Download Starlight & Stars - Wayne State University Physics and Astronomy

Analyzing
Starlight
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Image of
stars in the
direction of
the center of
the Milky
Way Galaxy,
taken by the
Hubble Space
Telescope
How do the
stars appear
different?
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Not All Stars are Alike
Stars appear different in
brightness, from very
bright to very faint
color, from red to bluewhite
size
A good constellation for
seeing star colors in the
winter sky is Orion
(the hunter)
Betelgeuse, a red
supergiant star
Rigel, a blue
supergiant star
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Betelgeuse
Brightness of Stars (1)
The total amount of energy at all wavelengths that a
star emits is called its luminosity
Note: this is how much energy the star gives off each
second, NOT how much energy ultimately reaches our
eyes or telescope
The luminosity of a star is perhaps its most important
characteristic
The amount of a star’s energy that actually reaches a
given area each second here on Earth is called the
star’s apparent brightness
If all stars had the same
luminosity, their apparent
brightnesses would tell us
how far they are from us
The inverse-square law of
light propagation: the apparent brightness of a light
source decreases as the square of the distance from it
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Brightness of Stars (2)
The inverse-square law
implies that
a star will appear 4 times
fainter if an observer’s
distance from it is
doubled, 9 times fainter
if the distance is tripled,
etc.
In reality, stars generally
do not have the same
luminosity
In other words, they are
not “standard bulbs”
Consequently, distance
is the among the most difficult quantities to measure in
astronomy
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Stars’ Apparent Magnitudes (1)
A star’s apparent brightness is described using
the magnitude system
The system was devised by the Greek
astronomer Hipparchus around 150 B.C.
He put the brightest stars into the firstmagnitude class, the next brightest stars into
second-magnitude class, and so on, until he
had all of the visible stars grouped into six
magnitude classes
Examples: a star of the 1st magnitude appears
2.5 times brighter than a star of the 2nd
magnitude, whereas a star of the 2nd
magnitude appears 40 times brighter than a
star of the 6th magnitude
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Stars’ Apparent Magnitudes (2)
Thus, the smaller the magnitude, the brighter the
object being observed!
The old magnitude-system was based on how bright a
star appeared to the unaided eye
Today’s magnitude system (based on more accurate
measurements) goes beyond Hipparchus' original
range of magnitudes 1 through 6
Very bright objects can have a magnitude of 0, or even
a negative number
Very faint objects have magnitudes greater than +10
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Stars’ Colors and Temperatures
A star is a ball of dense, hot gas that emits a
continuous spectrum of radiation
similar to the spectrum of blackbody radiation
The most intense color of a star is related to its
surface temperature by Wien’s law
The higher the temperature, the shorter the wavelength
of the most intense color
Thus
Blue colors dominate the light output of very hot stars
Cool stars emit most of their visible radiation at red
wavelengths
Our Sun’s surface temperature is about 6,000 K, with
the dominant color being a slightly greenish yellow
Hottest stars can have surface temperatures of 100,000
K, whereas coolest stars have surface temperatures of
about 2,000 K
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Star’s Color  Star’s Surface Temperature
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Determining Star’s Temperature
To determine the exact color of a star, astronomers
usually observe its brightness through filters
A filter allows only a
narrow range of
wavelengths (colors)
to pass through
Two commonly used
filters are
a blue (B) filter that
lets through only a
narrow band of blue
wavelengths
a “visual” (V) filter
that lets through only colors around the green-yellow band
The colored light transmitted by each filter has its own
brightness, usually expressed in magnitudes
The relative brightness of the transmitted colors can tell if the
star is hot, warm, or cool
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B-V Color Index
A B-V color index is defined as the difference
in magnitude between the B and V bands
A hot star has an
index of around 0
or a negative
number, while a
cool star has an
index close to 2.0
Other stars are
somewhere in
between
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Spectra of Stars
To analyze starlight, one can also use spectroscopy, instead
of filters
In general, the spectra of different stars look different
The primary reason is that stars have different temperatures
Most stars are very similar in composition to the Sun
Hydrogen is the most abundant element in stars
In the hottest stars, the hydrogen atoms are completely
ionized (no longer have their electrons attached) due to the
high temperature and, consequently, they cannot produce
hydrogen absorption lines in the spectra
In the coolest stars, the hydrogen atoms are all in lowest
state and, consequently, hydrogen transitions that can
occur do not produce absorption lines in the visible
spectrum
Only stars with intermediate surface temperatures (not too
hot, not too cool — about 10,000 K) have spectra with
hydrogen lines
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How Absorption Line is Produced
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Spectral Classes
Astronomers sort stars according to the
patterns of lines seen in their spectra into
seven principal spectral classes
From hottest to coldest, the classes are
designated O, B, A, F, G, K, and M
O
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B
A
F
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G
K
M
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Spectral
Class
O
B
A
Ionized helium and metals; hydrogen very weak
Neutral helium, ionized metals; hydrogen stronger
Hydrogen strongest; singly-ionized metals
F
Ionized metals; hydrogen weaker
G
Ionized and neutral metals; hydrogen very weak
K
M
Neutral metals; molecular lines begin to appear
Molecular titanium-oxide dominant, neutral metals
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Characteristics of Spectral Lines
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Spectral Classes L and T
Since 1995, astronomers have discovered objects
cooler than those in class M, but they are not
considered true stars because they are not
massive enough
Objects with masses less than 7.2% of or our Sun’s
mass (0.072 MSun) are not expected to become hot
enough for the nuclear fusion to take place
Those objects are called brown dwarfs
They are very faint and cool, emitting radiation in
the infrared part of the spectrum
The warmer brown dwarfs are assign to spectral
class L, and the cooler ones to spectral class T
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Spectral Classes of Stars
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Spectra of Stars in Different Spectral Classes
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Doppler Effect in Sound Waves
Case (a)
(a)
The source is moving
towards observer A
v
Observer A detects a
compressed wave, and
Observer A
hence a shorter wavelength
(or a higher frequency)
Observer B detects a
stretched wave, and hence
a longer wavelength (or a
(b)
lower frequency)
Observer B
Source
Case (b)
The source is stationary
Observers A and B both see
same wavelength
Observer B
Observer A
Source
Animations
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Doppler Effect in Starlight
The motion of a star causes its
spectral lines to shift positions
The shift depends on its speed
and direction of motion
If the star is moving towards
us, the wavelengths of its light
get shorter
Its spectral lines are shifted toward the shorterwavelength (blue) end of the spectrum
This is, therefore, called a blueshift
If the star is moving away from us, the wavelengths
of its light get longer
Its spectral lines are shifted toward
the longer-wavelength (red) end of
the visible spectrum
This is, thus, called a redshift
Doppler Effect in Stellar Spectra
The Doppler effect doesn’t affect the 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 near-infrared
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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Doppler Effect in
Stellar Rotation
The broadening of
spectral lines
indicates that the
star is rotating
The greater the
broadening, the
greater the speed
of rotation
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