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Transcript
Welcome to
Starry Monday at Otterbein
Astronomy Lecture Series
-every first Monday of the monthNovember 7, 2005
Dr. Uwe Trittmann
Today’s Topics
• Classification of Stars
• The Night Sky in November
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.)
Classification of Stars
• We can classify stars by many categories
–
–
–
–
–
–
–
–
–
–
Name
Position
Constellation
Distance
Color
Temperature
Size
Brightness
Spectra
Features: double stars, variable stars, …
How Stars Got Their Names
• Some have names that go back to ancient times
(e.g. Castor and Pollux, Greek mythology)
• Some were named by Arab astronomers (e.g.
Aldebaran, Algol, etc.)
• Since the 17th century we use a scheme that lists
stars by constellation
– in order of their apparent brightness
– labeled alphabetically in Greek alphabet
– Alpha Centauri is the brightest star in constellation
Centaurus
• Some dim stars have names according to their
place in a catalogue (e.g. Ross 154)
Positions of Stars
The Celestial Sphere
• An imaginary sphere
surrounding the earth,
on which we picture the
stars attached
• Axis through earth’s
north and south pole
goes through celestial
north and south pole
• Earth’s equator
Celestial equator
Celestial Coordinates
Earth: latitude, longitude
Sky:
• declination (dec)
[from equator,+/-90°]
• right ascension (RA)
[from vernal equinox,
0-24h; 6h=90°]
Examples:
• Westerville, OH
40.1°N, 88°W
• Betelgeuse (α Orionis)
dec = 7° 24’
RA = 5h 52m
But: What’s up for you…
Observer
Coordinates
• Horizon – the
plane you stand on
• Zenith – the point
right above you
• Meridian – the
line from North to
Zenith to south
…depends where you are!
• Your local sky –
your view depends on your location on earth
Constellations of Stars
• About 5000 stars visible with naked eye
• About 3500 of them from the northern hemisphere
• Stars that appear to be close are grouped together
into constellations since antiquity
• Officially 88 constellations
(with strict boundaries for classification of objects)
• Names range from mythological (Perseus,
Cassiopeia) to technical (Air Pump, Compass)
Constellations of Stars (cont’d)
Orion as seen at night
Orion as imagined by men
Constellations (cont’d)
Orion “from the side”
Stars in a constellation are not connected in any
real way; they aren’t even close together!
Distances to the Stars
• Parallax can be used out to about
100 light years
• The parsec:
– Distance in parsecs = 1/parallax (in
arc seconds)
– Thus a star with a measured
parallax of 1” is 1 parsec away
– 1 pc is about 3.3 light years
• The nearest star (Proxima
Centauri) is about 1.3 pc or 4.3
lyr away
– Solar system is less than 1/1000 lyr
Our Stellar Neighborhood
Scale Model
• If the Sun = a golf ball, then
–
–
–
–
–
Earth = a grain of sand
The Earth orbits the Sun at a distance of one meter
Proxima Centauri lies 270 kilometers (170 miles) away
Barnard’s Star lies 370 kilometers (230 miles) away
Less than 100 stars lie within 1000 kilometers (600 miles)
• The Universe is almost empty!
• Hipparcos satellite measured distances to nearly 1
million stars in the range of 100 pc
• almost all of the stars in our Galaxy are more distant
Brightness
• A measure of the apparent
brightness
• Logarithmic scale
• Notation: 1m.4 (smaller brighter)
• Originally six groupings
– 1st magnitude the brightest
– 6th magnitude the dimmest
• The modern scale is more complex
• The absolute magnitude is the
apparent magnitude a star would
have at a distance of 10 pc: 2M.8
Electromagnetic Spectrum
Three Things Light Tells Us
• Temperature
– from black body spectrum
• Chemical composition
– from spectral lines
• Radial velocity
– from Doppler shift
Peak frequency
Black Body
Spectrum
(gives away the
temperature)
• All objects - even you emit radiation of all
frequencies, but with
different intensities
Measuring Temperatures
• Find maximal intensity
 Temperature (Wien’s law)
Identify spectral lines
of ionized elements
 Temperature
Wien’s Law
• The peak of the intensity curve will move
with temperature, this is Wien’s law:
λ T = const. = 0.0029 m · K
So: the higher the temperature T, the smaller the
wavelength λ, i.e. the higher the energy of the
electromagnetic wave
Luminosity and Brightness
• Luminosity L is the total power
(energy per unit time) radiated
by the star
• Apparent brightness B is how
bright it appears from Earth
– Determined by the amount of
light per unit area reaching Earth
– B  L / d2
• Just by looking, we cannot tell
if a star is close and dim or far
away and bright
Measuring the Sizes of Stars
• Direct measurement is possible for a few
dozen relatively close, large stars
– Angular size of the disk and known distance
can be used to deduce diameter
Sizes of Stars
• Dwarfs
– Comparable in
size, or smaller
than, the Sun
• Giants
– Up to 100 times
the size of the Sun
• Supergiants
– Up to 1000 times
the size of the Sun
• Note: Temperature
changes!
Star Systems: Binary Stars
• Some stars form binary systems – stars that
orbit one another
– visual binaries
– spectroscopic binaries
– eclipsing binaries
• Beware of optical doubles
– stars that happen to lie along the same line of
sight from Earth
• We can’t determine the mass of an isolated
star, but of a binary star
Visual Binaries
• Members are well separated, distinguishable
Spectroscopic Binaries
• Too distant to resolve the individual stars
• Can be viewed indirectly by observing the
back-and-forth Doppler shifts of their
spectral lines
Eclipsing Binaries (Rare!)
