Chapter 16:
Analyzing Starlight
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Starlight
stars: bright lights in the sky
like our sun? different? in what ways? why?
Nearest: Proxima Centauri 4.2 LY
100,000 years for fastest spacecraft
red dwarf, 7% of Sun's diameter
1/18,000 as bright as Sun
nearest star of a triple system
observable from southern hemisphere
Alpha Centauri 4.3 LY
40% brighter than Sun (G2 V spectral class)
yellow orange (main sequence)
magnitude -0.3 apparent, +4.4 absolute
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All Stars are Different
colors: blue-white to red
brightness: bright to very faint
Orion: Constellation with many
different star types
Betelgeuse: orange-red
supergiant
Rigel: blue-white supergiant
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Betelgeuse
A red supergiant star
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Key Properties of Stars
Sun's key properties
mass = 333,400 x Earth mass
surface temperature: color yellow  5860 K
composition: spectrum  mostly hydrogen
size = 110 x Earth diameter
luminosity = 3.8 x 1026 watts (spectral type G2V)
Magnitude – 26.7 apparent, +4.8 absolute
Deduce: core “burns” hydrogen, converting it to
helium by thermonuclear fusion.
Stars: how do we infer mass, temperature,
chemical composition, size from observations?
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Key Properties of Stars
Stars: how do we infer mass, temperature, chemical
composition, size from observations?
once we know distance, we can say a lot
Chapter 18: how we measure distance
assume for now we know the distance
Distance Units:
1 AU handy unit for distances in Solar system
light year: distance light travels in one year, 9.46 x 1017 m
New unit: parsec (pc)
1 pc = 3.26 LY is roughly the average distance between
stars
1 kiloparsec = kpc = 1,000 parsecs is roughly the size of
galaxy
natural unit for measuring distances – see Ch. 18
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Star Brightness
Star brightness specified with the magnitude system.
Devised by the Greek astronomer Hipparchus around
150 B.C.E.
brightest stars into the first magnitude class,
next brightest stars into second magnitude class,
and so on, until he had all of the visible stars grouped into
six magnitude classes.
dimmest stars were of sixth magnitude.
brighter objects have smaller magnitudes than fainter
objects!
magnitude system was based on how bright a star
appeared to the unaided eye.
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Extra magnitude?
Some objects go beyond Hipparchus' original bounds of
magnitude 1 to 6.
Very bright objects can have magnitudes of 0 or even negative
numbers.
Very faint objects have magnitudes greater than +6.
Remember: brighter objects have smaller magnitudes than
fainter objects!
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Apparent Magnitude
Apparent brightness of a star observed from the Earth
is called the apparent magnitude.
The apparent magnitude is a measure of the star's flux
received by us.
Examples of apparent magnitudes:








Sun = -26.7,
Moon = -12.6,
Venus = -4.4,
Sirius = -1.4,
Vega = 0.00,
faintest naked eye star = +6.5,
brightest quasar = +12.8,
faintest object = +27 to +28.
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Absolute Magnitude
Measure of star luminosity.
Luminosity is the total amount of energy radiated by the star every
second
If you measure a star's apparent magnitude and know its
absolute magnitude, you can find the star's distance
If you know a star's apparent magnitude and distance, you can
find the star's luminosity
A quantity that depends on the star itself, not on how
far away it is
Provides information about the structure of the star –
this is the real luminosity
More important quantity than the apparent brightness
need the distance to determine the absolute magnitude
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Star Luminosity vs Temperature
stars are luminous
because
they are hot
they are large
or both!
Luminosity of an object = the amount of energy every
square meter produces multiplied by its surface area.
Luminosity = s x T4,
Luminosity of a star increases very quickly with even
slight increases in the temperature.
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Star Luminosity vs Size
Luminosity ~ surface area.
1,000 watt bulb has same luminosity as a row of ten 100 watt bulbs
Luminosity of a bigger star larger than a smaller star at the
same temperature.
From the apparent brightness, temperature, and distance of a
star, one can determine its size.
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Absolute vs
Apparent
Star brighter if
closer
brightness fades
with distance
inverse square
law
if stars were all
the same
brightness than
apparent
luminosity would
measure distance
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Famous stars
Most famous
apparently bright stars
are also intrinsically
bright (luminous).
Magnitudes and Distances of
some stars
(from the precise measurements of the Hipparcos mission)
Can be seen from great
distances away.
Most nearby stars are
intrinsically faint.
