Transcript Friday, November 7 - Otterbein University

Stars and HR Diagrams
Nuclear fusion reaction
–
–
–
In essence, 4 hydrogen nuclei combine (fuse) to
form a helium nucleus, plus some byproducts
(actually, a total of 6 nuclei are involved)
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)
Hydrogen fuses to Helium
Start: 4 protons  End: Helium + neutrinos + energy
Hydrogen
fuses to
Helium
The Standard Solar Model (SSM)
• Sun is a gas ball of hydrogen & helium
• Density and temperature increase towards
center
• Very hot & dense core produces all the
energy by hydrogen nuclear fusion
• Energy is released in the form of EM
radiation and particles (neutrinos)
• Energy transport well understood in physics
Standard Solar Model
Hydrostatic Equilibrium
• Two forces compete: gravity (inward) and energy
pressure due to heat generated (outward)
• Stars neither shrink nor expand, they are in
hydrostatic equilibrium, i.e. the forces are equally
strong
Gravity
Heat
Gravity
More Mass means more Energy
• More mass means more gravitational
pressure
• More pressure means higher density,
temperature
• Higher density, temp. means faster reactions
& more reactions per time
• This means more energy is produced
How do we know what happens in
the Sun?
• We can’t “look” into the Sun
• But: come up with theory that explains all the
features of the Sun and predicts new things
• Do more experiments to test predictions
• This lends plausibility to theory
Details
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Radiation Zone and Convection Zone
Chromosphere
Photosphere
Corona
Sunspots
Solar Cycle
Flares & Prominences
Understanding Stars
• “Understanding” in the scientific sense
means coming up with a model that
describes how they “work”:
– Collecting data (Identify the stars)
– Analyzing data (Classify the stars)
– Building a theory (Explain the classes and their
differences)
– Making predictions
– Testing predictions by more observations
Identifying Stars - Star 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)
Classification by Star Properties
• What properties can we measure?
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distance
velocity
temperature
size
luminosity
chemical composition
mass
Classification of the Stars:
Temperature
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
The Key Tool to understanding Stars: the
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, e.g.
K5 for Aldebaran. 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