• The orbital plane of the pair almost edge-on to our
line of sight
• We observe periodic changes in the starlight as one
member of the binary passes in front of the other
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
Spectral Lines – Fingerprints of the Elements
• Can use spectra
to identify
elements on
distant objects!
• Different
elements yield
different
emission spectra
Origin of
Spectral
Lines
• 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)
• The energy of the electron depends on orbit
• When an electron jumps from one orbital to
another, it emits (emission line) or absorbs
(absorption line) a photon of a certain energy
• The frequency of emitted or absorbed photon is
related to its energy
E=hf
(h is called Planck’s constant, f is frequency)
Hertzsprung-Russell-Diagram
• 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.
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
Hertzsprung-Russell diagrams
… of the closest stars
…of the brightest stars
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
Stellar Lifetimes
• From the luminosity, we can
determine the rate of energy
release, and thus rate of fuel
consumption
• Given the mass (amount of
fuel to burn) we can obtain
the lifetime
• Large hot blue stars: ~ 20 million
years
• The Sun: 10 billion years
• Small cool red dwarfs: trillions of
years
The hotter, the shorter the
life!
Preview: Stellar Lifecycle
• Next Starry Monday:
–
–
–
–
How stars are born and die
What makes stars “shine”
Planetary nebulae are dead stars!
…and much more
The Night Sky in November
• Back to standard time -> earlier observing!
• Autumn constellations are up: Cassiopeia, Pegasus,
Perseus, Andromeda, Pisces  lots of open star
clusters!
• Mars at opposition
• Saturn is visible later at night
Moon Phases
• Today (New Moon, 36%)
• 11 / 8 (First Quarter Moon)
• 11 / 15 (Full Moon)
• 11 / 23 (Last Quarter Moon)
• 12/ 1 (New Moon)
Today
at
Noon
Sun at
meridian,
i.e.
exactly
south
10 PM
Typical
observing
hour, early
November
Mars
Uranus at
meridian
Neptune
Moon
SouthEast
Plejades
Mars at its
brightest
in Aries
West
The summer
triangle is
still
hanging on
…
Due
North
Big Dipper
points to the
north pole
High up – the
Autumn
Constellations
• W of
Cassiopeia
• Big Square
of Pegasus
• Andromeda
Galaxy
Andromeda
Galaxy
• “PR” Foto
• Actual look
SouthEast
High in the
sky:
Perseus and
Auriga
with Plejades and
the Double
Cluster
SouthWest
• Planets
– Uranus
– Neptune
• Zodiac:
– Capricorn
– Aquarius
Mark your Calendars!
• Next Starry Monday: January 9, 2005, 7 pm
(this is a Monday
• Observing at Prairie Oaks Metro Park:
– Friday, November 18, 7:30 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.
It’s Nuclear
Fusion !
• 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
Could We Use This on Earth?
• Requirements:
– High temperature
– High density
– Very difficult to achieve on Earth!
Nuclear Fission
• Problems: limited fuel supply, dangerous
byproducts, expensive technology, limited
lifetime of power plant due to radiation
The Solar Neutrino Problem
• 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”
Homework: Luminosity and Distance
• Distance and brightness can be used to find
the luminosity:
L  d2 B
• So luminosity and brightness can be used to
find Distance of two stars 1 and 2:
d21 / d22 = L1 / L2
(since B1 = B2 )
Stars II - Lifecycle
The Fundamental Problem
• 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?
Star Formation
• A star’s existence is based on a competition
between gravity (inward) and pressure due to
energy production (outward)
• Stage 1: Contraction of a cold interstellar cloud
– Lasts about 2 million years
– Central temperature about 10 K
– Size ~ tens of parsecs
Star Formation (cont’d)
• Stage 2: Cloud contracts/warms, begins radiating;
almost all radiated energy escapes
– Duration ~ 30,000 years
– Temperature ~ 100 K at center, 10 K at surface
– Size about 100 times that of the solar system
• Stage 3: Cloud becomes dense  opaque to
radiation  radiated energy trapped  core
heats up
– Duration ~ 100,000 years
– Temperature ~ 10,000 K at center, 100 K at surface
– Size ~ the solar system
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
Possible T Tauri Stars
Jets from T Tauri Stars
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
– Duration ~ 30 million years
– Temperature ~ 10 million K at core, 4500 K at surface
– Size ~ slightly larger than the Sun
• 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
Path in the Hertzsprung-Russell
Diagram
• The new-born stars
‘hops’ onto the main
sequence
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
• Temperature ~ 50 million K
(core) to 4000 K (surface)
• Size ~ several Suns
Stage 9: The Red Giant Stage
• Luminosity huge (~ 100 Suns)
• Core contains 25% of the star’s
mass and continues to shrink
• Strong stellar winds eject up to
30% of stars mass from surface
• Duration: 100,000 years
• Temperature: 100  106 K
(core) to 4000 K (surface)
• Size ~ 70 Suns (orbit of
Mercury)
Lifecycle
• Lifecycle of a
main sequence G
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 enough
for atoms to recombine with
electrons, and becomes
opaque; solar radiation
Duration: 100,000 years
pushes it outward from the
Central Temperature: 300  106 K
star
Surface Temperature: 100,000 K
• A planetary nebula is formed
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
• Temperature: 100  106
K at core; 50,000 K at
surface
• 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