Transcript Measuring stars Part I

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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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
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 of Stars
Luminosity – the total amount of power radiated by a star into space.
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Apparent brightness (I) refers to
the amount of a star’s light which
reaches us per unit area.
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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]
I = 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.
Check out annimation at: http://instruct1.cit.cornell.edu/courses/astro101/java/parallax/parallax.html
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
Spectroscopic parllax
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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.
At the same time, we can obtain spectra and spectral
type for all these nearby stars (about 10,000!)
Now we know M from looking at the spectra!