Transcript February 2006 - Otterbein University

Welcome to
Starry Monday at Otterbein
Astronomy Lecture Series
-every first Monday of the monthFebruary 6, 2006
Dr. Uwe Trittmann
Today’s Topics
• Lifecycle of Stars
• The Night Sky in February
On the Web
• To learn more about astronomy and physics at
Otterbein, please visit
– http://www.otterbein.edu/dept/PHYS/weitkamp.a
sp (Observatory)
– 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
• Build up larger nuclei Fusion
• Decompose into smaller nuclei Fission
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 February
• Long nights, early observing!
• Winter constellations are up: Orion, Taurus,
Gemini, Auriga, Canis Major & Minor  lots of
deep sky objects!
• Saturn at its brightest
Moon Phases
• Today (Waxing Gibbous, 67%)
• 1/ 12 (Full Moon)
• 1 / 21 (Last Quarter Moon)
• 1 / 27 (New Moon)
• 3 / 6 (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 /
Taurus
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
South
The Winter
Constellations
–
–
–
–
–
Orion
Taurus
Canis Major
Gemini
Canis Minor
The
Winter
Hexagon
•
•
•
•
•
•
Sirius
Procyon
Pollux
Capella
Aldebaran
Rigel
East
• Saturn
near
Praesepe,
an open
star
cluster
Mark your Calendars!
• Next Starry Monday: March 6, 2005, 7 pm
(this is a Monday
• Observing at Prairie Oaks Metro Park:
– Friday, May 5, 9:00 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.