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Let’s review some important things we want
to know about stars…
Given enough time and information, we can figure
out their…
• Brightness - easily observed
• Parallax to measure distance
• Spectral type - can get from the spectrum
• Brightness + Distance = Luminosity
• Temperature - can get from spectrum
• Temperature + distance = Size
• Mass - hard to figure out, but there are binary stars
• Age - exact age is hard, but can estimate
What do you do when you have
data and you don’t know what to do
with it and you don’t understand it?
CLASSIFY!
HOPE:
We just might get to know THE universe better?
Stars can be arranged into categories
based on the features in their spectra…
This is called
“Spectral Classification”
How do we categorize stars?
A few options:
1. by the “strength” (depth) of the absorption lines in their
spectra
2. by their color as determined by their blackbody curve
3. by their temperature and luminosity
• Much of the work in classifying and explaining
stellar spectra and brightness was done by women
at Harvard around the turn of the century.
Harvard Computers (1890)
Annie Jump Cannon
(1863-1941)
• Single-handedly classified more
than 250,000 stellar spectra.
Henrietta Leavitt
(1868-1921)
Stars are classified
by their spectra as
O, B, A, F, G, K, and M
spectral types
• OBAFGKM
• hottest to coolest
• bluish to reddish
• An important sequence to remember:
–Oh Boy, An F Grade Kills Me
The Spectral Sequence
Class
Spectrum
Color
Temperature
O
ionized and neutral helium,
weakened hydrogen
bluish
31,000-49,000 K
B
A
F
neutral helium, stronger
hydrogen
blue-white
10,000-31,000 K
strong hydrogen, ionized
metals
white
7400-10,000 K
weaker hydrogen, ionized
metals
yellowish white
6000-7400 K
G
still weaker hydrogen, ionized
and neutral metals
yellowish
5300-6000 K
K
weak hydrogen, neutral
metals
orange
3900-5300 K
M
little or no hydrogen, neutral
metals, molecules
reddish
2200-3900 K
L
no hydrogen, metallic
hydrides, alkalai metals
red-infrared
1200-2200 K
T
methane bands
infrared
under 1200 K
Eventually, the
connection was made
between the observables
and the theory.
Observable:
• Strength of Hydrogen Absorption Lines
• Blackbody Curve (Color)
Cecilia Payne
Theoretical:
• Using observables to determine things
we can’t measure:
Temperature and Luminosity
Categorizing the stars…
Hertzsprung-Russell (H-R ) Diagram
• done independently by Enjar Hertzsprung
and Henry Norris Russell
• graph of luminosity versus temperature
(or spectral class)
Shematic H-R Diagram
O
B
A F G
K
M
– 10
SUPER GIANTS
106
– 5
104
GIANTS
L/LΘ
BRIGHT
5
102
0
10
10-2
15
10-4
40,000
20,000
10,000
5,000
2,500
Temperature
HOT
COOL
FAINT
Same
the stars aren’t randomly scattered on this graph-temperature,
they form a line!
but much
WHAT IS AMAZING: Stars of different masses fall along
a
brighter than
narrow path in L/T diagram
MS stars
→ Must be
much larger
► Giant Stars
“Red Giants”
“Supergiants”
None of
these “extra”
stars are
Hydrogen
burning!
If a random
star falls on
the Main
Sequence, you
also know that
it’s Hydrogen
burning!
If you measure
the luminosity
and the color
of a star, you
know its
mass!!!
The more massive a star is, the more luminous it is…
But a higher rate of fusion means it’s burning its
fuel faster!
More massive stars are…
Low mass stars have
lifetimes comparable to the
Age of the Universe
High mass stars have very short
lifetimes, and disappear quickly!
•
•
•
•
Hotter
Brighter
Bigger
Shorter-lived
The Mystery of Red Giants and White Dwarfs…
Many of these stars have the same temperature as normal Main
Sequence stars, but they’re much brighter or fainter!
How is this possible???
Same Temperature & Surface Brightness
Hot.
Cool.
Same Luminosity
Cool, but big
Luminous!
If the size of the star changes,
its luminosity changes
Hot, but tiny
Faint.
4
2
L
=
b
x
Area
L  T  4R
The high mass
stars are gone!
After time
passes…
Only long-lived
low mass stars
are left on the
main sequence!
Red Giants:
• Cool, but bright.
• Same temp as some
main sequence stars 
same surface
brightness!
Must be bigger AREA  BIGGER star!
(and thus the name, red giant)
L  T  4R
4
2
First, there was a nebula. Or: last, there was a nebula.
as gravity caused the collapse
Stars are formed by a cloud of gas and
dust that collapsed inward and began to
spin. These clouds are called nebula.
About 30 million years after the cloud
collapsed, its center has reached 15 million
kelvin and has become a protostar. As
stars continue to go through nuclear fusion
from hydrogen gas combining to make
deuterons and then two deuterons making
helium, the star will eventually run out of
hydrogen.
The birth of stars in the M16 Eagle Nebula
and the cycle starts again
RED GIANT PHASE of star’s existance
A star experiences an energy crisis and its core collapses when the star's basic, nonrenewable energy source - hydrogen - is used up. A shell of hydrogen on the edge of
the collapsed core will be compressed and heated. The nuclear fusion of the
hydrogen in the shell will produce a new surge of power that will cause the outer
layers of the star to expand until it has a diameter a hundred times its present value.
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