by
neutron pressure
– Most of the core is composed of
neutrons at this point
• Size ~ few km
• Density ~ 1018 kg/m3; 1 cubic cm
has a mass of 100 million kg!
Manhattan
Formation of the Elements
• Light elements (hydrogen, helium) formed in Big Bang
• Heavier elements formed by nuclear fusion in stars and
thrown into space by supernovae
– Condense into new stars and planets
– Elements heavier than iron form during supernovae explosions
• Evidence:
– Theory predicts the observed elemental abundance in the
universe very well
– Spectra of supernovae show the presence of unstable isotopes
like Nickel-56
– Older globular clusters are deficient in heavy elements
Review:
The life
of Stars
The Night Sky in January
• Long nights, early observing!
• Winter constellations are up: Orion, Taurus,
Gemini, Auriga, Canis Major & Minor  lots of
deep sky objects!
• Saturn at opposition
Moon Phases
• Today (Waxing Gibbous, 80%)
• 1/ 14 (Full Moon)
• 1 / 22 (Last Quarter Moon)
• 1 / 29 (New Moon)
• 2/ 5 (First Quarter Moon)
Today
at
Noon
Sun at
meridian,
i.e.
exactly
south
10 PM
Typical
observing
hour,
early
January
Saturn
Mars
Moon
SouthWest
Plejades
Mars in
Aries
Due
North
Big Dipper
points to the
north pole
West – the
Autumn
Constellations
• W of
Cassiopeia
• Big Square
of Pegasus
• Andromeda
Galaxy
Andromeda
Galaxy
• “PR” Foto
• Actual look
Zenith
High in the
sky:
Perseus and
Auriga
with Plejades and
the Double
Cluster
SouthWest
The Autumn
Constellations
• W of Cassiopeia
• Big Square of
Pegasus
• Andromeda
Galaxy
South
The Winter
Constellations
–
–
–
–
–
Orion
Taurus
Canis Major
Gemini
Canis Minor
Mark your Calendars!
• Next Starry Monday: February 6, 2005, 7 pm
(this is a Monday
• Observing at Prairie Oaks Metro Park:
– Friday, January 20, 6:30 pm
• Web pages:
– http://www.otterbein.edu/dept/PHYS/weitkamp.asp (Obs.)
– http://www.otterbein.edu/dept/PHYS/ (Physics Dept.)
)
Mark your Calendars II
•
•
•
•
Physics Coffee is every Wednesday, 3:30 pm
Open to the public, everyone welcome!
Location: across the hall, Science 256
Free coffee, cookies, etc.