Not necessarily
representative of all
stars…
Star
Apparent
Magnitude
Distance
(pc)
Absolute
Magnitude
Luminosity
(relative to
Sun)
Sun
-26.74
4.84813×10-6
4.83
1
Sirius
-1.44
2.6371
1.45
22.5
Arcturus
-0.05
11.25
-0.31
114
Vega
0.03
7.7561
0.58
50.1
Spica
0.98
80.39
-3.55
2250
Barnard's Star
9.54
1.8215
13.24
1/2310
Proxima
Centauri
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1.2948
15.45
1/17700
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Color and Temperature
Stars are dense hot balls of gas
Their spectrum is close to that of a
perfect thermal radiator
Which produces a smooth continuous
spectrum
So called blackbody spectrum.
Color of stars depends on their
temperature:
hotter stars are bluer
cooler stars are redder.
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Color and Temperature
One can observe the stars through different
filters to get an approximate temperature.
Filter allows only a narrow range of
wavelengths (colors) through.
By sampling the star's spectrum at two
different wavelength ranges (“bands”), one can
determine if the spectrum is that a hot, warm,
cool, or cold star.
Hot stars have surface temperatures around
60,000 K while cold stars have surface
temperatures around 3,000 K.
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Star’s Color Temperature
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B-V Color Index
Measure of the temperature based
on apparent color.
Based on two different filters.
A blue (B) filter that only lets a narrow
range of colors or wavelengths
through centered on the blue colors.
A “visible”' (V) filter that only lets the
wavelengths close to the greenyellow band through.
A hot star has a B-V color index close to
0 or negative, while a cool star has a B-V
color index close to 2.0. Other stars are
somewhere in between.
Defined as the difference in magnitude
between the B and V bands.
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Spectra of Stars
primary reason stellar spectra look different is
stars have different temperatures
hydrogen most abundant element – most stars
show hydrogen absorption lines
hottest may not
so hot that hydrogen is completely ionized
coolest stars
hydrogen atoms are all in lowest state
no hydrogen transitions seen
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Spectral Classes
O
B
A
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F
M
G
K
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Spectral
Class
Characteristics
O
B
Ionized Helium and metals; weak Hydrogen
Neutral Helium, ionized metals, stronger Hydrogen
A
F
G
K
M
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Balmer Hydrogen lines dominant, singly-ionized
metals
Hydrogen weaker, neutral and singly-ionized metals
Singly-ionized Calcium most prominent, Hydrogen
weaker, neutral metals
Neutral metals, molecular lines begin to appear
Titanium Oxide molecular lines dominant, neutral
metals
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Doppler Effect
Case (a)
(a)
Object (source) moving
towards observer A at velocity
“v”
Observer “A” sees compressed
wave, I.e. shorter wavelength,
higher frequency.
Observer “B” see stretched
wave, I.e. longer wavelength,
lower frequency.
v
Observer B
Observer A
Source
(b)
Case (b)
Stationary source
Observer “A” and “B” see same
wavelength.
Observer B
Observer A
Source
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Doppler Effect with Stars
Motion of the light source (star) causes the
spectral lines to shift positions.
An object's motion causes a wavelength shift
Dl= lnew - lrest
Depends on speed and direction of moving
object.
Shift given by:
Dl = lrest × Vradial / c,
c is the speed of light,
lrest is the wavelength measured if object is at rest.
Vradial is object speed along the line of sight.
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Red and Blue Shift
If object is moving toward
you, the waves are compressed,
So their wavelength is shorter.
Lines are shifted to shorter (bluer) wavelengths.
This is called a blueshift.
If the object is moving away from you, the waves
are stretched out,
So their wavelength is longer.
The lines are shifted to longer (redder) wavelengths.
This is called a redshift.
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Spectral Shifts
Doppler effect doesn’t affect overall color of an
object unless it is moving at a significant
fraction of the speed of light (VERY fast!).
For an object moving toward us, the red colors
will be shifted to the orange and the nearinfrared will be shifted to the red, etc. All of the
colors shift.
The overall color of the object depends on the
combined intensities of all of the wavelengths
(colors).
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Spectral Shifts
Sun spectrum at 3
speeds (0, 0.01c, 0.1c).
Hydrogen-alpha line (at
656.3nm) is shown.
Objects in our galaxy
move at speeds much
less than 0.01c.
Doppler-shifted
continuous spectrum for
the Sun moving at 0.01c
almost indistinguishable
from the Sun at rest.
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Spectral Shifts (cont’d)
Doppler shift of spectral
lines measurable even
for slow speed.
Astronomers can
detect spectral line
doppler shifts for
speeds as small as
1 km/sec or lower (less
than 3.33410-6 c).
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Proper Motion
Doppler effect
provides speed along
the line of sight.
Most stars move at
an angle to our line of
sight.
We measure this by
watching stars move
over time.
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Doppler Shift
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Summary
A wealth of information is contained in the
spectra of stars.
Astronomers can learn about:
luminosity
surface temperature
composition
radial motion from the doppler shift
rotation from broadening of spectral lines
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