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Understanding the Stars
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How can we learn about the lives of stars,
which last millions to billions of years?
Consider the story of the Ephemera
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Image from: http://www.startunzflutes.com/graphics
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How can we Study the Life Cycles of Stars?
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A star can live for millions to billions of years.
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we will never observe a particular star evolve from birth
to death
The key is that all stars were not born at the same time.
the stars which we see today are at different stages in their
lives
we observe only a brief moment in any one star’s life
by studying large numbers of stars, we get a “snapshot”
of one moment in the history of the stellar community
we can draw conclusions just like we would with human
census data…we do stellar demographics!
Classification of Stars
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Stars were originally classified based on:
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their brightness
their location in the sky
This classification is still reflected in the names of the
brightest stars…those we can see with our eyes:
Order of brightness
within a constellation
Latin Genitive of
the constellation
 Orionis
 Geminorum
Classification of Stars
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The old classification scheme told us little about a
star’s true (physical) nature.
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a star could be very bright because is was very close to
us; not because it was truly bright
two stars in the same constellation might not be close
to each other; one could be much farther away
In the 20th Century, astronomers developed a
more appropriate classification system based on:
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a star’s luminosity
a star’s surface temperature
A star’s stage of life
Summer Triangle
•Which of these three Stars in the Summer Triangle is the brightest (to the
naked eye)
•Which is the most Luminous?
•Which of these three are the closest?
Dark Sky image of Summer Triangle
Don’t always trust your eyes!
•Note that Deneb is a bit larger than the other two
•Its also a bit farther away—2250 light years!
•Vega is 22 light years away—its in the neighborhood
•Note that Altair (about 30 lyrs) is probably
flattened..its rotating very fast!
•Apparent brightness depends on size, temp, and
distance!
Luminosity and Intensity
Our goals for learning:
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What is luminosity and how do we determine it?
How do we measure the distance to nearby stars?
How does the magnitude of a star depend on its
apparent brightness?
Luminosity of Stars
Luminosity – the total amount of power radiated by a star into space.
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Apparent brightness refers to the amount of a
star’s light which reaches us per unit area.
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the farther away a star is, the fainter it appears to
us
how much fainter it gets obeys an inverse square
law
its apparent brightness decreases as the
(distance)2
Apprent brightness is also known as the
Intensity
• The apparent brightness of a star depends on two things:
• How much light is it emitting: luminosity (L) [watts]
• How far away is it: distance (d) [meters]
App Bright = L / 4d
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Apparent Brightness
The Inverse Square Law for Light
What Determines Apparent Brightness?
Measuring Distances to Stars
parallax – apparent wobble of a star due to the Earth’s
orbiting of the Sun
Measuring Distances to Stars
p = parallax angle
d = 1 AU/ p
Gives distance in
Parsecs.
convert p into arcsec
d = 206,265 AU/ p
Measuring Distances to Stars
let’s define
1 parsec  206,265 A.U. = 3.26 light years
d = 1/p
If p is in arcsec and d is in parsecs
A star with a parallax of 1 arcsec is 1 parsec distant
The Brightness of Stars
Astronomers still use an ancient method for measuring
stellar brightness which was proposed by the Greek
astronomer Hipparchus (c. 190 – 120 B.C.)
This scale runs backwards:
The bigger the number, the fainter the star
Brightest stars are #1, next brightest are #2, etc.
The Modern Magnitude System
apparent magnitude
= -2.5 log (app bright)
• brightness of a star as it appears from Earth
• each step in magnitude is 2.5 times in
brightness
absolute magnitude
• the apparent magnitude a star would have
if it were 10 pc away
What good is this?
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If you know apparent brightness, you can find
magnitude.
If you know magnitude, you can use another
relationship to find distance
M – m = 5 – 5log(d)
M= Absolute magitude
M = m when distance is ten parsecs.
An example of how this works!
Deneb has an apparent visual magnitude of
1.26 (see chart of brightest stars at end of your text)
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Deneb has an Absolute visual magnitude of -8.73
(this is about the same brightness as the quarter moon---but at
32.6 light years away!)
Using the weird equation, the distance to deneb can be calculated:
2500 light years (M – m = 5 – 5log(d))
One last obvious question: How did we ever know the Absolute
visual magnitude to Deneb without knowing its distance in the
first place?
16.3 Classifying Stars
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Hypothysis:
the Luminoisty and Abolsute Magnitude of Stars can
be known if we know their Classification.
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There Classification is completely revealed in their
Spectra.
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Comparing the spectra of nearby stars allows us to
test this hypothis.
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The Classification is known as Spectral Type
Spectral type is revealed in the Colors of Stars
Stars come in many
different colors.
The color tells us the
star’s temperature
according to Wien’s Law.
Bluer means hotter!
Spectral Type Classification System
O B A F G K M (L)
Oh Be A Fine Girl/Guy, Kiss Me!
50,000 K
3,000 K
Temperature
Spectral Types of Stars
Spectral Types of Stars
Spectral Types of Stars
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Spectral types are defined by the:
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However, spectral type is not determined by a star’s composition.
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existence of absorption lines belonging to various elements, ions, & molecules
in a star’s spectrum
the relative strengths of these line
all stars are made primarily of Hydrogen & Helium
Spectral type is determined by a star’s surface temperature.
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temperature dictates the energy states of electrons in atoms
temperature dictates the types of ions or molecules which exist
this, in turn, determines the number and relative strengths of absorption lines
in the star’s spectrum
this fact was discovered by Cecilia Payne-Gaposchkin in 1925
Spectroscopic parllax
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Summary:
If we know distance (d) from parallax measurements and..
If we know apparent visual magnitude (m) from photometry or image size
(apparent brightness is also measured in this way).
Then we can calculate Absolute visual magniutde (M). Luminosity is also
measured in this way.
We can obtain spectra and spectral type for all these nearby stars (about
10,000!)
We can make a table that provides Absolute visual magnitude for stars given
their spectral type.
With this table, we can find the distance to distant stars simply by obtaining
their spectra and apparent visual magnitude.
In a strange way, we have extended the parallax measurements out way
beyond the one hundred parsec limit!